https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68125
Bug ID: 68125
Summary: std::sqrt prevent use of associative math
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: vincenzo.innocente at cern dot ch
Target Milestone: ---
with -Ofast
the code generated differs
float rsqrt1(float a, float x, float y) {
return a/std::sqrt(x)/std::sqrt(y);
}
float rsqrt2(float a, float x, float y) {
return a/sqrtf(x)/sqrtf(y);
}
rsqrt1(float, float, float):
sqrtss %xmm2, %xmm2
sqrtss %xmm1, %xmm1
mulss %xmm2, %xmm1
divss %xmm1, %xmm0
ret
rsqrt2(float, float, float):
mulss %xmm1, %xmm2
rsqrtss %xmm2, %xmm1
mulss %xmm1, %xmm2
mulss %xmm1, %xmm2
mulss .LC9(%rip), %xmm1
addss .LC8(%rip), %xmm2
mulss %xmm1, %xmm2
mulss %xmm2, %xmm0
ret
