Re: [PATCH] [x86]Don't optimize cmp mem, 0 to load mem, reg + test reg, reg

[Uros Bizjak <ubizjak@gmail.com>]

On Fri, Sep 16, 2022 at 3:32 AM Jeff Law via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
>
> On 9/15/22 19:06, liuhongt via Gcc-patches wrote:
> > There's peephole2 submit in 1990s which split cmp mem, 0 to load mem,
> > reg + test reg, reg. I don't know exact reason why gcc do this.
> >
> > For latest x86 processors, ciscization should help processor frontend
> > also codesize, for processor backend, they should be the same(has same
> > uops).
> >
> > So the patch deleted the peephole2, and also modify another splitter to
> > generate more cmp mem, 0 for 32-bit target.
> >
> > It will help instruction fetch.
> >
> > for minmax-1.c minmax-2.c minmax-10, pr96891.c, it's supposed to scan there's no
> > comparison to 1 or -1, so adjust the testcase since under 32-bit
> > target, we now generate cmp mem, 0 instead of load + test.
> >
> > Similar for pr78035.c.
> >
> > Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}
> > No performance impact for SPEC2017 on ICX/Znver3.
> >
> > Ok for trunk?
> >
> > gcc/ChangeLog:
> >
> >       * config/i386/i386.md (*<code><mode>3_1): Replace
> >       register_operand with nonimmediate_operand for operand 1. Also
> >       force_reg it when mode is QImode.
> >       (define_peephole2): Deleted related peephole2.
> >
> > gcc/testsuite/ChangeLog:
> >
> >       * gcc.target/i386/minmax-1.c: Scan-assemble-not for cmp with 1
> >       or -1, also don't scan-assembler test for ia32.
> >       * gcc.target/i386/minmax-10.c: Ditto.
> >       * gcc.target/i386/minmax-2.c: Ditto.
> >       * gcc.target/i386/pr78035.c: Ditto.
> >       * gcc.target/i386/pr96861.c: Scan either cmp or test 3 times.
>
> It was almost certainly for PPro/P2 given it was rth's work from
> 1999.    Probably should have been conditionalized on PPro/P2 at the
> time.   No worries losing it now...

Please add a tune flag in x86-tune.def under "Historical relics" and
use it in the relevant peephole2 instead of deleting it.

Uros.