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# RTL Representation

Most of the work of the compiler is done on an intermediate representation called register transfer language. In this language, the instructions to be output are described, pretty much one by one, in an algebraic form that describes what the instruction does.

RTL is inspired by Lisp lists. It has both an internal form, made up of structures that point at other structures, and a textual form that is used in the machine description and in printed debugging dumps. The textual form uses nested parentheses to indicate the pointers in the internal form.

- RTL Objects: Expressions vs vectors vs strings vs integers.
- Accessors: Macros to access expression operands or vector elts.
- Flags: Other flags in an RTL expression.
- Machine Modes: Describing the size and format of a datum.
- Constants: Expressions with constant values.
- Regs and Memory: Expressions representing register contents or memory.
- Arithmetic: Expressions representing arithmetic on other expressions.
- Comparisons: Expressions representing comparison of expressions.
- Bit Fields: Expressions representing bitfields in memory or reg.
- Conversions: Extending, truncating, floating or fixing.
- RTL Declarations: Declaring volatility, constancy, etc.
- Side Effects: Expressions for storing in registers, etc.
- Incdec: Embedded side-effects for autoincrement addressing.
- Assembler: Representing `asm` with operands.
- Insns: Expression types for entire insns.
- Calls: RTL representation of function call insns.
- Sharing: Some expressions are unique; others *must* be copied.
- Reading RTL: Reading textual RTL from a file.

## RTL Object Types

RTL uses five kinds of objects: expressions, integers, wide integers, strings and vectors. Expressions are the most important ones. An RTL expression ("RTX", for short) is a C structure, but it is usually referred to with a pointer; a type that is given the typedef name `rtx`.

An integer is simply an `int`; their written form uses decimal digits. A wide integer is an integral object whose type is `HOST_WIDE_INT` (see section The Configuration File); their written form uses decimal digits.

A string is a sequence of characters. In core it is represented as a `char *` in usual C fashion, and it is written in C syntax as well. However, strings in RTL may never be null. If you write an empty string in a machine description, it is represented in core as a null pointer rather than as a pointer to a null character. In certain contexts, these null pointers instead of strings are valid. Within RTL code, strings are most commonly found inside `symbol_ref` expressions, but they appear in other contexts in the RTL expressions that make up machine descriptions.

A vector contains an arbitrary number of pointers to expressions. The number of elements in the vector is explicitly present in the vector. The written form of a vector consists of square brackets (``[...]'`) surrounding the elements, in sequence and with whitespace separating them. Vectors of length zero are not created; null pointers are used instead.

Expressions are classified by expression codes (also called RTX codes). The expression code is a name defined in ``rtl.def'`, which is also (in upper case) a C enumeration constant. The possible expression codes and their meanings are machine-independent. The code of an RTX can be extracted with the macro `GET_CODE (x)` and altered with `PUT_CODE (x, newcode)`.

The expression code determines how many operands the expression contains, and what kinds of objects they are. In RTL, unlike Lisp, you cannot tell by looking at an operand what kind of object it is. Instead, you must know from its context--from the expression code of the containing expression. For example, in an expression of code `subreg`, the first operand is to be regarded as an expression and the second operand as an integer. In an expression of code `plus`, there are two operands, both of which are to be regarded as expressions. In a `symbol_ref` expression, there is one operand, which is to be regarded as a string.

Expressions are written as parentheses containing the name of the expression type, its flags and machine mode if any, and then the operands of the expression (separated by spaces).

Expression code names in the ``md'` file are written in lower case, but when they appear in C code they are written in upper case. In this manual, they are shown as follows: `const_int`.

In a few contexts a null pointer is valid where an expression is normally wanted. The written form of this is `(nil)`.

## Access to Operands

For each expression type ``rtl.def'` specifies the number of contained objects and their kinds, with four possibilities: ``e'` for expression (actually a pointer to an expression), ``i'` for integer, ``w'` for wide integer, ``s'` for string, and ``E'` for vector of expressions. The sequence of letters for an expression code is called its format. Thus, the format of `subreg` is ``ei'`.

A few other format characters are used occasionally:

`u`
  ``u'` is equivalent to ``e'` except that it is printed differently in debugging dumps. It is used for pointers to insns.
`n`
  ``n'` is equivalent to ``i'` except that it is printed differently in debugging dumps. It is used for the line number or code number of a `note` insn.
`S`
  ``S'` indicates a string which is optional. In the RTL objects in core, ``S'` is equivalent to ``s'`, but when the object is read, from an ``md'` file, the string value of this operand may be omitted. An omitted string is taken to be the null string.
`V`
  ``V'` indicates a vector which is optional. In the RTL objects in core, ``V'` is equivalent to ``E'`, but when the object is read from an ``md'` file, the vector value of this operand may be omitted. An omitted vector is effectively the same as a vector of no elements.
`0`
  ``0'` means a slot whose contents do not fit any normal category. ``0'` slots are not printed at all in dumps, and are often used in special ways by small parts of the compiler.

There are macros to get the number of operands, the format, and the class of an expression code:

`GET_RTX_LENGTH (code)`
  Number of operands of an RTX of code code.
`GET_RTX_FORMAT (code)`
  The format of an RTX of code code, as a C string.
`GET_RTX_CLASS (code)`
  A single character representing the type of RTX operation that code code performs. The following classes are defined:
  `o`
    An RTX code that represents an actual object, such as `reg` or `mem`. `subreg` is not in this class.
  `<`
    An RTX code for a comparison. The codes in this class are `NE`, `EQ`, `LE`, `LT`, `GE`, `GT`, `LEU`, `LTU`, `GEU`, `GTU`.
  `1`
    An RTX code for a unary arithmetic operation, such as `neg`.
  `c`
    An RTX code for a commutative binary operation, other than `NE` and `EQ` (which have class ``<'`).
  `2`
    An RTX code for a noncommutative binary operation, such as `MINUS`.
  `b`
    An RTX code for a bitfield operation, either `ZERO_EXTRACT` or `SIGN_EXTRACT`.
  `3`
    An RTX code for other three input operations, such as `IF_THEN_ELSE`.
  `i`
    An RTX code for a machine insn (`INSN`, `JUMP_INSN`, and `CALL_INSN`).
  `m`
    An RTX code for something that matches in insns, such as `MATCH_DUP`.
  `x`
    All other RTX codes.

Operands of expressions are accessed using the macros `XEXP`, `XINT`, `XWINT` and `XSTR`. Each of these macros takes two arguments: an expression-pointer (RTX) and an operand number (counting from zero). Thus,

    XEXP (x, 2)

accesses operand 2 of expression x, as an expression.

    XINT (x, 2)

accesses the same operand as an integer. `XSTR`, used in the same fashion, would access it as a string.

Any operand can be accessed as an integer, as an expression or as a string. You must choose the correct method of access for the kind of value actually stored in the operand. You would do this based on the expression code of the containing expression. That is also how you would know how many operands there are.

For example, if x is a `subreg` expression, you know that it has two operands which can be correctly accessed as `XEXP (x, 0)` and `XINT (x, 1)`. If you did `XINT (x, 0)`, you would get the address of the expression operand but cast as an integer; that might occasionally be useful, but it would be cleaner to write `(int) XEXP (x, 0)`. `XEXP (x, 1)` would also compile without error, and would return the second, integer operand cast as an expression pointer, which would probably result in a crash when accessed. Nothing stops you from writing `XEXP (x, 28)` either, but this will access memory past the end of the expression with unpredictable results.

Access to operands which are vectors is more complicated. You can use the macro `XVEC` to get the vector-pointer itself, or the macros `XVECEXP` and `XVECLEN` to access the elements and length of a vector.

`XVEC (exp, idx)`
  Access the vector-pointer which is operand number idx in exp.
`XVECLEN (exp, idx)`
  Access the length (number of elements) in the vector which is in operand number idx in exp. This value is an `int`.
`XVECEXP (exp, idx, eltnum)`
  Access element number eltnum in the vector which is in operand number idx in exp. This value is an RTX. It is up to you to make sure that eltnum is not negative and is less than `XVECLEN (exp, idx)`.

All the macros defined in this section expand into lvalues and therefore can be used to assign the operands, lengths and vector elements as well as to access them.

## Flags in an RTL Expression

RTL expressions contain several flags (one-bit bitfields) that are used in certain types of expression. Most often they are accessed with the following macros:

