On Thu, Sep 30, 2010 at 9:54 AM, Albert Cahalan <acaha...@gmail.com> wrote:
> int weird(float *fp){
> // access an int as an int (see caller),
> // so not an aliasing violation
> return *(int*)fp;
> }
> int main(int argc, char *argv[]){
> return weird((float*)&argc);
> }
>
> I just tried this code with gcc 4.4.5 on 32-bit powerpc using -O2 -W -Wall.
> Assembly code for the weird function looks OK, both inlined and not, but
> that certainly isn't proof that gcc will always tolerate such code.
> I recall that there were problems handling this type of code. (never mind
> any non-conformant callers that actually pass a pointer to a float -- not
> that gcc would be able to see them in separately compiled files)
>
> So, is it fixed now? (what gcc version?) If not, is it at least fixed
> if I change "float" to "void" and/or "unsigned char"?
>
> BTW, oddly it looks like gcc tolerates a genuine aliasing violation
> as well now. (passing the value as a float) Of course, that may just
> be my luck with the optimizer.
I indeed fixed the above problem at some point (4.1 may be still
broken, 4.3 should be fixed I think).
We're trying to tolerate genuine alias violations if we can see
what the user intended (in compiler-speak, when we detect
a must-alias relationship we do not try to disabiguate using
type-based alias analysis). That's just being nice to users and
not breaking their code just because we can.
Richard.
