[Bug target/51954] __int128_t (and long long on x86) negation can be optimized

[crazylht at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51954

--- Comment #4 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to Andrew Pinski from comment #2)
> We get this now:
> 
>         movq    %rdi, %rax
>         movq    %rsi, %rdx
>         negq    %rax
>         adcq    $0, %rdx
>         negq    %rdx
> 
> ICC produces:
>         xorl      %edx, %edx                                    #3.10
>         xorl      %eax, %eax                                    #3.10
>         subq      %rdi, %rax                                    #3.10
>         sbbq      %rsi, %rdx                                    #3.10
>         ret                                                     #3.10
> 
> Which seems even better.
> 
> LLVM does produce:
>         movq    %rdi, %rax
>         xorl    %edx, %edx
>         negq    %rax
>         sbbq    %rsi, %rdx
> 
> Which basically combines the addcq and negq together and changes one mov
> into an exor.

Yes, neg rdx = (0 - rdx) = (0 - (rdx + CF)(last def)) = (sbb 0, rdx)