#include <stdio.h>
int main() {
int x = 0x87654321;
switch (3*x % 3) {
case 2: puts("SUCCESS"); break;
default: puts("FAILURE"); break;
}
return 0;
}
gcc -fwrapv test.c
./a.out
FAILURE
Expected output: SUCCESS
This would not be a bug, since integer overflow is undefined in C, *except* for
the fact that I passed -fwrapv. I was under the impression that -fwrapv would
enforce integer arithmetic modulo 2**32, and not incorrectly optimize (x*n%x)
as if it were equivalent to 0.
If -fwrapv does not strictly enforce arithmetic mod 2**32, then it could use
some better documentation. If it *is* supposed to enforce arithmetic mod 2**32,
then this is a bug that ought to be fixed.
--
Summary: Even with -fwrapv, (3*x%3) is assumed to always be equal
to 0
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: foo at mailinator dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42615
