Re: optimization/6985: GCC-3.x incorrectly initializes local arrays

[Andrew Pinski <pinskia@physics.uc.edu>]

The following reply was made to PR optimization/6985; it has been noted by GNATS.

From: Andrew Pinski <pinskia@physics.uc.edu>
To: lucho@haemimont.bg
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: optimization/6985: GCC-3.x incorrectly initializes local arrays
Date: Mon, 10 Jun 2002 21:22:47 -0400

 This is not a bug but your code violates ISO C's aliasing rules 
 (C89, ANSI C).
 To work around this use an union or use the option 
 `-fno-strict-aliasing'.
 Thanks,
 Andrew Pinski
 
 On Monday, June 10, 2002, at 08:48 , lucho@haemimont.bg wrote:
 
 > /*
 >  * GCC-3.1 bug
 >  * compile with -O2 to let the bug to appear.
 >  *
 >  * The test should output (and does so when compiled with -O1 or less):
 >  *
 >  * aaaaa
 >  * a=13
 >  *
 >  * but when compiled with -O2 or higher, instead it outputs:
 >  *
 >  * aaaaa
 >  * a=0
 >  *
 >  *
 >  * I've tested this on 2.95.2, 3.0, 3.0.4 and 3.1.
 >  * 2.95.2 doesn't have this bug and all 3.x have it.
 >  */
 >
 > unsigned a = 13, b = 17;
 >
 > int main()
 > {
 > 	unsigned char buf[18] = {0};
 > 	*(unsigned short *)(buf + 12) = a;
 > 	*(unsigned short *)(buf + 14) = b;
 > 	printf("aaaaa\n");
 > 	printf("a=%i\n", *(unsigned short *)(buf + 12));
 > 	return 0;
 > }