http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56876
Bug #: 56876
Summary: Combine does not invent new moves
Classification: Unclassified
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: enhancement
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: gli...@gcc.gnu.org
Target: x86_64-linux-gnu
Hello,
I am looking at this testcase:
typedef unsigned long long vec __attribute__((vector_size(16)));
vec g;
vec f1(vec a, vec b){
return ~a&b;
}
vec f2(vec a, vec b){
return ~g&b;
}
which compiles to:
f1:
pandn %xmm1, %xmm0
f2:
pcmpeqd %xmm0, %xmm0
pxor g(%rip), %xmm0
pand %xmm1, %xmm0
whereas I would like to get, like I do with the _mm_andnot_si128 builtin:
movdqa g(%rip), %xmm0
pandn %xmm1, %xmm0
It seems that combine cannot match the pandn pattern because the first argument
is a memory load and not a register. In this case, it would be better if it
emitted a move to put it in a register so it can match, instead of giving up. I
don't know if there is a good way to characterize such situations where an
extra move is worth it.
