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From: "gabravier at gmail dot com" <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
Date: Mon, 27 Apr 2020 20:50:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94804

--- Comment #1 from Gabriel Ravier <gabravier at gmail dot com> ---
For subtraction, it's even worse.

using i128 =3D __int128;

i128 sub128(i128 a, i128 b)
{
    return a - b;
}

results in=20

sub128(__int128, __int128):
  mov rax, rdi
  sub rax, rdx
  sbb rsi, rcx
  mov rdx, rsi
  ret

with LLVM and=20

sub128(__int128, __int128):
  mov r9, rdi
  mov r8, rsi
  mov rdi, r8
  mov rax, r9
  mov r8, rdx
  sub rax, r8
  mov rdx, rdi
  sbb rdx, rcx
  ret

with GCC.

The excess of `mov`s feels to me like there is some sort of bug in the 128-=
bit
register allocator or something like that.=