`MEM_VOLATILE_P (x)`
  In `mem` expressions, nonzero for volatile memory references. Stored in the `volatil` field and printed as ``/v'`.
`MEM_IN_STRUCT_P (x)`
  In `mem` expressions, nonzero for reference to an entire structure, union or array, or to a component of one. Zero for references to a scalar variable or through a pointer to a scalar. Stored in the `in_struct` field and printed as ``/s'`.
`REG_LOOP_TEST_P`
  In `reg` expressions, nonzero if this register's entire life is contained in the exit test code for some loop. Stored in the `in_struct` field and printed as ``/s'`.
`REG_USERVAR_P (x)`
  In a `reg`, nonzero if it corresponds to a variable present in the user's source code. Zero for temporaries generated internally by the compiler. Stored in the `volatil` field and printed as ``/v'`.
`REG_FUNCTION_VALUE_P (x)`
  Nonzero in a `reg` if it is the place in which this function's value is going to be returned. (This happens only in a hard register.) Stored in the `integrated` field and printed as ``/i'`. The same hard register may be used also for collecting the values of functions called by this one, but `REG_FUNCTION_VALUE_P` is zero in this kind of use.
`SUBREG_PROMOTED_VAR_P`
  Nonzero in a `subreg` if it was made when accessing an object that was promoted to a wider mode in accord with the `PROMOTED_MODE` machine description macro (see section Storage Layout). In this case, the mode of the `subreg` is the declared mode of the object and the mode of `SUBREG_REG` is the mode of the register that holds the object. Promoted variables are always either sign- or zero-extended to the wider mode on every assignment. Stored in the `in_struct` field and printed as ``/s'`.
`SUBREG_PROMOTED_UNSIGNED_P`
  Nonzero in a `subreg` that has `SUBREG_PROMOTED_VAR_P` nonzero if the object being referenced is kept zero-extended and zero if it is kept sign-extended. Stored in the `unchanging` field and printed as ``/u'`.
`RTX_UNCHANGING_P (x)`
  Nonzero in a `reg` or `mem` if the value is not changed. (This flag is not set for memory references via pointers to constants. Such pointers only guarantee that the object will not be changed explicitly by the current function. The object might be changed by other functions or by aliasing.) Stored in the `unchanging` field and printed as ``/u'`.
`RTX_INTEGRATED_P (insn)`
  Nonzero in an insn if it resulted from an in-line function call. Stored in the `integrated` field and printed as ``/i'`. This may be deleted; nothing currently depends on it.
`SYMBOL_REF_USED (x)`
  In a `symbol_ref`, indicates that x has been used. This is normally only used to ensure that x is only declared external once. Stored in the `used` field.
`SYMBOL_REF_FLAG (x)`
  In a `symbol_ref`, this is used as a flag for machine-specific purposes. Stored in the `volatil` field and printed as ``/v'`.
`LABEL_OUTSIDE_LOOP_P`
  In `label_ref` expressions, nonzero if this is a reference to a label that is outside the innermost loop containing the reference to the label. Stored in the `in_struct` field and printed as ``/s'`.
`INSN_DELETED_P (insn)`
  In an insn, nonzero if the insn has been deleted. Stored in the `volatil` field and printed as ``/v'`.
`INSN_ANNULLED_BRANCH_P (insn)`
  In an `insn` in the delay slot of a branch insn, indicates that an annulling branch should be used. See the discussion under `sequence` below. Stored in the `unchanging` field and printed as ``/u'`.
`INSN_FROM_TARGET_P (insn)`
  In an `insn` in a delay slot of a branch, indicates that the insn is from the target of the branch. If the branch insn has `INSN_ANNULLED_BRANCH_P` set, this insn should only be executed if the branch is taken. For annulled branches with this bit clear, the insn should be executed only if the branch is not taken. Stored in the `in_struct` field and printed as ``/s'`.
`CONSTANT_POOL_ADDRESS_P (x)`
  Nonzero in a `symbol_ref` if it refers to part of the current function's "constants pool". These are addresses close to the beginning of the function, and GNU CC assumes they can be addressed directly (perhaps with the help of base registers). Stored in the `unchanging` field and printed as ``/u'`.
`CONST_CALL_P (x)`
  In a `call_insn`, indicates that the insn represents a call to a const function. Stored in the `unchanging` field and printed as ``/u'`.
`LABEL_PRESERVE_P (x)`
  In a `code_label`, indicates that the label can never be deleted. Labels referenced by a non-local goto will have this bit set. Stored in the `in_struct` field and printed as ``/s'`.
`SCHED_GROUP_P (insn)`
  During instruction scheduling, in an insn, indicates that the previous insn must be scheduled together with this insn. This is used to ensure that certain groups of instructions will not be split up by the instruction scheduling pass, for example, `use` insns before a `call_insn` may not be separated from the `call_insn`. Stored in the `in_struct` field and printed as ``/s'`.

These are the fields which the above macros refer to:

`used`
  Normally, this flag is used only momentarily, at the end of RTL generation for a function, to count the number of times an expression appears in insns. Expressions that appear more than once are copied, according to the rules for shared structure (see section Structure Sharing Assumptions). In a `symbol_ref`, it indicates that an external declaration for the symbol has already been written. In a `reg`, it is used by the leaf register renumbering code to ensure that each register is only renumbered once.
`volatil`
  This flag is used in `mem`, `symbol_ref` and `reg` expressions and in insns. In RTL dump files, it is printed as ``/v'`. In a `mem` expression, it is 1 if the memory reference is volatile. Volatile memory references may not be deleted, reordered or combined. In a `symbol_ref` expression, it is used for machine-specific purposes. In a `reg` expression, it is 1 if the value is a user-level variable. 0 indicates an internal compiler temporary. In an insn, 1 means the insn has been deleted.
`in_struct`
  In `mem` expressions, it is 1 if the memory datum referred to is all or part of a structure or array; 0 if it is (or might be) a scalar variable. A reference through a C pointer has 0 because the pointer might point to a scalar variable. This information allows the compiler to determine something about possible cases of aliasing. In an insn in the delay slot of a branch, 1 means that this insn is from the target of the branch. During instruction scheduling, in an insn, 1 means that this insn must be scheduled as part of a group together with the previous insn. In `reg` expressions, it is 1 if the register has its entire life contained within the test expression of some loop. In `subreg` expressions, 1 means that the `subreg` is accessing an object that has had its mode promoted from a wider mode. In `label_ref` expressions, 1 means that the referenced label is outside the innermost loop containing the insn in which the `label_ref` was found. In `code_label` expressions, it is 1 if the label may never be deleted. This is used for labels which are the target of non-local gotos. In an RTL dump, this flag is represented as ``/s'`.
`unchanging`
  In `reg` and `mem` expressions, 1 means that the value of the expression never changes. In `subreg` expressions, it is 1 if the `subreg` references an unsigned object whose mode has been promoted to a wider mode. In an insn, 1 means that this is an annulling branch. In a `symbol_ref` expression, 1 means that this symbol addresses something in the per-function constants pool. In a `call_insn`, 1 means that this instruction is a call to a const function. In an RTL dump, this flag is represented as ``/u'`.
`integrated`
  In some kinds of expressions, including insns, this flag means the rtl was produced by procedure integration. In a `reg` expression, this flag indicates the register containing the value to be returned by the current function. On machines that pass parameters in registers, the same register number may be used for parameters as well, but this flag is not set on such uses.

## Machine Modes

A machine mode describes a size of data object and the representation used for it. In the C code, machine modes are represented by an enumeration type, `enum machine_mode`, defined in ``machmode.def'`. Each RTL expression has room for a machine mode and so do certain kinds of tree expressions (declarations and types, to be precise).

In debugging dumps and machine descriptions, the machine mode of an RTL expression is written after the expression code with a colon to separate them. The letters ``mode'` which appear at the end of each machine mode name are omitted. For example, `(reg:SI 38)` is a `reg` expression with machine mode `SImode`. If the mode is `VOIDmode`, it is not written at all.

Here is a table of machine modes. The term "byte" below refers to an object of `BITS_PER_UNIT` bits (see section Storage Layout).

`QImode`
  "Quarter-Integer" mode represents a single byte treated as an integer.
`HImode`
  "Half-Integer" mode represents a two-byte integer.
`PSImode`
  "Partial Single Integer" mode represents an integer which occupies four bytes but which doesn't really use all four. On some machines, this is the right mode to use for pointers.
`SImode`
  "Single Integer" mode represents a four-byte integer.
`PDImode`
  "Partial Double Integer" mode represents an integer which occupies eight bytes but which doesn't really use all eight. On some machines, this is the right mode to use for certain pointers.
`DImode`
  "Double Integer" mode represents an eight-byte integer.
`TImode`
  "Tetra Integer" (?) mode represents a sixteen-byte integer.
`SFmode`
  "Single Floating" mode represents a single-precision (four byte) floating point number.
`DFmode`
  "Double Floating" mode represents a double-precision (eight byte) floating point number.
`XFmode`
  "Extended Floating" mode represents a triple-precision (twelve byte) floating point number. This mode is used for IEEE extended floating point.
`TFmode`
  "Tetra Floating" mode represents a quadruple-precision (sixteen byte) floating point number.
`CCmode`
  "Condition Code" mode represents the value of a condition code, which is a machine-specific set of bits used to represent the result of a comparison operation. Other machine-specific modes may also be used for the condition code. These modes are not used on machines that use `cc0` (see see section Condition Code Status).
`BLKmode`
  "Block" mode represents values that are aggregates to which none of the other modes apply. In RTL, only memory references can have this mode, and only if they appear in string-move or vector instructions. On machines which have no such instructions, `BLKmode` will not appear in RTL.
`VOIDmode`
  Void mode means the absence of a mode or an unspecified mode. For example, RTL expressions of code `const_int` have mode `VOIDmode` because they can be taken to have whatever mode the context requires. In debugging dumps of RTL, `VOIDmode` is expressed by the absence of any mode.
`SCmode, DCmode, XCmode, TCmode`
  These modes stand for a complex number represented as a pair of floating point values. The floating point values are in `SFmode`, `DFmode`, `XFmode`, and `TFmode`, respectively.
`CQImode, CHImode, CSImode, CDImode, CTImode, COImode`
  These modes stand for a complex number represented as a pair of integer values. The integer values are in `QImode`, `HImode`, `SImode`, `DImode`, `TImode`, and `OImode`, respectively.

The machine description defines `Pmode` as a C macro which expands into the machine mode used for addresses. Normally this is the mode whose size is `BITS_PER_WORD`, `SImode` on 32-bit machines.

The only modes which a machine description must support are `QImode`, and the modes corresponding to `BITS_PER_WORD`, `FLOAT_TYPE_SIZE` and `DOUBLE_TYPE_SIZE`. The compiler will attempt to use `DImode` for 8-byte structures and unions, but this can be prevented by overriding the definition of `MAX_FIXED_MODE_SIZE`. Alternatively, you can have the compiler use `TImode` for 16-byte structures and unions. Likewise, you can arrange for the C type `short int` to avoid using `HImode`.

