Re: optimization/9052: in C code, "if" statement fails to execute if optimized

[Steven Bosscher <s.bosscher@student.tudelft.nl>]

The following reply was made to PR optimization/9052; it has been noted by GNATS.

From: Steven Bosscher <s.bosscher@student.tudelft.nl>
To: gcc-gnats@gcc.gnu.org, gcc-bugs@gcc.gnu.org, nobody@gcc.gnu.org,
	gcc-prs@gcc.gnu.org, phama@webjockey.net
Cc: Eric Botcazou <ebotcazou@libertysurf.fr>
Subject: Re: optimization/9052: in C code, "if" statement fails to execute
	if optimized
Date: 12 Feb 2003 16:25:49 +0100

 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9052
 
 A small test case for this PR is the following:
 
 #define N 256
 
 int
 main (void)
 {
   float r[N];
   int n;
 
   r[0] = 0;
   for (n = 1; n < N; n++)
     {
       r[n] = r[n - 1] * r[n - 1] - .125;
 
 //    printf("%f\n",r[n]-r[n/2]);
 
       if (r[n / 2] == r[n])
 	break;
     }
 
   printf ("%d\n", n);
   return 0;
 }
 
 I have tested this with two different GCC versions:
 
 Reading specs from /usr/lib/gcc-lib/i486-suse-linux/2.95.3/specs
 gcc version 2.95.3 20010315 (SuSE
 
 Reading specs from
 /opt/experimental/lib/gcc-lib/i586-pc-linux-gnu/3.4/specs
 Configured with: ../gcc-trunk/configure --disable-nls --with-gnu-as
 --with-gnu-ld --prefix=/opt/experimental --program-suffix=-3.4
 --enable-languages=c
 Thread model: posix
 gcc version 3.4 20030211 (experimental)
 
 Output from this testcase:
 gcc version:	at -O0		at -O1		at -O2
 2.95.3		 22		 22		 22
 3.4 exp.	 22		256		256
 
 After uncommenting the printf, gcc 3.4 also prints 22 at -O1 and -O2.
 
 I have looked for documentation about this change in behavior but it
 doesn't seem to exist. I'm not a floating point specialist, but
 generally, comparing floats for equality doesn't seem like the best
 thing to do.  However, since it apparently worked with older gcc
 versions, I suppose one could qualify this as a regression...
 
 Eric, I CC'ed you as the Great C Bug Squasher.  Do you think this is a
 regression, and if so, change the status in GNATS?
 
 Greetz
 Steven