https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84891
Bug ID: 84891
Summary: -fno-signed-zeros leads to optimization which should
be possible only if also -ffinite-math-only is on
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: no...@turm-lahnstein.de
Target Milestone: ---
Please consider the following example
#include <complex>
#include <iostream>
#include <cmath>
std::complex<double> mult(std::complex<double> c, double im){
std::complex<double> jomega(0.0, im);
return c*jomega;
}
int main(){
//(-nan,-nan) expected:
std::cout<<"case INF:
"<<mult(std::complex<double>(INFINITY,0.0),0.0<<"\n";
//(nan,nan) expected:
std::cout<<"case NAN: "<<mult(std::complex<double>(NAN,0.0), 0.0)<<"\n";
}
when compiled with -fno-signed-zeros the compiler seems to make some
optimizations which should not be possible without -ffinite-math-only, because
the result of 0.0*nan and 0.0*inf isn't 0.0. Live here
http://coliru.stacked-crooked.com/a/bca026da888d5b5d
echo "IEEE 754"; g++ -std=c++17 -O2 -Wall -pedantic -pthread main.cpp &&
./a.out
echo "Non IEEE 754"; g++ -std=c++17 -O2 -Wall -fno-signed-zeros -pedantic
-pthread main.cpp && ./a.out
gives us:
IEEE 754
case INF: (-nan,-nan)
case NAN: (nan,nan)
Non IEEE 754
case INF: (nan,0)
case NAN: (nan,0)
The resulting assembler is (see also https://godbolt.org/g/TSvcSp)
mult(std::complex<double>, double):
mulsd %xmm2, %xmm1
movapd %xmm0, %xmm3
mulsd %xmm2, %xmm3
movapd %xmm1, %xmm0
movapd %xmm3, %xmm1
xorpd .LC0(%rip), %xmm0
ret
.LC0:
.long 0
.long -2147483648
.long 0
.long 0
with only two multiplication instead of four. The clang-behavior is more
similar to gcc-version 4.6.
