10.8 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 bytenum)

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-part 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, bytenum 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 bytenum offset 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 byte number zero is part of the most significant word; otherwise, it is part of the least significant word.

The compilation parameter BYTES_BIG_ENDIAN, if set to 1, says that byte number zero is the most significant byte within a word; otherwise, it is the least significant byte within a word.

On a few targets, FLOAT_WORDS_BIG_ENDIAN disagrees with WORDS_BIG_ENDIAN. However, most parts of the compiler treat floating point values as if they had the same endianness as integer values. This works because they handle them solely as a collection of integer values, with no particular numerical value. Only real.c and the runtime libraries care about FLOAT_WORDS_BIG_ENDIAN.

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) 4) 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_BYTE 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 Side Effects).

(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 Condition Code. 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 alias)

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. alias specifies an alias set for the reference. In general two items are in different alias sets if they cannot reference the same memory address.

The construct (mem:BLK (scratch)) is considered to alias all other memories. Thus it may be used as a memory barrier in epilogue stack deallocation patterns.

(addressof:m reg)

This RTX represents a request for the address of register reg. Its mode is always Pmode. If there are any addressof expressions left in the function after CSE, reg is forced into the stack and the addressof expression is replaced with a plus expression for the address of its stack slot.