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SUMMARY: Another "cc -O2" compiler bug?


Thanks to all those who replied.

The problem was compiling with -O2 (and higher) optimization.
The code below is an example where -O2 gives the wrong answer.

------------------
#include <stdio.h>
#include <math.h>
void tmp( double );
void main()
{
  double x;
  x = 1.0;
  tmp(x);
}
void tmp( double xx )
{
  double fkn1, fkn2, arg, aux1, aux2; 
  aux2 = xx;
  aux1 = 1.0;
  /** original definition - gives the inverse of the correct thing **/
  fkn1 = exp(-aux1 * aux2);
  /** alternative definition - gives the correct result **/
  arg = aux1 * aux2;
  fkn2 = exp(-arg);
  printf("%f  %f  %f  %f\n", xx, fkn1, fkn2, fkn1 * fkn2);
}
------------------

I got 6 answers all confirming the bug. 
One can, however use the -migrate compiler option
to get around the problem. 

>> cc -migrate -O2 -o bug bug.c -lm

gives you the right answer.

Thanks again,

/Mattias

---------------------------------
Mattias Ohlsson
Department of Theoretical Physics
Lund University
Sweden
---------------------------------