http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55912
Bug #: 55912
Summary: missing optimization of x/x and x/std::abs(x)
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: vincenzo.innoce...@cern.ch
in the following examples I'm quite surprised that only (x/x)is optimized to
"!".
With Ofast (and even more with -mrecip) I would have expected that
commutativity of multiplication to be used at least to optimized x*a/x
Optimizing x/x and x/std::abs(x) would be beneficial with -mrecip to amortize a
trivial loss of precision.
cat one.cc;c++ -Ofast -S one.cc -msse4.2; cat one.s
#include<cmath>
int one1(float a, float x) {
return x*a/x;
}
#include<cmath>
int one2(float a, float x) {
return a*(x/x);
}
int sign1(float a, float x) {
return x*a/std::abs(x);
}
int sign2(float a, float x) {
return a*(x/std::abs(x));
}
.text
.align 4,0x90
.globl __Z4one1ff
__Z4one1ff:
LFB86:
mulss %xmm1, %xmm0
divss %xmm1, %xmm0
cvttss2si %xmm0, %eax
ret
LFE86:
.align 4,0x90
.globl __Z4one2ff
__Z4one2ff:
LFB87:
cvttss2si %xmm0, %eax
ret
LFE87:
.align 4,0x90
.globl __Z5sign1ff
__Z5sign1ff:
LFB88:
mulss %xmm1, %xmm0
movss LC0(%rip), %xmm2
andps %xmm2, %xmm1
divss %xmm1, %xmm0
cvttss2si %xmm0, %eax
ret
LFE88:
.align 4,0x90
.globl __Z5sign2ff
__Z5sign2ff:
LFB89:
movss LC0(%rip), %xmm2
andps %xmm1, %xmm2
divss %xmm2, %xmm1
mulss %xmm0, %xmm1
cvttss2si %xmm1, %eax
ret
