On Thu, Nov 12, 2020 at 2:53 AM Eugene Rozenfeld via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Simplify (a ^ b) & ((b ^ c) ^ a) --> (a ^ b) & ~c.
>
> int f(int a, int b, int c)
> {
> return (a ^ b) & ((b ^ c) ^ a);
> }
>
> Code without the patch:
> mov eax,edx
> xor eax,esi
> xor eax,edi
> xor edi,esi
> and eax,edi
> ret
>
> Code with the patch:
> xor edi,esi
> andn eax,edx,edi
> ret
>
> Simplify (a ^ b) | ((b ^ c) ^ a) --> (a ^ b) | c.
> int g(int a, int b, int c)
> {
> return (a ^ b) | ((b ^ c) ^ a);
> }
>
> Code without the patch:
> mov eax,edx
> xor eax,esi
> xor eax,edi
> xor edi,esi
> or eax,edi
> ret
>
> Code with the patch:
> xor edi,esi
> mov eax,edi
> or eax,edx
> ret
>
> This fixes PR96671.
+/* (a ^ b) & ((b ^ c) ^ a) --> (a ^ b) & ~c */
+(simplify
+ (bit_and:c (bit_xor:c @0 @1) (bit_xor:cs (bit_xor:c @1 @2) @0))
+ (bit_and (bit_xor @0 @1) (bit_not @2)))
you should be able to re-use the first bit_xor by doing
+/* (a ^ b) & ((b ^ c) ^ a) --> (a ^ b) & ~c */
+(simplify
+ (bit_and:c (bit_xor:c@3 @0 @1) (bit_xor:cs (bit_xor:c @1 @2) @0))
+ (bit_and @3 (bit_not @2)))
For completeness the last (bit_xor:c @1 @2) should also have a :s
+/* (a ^ b) | ((b ^ c) ^ a) --> (a ^ b) | c */
+(simplify
+ (bit_ior:c (bit_xor:c @0 @1) (bit_xor:c (bit_xor:c @1 @2) @0))
+ (bit_ior (bit_xor @0 @1) @2))
completely misses :s here and the same comment about the re-use
of the existing xor applies.
Otherwise looks OK to me, sorry for the delay.
Thanks,
Richard.
> Tested on x86_64-pc-linux-gnu.
>
