Segher Boessenkool <seg...@kernel.crashing.org> writes: > Hi! > > On Thu, Sep 01, 2022 at 11:24:07AM +0800, Jiufu Guo wrote: >> Currently, these two splitters (touched in this patch) are using predicate >> `int_reg_operand_not_pseudo`, then they work in split2 pass after RA in >> most times, and can not run before RA. >> >> It would not be a bad idea to allow these splitters before RA. Then more >> passes (between split1 and split2) could optimize the emitted instructions. > > The splitters can be used earlier even. For example, often combine will > use them. Oh, yes! > >> And if splitting before RA, for these constant splitters, we may have more >> freedom to create pseduo to generate more parallel instructions. >> >> For the example in the leading patch [PATCH 1/2]: pli+plit+rldimi would be >> better than pli+sldi+paddi. > > Yes. If you split after reload you have to do all local optimisations > (that would have been done in earlier passes) manually. And all more > global ones (involving just one or two more insns already) are > essentially impossible to do. > > Splitting after reload is necessary in some cases. For example, all the > integer "dot" insns split to the base insn and an explicit compare, if > for some reason RA did not get cr0 here. Importantly, this happens very > seldomly: RA knows it is two insns instead of one, and it chooses > accordingly. Also it *has* to be after reload, it directly depends on > what RA chose to do. > > Splitting dependent on if a VSR or a GPR (pair) was used is a losing > proposition. It usually costs much more than it can gain. > Thanks for your detailed explain which makes more reasonable!
>> gcc/ChangeLog: >> >> * config/rs6000/rs6000.md (const splitter): Update predicate. > > * config/rs6000/rs6000.md (splitter for set to and_mask constants): > Use int_reg_operand (instead of int_reg_operand_not_pseudo). > (splitter for multi-insn constant loads): Ditto. > > You should mention the changed to *both* splitters. For nameless > splitters it helps if you can describe it a bit. This is hard, yes :-/ > Thanks for your always helpful comments! BR, Jeff(Jiufu) > Okay for trunk like that. Thanks! > > > Segher
