optimization/10087: [3.3/3.4 regression] optimizer produces wrong code when indexing 2D array

[Steven Bosscher]

The following reply was made to PR optimization/10087; it has been noted by GNATS.

From: Steven Bosscher <s.bosscher@student.tudelft.nl>
To: p.van-hoof@qub.ac.uk, gcc-gnats@gcc.gnu.org,
	gcc-bugs@gcc.gnu.org, nobody@gcc.gnu.org, gcc-prs@gcc.gnu.org
Cc:  
Subject: Re: optimization/10087: [3.3/3.4 regression] optimizer produces wrong
 code when indexing 2D array
Date: Sat, 15 Mar 2003 01:48:02 +0100

 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10087
 
 A little more information...
 
 A slightly smaller test case is:
 ----------------------------
 static void
 b (int *i, int *j)
 {
 }
 
 int
 main (void)
 {
   int i, j;
 
   S x1[2][2];
   S *x[2] = { x1[0], x1[1] };
   S **E = x;
 
   for (i = 0; i < 2; i++)
     for (j = 0; j < 2; j++)
       E[j][i].T1 = 1;
 
   b (&j, &i);
   printf ("result %.6e\n", E[1][1].T1);
   return 0;
 }
 ----------------------------
 
 The line "E[j][i] =..." seems to be the problem:
 With j first, then i, the output is wrong (and you get a segfault as a 
 bonus).
 With i first, then j ("E[i][j]=..")  you get the  correct result.
 
 # gcc-3.3 -O t.c
 # a.out
 result 1.000000e+00
 # gcc-3.3 t.c -O -fstrength-reduce -fstrict-aliasing -fforce-mem -fgcse
 # a.out
 result 2.676500e+00
 Segmentation fault
 
 Again, disable any of these four options and you get the expected output.
 
 (For the record, this is on i586-pc-linux-gnu.)
 
 Greetz
 Steven