GNU Compiler Collection (GCC) Internals: Side Effects

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13.15 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, strict_low_part or zero_extract), mem, pc, parallel, 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 a zero_extract, then the referenced part of the bit-field (a memory or register reference) specified by the zero_extract is given the value x and the rest of the bit-field is not changed. Note that sign_extract can not appear in lval.

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 a parallel, it is used to represent the case of a function returning a structure in multiple registers. Each element of the parallel is an expr_list whose first operand is a reg and whose second operand is a const_int representing the offset (in bytes) into the structure at which the data in that register corresponds. The first element may be null to indicate that the structure is also passed partly in memory.

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 VAXen. 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.

(simple_return)

Like (return), but truly represents only a function return, while (return) may represent an insn that also performs other functions of the function epilogue. Like (return), this may also occur in conditional jumps.

(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, parallel 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)) or (mem:BLK (scratch)), it means that all memory locations must be presumed clobbered. If x is a parallel, it has the same meaning as a parallel in a set expression.

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 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 if it is a hard register clobber. For pseudo-register clobber, the register allocator and the reload pass do not assign the same hard register to the clobber and the input operands if there is an insn alternative containing the ‘&’ constraint (see Modifiers) for the clobber and the hard register is in register classes of the clobber in the alternative. You can clobber either a specific hard register, a pseudo register, or a scratch expression; in the latter two cases, GCC 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.

In some situations, it may be tempting to add a use of a register in a parallel to describe a situation where the value of a special register will modify the behavior of the instruction. A hypothetical example might be a pattern for an addition that can either wrap around or use saturating addition depending on the value of a special control register:

(parallel [(set (reg:SI 2) (unspec:SI [(reg:SI 3)
                                       (reg:SI 4)] 0))
           (use (reg:SI 1))])

This will not work, several of the optimizers only look at expressions locally; it is very likely that if you have multiple insns with identical inputs to the unspec, they will be optimized away even if register 1 changes in between.

This means that use can only be used to describe that the register is live. You should think twice before adding use statements, more often you will want to use unspec instead. The use RTX is most commonly useful to describe that a fixed register is implicitly used in an insn. It is also safe to use in patterns where the compiler knows for other reasons that the result of the whole pattern is variable, such as ‘movmemm ’ or ‘call’ patterns.

During the reload phase, an insn that has a use as pattern can carry a reg_equal note. These use insns will be deleted before the reload phase exits.

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, simple_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.

(cond_exec [cond expr])

Represents a conditionally executed expression. The expr is executed only if the cond is nonzero. The cond expression must not have side-effects, but the expr may very well have side-effects.

(sequence [insns …])

Represents a sequence of insns. If a sequence appears in the chain of insns, then each of the insns that appears in the sequence must be suitable for appearing in the chain of insns, i.e. must satisfy the INSN_P predicate.

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 Delay Slots.

Some back ends also use sequence objects for purposes other than delay-slot groups. This is not supported in the common parts of the compiler, which treat such sequences as delay-slot groups.

DWARF2 Call Frame Address (CFA) adjustments are sometimes also expressed using sequence objects as the value of a RTX_FRAME_RELATED_P note. This only happens if the CFA adjustments cannot be easily derived from the pattern of the instruction to which the note is attached. In such cases, the value of the note is used instead of best-guesing the semantics of the instruction. The back end can attach notes containing a sequence of set patterns that express the effect of the parent instruction.

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 …] min max flags)

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. min and max are set up by branch shortening and hold a label with a minimum and a maximum address, respectively. flags indicates the relative position of base, min and max to the containing insn and of min and max to base. See rtl.def for details.

(prefetch:m addr rw locality)

Represents prefetch of memory at address addr. Operand rw is 1 if the prefetch is for data to be written, 0 otherwise; targets that do not support write prefetches should treat this as a normal prefetch. Operand locality specifies the amount of temporal locality; 0 if there is none or 1, 2, or 3 for increasing levels of temporal locality; targets that do not support locality hints should ignore this.

This insn is used to minimize cache-miss latency by moving data into a cache before it is accessed. It should use only non-faulting data prefetch instructions.

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