Bug#179712: gcc-3.2 -O2 miscompilation (i386)

[Daniel Jacobowitz <dan@debian.org>]

On Tue, Feb 04, 2003 at 05:02:10AM +0200, Marko Kreen wrote:
> Package: gcc-3.2
> Version: 1:3.2.2-0pre8
> Severity: normal
> 
> 
> gcc-3.2 -O2 miscompiles following program:
> 
> ---------------------------------------------
> extern int write(int fd, void *buf, int len);
> extern int toupper(int c);
> 
> void strupr (char *s)
> {
>         while (*s)
>                 *s++ = toupper(*s);
> }
> 
> int main(int argc, char *argv[])
> {
>         char s[] = "foo";
>         strupr(s);
>         write(1, s, 3);
>         write(1, "\n", 1);
>         return 0;
> }
> 
> ---------------------------------------------
> $ gcc-3.2 -Wall -O2 -o test test.c 
> test.c: In function `strupr':
> test.c:7: warning: operation on `s' may be undefined
> $ ./test
> OO
> $
> ---------------------------------------------
> 
> gcc-2.95 compiles it correctly, also gcc-3.2 when optimizing 
> less than -O2.  When the "toupper(*s)" is replaced with "*s & 0x5f"
> gcc-3.2 -O2 also works and without warning.

I think the lack of warning in that case is actually the bug, but I'm
not sure.  C doesn't guarantee order-of-operation across an equals
sign; it's not a sequence point.  The function call is a sequence point
but it isn't specified which side of the sequence point the increment
will happen on.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer