[Valgrind-users] Possible bug in valgrind-1.9.4

[Dominic M. <dma...@ai...>]

Hi,

I've been using valgrind for about six months, and it's been wonderful to 
have.  I was spoiled having access to purify on Solaris machines for a 
while, and missed having something similar on Linux.  Many thanks to 
Julian Seward and everyone else who contributed to its development.

I've included a very small program that generates different output when 
it's run through valgrind.  I noticed the error while I was debugging a 
function to quickly check if an array of single-precision floats has any 
NaNs in it - it turns out that doing a bitwise test is 2-3x faster than 
calling the finite() function.

Anyway, the error only occurs if you compile it with the options:

  gcc -O2 -mcpu=pentiumpro -march=pentiumpro

However, the exact same error occurs whether I compile the program with 
gcc 2.96 (RedHat 7.3's version) or gcc 3.2.

The correct output of the program is "0.000000".  When run under valgrind 
1.9.4, it outputs "1.000000".

I hope this helps!  It's easy enough for me to work around, but I can only 
guess that this is probably the symptom of a bug that could manifest 
itself in other ways.

- Dominic

#include <stdio.h>

int main(int argc, char **argv)
{
   union {
      float a[2];
      int b[2];
   } u;

   u.a[0] = 0.0 / 0.0;
   u.a[1] = ((*u.b & 0x7FC00000) != 0x7FC00000);
   printf("%f\n", u.a[1]);

   return 0;
}