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and subject line Bug#40189: c/6898: gcc produces incorrect minimization code
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From: <[EMAIL PROTECTED]>
Subject: egcc: egcc produces incorrect minimization code
To: [EMAIL PROTECTED]
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Message-Id: <[EMAIL PROTECTED]>
Date: Fri, 25 Jun 1999 15:34:24 -0400
Package: egcc
Version: 2.91.60-5
The following program inconsistently chooses minimal
array elements in case of equal elements being present.
Here is the code (ttr.c):
------------------------
#include <stdio.h>
#include <math.h>
int
main() {
double
a[4]={0.006776,2.5,3.0,0.006776},w[4]={0.000030,3.7,3.0,0.000030},t,amin;
int ir,i,nr=4;
double tol=9.31322574615479e-10*3.0;
for (i=0,ir=-1,amin=DBL_MAX;i<nr;i++)
if (a[i]>tol) {
t=w[i]/a[i];
if (t<amin || ir==-1) {
amin=t;
ir=i;
}
}
printf("ir=%d\n",ir);
for (i=0,ir=-1,amin=DBL_MAX;i<nr;i++)
if (a[i]>tol && ((t=w[i]/a[i])<amin || ir==-1)) {
amin=t;
ir=i;
}
printf("ir=%d\n",ir);
exit(0);
}
---------------
and here is the execution:
---------------
intech19:~$ egcc -g -Wall ttr.c -o ttr -lm
intech19:~$ ./ttr
ir=0
ir=3
-----------------
-- System Information
Debian Release: 2.1
Kernel Version: Linux intech19 2.2.7 #1 SMP Thu May 27 11:53:43 EDT 1999 i686
unknown
Versions of the packages egcc depends on:
ii libc6 2.0.7.19981211 GNU C Library: shared libraries
ii g++ 2.91.60-5 The GNU (egcs) C++ compiler.
ii g++ 2.91.60-5 The GNU (egcs) C++ compiler.
---------------------------------------
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From: Matthias Klose <[EMAIL PROTECTED]>
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Date: Fri, 27 Dec 2002 17:05:03 +0100
To: [EMAIL PROTECTED]
Subject: Re: Bug#40189: c/6898: gcc produces incorrect minimization code
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therefore closing ...
[EMAIL PROTECTED] writes:
> Synopsis: gcc produces incorrect minimization code
>
> State-Changed-From-To: open->closed
> State-Changed-By: reichelt
> State-Changed-When: Fri Nov 22 08:47:00 2002
> State-Changed-Why:
> Not a bug.
>
> Whether you get ir=0 or ir=3 is just a matter when rounding will take
> place.
> Since the code is rearranged in optimization steps, you'll get varying
> results with different architectures and optimization options. Similar
> but not identical code might also lead to different results.
> Welcome to the world of floating point arithmetic.
>
> BTW, declaring t and amin as "volatile double" leads to your "expected"
> result even for -O0.
>
> http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=6898