Very few explicit references to machine modes remain in the compiler and these few references will soon be removed. Instead, the machine modes are divided into mode classes. These are represented by the enumeration type `enum mode_class` defined in ``machmode.h'`. The possible mode classes are:

`MODE_INT`
  Integer modes. By default these are `QImode`, `HImode`, `SImode`, `DImode`, and `TImode`.
`MODE_PARTIAL_INT`
  The "partial integer" modes, `PSImode` and `PDImode`.
`MODE_FLOAT`
  floating point modes. By default these are `SFmode`, `DFmode`, `XFmode` and `TFmode`.
`MODE_COMPLEX_INT`
  Complex integer modes. (These are not currently implemented).
`MODE_COMPLEX_FLOAT`
  Complex floating point modes. By default these are `SCmode`, `DCmode`, `XCmode`, and `TCmode`.
`MODE_FUNCTION`
  Algol or Pascal function variables including a static chain. (These are not currently implemented).
`MODE_CC`
  Modes representing condition code values. These are `CCmode` plus any modes listed in the `EXTRA_CC_MODES` macro. See section Defining Jump Instruction Patterns, also see section Condition Code Status.
`MODE_RANDOM`
  This is a catchall mode class for modes which don't fit into the above classes. Currently `VOIDmode` and `BLKmode` are in `MODE_RANDOM`.

Here are some C macros that relate to machine modes:

`GET_MODE (x)`
  Returns the machine mode of the RTX x.
`PUT_MODE (x, newmode)`
  Alters the machine mode of the RTX x to be newmode.
`NUM_MACHINE_MODES`
  Stands for the number of machine modes available on the target machine. This is one greater than the largest numeric value of any machine mode.
`GET_MODE_NAME (m)`
  Returns the name of mode m as a string.
`GET_MODE_CLASS (m)`
  Returns the mode class of mode m.
`GET_MODE_WIDER_MODE (m)`
  Returns the next wider natural mode. For example, the expression `GET_MODE_WIDER_MODE (QImode)` returns `HImode`.
`GET_MODE_SIZE (m)`
  Returns the size in bytes of a datum of mode m.
`GET_MODE_BITSIZE (m)`
  Returns the size in bits of a datum of mode m.
`GET_MODE_MASK (m)`
  Returns a bitmask containing 1 for all bits in a word that fit within mode m. This macro can only be used for modes whose bitsize is less than or equal to `HOST_BITS_PER_INT`.
`GET_MODE_ALIGNMENT (m))`
  Return the required alignment, in bits, for an object of mode m.
`GET_MODE_UNIT_SIZE (m)`
  Returns the size in bytes of the subunits of a datum of mode m. This is the same as `GET_MODE_SIZE` except in the case of complex modes. For them, the unit size is the size of the real or imaginary part.
`GET_MODE_NUNITS (m)`
  Returns the number of units contained in a mode, i.e., `GET_MODE_SIZE` divided by `GET_MODE_UNIT_SIZE`.
`GET_CLASS_NARROWEST_MODE (c)`
  Returns the narrowest mode in mode class c.

The global variables `byte_mode` and `word_mode` contain modes whose classes are `MODE_INT` and whose bitsizes are either `BITS_PER_UNIT` or `BITS_PER_WORD`, respectively. On 32-bit machines, these are `QImode` and `SImode`, respectively.

## Constant Expression Types

The simplest RTL expressions are those that represent constant values.

`(const_int i)`
  This type of expression represents the integer value i. i is customarily accessed with the macro `INTVAL` as in `INTVAL (exp)`, which is equivalent to `XWINT (exp, 0)`. There is only one expression object for the integer value zero; it is the value of the variable `const0_rtx`. Likewise, the only expression for integer value one is found in `const1_rtx`, the only expression for integer value two is found in `const2_rtx`, and the only expression for integer value negative one is found in `constm1_rtx`. Any attempt to create an expression of code `const_int` and value zero, one, two or negative one will return `const0_rtx`, `const1_rtx`, `const2_rtx` or `constm1_rtx` as appropriate. Similarly, there is only one object for the integer whose value is `STORE_FLAG_VALUE`. It is found in `const_true_rtx`. If `STORE_FLAG_VALUE` is one, `const_true_rtx` and `const1_rtx` will point to the same object. If `STORE_FLAG_VALUE` is -1, `const_true_rtx` and `constm1_rtx` will point to the same object.
`(const_double:m addr i0 i1 ...)`
  Represents either a floating-point constant of mode m or an integer constant too large to fit into `HOST_BITS_PER_WIDE_INT` bits but small enough to fit within twice that number of bits (GNU CC does not provide a mechanism to represent even larger constants). In the latter case, m will be `VOIDmode`. addr is used to contain the `mem` expression that corresponds to the location in memory that at which the constant can be found. If it has not been allocated a memory location, but is on the chain of all `const_double` expressions in this compilation (maintained using an undisplayed field), addr contains `const0_rtx`. If it is not on the chain, addr contains `cc0_rtx`. addr is customarily accessed with the macro `CONST_DOUBLE_MEM` and the chain field via `CONST_DOUBLE_CHAIN`. If m is `VOIDmode`, the bits of the value are stored in i0 and i1. i0 is customarily accessed with the macro `CONST_DOUBLE_LOW` and i1 with `CONST_DOUBLE_HIGH`. If the constant is floating point (regardless of its precision), then the number of integers used to store the value depends on the size of `REAL_VALUE_TYPE` (see section Cross Compilation and Floating Point). The integers represent a floating point number, but not precisely in the target machine's or host machine's floating point format. To convert them to the precise bit pattern used by the target machine, use the macro `REAL_VALUE_TO_TARGET_DOUBLE` and friends (see section Output of Data). The macro `CONST0_RTX (mode)` refers to an expression with value 0 in mode mode. If mode mode is of mode class `MODE_INT`, it returns `const0_rtx`. Otherwise, it returns a `CONST_DOUBLE` expression in mode mode. Similarly, the macro `CONST1_RTX (mode)` refers to an expression with value 1 in mode mode and similarly for `CONST2_RTX`.
`(const_string str)`
  Represents a constant string with value str. Currently this is used only for insn attributes (see section Instruction Attributes) since constant strings in C are placed in memory.
`(symbol_ref:mode symbol)`
  Represents the value of an assembler label for data. symbol is a string that describes the name of the assembler label. If it starts with a ``*'`, the label is the rest of symbol not including the ``*'`. Otherwise, the label is symbol, usually prefixed with ``_'`. The `symbol_ref` contains a mode, which is usually `Pmode`. Usually that is the only mode for which a symbol is directly valid.
`(label_ref label)`
  Represents the value of an assembler label for code. It contains one operand, an expression, which must be a `code_label` that appears in the instruction sequence to identify the place where the label should go. The reason for using a distinct expression type for code label references is so that jump optimization can distinguish them.
`(const:m exp)`
  Represents a constant that is the result of an assembly-time arithmetic computation. The operand, exp, is an expression that contains only constants (`const_int`, `symbol_ref` and `label_ref` expressions) combined with `plus` and `minus`. However, not all combinations are valid, since the assembler cannot do arbitrary arithmetic on relocatable symbols. m should be `Pmode`.
`(high:m exp)`
  Represents the high-order bits of exp, usually a `symbol_ref`. The number of bits is machine-dependent and is normally the number of bits specified in an instruction that initializes the high order bits of a register. It is used with `lo_sum` to represent the typical two-instruction sequence used in RISC machines to reference a global memory location. m should be `Pmode`.

## Registers and Memory

Here are the RTL expression types for describing access to machine registers and to main memory.

