Re: [x86 PATCH] PR target/47949: Use xchg to move from/to AX_REG with -Oz.

[Uros Bizjak <ubizjak@gmail.com>]

On Tue, Aug 2, 2022 at 4:59 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch adds a peephole2 to i386.md to implement the suggestion in
> PR target/47949, of using xchg instead of mov for moving values to/from
> the %rax/%eax register, controlled by -Oz, as the xchg instruction is
> one byte shorter than the move it is replacing.
>
> The new test case is taken from the PR:
> int foo(int x) { return x; }
>
> where previously we'd generate:
> foo:    mov %edi,%eax  // 2 bytes
>         ret
>
> but with this patch, using -Oz, we generate:
> foo:    xchg %eax,%edi  // 1 byte
>         ret
>
> On the CSiBE benchmark, this saves a total of 10238 bytes (reducing
> the -Oz total from 3661796 bytes to 3651558 bytes, a 0.28% saving).
>
> Interestingly, some modern architectures (such as Zen 3) implement
> xchg using zero latency register renaming (just like mov), so in theory
> this transformation could be enabled when optimizing for speed, if
> benchmarking shows the improved code density produces consistently
> better performance.  However, this is architecture dependent, and
> there may be interactions using xchg (instead a single_set) in the
> late RTL passes (such as cprop_hardreg), so for now I've restricted
> this to -Oz.
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32},
> with no new failures.  Ok for mainline?
>
>
> 2022-08-02  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         PR target/47949
>         * config/i386/i386.md (peephole2): New peephole2 to convert
>         SWI48 moves to/from %rax/%eax where the src is dead to xchg,
>         when optimizing for minimal size with -Oz.
>
> gcc/testsuite/ChangeLog
>         PR target/47949
>         * gcc.target/i386/pr47949.c: New test case.

OK.

BTW: The testcase can be improved to cover Dimode as well as SImode case:

+/* { dg-do compile { target { ! ia32 } } } */
+/* { dg-options "-Oz" } */
+
+int foo(int x)

long foo (long x, long y)
{
   return y;
}

And add additional options of -mregparm=2 for ia32. This will move
%edx to %eax for 32bit and %rsi to %rax for 64bit targets.

Thanks,
Uros.