> Are there any technical obstacles to introduce subst to
> define_{,insn_and_}split?gccint says: define_subst can be used only in define_insn and define_expand, it cannot be used in other expressions (e.g. in define_insn_and_split). I have no idea how to implement it in current infrastructure. > In the proposed patch, if the output register is also mentioned in the > input, then clearing before insn will clear the value in the input > register. The solution in the i386.md also takes care of this issue. > For this, I think we can add REGNO checks for operands in condition (which means there is true dependency). Uros Bizjak <ubiz...@gmail.com> 于2022年1月14日周五 16:37写道： > > On Fri, Jan 14, 2022 at 7:11 AM Hongyu Wang <wwwhhhyyy...@gmail.com> wrote: > > > > > > No, the approach is wrong. You have to solve output clearing on RTL > > > > level, please look at how e.g. tzcnt false dep is solved: > > > > > > Actually we have considered such approach before, but we found we need > > > to break original define_insn to remove the mask/rounding subst, > > > since define_split could not adopt subst, and that would add 6 more > > > define_insn_and_split and 4 define_insn for each instruction. We think > > > such approach would introduce too much redundant code. > > > > > > Do you think the code size increment is acceptable? > > > > Also that 100+ more patterns increases maintenance effort. If we split > > them at epilogue_complete stage, > > it seems not much difference to put it under output template... > > In the proposed patch, if the output register is also mentioned in the > input, then clearing before insn will clear the value in the input > register. The solution in the i386.md also takes care of this issue. > > Uros.
