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14.14 Condition Code Status

This describes the condition code status.

The file conditions.h defines a variable cc_status to describe how the condition code was computed (in case the interpretation of the condition code depends on the instruction that it was set by). This variable contains the RTL expressions on which the condition code is currently based, and several standard flags.

Sometimes additional machine-specific flags must be defined in the machine description header file. It can also add additional machine-specific information by defining CC_STATUS_MDEP.

— Macro: CC_STATUS_MDEP

C code for a data type which is used for declaring the mdep component of cc_status. It defaults to int.

This macro is not used on machines that do not use cc0.

— Macro: CC_STATUS_MDEP_INIT

A C expression to initialize the mdep field to “empty”. The default definition does nothing, since most machines don't use the field anyway. If you want to use the field, you should probably define this macro to initialize it.

This macro is not used on machines that do not use cc0.

— Macro: NOTICE_UPDATE_CC (exp, insn)

A C compound statement to set the components of cc_status appropriately for an insn insn whose body is exp. It is this macro's responsibility to recognize insns that set the condition code as a byproduct of other activity as well as those that explicitly set (cc0).

This macro is not used on machines that do not use cc0.

If there are insns that do not set the condition code but do alter other machine registers, this macro must check to see whether they invalidate the expressions that the condition code is recorded as reflecting. For example, on the 68000, insns that store in address registers do not set the condition code, which means that usually NOTICE_UPDATE_CC can leave cc_status unaltered for such insns. But suppose that the previous insn set the condition code based on location `a4@(102)' and the current insn stores a new value in `a4'. Although the condition code is not changed by this, it will no longer be true that it reflects the contents of `a4@(102)'. Therefore, NOTICE_UPDATE_CC must alter cc_status in this case to say that nothing is known about the condition code value.

The definition of NOTICE_UPDATE_CC must be prepared to deal with the results of peephole optimization: insns whose patterns are parallel RTXs containing various reg, mem or constants which are just the operands. The RTL structure of these insns is not sufficient to indicate what the insns actually do. What NOTICE_UPDATE_CC should do when it sees one is just to run CC_STATUS_INIT.

A possible definition of NOTICE_UPDATE_CC is to call a function that looks at an attribute (see Insn Attributes) named, for example, `cc'. This avoids having detailed information about patterns in two places, the md file and in NOTICE_UPDATE_CC.

— Macro: SELECT_CC_MODE (op, x, y)

Returns a mode from class MODE_CC to be used when comparison operation code op is applied to rtx x and y. For example, on the SPARC, SELECT_CC_MODE is defined as (see see Jump Patterns for a description of the reason for this definition)

          #define SELECT_CC_MODE(OP,X,Y) \
            (GET_MODE_CLASS (GET_MODE (X)) == MODE_FLOAT          \
             ? ((OP == EQ || OP == NE) ? CCFPmode : CCFPEmode)    \
             : ((GET_CODE (X) == PLUS || GET_CODE (X) == MINUS    \
                 || GET_CODE (X) == NEG) \
                ? CC_NOOVmode : CCmode))
     

You should define this macro if and only if you define extra CC modes in machine-modes.def.

— Macro: CANONICALIZE_COMPARISON (code, op0, op1)

On some machines not all possible comparisons are defined, but you can convert an invalid comparison into a valid one. For example, the Alpha does not have a GT comparison, but you can use an LT comparison instead and swap the order of the operands.

On such machines, define this macro to be a C statement to do any required conversions. code is the initial comparison code and op0 and op1 are the left and right operands of the comparison, respectively. You should modify code, op0, and op1 as required.

GCC will not assume that the comparison resulting from this macro is valid but will see if the resulting insn matches a pattern in the md file.

You need not define this macro if it would never change the comparison code or operands.

— Macro: REVERSIBLE_CC_MODE (mode)

A C expression whose value is one if it is always safe to reverse a comparison whose mode is mode. If SELECT_CC_MODE can ever return mode for a floating-point inequality comparison, then REVERSIBLE_CC_MODE (mode) must be zero.

You need not define this macro if it would always returns zero or if the floating-point format is anything other than IEEE_FLOAT_FORMAT. For example, here is the definition used on the SPARC, where floating-point inequality comparisons are always given CCFPEmode:

          #define REVERSIBLE_CC_MODE(MODE)  ((MODE) != CCFPEmode)
     
— Macro: REVERSE_CONDITION (code, mode)

A C expression whose value is reversed condition code of the code for comparison done in CC_MODE mode. The macro is used only in case REVERSIBLE_CC_MODE (mode) is nonzero. Define this macro in case machine has some non-standard way how to reverse certain conditionals. For instance in case all floating point conditions are non-trapping, compiler may freely convert unordered compares to ordered one. Then definition may look like:

          #define REVERSE_CONDITION(CODE, MODE) \
             ((MODE) != CCFPmode ? reverse_condition (CODE) \
              : reverse_condition_maybe_unordered (CODE))
     
— Macro: REVERSE_CONDEXEC_PREDICATES_P (op1, op2)

A C expression that returns true if the conditional execution predicate op1, a comparison operation, is the inverse of op2 and vice versa. Define this to return 0 if the target has conditional execution predicates that cannot be reversed safely. There is no need to validate that the arguments of op1 and op2 are the same, this is done separately. If no expansion is specified, this macro is defined as follows:

          #define REVERSE_CONDEXEC_PREDICATES_P (x, y) \
             (GET_CODE ((x)) == reversed_comparison_code ((y), NULL))
     
— Target Hook: bool TARGET_FIXED_CONDITION_CODE_REGS (unsigned int *, unsigned int *)

On targets which do not use (cc0), and which use a hard register rather than a pseudo-register to hold condition codes, the regular CSE passes are often not able to identify cases in which the hard register is set to a common value. Use this hook to enable a small pass which optimizes such cases. This hook should return true to enable this pass, and it should set the integers to which its arguments point to the hard register numbers used for condition codes. When there is only one such register, as is true on most systems, the integer pointed to by the second argument should be set to INVALID_REGNUM.

The default version of this hook returns false.

— Target Hook: enum machine_mode TARGET_CC_MODES_COMPATIBLE (enum machine_mode, enum machine_mode)

On targets which use multiple condition code modes in class MODE_CC, it is sometimes the case that a comparison can be validly done in more than one mode. On such a system, define this target hook to take two mode arguments and to return a mode in which both comparisons may be validly done. If there is no such mode, return VOIDmode.

The default version of this hook checks whether the modes are the same. If they are, it returns that mode. If they are different, it returns VOIDmode.