On many machines, the condition code may be produced by other instructions than compares, for example the branch can use directly the condition code set by a subtract instruction. However, on some machines when the condition code is set this way some bits (such as the overflow bit) are not set in the same way as a test instruction, so that a different branch instruction must be used for some conditional branches. When this happens, use the machine mode of the condition code register to record different formats of the condition code register. Modes can also be used to record which compare instruction (e.g. a signed or an unsigned comparison) produced the condition codes.
If other modes than
CCmodeare required, add them to machine-modes.def and define
SELECT_CC_MODEto choose a mode given an operand of a compare. This is needed because the modes have to be chosen not only during RTL generation but also, for example, by instruction combination. The result of
SELECT_CC_MODEshould be consistent with the mode used in the patterns; for example to support the case of the add on the SPARC discussed above, we have the pattern(define_insn "" [(set (reg:CC_NOOV 0) (compare:CC_NOOV (plus:SI (match_operand:SI 0 "register_operand" "%r") (match_operand:SI 1 "arith_operand" "rI")) (const_int 0)))] "" "...")
together with a
CC_NOOVmodefor comparisons whose argument is a
plus:#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))
Another reason to use modes is to retain information on which operands were used by the comparison; see
REVERSIBLE_CC_MODElater in this section.
You should define this macro if and only if you define extra CC modes in machine-modes.def.
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
GTcomparison, but you can use an
LTcomparison 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.
A C expression whose value is one if it is always safe to reverse a comparison whose mode is mode. If
SELECT_CC_MODEcan ever return mode for a floating-point inequality comparison, then
)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)
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
)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))
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 p2 should be set to
The default version of this hook returns false.
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
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