`(reg:m n)`
  For small values of the integer n (those that are less than `FIRST_PSEUDO_REGISTER`), this stands for a reference to machine register number n: a hard register. For larger values of n, it stands for a temporary value or pseudo register. The compiler's strategy is to generate code assuming an unlimited number of such pseudo registers, and later convert them into hard registers or into memory references. m is the machine mode of the reference. It is necessary because machines can generally refer to each register in more than one mode. For example, a register may contain a full word but there may be instructions to refer to it as a half word or as a single byte, as well as instructions to refer to it as a floating point number of various precisions. Even for a register that the machine can access in only one mode, the mode must always be specified. The symbol `FIRST_PSEUDO_REGISTER` is defined by the machine description, since the number of hard registers on the machine is an invariant characteristic of the machine. Note, however, that not all of the machine registers must be general registers. All the machine registers that can be used for storage of data are given hard register numbers, even those that can be used only in certain instructions or can hold only certain types of data. A hard register may be accessed in various modes throughout one function, but each pseudo register is given a natural mode and is accessed only in that mode. When it is necessary to describe an access to a pseudo register using a nonnatural mode, a `subreg` expression is used. A `reg` expression with a machine mode that specifies more than one word of data may actually stand for several consecutive registers. If in addition the register number specifies a hardware register, then it actually represents several consecutive hardware registers starting with the specified one. Each pseudo register number used in a function's RTL code is represented by a unique `reg` expression. Some pseudo register numbers, those within the range of `FIRST_VIRTUAL_REGISTER` to `LAST_VIRTUAL_REGISTER` only appear during the RTL generation phase and are eliminated before the optimization phases. These represent locations in the stack frame that cannot be determined until RTL generation for the function has been completed. The following virtual register numbers are defined:
  `VIRTUAL_INCOMING_ARGS_REGNUM`
    This points to the first word of the incoming arguments passed on the stack. Normally these arguments are placed there by the caller, but the callee may have pushed some arguments that were previously passed in registers. When RTL generation is complete, this virtual register is replaced by the sum of the register given by `ARG_POINTER_REGNUM` and the value of `FIRST_PARM_OFFSET`.
  `VIRTUAL_STACK_VARS_REGNUM`
    If `FRAME_GROWS_DOWNWARD` is defined, this points to immediately above the first variable on the stack. Otherwise, it points to the first variable on the stack. `VIRTUAL_STACK_VARS_REGNUM` is replaced with the sum of the register given by `FRAME_POINTER_REGNUM` and the value `STARTING_FRAME_OFFSET`.
  `VIRTUAL_STACK_DYNAMIC_REGNUM`
    This points to the location of dynamically allocated memory on the stack immediately after the stack pointer has been adjusted by the amount of memory desired. This virtual register is replaced by the sum of the register given by `STACK_POINTER_REGNUM` and the value `STACK_DYNAMIC_OFFSET`.
  `VIRTUAL_OUTGOING_ARGS_REGNUM`
    This points to the location in the stack at which outgoing arguments should be written when the stack is pre-pushed (arguments pushed using push insns should always use `STACK_POINTER_REGNUM`). This virtual register is replaced by the sum of the register given by `STACK_POINTER_REGNUM` and the value `STACK_POINTER_OFFSET`.
`(subreg:m reg wordnum)`
  `subreg` expressions are used to refer to a register in a machine mode other than its natural one, or to refer to one register of a multi-word `reg` that actually refers to several registers. Each pseudo-register has a natural mode. If it is necessary to operate on it in a different mode--for example, to perform a fullword move instruction on a pseudo-register that contains a single byte--the pseudo-register must be enclosed in a `subreg`. In such a case, wordnum is zero. Usually m is at least as narrow as the mode of reg, in which case it is restricting consideration to only the bits of reg that are in m. Sometimes m is wider than the mode of reg. These `subreg` expressions are often called paradoxical. They are used in cases where we want to refer to an object in a wider mode but do not care what value the additional bits have. The reload pass ensures that paradoxical references are only made to hard registers. The other use of `subreg` is to extract the individual registers of a multi-register value. Machine modes such as `DImode` and `TImode` can indicate values longer than a word, values which usually require two or more consecutive registers. To access one of the registers, use a `subreg` with mode `SImode` and a wordnum that says which register. Storing in a non-paradoxical `subreg` has undefined results for bits belonging to the same word as the `subreg`. This laxity makes it easier to generate efficient code for such instructions. To represent an instruction that preserves all the bits outside of those in the `subreg`, use `strict_low_part` around the `subreg`. The compilation parameter `WORDS_BIG_ENDIAN`, if set to 1, says that word number zero is the most significant part; otherwise, it is the least significant part. Between the combiner pass and the reload pass, it is possible to have a paradoxical `subreg` which contains a `mem` instead of a `reg` as its first operand. After the reload pass, it is also possible to have a non-paradoxical `subreg` which contains a `mem`; this usually occurs when the `mem` is a stack slot which replaced a pseudo register. Note that it is not valid to access a `DFmode` value in `SFmode` using a `subreg`. On some machines the most significant part of a `DFmode` value does not have the same format as a single-precision floating value. It is also not valid to access a single word of a multi-word value in a hard register when less registers can hold the value than would be expected from its size. For example, some 32-bit machines have floating-point registers that can hold an entire `DFmode` value. If register 10 were such a register `(subreg:SI (reg:DF 10) 1)` would be invalid because there is no way to convert that reference to a single machine register. The reload pass prevents `subreg` expressions such as these from being formed. The first operand of a `subreg` expression is customarily accessed with the `SUBREG_REG` macro and the second operand is customarily accessed with the `SUBREG_WORD` macro.
`(scratch:m)`
  This represents a scratch register that will be required for the execution of a single instruction and not used subsequently. It is converted into a `reg` by either the local register allocator or the reload pass. `scratch` is usually present inside a `clobber` operation (see section Side Effect Expressions).
`(cc0)`
  This refers to the machine's condition code register. It has no operands and may not have a machine mode. There are two ways to use it:
  - To stand for a complete set of condition code flags. This is best on most machines, where each comparison sets the entire series of flags. With this technique, `(cc0)` may be validly used in only two contexts: as the destination of an assignment (in test and compare instructions) and in comparison operators comparing against zero (`const_int` with value zero; that is to say, `const0_rtx`).
  - To stand for a single flag that is the result of a single condition. This is useful on machines that have only a single flag bit, and in which comparison instructions must specify the condition to test. With this technique, `(cc0)` may be validly used in only two contexts: as the destination of an assignment (in test and compare instructions) where the source is a comparison operator, and as the first operand of `if_then_else` (in a conditional branch).
  There is only one expression object of code `cc0`; it is the value of the variable `cc0_rtx`. Any attempt to create an expression of code `cc0` will return `cc0_rtx`. Instructions can set the condition code implicitly. On many machines, nearly all instructions set the condition code based on the value that they compute or store. It is not necessary to record these actions explicitly in the RTL because the machine description includes a prescription for recognizing the instructions that do so (by means of the macro `NOTICE_UPDATE_CC`). See section Condition Code Status. Only instructions whose sole purpose is to set the condition code, and instructions that use the condition code, need mention `(cc0)`. On some machines, the condition code register is given a register number and a `reg` is used instead of `(cc0)`. This is usually the preferable approach if only a small subset of instructions modify the condition code. Other machines store condition codes in general registers; in such cases a pseudo register should be used. Some machines, such as the Sparc and RS/6000, have two sets of arithmetic instructions, one that sets and one that does not set the condition code. This is best handled by normally generating the instruction that does not set the condition code, and making a pattern that both performs the arithmetic and sets the condition code register (which would not be `(cc0)` in this case). For examples, search for ``addcc'` and ``andcc'` in ``sparc.md'`.
`(pc)`
  This represents the machine's program counter. It has no operands and may not have a machine mode. `(pc)` may be validly used only in certain specific contexts in jump instructions. There is only one expression object of code `pc`; it is the value of the variable `pc_rtx`. Any attempt to create an expression of code `pc` will return `pc_rtx`. All instructions that do not jump alter the program counter implicitly by incrementing it, but there is no need to mention this in the RTL.
`(mem:m addr)`
  This RTX represents a reference to main memory at an address represented by the expression addr. m specifies how large a unit of memory is accessed.

## RTL Expressions for Arithmetic

Unless otherwise specified, all the operands of arithmetic expressions must be valid for mode m. An operand is valid for mode m if it has mode m, or if it is a `const_int` or `const_double` and m is a mode of class `MODE_INT`.

For commutative binary operations, constants should be placed in the second operand.

`(plus:m x y)`
  Represents the sum of the values represented by x and y carried out in machine mode m.
`(lo_sum:m x y)`
  Like `plus`, except that it represents that sum of x and the low-order bits of y. The number of low order bits is machine-dependent but is normally the number of bits in a `Pmode` item minus the number of bits set by the `high` code (see section Constant Expression Types). m should be `Pmode`.
`(minus:m x y)`
  Like `plus` but represents subtraction.
`(compare:m x y)`
  Represents the result of subtracting y from x for purposes of comparison. The result is computed without overflow, as if with infinite precision. Of course, machines can't really subtract with infinite precision. However, they can pretend to do so when only the sign of the result will be used, which is the case when the result is stored in the condition code. And that is the only way this kind of expression may validly be used: as a value to be stored in the condition codes. The mode m is not related to the modes of x and y, but instead is the mode of the condition code value. If `(cc0)` is used, it is `VOIDmode`. Otherwise it is some mode in class `MODE_CC`, often `CCmode`. See section Condition Code Status. Normally, x and y must have the same mode. Otherwise, `compare` is valid only if the mode of x is in class `MODE_INT` and y is a `const_int` or `const_double` with mode `VOIDmode`. The mode of x determines what mode the comparison is to be done in; thus it must not be `VOIDmode`. If one of the operands is a constant, it should be placed in the second operand and the comparison code adjusted as appropriate. A `compare` specifying two `VOIDmode` constants is not valid since there is no way to know in what mode the comparison is to be performed; the comparison must either be folded during the compilation or the first operand must be loaded into a register while its mode is still known.
`(neg:m x)`
  Represents the negation (subtraction from zero) of the value represented by x, carried out in mode m.
`(mult:m x y)`
  Represents the signed product of the values represented by x and y carried out in machine mode m. Some machines support a multiplication that generates a product wider than the operands. Write the pattern for this as

      (mult:m (sign_extend:m x) (sign_extend:m y))

  where m is wider than the modes of x and y, which need not be the same. Write patterns for unsigned widening multiplication similarly using `zero_extend`.
`(div:m x y)`
  Represents the quotient in signed division of x by y, carried out in machine mode m. If m is a floating point mode, it represents the exact quotient; otherwise, the integerized quotient. Some machines have division instructions in which the operands and quotient widths are not all the same; you should represent such instructions using `truncate` and `sign_extend` as in,

      (truncate:m1 (div:m2 x (sign_extend:m2 y)))

`(udiv:m x y)`
  Like `div` but represents unsigned division.
`(mod:m x y)`
`(umod:m x y)`
  Like `div` and `udiv` but represent the remainder instead of the quotient.
`(smin:m x y)`
`(smax:m x y)`
  Represents the smaller (for `smin`) or larger (for `smax`) of x and y, interpreted as signed integers in mode m.
`(umin:m x y)`
`(umax:m x y)`
  Like `smin` and `smax`, but the values are interpreted as unsigned integers.
`(not:m x)`
  Represents the bitwise complement of the value represented by x, carried out in mode m, which must be a fixed-point machine mode.
`(and:m x y)`
  Represents the bitwise logical-and of the values represented by x and y, carried out in machine mode m, which must be a fixed-point machine mode.
`(ior:m x y)`
  Represents the bitwise inclusive-or of the values represented by x and y, carried out in machine mode m, which must be a fixed-point mode.
`(xor:m x y)`
  Represents the bitwise exclusive-or of the values represented by x and y, carried out in machine mode m, which must be a fixed-point mode.
`(ashift:m x c)`
  Represents the result of arithmetically shifting x left by c places. x have mode m, a fixed-point machine mode. c be a fixed-point mode or be a constant with mode `VOIDmode`; which mode is determined by the mode called for in the machine description entry for the left-shift instruction. For example, on the Vax, the mode of c is `QImode` regardless of m.
`(lshiftrt:m x c)`
`(ashiftrt:m x c)`
  Like `ashift` but for right shift. Unlike the case for left shift, these two operations are distinct.
`(rotate:m x c)`
`(rotatert:m x c)`
  Similar but represent left and right rotate. If c is a constant, use `rotate`.
`(abs:m x)`
  Represents the absolute value of x, computed in mode m.
`(sqrt:m x)`
  Represents the square root of x, computed in mode m. Most often m will be a floating point mode.
`(ffs:m x)`
  Represents one plus the index of the least significant 1-bit in x, represented as an integer of mode m. (The value is zero if x is zero.) The mode of x need not be m; depending on the target machine, various mode combinations may be valid.

