[Bug middle-end/63568] Missed optimization (a & ~mask) | (b & mask) = a ^ ((a ^ b) & mask)

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63568

--- Comment #6 from rguenther at suse dot de <rguenther at suse dot de> ---
On Tue, 16 Dec 2014, mpolacek at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63568
> 
> --- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
> True.  E.g. on my x86_64 i7 Nehalem I see (using ./cc1 -quiet -O2 qq.c -mbmi) 
> 
>         andn    %edi, %edx, %edi
>         andl    %edx, %esi
>         movl    %edi, %eax
>         orl     %esi, %eax
>         ret
> 
> for return (a & ~m) | (b & m); and
> 
>         xorl    %edi, %esi
>         movl    %edi, %eax
>         andl    %esi, %edx
>         xorl    %edx, %eax
>         ret
> 
> for return a ^ ((a ^ b) & m);

The former is also better for instruction level parallelism - but
that just asks for a clever enough expander / combiner that can
generate that from the latter.

I think on GIMPLE we want to canoncalize to the variant with less
(gimple) operations.  single-use restrictions may apply (with
the lack of a global unified combine / CSE phase)