On Wed, Sep 26, 2012 at 1:11 PM, Cynbe ru Taren <cynbe@xxxxxxx> wrote: > > Ian Lance Taylor <iant@xxxxxxxxxx> writes: > >> No, addsi3 is a named pattern and you could and should use define_expand. >> >> Ian > > That is an excellent hint -- thank you! > > Might I ask what the advantage of define_expand is in this case? You can generate a different insn sequence if the constant is too large. > Is it that match_scratch really works only in define_expand and > not in define_insn, say? No, match_scratch can work. But the scheduler won't be able to see the sequence of instructions, and neither will CSE, etc. > And if I want to generate the single-instruction form 99.999% of > the time, using define_expand to actually expand into multiple > insns seems chancy -- isn't there a risk that the insns might get > re-ordered such that they don't get combined back into a > single-instruction solution in combine()? No. A define_expand generates one or more insns that must match define_insn patterns. Your define_insn pattern will use a constraint that only permits constants that are in range for the instruction. Ian