## Comparison Operations

Comparison operators test a relation on two operands and are considered to represent a machine-dependent nonzero value described by, but not necessarily equal to, `STORE_FLAG_VALUE` (see section Miscellaneous Parameters) if the relation holds, or zero if it does not. The mode of the comparison operation is independent of the mode of the data being compared. If the comparison operation is being tested (e.g., the first operand of an `if_then_else`), the mode must be `VOIDmode`. If the comparison operation is producing data to be stored in some variable, the mode must be in class `MODE_INT`. All comparison operations producing data must use the same mode, which is machine-specific.

There are two ways that comparison operations may be used. The comparison operators may be used to compare the condition codes `(cc0)` against zero, as in `(eq (cc0) (const_int 0))`. Such a construct actually refers to the result of the preceding instruction in which the condition codes were set. The instructing setting the condition code must be adjacent to the instruction using the condition code; only `note` insns may separate them.

Alternatively, a comparison operation may directly compare two data objects. The mode of the comparison is determined by the operands; they must both be valid for a common machine mode. A comparison with both operands constant would be invalid as the machine mode could not be deduced from it, but such a comparison should never exist in RTL due to constant folding.

In the example above, if `(cc0)` were last set to `(compare x y)`, the comparison operation is identical to `(eq x y)`. Usually only one style of comparisons is supported on a particular machine, but the combine pass will try to merge the operations to produce the `eq` shown in case it exists in the context of the particular insn involved.

Inequality comparisons come in two flavors, signed and unsigned. Thus, there are distinct expression codes `gt` and `gtu` for signed and unsigned greater-than. These can produce different results for the same pair of integer values: for example, 1 is signed greater-than -1 but not unsigned greater-than, because -1 when regarded as unsigned is actually `0xffffffff` which is greater than 1.

The signed comparisons are also used for floating point values. Floating point comparisons are distinguished by the machine modes of the operands.

`(eq:m x y)`
  1 if the values represented by x and y are equal, otherwise 0.
`(ne:m x y)`
  1 if the values represented by x and y are not equal, otherwise 0.
`(gt:m x y)`
  1 if the x is greater than y. If they are fixed-point, the comparison is done in a signed sense.
`(gtu:m x y)`
  Like `gt` but does unsigned comparison, on fixed-point numbers only.
`(lt:m x y)`
`(ltu:m x y)`
  Like `gt` and `gtu` but test for "less than".
`(ge:m x y)`
`(geu:m x y)`
  Like `gt` and `gtu` but test for "greater than or equal".
`(le:m x y)`
`(leu:m x y)`
  Like `gt` and `gtu` but test for "less than or equal".
`(if_then_else cond then else)`
  This is not a comparison operation but is listed here because it is always used in conjunction with a comparison operation. To be precise, cond is a comparison expression. This expression represents a choice, according to cond, between the value represented by then and the one represented by else. On most machines, `if_then_else` expressions are valid only to express conditional jumps.
`(cond [test1 value1 test2 value2 ...] default)`
  Similar to `if_then_else`, but more general. Each of test1, test2, ... is performed in turn. The result of this expression is the value corresponding to the first non-zero test, or default if none of the tests are non-zero expressions. This is currently not valid for instruction patterns and is supported only for insn attributes. See section Instruction Attributes.

## Bit Fields

Special expression codes exist to represent bitfield instructions. These types of expressions are lvalues in RTL; they may appear on the left side of an assignment, indicating insertion of a value into the specified bit field.

`(sign_extract:m loc size pos)`
  This represents a reference to a sign-extended bit field contained or starting in loc (a memory or register reference). The bit field is size bits wide and starts at bit pos. The compilation option `BITS_BIG_ENDIAN` says which end of the memory unit pos counts from. If loc is in memory, its mode must be a single-byte integer mode. If loc is in a register, the mode to use is specified by the operand of the `insv` or `extv` pattern (see section Standard Pattern Names For Generation) and is usually a full-word integer mode. The mode of pos is machine-specific and is also specified in the `insv` or `extv` pattern. The mode m is the same as the mode that would be used for loc if it were a register.
`(zero_extract:m loc size pos)`
  Like `sign_extract` but refers to an unsigned or zero-extended bit field. The same sequence of bits are extracted, but they are filled to an entire word with zeros instead of by sign-extension.

## Conversions

All conversions between machine modes must be represented by explicit conversion operations. For example, an expression which is the sum of a byte and a full word cannot be written as `(plus:SI (reg:QI 34) (reg:SI 80))` because the `plus` operation requires two operands of the same machine mode. Therefore, the byte-sized operand is enclosed in a conversion operation, as in

    (plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80))

The conversion operation is not a mere placeholder, because there may be more than one way of converting from a given starting mode to the desired final mode. The conversion operation code says how to do it.

For all conversion operations, x must not be `VOIDmode` because the mode in which to do the conversion would not be known. The conversion must either be done at compile-time or x must be placed into a register.

`(sign_extend:m x)`
  Represents the result of sign-extending the value x to machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode narrower than m.
`(zero_extend:m x)`
  Represents the result of zero-extending the value x to machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode narrower than m.
`(float_extend:m x)`
  Represents the result of extending the value x to machine mode m. m must be a floating point mode and x a floating point value of a mode narrower than m.
`(truncate:m x)`
  Represents the result of truncating the value x to machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode wider than m.
`(float_truncate:m x)`
  Represents the result of truncating the value x to machine mode m. m must be a floating point mode and x a floating point value of a mode wider than m.
`(float:m x)`
  Represents the result of converting fixed point value x, regarded as signed, to floating point mode m.
`(unsigned_float:m x)`
  Represents the result of converting fixed point value x, regarded as unsigned, to floating point mode m.
`(fix:m x)`
  When m is a fixed point mode, represents the result of converting floating point value x to mode m, regarded as signed. How rounding is done is not specified, so this operation may be used validly in compiling C code only for integer-valued operands.
`(unsigned_fix:m x)`
  Represents the result of converting floating point value x to fixed point mode m, regarded as unsigned. How rounding is done is not specified.
`(fix:m x)`
  When m is a floating point mode, represents the result of converting floating point value x (valid for mode m) to an integer, still represented in floating point mode m, by rounding towards zero.

## Declarations

Declaration expression codes do not represent arithmetic operations but rather state assertions about their operands.

`(strict_low_part (subreg:m (reg:n r) 0))`
  This expression code is used in only one context: as the destination operand of a `set` expression. In addition, the operand of this expression must be a non-paradoxical `subreg` expression. The presence of `strict_low_part` says that the part of the register which is meaningful in mode n, but is not part of mode m, is not to be altered. Normally, an assignment to such a subreg is allowed to have undefined effects on the rest of the register when m is less than a word.

## Side Effect Expressions

The expression codes described so far represent values, not actions. But machine instructions never produce values; they are meaningful only for their side effects on the state of the machine. Special expression codes are used to represent side effects.

The body of an instruction is always one of these side effect codes; the codes described above, which represent values, appear only as the operands of these.

