http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56863
Bug #: 56863
Summary: cmpnltpd recognition
Classification: Unclassified
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: gli...@gcc.gnu.org
Target: x86_64-linux-gnu
Hello,
I was looking at this program, trying to get it to generate cmpnltpd:
#include <x86intrin.h>
__m128d f(__m128d a, __m128d b){
return (__m128d)(~(a<b));
}
but it doesn't:
cmpltpd %xmm1, %xmm0
pcmpeqd %xmm1, %xmm1
pxor %xmm0, %xmm1
movapd %xmm1, %xmm0
One reason seems to be that the instruction is modeled as (unge a b), whereas
from a trapping point of view I think it should be (not (lt a b)). But even
-ffast-math does not help.
As a side note, notice that:
return (__m128d)((a<b)?0:(__m128i){-1,-1});
uses pandn instead of pxor to generate the negation. I don't think one is
better than the other, I was just surprised to see the difference.
