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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/94804] Failure to elide useless movs in
 128-bit addition with __int128_t arguments
Date: Thu, 19 Jan 2023 23:20:29 +0000
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X-Bugzilla-Type: changed
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 10.0
X-Bugzilla-Keywords: missed-optimization, ra
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94804

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |101926
            Summary|Failure to elide useless    |Failure to elide useless
                   |movs in 128-bit addition    |movs in 128-bit addition
                   |                            |with __int128_t arguments

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is ra issue with arguments really.
We get good code with:
```
using i128 =3D __int128;
i128 sub128(i128 *a, i128 *b)
{
    return *a - *b;
}
```
```
        movq    (%rdi), %rax
        movq    8(%rdi), %rdx
        subq    (%rsi), %rax
        sbbq    8(%rsi), %rdx
```

With:
```
using i128 =3D __int128;

void sub128(i128 a, i128 b, i128 *c)
{
    *c =3D  a - b;
}
```
We get not so good code (extra movs):
```
        movq    %rsi, %rax
        movq    %rdi, %rsi
        movq    %rax, %rdi
        subq    %rdx, %rsi
        sbbq    %rcx, %rdi
        movq    %rsi, (%r8)
        movq    %rdi, 8(%r8)
```


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101926
[Bug 101926] [meta-bug] struct/complex argument passing and return should be
improved=