>Number: 156082
>Category: misc
>Synopsis: gcc makes incorrect code when swapping array elements by xor
>without optimization
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: freebsd-bugs
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Thu Mar 31 08:50:12 UTC 2011
>Closed-Date:
>Last-Modified:
>Originator: Dmitry Afanasiev
>Release: FreeBSD 9.0-CURRENT i386
>Organization:
>Environment:
FreeBSD orion 9.0-CURRENT FreeBSD 9.0-CURRENT #6: Sun Mar 27 03:24:55 MSD 2011
root@orion:/usr/obj/usr/src/sys/orion amd64
>Description:
Base gcc 4.2.1 makes incorrect code when swapping array elements by xor without
optimization.
Simple .c code attached as xor.txt.
Checked on FreeBSD 9.0 on i386/amd64/sparc64 platforms.
>How-To-Repeat:
# gcc -o xor xor.txt
# gcc -o xoro1 -O1 xor.txt
# ./xor
a0 = 127 b0 = 33
a1 = 127 b1 = 33
a2 = 127 b2[0] = 33
a3 = 127 b3[0] = 0
# ./xoro1
a0 = 127 b0 = 33
a1 = 127 b1 = 33
a2 = 127 b2[0] = 33
a3 = 127 b3[0] = 33
But in theory output must be identical.
>Fix:
Patch attached with submission follows:
#include <stdio.h>
#include <sysexits.h>
int
main(int argc, char *argv[])
{
int a0 = 33;
int b0 = 127;
int a1 = 33;
int b1 = 127;
int a2 = 33;
int b2[] = {127};
int a3 = 33;
int b3[] = {127};
a0 ^= b0 ^= a0 ^= b0;
b1 ^= a1 ^= b1 ^= a1;
a2 ^= b2[0] ^= a2 ^= b2[0];
b3[0] ^= a3 ^= b3[0] ^= a3;
printf("a0 = %i b0 = %i\n", a0, b0);
printf("a1 = %i b1 = %i\n", a1, b1);
printf("a2 = %i b2[0] = %i\n", a2, b2[0]);
printf("a3 = %i b3[0] = %i\n", a3, b3[0]);
return EX_OK;
}
>Release-Note:
>Audit-Trail:
>Unformatted:
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