[PATCH] [i386] GLC tuning: Break false dependency for dest register.

[Hongyu Wang]

> 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 <ubizjak@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 <wwwhhhyyy333@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.