`(set lval x)`
  Represents the action of storing the value of x into the place represented by lval. lval must be an expression representing a place that can be stored in: `reg` (or `subreg` or `strict_low_part`), `mem`, `pc` or `cc0`. If lval is a `reg`, `subreg` or `mem`, it has a machine mode; then x must be valid for that mode. If lval is a `reg` whose machine mode is less than the full width of the register, then it means that the part of the register specified by the machine mode is given the specified value and the rest of the register receives an undefined value. Likewise, if lval is a `subreg` whose machine mode is narrower than the mode of the register, the rest of the register can be changed in an undefined way. If lval is a `strict_low_part` of a `subreg`, then the part of the register specified by the machine mode of the `subreg` is given the value x and the rest of the register is not changed. If lval is `(cc0)`, it has no machine mode, and x may be either a `compare` expression or a value that may have any mode. The latter case represents a "test" instruction. The expression `(set (cc0) (reg:m n))` is equivalent to `(set (cc0) (compare (reg:m n) (const_int 0)))`. Use the former expression to save space during the compilation. If lval is `(pc)`, we have a jump instruction, and the possibilities for x are very limited. It may be a `label_ref` expression (unconditional jump). It may be an `if_then_else` (conditional jump), in which case either the second or the third operand must be `(pc)` (for the case which does not jump) and the other of the two must be a `label_ref` (for the case which does jump). x may also be a `mem` or `(plus:SI (pc) y)`, where y may be a `reg` or a `mem`; these unusual patterns are used to represent jumps through branch tables. If lval is neither `(cc0)` nor `(pc)`, the mode of lval must not be `VOIDmode` and the mode of x must be valid for the mode of lval. lval is customarily accessed with the `SET_DEST` macro and x with the `SET_SRC` macro.
`(return)`
  As the sole expression in a pattern, represents a return from the current function, on machines where this can be done with one instruction, such as Vaxes. On machines where a multi-instruction "epilogue" must be executed in order to return from the function, returning is done by jumping to a label which precedes the epilogue, and the `return` expression code is never used. Inside an `if_then_else` expression, represents the value to be placed in `pc` to return to the caller. Note that an insn pattern of `(return)` is logically equivalent to `(set (pc) (return))`, but the latter form is never used.
`(call function nargs)`
  Represents a function call. function is a `mem` expression whose address is the address of the function to be called. nargs is an expression which can be used for two purposes: on some machines it represents the number of bytes of stack argument; on others, it represents the number of argument registers. Each machine has a standard machine mode which function must have. The machine description defines macro `FUNCTION_MODE` to expand into the requisite mode name. The purpose of this mode is to specify what kind of addressing is allowed, on machines where the allowed kinds of addressing depend on the machine mode being addressed.
`(clobber x)`
  Represents the storing or possible storing of an unpredictable, undescribed value into x, which must be a `reg`, `scratch` or `mem` expression. One place this is used is in string instructions that store standard values into particular hard registers. It may not be worth the trouble to describe the values that are stored, but it is essential to inform the compiler that the registers will be altered, lest it attempt to keep data in them across the string instruction. If x is `(mem:BLK (const_int 0))`, it means that all memory locations must be presumed clobbered. Note that the machine description classifies certain hard registers as "call-clobbered". All function call instructions are assumed by default to clobber these registers, so there is no need to use `clobber` expressions to indicate this fact. Also, each function call is assumed to have the potential to alter any memory location, unless the function is declared `const`. If the last group of expressions in a `parallel` are each a `clobber` expression whose arguments are `reg` or `match_scratch` (see section RTL Template) expressions, the combiner phase can add the appropriate `clobber` expressions to an insn it has constructed when doing so will cause a pattern to be matched. This feature can be used, for example, on a machine that whose multiply and add instructions don't use an MQ register but which has an add-accumulate instruction that does clobber the MQ register. Similarly, a combined instruction might require a temporary register while the constituent instructions might not. When a `clobber` expression for a register appears inside a `parallel` with other side effects, the register allocator guarantees that the register is unoccupied both before and after that insn. However, the reload phase may allocate a register used for one of the inputs unless the ``&'` constraint is specified for the selected alternative (see section Constraint Modifier Characters). You can clobber either a specific hard register, a pseudo register, or a `scratch` expression; in the latter two cases, GNU CC will allocate a hard register that is available there for use as a temporary. For instructions that require a temporary register, you should use `scratch` instead of a pseudo-register because this will allow the combiner phase to add the `clobber` when required. You do this by coding (`clobber` (`match_scratch` ...)). If you do clobber a pseudo register, use one which appears nowhere else--generate a new one each time. Otherwise, you may confuse CSE. There is one other known use for clobbering a pseudo register in a `parallel`: when one of the input operands of the insn is also clobbered by the insn. In this case, using the same pseudo register in the clobber and elsewhere in the insn produces the expected results.
`(use x)`
  Represents the use of the value of x. It indicates that the value in x at this point in the program is needed, even though it may not be apparent why this is so. Therefore, the compiler will not attempt to delete previous instructions whose only effect is to store a value in x. x must be a `reg` expression. During the delayed branch scheduling phase, x may be an insn. This indicates that x previously was located at this place in the code and its data dependencies need to be taken into account. These `use` insns will be deleted before the delayed branch scheduling phase exits.
`(parallel [x0 x1 ...])`
  Represents several side effects performed in parallel. The square brackets stand for a vector; the operand of `parallel` is a vector of expressions. x0, x1 and so on are individual side effect expressions--expressions of code `set`, `call`, `return`, `clobber` or `use`. "In parallel" means that first all the values used in the individual side-effects are computed, and second all the actual side-effects are performed. For example,

      (parallel [(set (reg:SI 1) (mem:SI (reg:SI 1)))
                 (set (mem:SI (reg:SI 1)) (reg:SI 1))])

  says unambiguously that the values of hard register 1 and the memory location addressed by it are interchanged. In both places where `(reg:SI 1)` appears as a memory address it refers to the value in register 1 before the execution of the insn. It follows that it is incorrect to use `parallel` and expect the result of one `set` to be available for the next one. For example, people sometimes attempt to represent a jump-if-zero instruction this way:

      (parallel [(set (cc0) (reg:SI 34))
                 (set (pc) (if_then_else
                              (eq (cc0) (const_int 0))
                              (label_ref ...)
                              (pc)))])

  But this is incorrect, because it says that the jump condition depends on the condition code value before this instruction, not on the new value that is set by this instruction. Peephole optimization, which takes place together with final assembly code output, can produce insns whose patterns consist of a `parallel` whose elements are the operands needed to output the resulting assembler code--often `reg`, `mem` or constant expressions. This would not be well-formed RTL at any other stage in compilation, but it is ok then because no further optimization remains to be done. However, the definition of the macro `NOTICE_UPDATE_CC`, if any, must deal with such insns if you define any peephole optimizations.
`(sequence [insns ...])`
  Represents a sequence of insns. Each of the insns that appears in the vector is suitable for appearing in the chain of insns, so it must be an `insn`, `jump_insn`, `call_insn`, `code_label`, `barrier` or `note`. A `sequence` RTX is never placed in an actual insn during RTL generation. It represents the sequence of insns that result from a `define_expand` before those insns are passed to `emit_insn` to insert them in the chain of insns. When actually inserted, the individual sub-insns are separated out and the `sequence` is forgotten. After delay-slot scheduling is completed, an insn and all the insns that reside in its delay slots are grouped together into a `sequence`. The insn requiring the delay slot is the first insn in the vector; subsequent insns are to be placed in the delay slot. `INSN_ANNULLED_BRANCH_P` is set on an insn in a delay slot to indicate that a branch insn should be used that will conditionally annul the effect of the insns in the delay slots. In such a case, `INSN_FROM_TARGET_P` indicates that the insn is from the target of the branch and should be executed only if the branch is taken; otherwise the insn should be executed only if the branch is not taken. See section Delay Slot Scheduling.

These expression codes appear in place of a side effect, as the body of an insn, though strictly speaking they do not always describe side effects as such:

`(asm_input s)`
  Represents literal assembler code as described by the string s.
`(unspec [operands ...] index)`
`(unspec_volatile [operands ...] index)`
  Represents a machine-specific operation on operands. index selects between multiple machine-specific operations. `unspec_volatile` is used for volatile operations and operations that may trap; `unspec` is used for other operations. These codes may appear inside a `pattern` of an insn, inside a `parallel`, or inside an expression.
`(addr_vec:m [lr0 lr1 ...])`
  Represents a table of jump addresses. The vector elements lr0, etc., are `label_ref` expressions. The mode m specifies how much space is given to each address; normally m would be `Pmode`.
`(addr_diff_vec:m base [lr0 lr1 ...])`
  Represents a table of jump addresses expressed as offsets from base. The vector elements lr0, etc., are `label_ref` expressions and so is base. The mode m specifies how much space is given to each address-difference.

## Embedded Side-Effects on Addresses

Four special side-effect expression codes appear as memory addresses.

`(pre_dec:m x)`
  Represents the side effect of decrementing x by a standard amount and represents also the value that x has after being decremented. x must be a `reg` or `mem`, but most machines allow only a `reg`. m must be the machine mode for pointers on the machine in use. The amount x is decremented by is the length in bytes of the machine mode of the containing memory reference of which this expression serves as the address. Here is an example of its use:

      (mem:DF (pre_dec:SI (reg:SI 39)))

  This says to decrement pseudo register 39 by the length of a `DFmode` value and use the result to address a `DFmode` value.
`(pre_inc:m x)`
  Similar, but specifies incrementing x instead of decrementing it.
`(post_dec:m x)`
  Represents the same side effect as `pre_dec` but a different value. The value represented here is the value x has before being decremented.
`(post_inc:m x)`
  Similar, but specifies incrementing x instead of decrementing it.

These embedded side effect expressions must be used with care. Instruction patterns may not use them. Until the ``flow'` pass of the compiler, they may occur only to represent pushes onto the stack. The ``flow'` pass finds cases where registers are incremented or decremented in one instruction and used as an address shortly before or after; these cases are then transformed to use pre- or post-increment or -decrement.

If a register used as the operand of these expressions is used in another address in an insn, the original value of the register is used. Uses of the register outside of an address are not permitted within the same insn as a use in an embedded side effect expression because such insns behave differently on different machines and hence must be treated as ambiguous and disallowed.

An instruction that can be represented with an embedded side effect could also be represented using `parallel` containing an additional `set` to describe how the address register is altered. This is not done because machines that allow these operations at all typically allow them wherever a memory address is called for. Describing them as additional parallel stores would require doubling the number of entries in the machine description.

## Assembler Instructions as Expressions

The RTX code `asm_operands` represents a value produced by a user-specified assembler instruction. It is used to represent an `asm` statement with arguments. An `asm` statement with a single output operand, like this:

    asm ("foo %1,%2,%0" : "=a" (outputvar) : "g" (x + y), "di" (*z));

is represented using a single `asm_operands` RTX which represents the value that is stored in `outputvar`:

    (set rtx-for-outputvar
         (asm_operands "foo %1,%2,%0" "a" 0
                       [rtx-for-addition-result rtx-for-*z]
                       [(asm_input:m1 "g")
                        (asm_input:m2 "di")]))

Here the operands of the `asm_operands` RTX are the assembler template string, the output-operand's constraint, the index-number of the output operand among the output operands specified, a vector of input operand RTX's, and a vector of input-operand modes and constraints. The mode m1 is the mode of the sum `x+y`; m2 is that of `*z`.

When an `asm` statement has multiple output values, its insn has several such `set` RTX's inside of a `parallel`. Each `set` contains a `asm_operands`; all of these share the same assembler template and vectors, but each contains the constraint for the respective output operand. They are also distinguished by the output-operand index number, which is 0, 1, ... for successive output operands.

## Insns

The RTL representation of the code for a function is a doubly-linked chain of objects called insns. Insns are expressions with special codes that are used for no other purpose. Some insns are actual instructions; others represent dispatch tables for `switch` statements; others represent labels to jump to or various sorts of declarative information.

In addition to its own specific data, each insn must have a unique id-number that distinguishes it from all other insns in the current function (after delayed branch scheduling, copies of an insn with the same id-number may be present in multiple places in a function, but these copies will always be identical and will only appear inside a `sequence`), and chain pointers to the preceding and following insns. These three fields occupy the same position in every insn, independent of the expression code of the insn. They could be accessed with `XEXP` and `XINT`, but instead three special macros are always used:

`INSN_UID (i)`
  Accesses the unique id of insn i.
`PREV_INSN (i)`
  Accesses the chain pointer to the insn preceding i. If i is the first insn, this is a null pointer.
`NEXT_INSN (i)`
  Accesses the chain pointer to the insn following i. If i is the last insn, this is a null pointer.

