Consider the following program:
//----------------------------------------------------------------------
#include <iostream>
#include <cmath>
int main()
{
for(double p = .25; p <= .5; p += .25)
for(double x = -10.0; x <= 10.0; x += 20.0)
{
std::cout
<< "\nx*x = " << x*x
<< "\npow(100, " << p << ") = " << std::pow(100.0, p)
<< "\npow(x*x, " << p << ") = " << std::pow(x*x, p)
<< "\n";
}
}
//----------------------------------------------------------------------
When compiling without -ffast-math, the output is the expected one.
However, if compiled with -ffast-math (other optimization options don't
seem to matter), it produces incorrectly "nan" and "-10" for the third
line ("std::pow(x*x, p") when 'x' is negative.
If the -fno-unsafe-math-optimizations option is specified in addition
to -ffast-math, the bug is not triggered.
If "double xx = x*x;" is added inside the loop and then 'xx' is used
instead of 'x*x', the bug is not triggered either.
--
Summary: Erroneous optimization of pow() with -ffast-math
Product: gcc
Version: 4.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: warp at iki dot fi
GCC build triplet: i586-suse-linux
GCC host triplet: i586-suse-linux
GCC target triplet: i586-suse-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41094
