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16.12 Defining Jump Instruction Patterns

GCC does not assume anything about how the machine realizes jumps. The machine description should define a single pattern, usually a define_expand, which expands to all the required insns.

Usually, this would be a comparison insn to set the condition code and a separate branch insn testing the condition code and branching or not according to its value. For many machines, however, separating compares and branches is limiting, which is why the more flexible approach with one define_expand is used in GCC. The machine description becomes clearer for architectures that have compare-and-branch instructions but no condition code. It also works better when different sets of comparison operators are supported by different kinds of conditional branches (e.g. integer vs. floating-point), or by conditional branches with respect to conditional stores.

Two separate insns are always used if the machine description represents a condition code register using the legacy RTL expression (cc0), and on most machines that use a separate condition code register (see Condition Code). For machines that use (cc0), in fact, the set and use of the condition code must be separate and adjacent1, thus allowing flags in cc_status to be used (see Condition Code) and so that the comparison and branch insns could be located from each other by using the functions prev_cc0_setter and next_cc0_user.

Even in this case having a single entry point for conditional branches is advantageous, because it handles equally well the case where a single comparison instruction records the results of both signed and unsigned comparison of the given operands (with the branch insns coming in distinct signed and unsigned flavors) as in the x86 or SPARC, and the case where there are distinct signed and unsigned compare instructions and only one set of conditional branch instructions as in the PowerPC.


[1] note insns can separate them, though.