The first insn in the chain is obtained by calling `get_insns`; the last insn is the result of calling `get_last_insn`. Within the chain delimited by these insns, the `NEXT_INSN` and `PREV_INSN` pointers must always correspond: if insn is not the first insn,

    NEXT_INSN (PREV_INSN (insn)) == insn

is always true and if insn is not the last insn,

    PREV_INSN (NEXT_INSN (insn)) == insn

is always true.

After delay slot scheduling, some of the insns in the chain might be `sequence` expressions, which contain a vector of insns. The value of `NEXT_INSN` in all but the last of these insns is the next insn in the vector; the value of `NEXT_INSN` of the last insn in the vector is the same as the value of `NEXT_INSN` for the `sequence` in which it is contained. Similar rules apply for `PREV_INSN`.

This means that the above invariants are not necessarily true for insns inside `sequence` expressions. Specifically, if insn is the first insn in a `sequence`, `NEXT_INSN (PREV_INSN (insn))` is the insn containing the `sequence` expression, as is the value of `PREV_INSN (NEXT_INSN (insn))` is insn is the last insn in the `sequence` expression. You can use these expressions to find the containing `sequence` expression.

Every insn has one of the following six expression codes:

`insn`
  The expression code `insn` is used for instructions that do not jump and do not do function calls. `sequence` expressions are always contained in insns with code `insn` even if one of those insns should jump or do function calls. Insns with code `insn` have four additional fields beyond the three mandatory ones listed above. These four are described in a table below.
`jump_insn`
  The expression code `jump_insn` is used for instructions that may jump (or, more generally, may contain `label_ref` expressions). If there is an instruction to return from the current function, it is recorded as a `jump_insn`. `jump_insn` insns have the same extra fields as `insn` insns, accessed in the same way and in addition contain a field `JUMP_LABEL` which is defined once jump optimization has completed. For simple conditional and unconditional jumps, this field contains the `code_label` to which this insn will (possibly conditionally) branch. In a more complex jump, `JUMP_LABEL` records one of the labels that the insn refers to; the only way to find the others is to scan the entire body of the insn. Return insns count as jumps, but since they do not refer to any labels, they have zero in the `JUMP_LABEL` field.
`call_insn`
  The expression code `call_insn` is used for instructions that may do function calls. It is important to distinguish these instructions because they imply that certain registers and memory locations may be altered unpredictably. `call_insn` insns have the same extra fields as `insn` insns, accessed in the same way and in addition contain a field `CALL_INSN_FUNCTION_USAGE`, which contains a list (chain of `expr_list` expressions) containing `use` and `clobber` expressions that denote hard registers used or clobbered by the called function. A register specified in a `clobber` in this list is modified after the execution of the `call_insn`, while a register in a `clobber` in the body of the `call_insn` is clobbered before the insn completes execution. `clobber` expressions in this list augment registers specified in `CALL_USED_REGISTERS` (see section Basic Characteristics of Registers).
`code_label`
  A `code_label` insn represents a label that a jump insn can jump to. It contains two special fields of data in addition to the three standard ones. `CODE_LABEL_NUMBER` is used to hold the label number, a number that identifies this label uniquely among all the labels in the compilation (not just in the current function). Ultimately, the label is represented in the assembler output as an assembler label, usually of the form ``Ln'` where n is the label number. When a `code_label` appears in an RTL expression, it normally appears within a `label_ref` which represents the address of the label, as a number. The field `LABEL_NUSES` is only defined once the jump optimization phase is completed and contains the number of times this label is referenced in the current function.
`barrier`
  Barriers are placed in the instruction stream when control cannot flow past them. They are placed after unconditional jump instructions to indicate that the jumps are unconditional and after calls to `volatile` functions, which do not return (e.g., `exit`). They contain no information beyond the three standard fields.
`note`
  `note` insns are used to represent additional debugging and declarative information. They contain two nonstandard fields, an integer which is accessed with the macro `NOTE_LINE_NUMBER` and a string accessed with `NOTE_SOURCE_FILE`. If `NOTE_LINE_NUMBER` is positive, the note represents the position of a source line and `NOTE_SOURCE_FILE` is the source file name that the line came from. These notes control generation of line number data in the assembler output. Otherwise, `NOTE_LINE_NUMBER` is not really a line number but a code with one of the following values (and `NOTE_SOURCE_FILE` must contain a null pointer):
  `NOTE_INSN_DELETED`
    Such a note is completely ignorable. Some passes of the compiler delete insns by altering them into notes of this kind.
  `NOTE_INSN_BLOCK_BEG`
  `NOTE_INSN_BLOCK_END`
    These types of notes indicate the position of the beginning and end of a level of scoping of variable names. They control the output of debugging information.
  `NOTE_INSN_LOOP_BEG`
  `NOTE_INSN_LOOP_END`
    These types of notes indicate the position of the beginning and end of a `while` or `for` loop. They enable the loop optimizer to find loops quickly.
  `NOTE_INSN_LOOP_CONT`
    Appears at the place in a loop that `continue` statements jump to.
  `NOTE_INSN_LOOP_VTOP`
    This note indicates the place in a loop where the exit test begins for those loops in which the exit test has been duplicated. This position becomes another virtual start of the loop when considering loop invariants.
  `NOTE_INSN_FUNCTION_END`
    Appears near the end of the function body, just before the label that `return` statements jump to (on machine where a single instruction does not suffice for returning). This note may be deleted by jump optimization.
  `NOTE_INSN_SETJMP`
    Appears following each call to `setjmp` or a related function.
  These codes are printed symbolically when they appear in debugging dumps.

The machine mode of an insn is normally `VOIDmode`, but some phases use the mode for various purposes; for example, the reload pass sets it to `HImode` if the insn needs reloading but not register elimination and `QImode` if both are required. The common subexpression elimination pass sets the mode of an insn to `QImode` when it is the first insn in a block that has already been processed.

Here is a table of the extra fields of `insn`, `jump_insn` and `call_insn` insns:

`PATTERN (i)`
  An expression for the side effect performed by this insn. This must be one of the following codes: `set`, `call`, `use`, `clobber`, `return`, `asm_input`, `asm_output`, `addr_vec`, `addr_diff_vec`, `trap_if`, `unspec`, `unspec_volatile`, `parallel`, or `sequence`. If it is a `parallel`, each element of the `parallel` must be one these codes, except that `parallel` expressions cannot be nested and `addr_vec` and `addr_diff_vec` are not permitted inside a `parallel` expression.
`INSN_CODE (i)`
  An integer that says which pattern in the machine description matches this insn, or -1 if the matching has not yet been attempted. Such matching is never attempted and this field remains -1 on an insn whose pattern consists of a single `use`, `clobber`, `asm_input`, `addr_vec` or `addr_diff_vec` expression. Matching is also never attempted on insns that result from an `asm` statement. These contain at least one `asm_operands` expression. The function `asm_noperands` returns a non-negative value for such insns. In the debugging output, this field is printed as a number followed by a symbolic representation that locates the pattern in the ``md'` file as some small positive or negative offset from a named pattern.
`LOG_LINKS (i)`
  A list (chain of `insn_list` expressions) giving information about dependencies between instructions within a basic block. Neither a jump nor a label may come between the related insns.
`REG_NOTES (i)`
  A list (chain of `expr_list` and `insn_list` expressions) giving miscellaneous information about the insn. It is often information pertaining to the registers used in this insn.

The `LOG_LINKS` field of an insn is a chain of `insn_list` expressions. Each of these has two operands: the first is an insn, and the second is another `insn_list` expression (the next one in the chain). The last `insn_list` in the chain has a null pointer as second operand. The significant thing about the chain is which insns appear in it (as first operands of `insn_list` expressions). Their order is not significant.

This list is originally set up by the flow analysis pass; it is a null pointer until then. Flow only adds links for those data dependencies which can be used for instruction combination. For each insn, the flow analysis pass adds a link to insns which store into registers values that are used for the first time in this insn. The instruction scheduling pass adds extra links so that every dependence will be represented. Links represent data dependencies, antidependencies and output dependencies; the machine mode of the link distinguishes these three types: antidependencies have mode `REG_DEP_ANTI`, output dependencies have mode `REG_DEP_OUTPUT`, and data dependencies have mode `VOIDmode`.

The `REG_NOTES` field of an insn is a chain similar to the `LOG_LINKS` field but it includes `expr_list` expressions in addition to `insn_list` expressions. There are several kinds of register notes, which are distinguished by the machine mode, which in a register note is really understood as being an `enum reg_note`. The first operand op of the note is data whose meaning depends on the kind of note.

The macro `REG_NOTE_KIND (x)` returns the kind of register note. Its counterpart, the macro `PUT_REG_NOTE_KIND (x, newkind)` sets the register note type of x to be newkind.

Register notes are of three classes: They may say something about an input to an insn, they may say something about an output of an insn, or they may create a linkage between two insns. There are also a set of values that are only used in `LOG_LINKS`.

These register notes annotate inputs to an insn:

`REG_DEAD`
  The value in op dies in this insn; that is to say, altering the value immediately after this insn would not affect the future behavior of the program. This does not necessarily mean that the register op has no useful value after this insn since it may also be an output of the insn. In such a case, however, a `REG_DEAD` note would be redundant and is usually not present until after the reload pass, but no code relies on this fact.
`REG_INC`
  The register op is incremented (or decremented; at this level there is no distinction) by an embedded side effect inside this insn. This means it appears in a `post_inc`, `pre_inc`, `post_dec` or `pre_dec` expression.
`REG_NONNEG`
  The register op is known to have a nonnegative value when this insn is reached. This is used so that decrement and branch until zero instructions, such as the m68k dbra, can be matched. The `REG_NONNEG` note is added to insns only if the machine description has a ``decrement_and_branch_until_zero'` pattern.
`REG_NO_CONFLICT`
  This insn does not cause a conflict between op and the item being set by this insn even though it might appear that it does. In other words, if the destination register and op could otherwise be assigned the same register, this insn does not prevent that assignment. Insns with this note are usually part of a block that begins with a `clobber` insn specifying a multi-word pseudo register (which will be the output of the block), a group of insns that each set one word of the value and have the `REG_NO_CONFLICT` note attached, and a final insn that copies the output to itself with an attached `REG_EQUAL` note giving the expression being computed. This block is encapsulated with `REG_LIBCALL` and `REG_RETVAL` notes on the first and last insns, respectively.
`REG_LABEL`
  This insn uses op, a `code_label`, but is not a `jump_insn`. The presence of this note allows jump optimization to be aware that op is, in fact, being used.

