[Bug c++/41094] New: Erroneous optimization of pow() with -ffast-math

["warp at iki dot fi" <gcc-bugzilla at gcc dot gnu dot org>]

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