On Wed, Jun 28, 2023 at 8:23 PM Jeff Law <jeffreya...@gmail.com> wrote: > > > > On 6/28/23 06:39, Christoph Müllner wrote: > > >>> +;; XTheadMemIdx overview: > >>> +;; All peephole passes attempt to improve the operand utilization of > >>> +;; XTheadMemIdx instructions, where one sign or zero extended > >>> +;; register-index-operand can be shifted left by a 2-bit immediate. > >>> +;; > >>> +;; The basic idea is the following optimization: > >>> +;; (set (reg 0) (op (reg 1) (imm 2))) > >>> +;; (set (reg 3) (mem (plus (reg 0) (reg 4))) > >>> +;; ==> > >>> +;; (set (reg 3) (mem (plus (reg 4) (op2 (reg 1) (imm 2)))) > >>> +;; This optimization only valid if (reg 0) has no further uses. > >> Couldn't this be done by combine if you created define_insn patterns > >> rather than define_peephole2 patterns? Similarly for the other cases > >> handled here. > > > > I was inspired by XTheadMemPair, which merges two memory accesses > > into a mem-pair instruction (and which got inspiration from > > gcc/config/aarch64/aarch64-ldpstp.md). > Right. I'm pretty familiar with those. They cover a different case, > specifically the two insns being optimized don't have a true data > dependency between them. ie, the first instruction does not produce a > result used in the second insn. > > > In the case above there is a data dependency on reg0. ie, the first > instruction generates a result used in the second instruction. combine > is usually the best place to handle the data dependency case.
Ok, understood. It is a bit of a special case here, because the peephole is restricted to those cases, where reg0 is not used elsewhere (peep2_reg_dead_p()). I have not seen how to do this for combiner optimizations. I found sh_remove_reg_dead_or_unused_notes(), which tests for reg notes on a given rtx_insn. In our case we have a pattern that matches two insns, where we have to test if one operand (reg0) is dead or unused after the second insn. The first insn can be accessed with "curr_insn", but I did not see how to access the second matching insn. Any ideas or hints? Thanks, Christoph > > > > > > I don't see the benefit of using combine or peephole, but I can change > > if necessary. At least for the provided test cases, the implementation > > works quite well. > Peepholes require the instructions to be consecutive in the stream while > combine relies on data dependence links and can thus find these > opportunities even when the two insn we care about are separated by > unrelated other insns. > > > Jeff
