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14.7.1 Machine-Independent Predicates

These are the generic predicates available to all back ends. They are defined in recog.c. The first category of predicates allow only constant, or immediate, operands.

— Function: immediate_operand

This predicate allows any sort of constant that fits in mode. It is an appropriate choice for instructions that take operands that must be constant.

— Function: const_int_operand

This predicate allows any CONST_INT expression that fits in mode. It is an appropriate choice for an immediate operand that does not allow a symbol or label.

— Function: const_double_operand

This predicate accepts any CONST_DOUBLE expression that has exactly mode. If mode is VOIDmode, it will also accept CONST_INT. It is intended for immediate floating point constants.

The second category of predicates allow only some kind of machine register.

— Function: register_operand

This predicate allows any REG or SUBREG expression that is valid for mode. It is often suitable for arithmetic instruction operands on a RISC machine.

— Function: pmode_register_operand

This is a slight variant on register_operand which works around a limitation in the machine-description reader. 

          (match_operand n "pmode_register_operand" constraint)

means exactly what 

          (match_operand:P n "register_operand" constraint)

would mean, if the machine-description reader accepted `:P' mode suffixes. Unfortunately, it cannot, because Pmode is an alias for some other mode, and might vary with machine-specific options. See Misc.

— Function: scratch_operand

This predicate allows hard registers and SCRATCH expressions, but not pseudo-registers. It is used internally by match_scratch; it should not be used directly.

The third category of predicates allow only some kind of memory reference.

— Function: memory_operand

This predicate allows any valid reference to a quantity of mode mode in memory, as determined by the weak form of GO_IF_LEGITIMATE_ADDRESS (see Addressing Modes).

— Function: address_operand

This predicate is a little unusual; it allows any operand that is a valid expression for the address of a quantity of mode mode, again determined by the weak form of GO_IF_LEGITIMATE_ADDRESS. To first order, if `(mem:mode (exp))' is acceptable to memory_operand, then exp is acceptable to address_operand. Note that exp does not necessarily have the mode mode.

— Function: indirect_operand

This is a stricter form of memory_operand which allows only memory references with a general_operand as the address expression. New uses of this predicate are discouraged, because general_operand is very permissive, so it's hard to tell what an indirect_operand does or does not allow. If a target has different requirements for memory operands for different instructions, it is better to define target-specific predicates which enforce the hardware's requirements explicitly.

— Function: push_operand

This predicate allows a memory reference suitable for pushing a value onto the stack. This will be a MEM which refers to stack_pointer_rtx, with a side-effect in its address expression (see Incdec); which one is determined by the STACK_PUSH_CODE macro (see Frame Layout).

— Function: pop_operand

This predicate allows a memory reference suitable for popping a value off the stack. Again, this will be a MEM referring to stack_pointer_rtx, with a side-effect in its address expression. However, this time STACK_POP_CODE is expected.

The fourth category of predicates allow some combination of the above operands.

— Function: nonmemory_operand

This predicate allows any immediate or register operand valid for mode.

— Function: nonimmediate_operand

This predicate allows any register or memory operand valid for mode.

— Function: general_operand

This predicate allows any immediate, register, or memory operand valid for mode.

Finally, there is one generic operator predicate.

— Function: comparison_operator

This predicate matches any expression which performs an arithmetic comparison in mode; that is, COMPARISON_P is true for the expression code.