Re: Testcase for strict-aliasing wanted

[Geoff Keating <geoffk at geoffk dot org>]

Osku Salerma <osku@iki.fi> writes:

> Does anybody have a simple testcase available that breaks with strict
> aliasing enabled? I tried to construct my own, but I can't get either
> 2.95.2 or the latest CVS version to break intentionally. Either I'm
> misunderstanding some of the aliasing rules or gcc isn't optimizing as
> aggressively in this case as would be needed for the aliasing issues
> to surface.
> 
> For reference, here's my (not-working) effort:
> 
> #include <stdio.h>
> 
> void f(char* p1, int* p2)
> {
>     char c1,c2;
> 
>     c1 = *p1;
>     *p2 = 0;
>     c2 = *p1;
> 
>     printf("(%d,%d)\n", c1,c2);
> }
> 
> int main(void)
> {
>     char a[4];
> 
>     a[0] = 1;
> 
>     f(a, (int*)a);
> 
>     return 0;
> }

`char' is a special case; ISO C requires that it be allowed to alias
with anything.  Try the same testcase with `short' instead of `char'.

-- 
- Geoffrey Keating <geoffk@geoffk.org> <geoffk@redhat.com>