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From: "unlvsur at live dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/107167] New: It looks like GCC wastes
 registers on trivial computations when result can be cached
Date: Thu, 06 Oct 2022 05:33:59 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 13.0
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X-Bugzilla-Who: unlvsur at live dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107167

            Bug ID: 107167
           Summary: It looks like GCC wastes registers on trivial
                    computations when result can be cached
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: unlvsur at live dot com
  Target Milestone: ---

I do not know whether it is a big issue or not with targets that provide to=
ns
of available registers (like aarch64 or loongarch64). However, this looks l=
ike
a big issue for x86_64 which only provides 16 general purpose registers (pl=
us
%rsp is reserved, so 15 available registers)
Take the example like this:

https://godbolt.org/z/77rEsr1PG

#include<bit>

unsigned Sigma1(unsigned x) noexcept
{
    return std::rotr(x,6)^std::rotr(x,11)^std::rotr(x,25);
}


GCC generates code like this to avoid dependencies.
Sigma1m(unsigned int):
        movl    %edi, %eax
        movl    %edi, %edx
        roll    $7, %edi
        rorl    $6, %eax
        rorl    $11, %edx
        xorl    %edx, %eax
        xorl    %edi, %eax
        ret

However:
mySigma1m(unsigned int):
        movl    %edi, %eax
        rorl    $6, %edi
        rorl    $11, %eax
        xorl    %edi, %eax
        rorl    $19, %edi
        xorl    %edi, %eax
        ret

Saves one register in this task. That becomes a huge problem when tons of
computation are involved where registers are in a position of shortage.

1st one also generates 1 more instruction and it can affect the code cache.

Aggressively utilizing all registers may not give the best results. Local
maximum =3D/=3D Global maximum.
I don't know.=