c/5926: gcc 3.0.3 generates incorrect code with -O3 option

[gbv@oxixares.com]

>Number:         5926
>Category:       c
>Synopsis:       gcc 3.0.3 generates incorrect code with -O3 option
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Tue Mar 12 12:06:00 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     Guillermo Ballester Valor
>Release:        3.0.3
>Organization:
>Environment:
System: Linux mozart 2.2.19 #2 Sat Jun 2 21:32:31 CEST 2001 i586 unknown
Architecture: i586

	
host: i586-pc-linux-gnu
build: i586-pc-linux-gnu
target: i586-pc-linux-gnu
configured with: ./configure 
>Description:
	I've detected a problem when updated from gcc 2.95.2 . A result was different depending on the 
	order of a line of code were written, without aparent cause, when using -O3 
>How-To-Repeat:
	See the problem compiling the following code with -O3 option. The two routines are the same but
	a code of line changed. I think both results would be the same but I get different resuts.

-----------------------------------
 
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

size_t last_nsize=0;
#define BITS_PER_LIMB 32

unsigned long Y_MASQ,Y_LIMB,Y_MAX,Y_BITS;
double Y_TINV;

int bugy_routine(size_t n1, size_t n2)
{
  size_t n,nsize,needed=0,i;
 
  nsize= n1 + n2;

  /*See whether all the initialization work con be avoided */
  needed=(nsize*BITS_PER_LIMB);
  Y_BITS = (unsigned long)((double)24.00 - 0.5*log((double)needed));
  Y_MASQ=((1U<<Y_BITS)-1L);
  Y_LIMB=(1U<<Y_BITS);
  Y_MAX=(1U<<(Y_BITS-1));
  Y_TINV=1.0/(double)Y_LIMB;
  needed=(needed-1)/(size_t)Y_BITS+3; /****** LINE CHANGED *******/
  return needed;
}

int nobugy_routine(size_t n1, size_t n2)
{
  size_t n,nsize,needed=0,i;
  int save;
  
  nsize= n1 + n2;

  /*See whether all the initialization work con be avoided */
  needed=(nsize*BITS_PER_LIMB);
  Y_BITS = (unsigned long)((double)24.00 - 0.5*log((double)needed));
  needed=(needed-1)/(size_t)Y_BITS+3; /****** LINE CHANGED *******/
  Y_MASQ=((1U<<Y_BITS)-1L);
  Y_LIMB=(1U<<Y_BITS);
  Y_MAX=(1U<<(Y_BITS-1));
  Y_TINV=1.0/(double)Y_LIMB;
  return needed;
}

int main(int argc, char **argv)
{
  size_t n1=500, n2=500, res1, res2;
  
  res1=nobugy_routine(n1,n2);
  printf("Results of no bugy routine %i\n",res1);
  res2=bugy_routine(n1,n2);
  printf("Results of bugy routine %i\n",res2);
}

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>Fix:

I don't know
>Release-Note:
>Audit-Trail:
>Unformatted: