Re: GCC optimizes integer overflow: bug or feature?

[Robert Dewar]

Joseph S. Myers wrote:
> On Tue, 19 Dec 2006, Florian Weimer wrote:
>
> > * Paolo Bonzini:
> >
> > > Interesting read.  I agree with the proposed fix; however, note that
> > > GCC does not make the result of overflowing signed left-shifts
> > > undefined, exactly because in this case the overflow is relied upon by
> > > too many existing programs
> > Is this documented somewhere?  Without documentation, it could also be
> > an oversight (lack of optimization) from a programmer perspective.
>
> Certainly, in implement-c.texi:
>
>         GCC does not use the latitude given in C99 only to treat certain
>         aspects of signed @samp{<<} as undefined, but this is subject to
>         change.

That hardly seems sufficient documentation, when documenting undefined,
you had better say what the semantics is. Saying it is not treated as
undefined, and then failing to define it is a bit of a contradiction
in terms :-)
> This particular case has the special property that signed << was 
> implementation-defined in C90 (DR#081) and became undefined in some cases 
> in C99.
>
> We've optimized expressions such as (a*2)/2 on the basis of overflow being 
> undefined for a very long time, not just loops.

What is (a*2)/2 optimized to? certainly it has the value a if you wrap, 
so you are not necessarily depending on undefined here.
>