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(gcc-295.info)Side Effects
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'
(Note: 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 (Note: Modifiers.). 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 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',
`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. Note: Delay Slots.
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 cointaining insn and of MIN and MAX to BASE. See
rtl.def for details.
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