Thank you for the review Richard! I re-worked the patch based on your suggestions (attached). I made the change to reuse the first bit_xor in both patterns and I added :s to the last xor in the first pattern. For the second pattern I didn't add :s because I think the simplification is beneficial even if the second or third bit_xor has more than one use since we are simplifying them to just a single operand (@2). If that is incorrect, please explain why.
Eugene
-----Original Message-----
From: Richard Biener <richard.guent...@gmail.com>
Sent: Monday, November 16, 2020 4:11 AM
To: Eugene Rozenfeld <eugene.rozenf...@microsoft.com>
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] [tree-optimization] Optimize two patterns with three xors.
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.
>
0001-Optimize-two-patterns-with-three-xors.patch
Description: 0001-Optimize-two-patterns-with-three-xors.patch
