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It may not be worth thetrouble to describe the values that are stored, but it is essential toinform the compiler that the registers will be altered, lest itattempt to keep data in them across the string instruction. One place this is used is in string instructions that store standardvalues into particular hard registers. (clobber x) Represents the storing or possible storing of an unpredictable,undescribed value into x, which must be a reg, scratch or mem expression. The purpose of this mode is tospecify what kind of addressing is allowed, on machines where theallowed kinds of addressing depend on the machine mode beingaddressed. The machine description defines macro FUNCTION_MODE toexpand into the requisite mode name. nargs is an expression which can be used for two purposes: onsome machines it represents the number of bytes of stack argument onothers, it represents the number of argument registers.Įach machine has a standard machine mode which function musthave.
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function is a mem expressionwhose address is the address of the function to be called. (call function nargs) Represents a function call. Note that an insn pattern of (return) is logically equivalent to (set (pc) (return)), but the latter form is never used.
SIDE EFFECTS OF PARALLEL ON MAC PC
Inside an if_then_else expression, represents the value to beplaced in pc to return to the caller.
SIDE EFFECTS OF PARALLEL ON MAC CODE
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, andthe return expression code is never used. (return) As the sole expression in a pattern, represents a return from thecurrent function, on machines where this can be done with oneinstruction, such as Vaxes. Lval is customarily accessed with the SET_DEST macro and x with the SET_SRC macro. If lval is neither (cc0) nor (pc), the mode of lval must not be VOIDmode and the mode of x must bevalid for the mode of lval. 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 throughbranch tables. It may be an if_then_else (conditional jump), in which case either thesecond or the third operand must be (pc) (for the case whichdoes not jump) and the other of the two must be a label_ref(for the case which does jump). It may be a label_ref expression (unconditional jump). If lval is (pc), we have a jump instruction, and thepossibilities for x are very limited. 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.
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If lval is (cc0), it has no machine mode, and x maybe either a compare expression or a value that may have any mode.The latter case represents a ``test'' instruction. If lval is a strict_low_part of a subreg, then thepart of the register specified by the machine mode of the subreg is given the value x and the rest of the registeris not changed. Likewise, if lval is a subreg whose machine mode is narrower thanthe mode of the register, the rest of the register can be changed inan undefined way. If lval is a reg whose machine mode is less than the fullwidth of the register, then it means that the part of the registerspecified by the machine mode is given the specified value and therest of the register receives an undefined value. If lval is a reg, subreg or mem, it has amachine mode then x must be valid for that mode. lval must be an expressionrepresenting a place that can be stored in: reg (or subreg or strict_low_part), mem, pc or cc0.
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(set lval x) Represents the action of storing the value of x into the placerepresented by lval. The body of an instruction is always one of these side effect codes the codes described above, which represent values, appear only asthe operands of these. Specialexpression codes are used to represent side effects. The expression codes described so far represent values, not actions.But machine instructions never produce values they are meaningfulonly for their side effects on the state of the machine. Using and Porting GNU CC 14.12: Side Effect Expressions