[Bug rtl-optimization/94804] Failure to elide useless movs in 128-bit addition

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94804

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

using i128 = __int128;

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

results in 

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

with LLVM and 

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.