The following notes describe attributes of outputs of an insn:

`REG_EQUIV`
`REG_EQUAL`
  This note is only valid on an insn that sets only one register and indicates that that register will be equal to op at run time; the scope of this equivalence differs between the two types of notes. The value which the insn explicitly copies into the register may look different from op, but they will be equal at run time. If the output of the single `set` is a `strict_low_part` expression, the note refers to the register that is contained in `SUBREG_REG` of the `subreg` expression. For `REG_EQUIV`, the register is equivalent to op throughout the entire function, and could validly be replaced in all its occurrences by op. ("Validly" here refers to the data flow of the program; simple replacement may make some insns invalid.) For example, when a constant is loaded into a register that is never assigned any other value, this kind of note is used. When a parameter is copied into a pseudo-register at entry to a function, a note of this kind records that the register is equivalent to the stack slot where the parameter was passed. Although in this case the register may be set by other insns, it is still valid to replace the register by the stack slot throughout the function. In the case of `REG_EQUAL`, the register that is set by this insn will be equal to op at run time at the end of this insn but not necessarily elsewhere in the function. In this case, op is typically an arithmetic expression. For example, when a sequence of insns such as a library call is used to perform an arithmetic operation, this kind of note is attached to the insn that produces or copies the final value. These two notes are used in different ways by the compiler passes. `REG_EQUAL` is used by passes prior to register allocation (such as common subexpression elimination and loop optimization) to tell them how to think of that value. `REG_EQUIV` notes are used by register allocation to indicate that there is an available substitute expression (either a constant or a `mem` expression for the location of a parameter on the stack) that may be used in place of a register if insufficient registers are available. Except for stack homes for parameters, which are indicated by a `REG_EQUIV` note and are not useful to the early optimization passes and pseudo registers that are equivalent to a memory location throughout there entire life, which is not detected until later in the compilation, all equivalences are initially indicated by an attached `REG_EQUAL` note. In the early stages of register allocation, a `REG_EQUAL` note is changed into a `REG_EQUIV` note if op is a constant and the insn represents the only set of its destination register. Thus, compiler passes prior to register allocation need only check for `REG_EQUAL` notes and passes subsequent to register allocation need only check for `REG_EQUIV` notes.
`REG_UNUSED`
  The register op being set by this insn will not be used in a subsequent insn. This differs from a `REG_DEAD` note, which indicates that the value in an input will not be used subsequently. These two notes are independent; both may be present for the same register.
`REG_WAS_0`
  The single output of this insn contained zero before this insn. op is the insn that set it to zero. You can rely on this note if it is present and op has not been deleted or turned into a `note`; its absence implies nothing.

These notes describe linkages between insns. They occur in pairs: one insn has one of a pair of notes that points to a second insn, which has the inverse note pointing back to the first insn.

`REG_RETVAL`
  This insn copies the value of a multi-insn sequence (for example, a library call), and op is the first insn of the sequence (for a library call, the first insn that was generated to set up the arguments for the library call). Loop optimization uses this note to treat such a sequence as a single operation for code motion purposes and flow analysis uses this note to delete such sequences whose results are dead. A `REG_EQUAL` note will also usually be attached to this insn to provide the expression being computed by the sequence.
`REG_LIBCALL`
  This is the inverse of `REG_RETVAL`: it is placed on the first insn of a multi-insn sequence, and it points to the last one.
`REG_CC_SETTER`
`REG_CC_USER`
  On machines that use `cc0`, the insns which set and use `cc0` set and use `cc0` are adjacent. However, when branch delay slot filling is done, this may no longer be true. In this case a `REG_CC_USER` note will be placed on the insn setting `cc0` to point to the insn using `cc0` and a `REG_CC_SETTER` note will be placed on the insn using `cc0` to point to the insn setting `cc0`.

These values are only used in the `LOG_LINKS` field, and indicate the type of dependency that each link represents. Links which indicate a data dependence (a read after write dependence) do not use any code, they simply have mode `VOIDmode`, and are printed without any descriptive text.

`REG_DEP_ANTI`
  This indicates an anti dependence (a write after read dependence).
`REG_DEP_OUTPUT`
  This indicates an output dependence (a write after write dependence).

For convenience, the machine mode in an `insn_list` or `expr_list` is printed using these symbolic codes in debugging dumps.

The only difference between the expression codes `insn_list` and `expr_list` is that the first operand of an `insn_list` is assumed to be an insn and is printed in debugging dumps as the insn's unique id; the first operand of an `expr_list` is printed in the ordinary way as an expression.

## RTL Representation of Function-Call Insns

Insns that call subroutines have the RTL expression code `call_insn`. These insns must satisfy special rules, and their bodies must use a special RTL expression code, `call`.

A `call` expression has two operands, as follows:

    (call (mem:fm addr) nbytes)

Here nbytes is an operand that represents the number of bytes of argument data being passed to the subroutine, fm is a machine mode (which must equal as the definition of the `FUNCTION_MODE` macro in the machine description) and addr represents the address of the subroutine.

For a subroutine that returns no value, the `call` expression as shown above is the entire body of the insn, except that the insn might also contain `use` or `clobber` expressions.

For a subroutine that returns a value whose mode is not `BLKmode`, the value is returned in a hard register. If this register's number is r, then the body of the call insn looks like this:

    (set (reg:m r)
         (call (mem:fm addr) nbytes))

This RTL expression makes it clear (to the optimizer passes) that the appropriate register receives a useful value in this insn.

When a subroutine returns a `BLKmode` value, it is handled by passing to the subroutine the address of a place to store the value. So the call insn itself does not "return" any value, and it has the same RTL form as a call that returns nothing.

On some machines, the call instruction itself clobbers some register, for example to contain the return address. `call_insn` insns on these machines should have a body which is a `parallel` that contains both the `call` expression and `clobber` expressions that indicate which registers are destroyed. Similarly, if the call instruction requires some register other than the stack pointer that is not explicitly mentioned it its RTL, a `use` subexpression should mention that register.

Functions that are called are assumed to modify all registers listed in the configuration macro `CALL_USED_REGISTERS` (see section Basic Characteristics of Registers) and, with the exception of `const` functions and library calls, to modify all of memory.

Insns containing just `use` expressions directly precede the `call_insn` insn to indicate which registers contain inputs to the function. Similarly, if registers other than those in `CALL_USED_REGISTERS` are clobbered by the called function, insns containing a single `clobber` follow immediately after the call to indicate which registers.

## Structure Sharing Assumptions

The compiler assumes that certain kinds of RTL expressions are unique; there do not exist two distinct objects representing the same value. In other cases, it makes an opposite assumption: that no RTL expression object of a certain kind appears in more than one place in the containing structure.

These assumptions refer to a single function; except for the RTL objects that describe global variables and external functions, and a few standard objects such as small integer constants, no RTL objects are common to two functions.

- Each pseudo-register has only a single `reg` object to represent it, and therefore only a single machine mode.
- For any symbolic label, there is only one `symbol_ref` object referring to it.
- There is only one `const_int` expression with value 0, only one with value 1, and only one with value -1. Some other integer values are also stored uniquely.
- There is only one `pc` expression.
- There is only one `cc0` expression.
- There is only one `const_double` expression with value 0 for each floating point mode. Likewise for values 1 and 2.
- No `label_ref` or `scratch` appears in more than one place in the RTL structure; in other words, it is safe to do a tree-walk of all the insns in the function and assume that each time a `label_ref` or `scratch` is seen it is distinct from all others that are seen.
- Only one `mem` object is normally created for each static variable or stack slot, so these objects are frequently shared in all the places they appear. However, separate but equal objects for these variables are occasionally made.
- When a single `asm` statement has multiple output operands, a distinct `asm_operands` expression is made for each output operand. However, these all share the vector which contains the sequence of input operands. This sharing is used later on to test whether two `asm_operands` expressions come from the same statement, so all optimizations must carefully preserve the sharing if they copy the vector at all.
- No RTL object appears in more than one place in the RTL structure except as described above. Many passes of the compiler rely on this by assuming that they can modify RTL objects in place without unwanted side-effects on other insns.
- During initial RTL generation, shared structure is freely introduced. After all the RTL for a function has been generated, all shared structure is copied by `unshare_all_rtl` in ``emit-rtl.c'`, after which the above rules are guaranteed to be followed.
- During the combiner pass, shared structure within an insn can exist temporarily. However, the shared structure is copied before the combiner is finished with the insn. This is done by calling `copy_rtx_if_shared`, which is a subroutine of `unshare_all_rtl`.

## Reading RTL

To read an RTL object from a file, call `read_rtx`. It takes one argument, a stdio stream, and returns a single RTL object.

Reading RTL from a file is very slow. This is no currently not a problem because reading RTL occurs only as part of building the compiler.

People frequently have the idea of using RTL stored as text in a file as an interface between a language front end and the bulk of GNU CC. This idea is not feasible.

GNU CC was designed to use RTL internally only. Correct RTL for a given program is very dependent on the particular target machine. And the RTL does not contain all the information about the program.

The proper way to interface GNU CC to a new language front end is with the "tree" data structure. There is no manual for this data structure, but it is described in the files ``tree.h'` and ``tree.def'`.

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