[plt-dev] Re: [plt] Push #20286: master branch updated

On Sat, May 22, 2010 at 6:42 PM, Eli Barzilay <eli at barzilay.org> wrote:
> On May 22, Jay McCarthy wrote:
>> On Sat, May 22, 2010 at 6:03 PM, Eli Barzilay <eli at barzilay.org> wrote:
>> > On May 22, Matthew Flatt wrote:
>> >> At Sat, 22 May 2010 06:47:44 -0400, Eli Barzilay wrote:
>> >> > On May 21, jay at racket-lang.org wrote:
>> >> > >
>> >> > > src/racket/gc/new_hblk.c
>> >> > > ~~~~~~~~~~~~~~~~~~~~~~~~
>> >> > > --- OLD/src/racket/gc/new_hblk.c
>> >> > > +++ NEW/src/racket/gc/new_hblk.c
>> >> > > @@ -164,7 +164,8 @@ ptr_t GC_build_fl(struct hblk *h, size_t sz, GC_bool
>> >> > clear, ptr_t list)
>> >> > >     * put p (which is now head of list of objects in *h) as first
>> >> > >     * pointer in the appropriate free list for this size.
>> >> > >     */
>> >> > > -      obj_link(h -> hb_body) = list;
>> >> > > +   word *tmp = (word *)h->hb_body;
>> >> > > +      obj_link(tmp) = list;
>> >> > >        return ((ptr_t)p);
>> >> > >  }
>> >> >
>> >> > It's not clear to me what this is trying to do, but this breaks on
>> >> > windows.
>> >>
>> >> The problem is the variable declaration in the middle of a
>> >> block. I'll fix it.
>> >
>> > (Yeah, that was clear -- but why is it needed in the first place?  The
>> > only place I've seen this was due to xform limitations.)
>>
>> If "it" = this patch, then it removes this error:
>>
>> .../src/racket/gc/new_hblk.c:167: warning: dereferencing type-punned
>> pointer will break strict-aliasing rules
>
> Clarification: why does the temporary variable prevent that warning?
> (It's a C question -- I never got the type-punned thing beyond a vague
> "something related to C99".)

[I only vaguely understand it.] I think the main thing is the cast,
that gets rid of the type-punned-ness, but if you just add the cast,
you get a different error, so by adding the temporary variable,
there's no worry that it will alias. (Also, obj_link is a complicated
macro that hides some more casts and derefs.)

Jay

-- 
Jay McCarthy <jay at cs.byu.edu>
Assistant Professor / Brigham Young University
http://teammccarthy.org/jay

"The glory of God is Intelligence" - D&C 93