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

[lucho at haemimont dot bg]

>Number:         6985
>Category:       optimization
>Synopsis:       GCC-3.x incorrectly initializes local arrays
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Mon Jun 10 17:56:00 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     Luchezar Belev
>Release:        GCC-3.1
>Organization:
>Environment:
linux-2.4.18 pc-i686 glibc-2.1 gcc-3.1
>Description:
It seems that the compiler reorders the initialization-to-zero
with the following assignment operation so that the initialization comes last.
>How-To-Repeat:
/*
 * 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;
}

>Fix:
To work around: make little changes to the source and the compiler won't confuse anymore.
>Release-Note:
>Audit-Trail:
>Unformatted: