On Fri, 1 May 2020, Segher Boessenkool wrote:
> On Mon, Apr 27, 2020 at 05:01:34PM -0500, will schmidt wrote:
> > On Mon, 2020-04-27 at 15:53 -0400, Michael Meissner via Gcc-patches wrote:
> > > +unsigned long
> > > +load_us_offset1 (unsigned char *p)
> > > +{
> > > + return *(unsigned short *)(p + 1); /* should generate LHZ. */
> > > +}
>
> This violates aliasing rules (without -fno-strict-aliasing), unless p
> points to some "short" object -- in which case it has alignment 2
> already!
Ackchyually... that'd be 'unless p+1 points to some "short" object'.
You're allowed to do weird things passing char pointers around
(at least), as long as you point to the right-type object (the
one used when it was assigned) when you dereference it.
Though, I don't know if the same goes if p was something other
than a "char *" or "unsigned char *".
> So this testcase needs -fno-strict-aliasing, but also, you need to make
> sure GCC does not assume alignment 2 (after the cast) already. There
> are attributes for this.
FWIW, if it needs -fno-strict-aliasing it's for some other
reason than the above (we don't see the assigner in the test).
brgds, H-P
