Re: GCC aliasing rules: more aggressive than C99?

[Nick Stoughton <nick at msbit dot com>]

On Sun, 2010-01-03 at 10:31 -0800, Patrick Horgan wrote:
> Richard Guenther wrote:
> > On Sun, Jan 3, 2010 at 6:46 AM, Joshua Haberman <jhaberman@gmail.com> wrote:
> >   
> >> ... elision by patrick of part of a quote of 6.5 Expressions #7...
> >>  * an aggregate or union type that includes one of the aforementioned
> >>    types among its members (including, recursively, a member of a
> >>    subaggregate or contained union), or
> >>     
> >
> > Literally interpreting this sentence the way you do removes nearly all
> > advantages of type-based aliasing that you have when dealing with
> > disambiguating a pointer dereference vs. an object reference
> > and thus cannot be the desired interpretation (and thus we do not allow this).
> >   
> Since it would be hard to read this any other way, it seems then you 
> maintain that you found a bug in the standard, has it been brought up to 
> the committee?  The latest draft I see still has that wording.  Doesn't 
> seem to be on the radar for C1x.  This same thing has bitten me, though 
> I agree with your rationale about how it would be a bad idea, still 
> strictly speaking, gcc is not standards compliant on this one point, and 
> rather than change gcc, the defect in the standard could be changed.  If 
> you don't have anyone participating on the committee right now, you only 
> have to convince some one who is, e.g. Nick Stoughton or P. J. Plauger 
> or Herb Sutter, right?

Herb is C++ ...

The C1x timetable has us finishing the draft for an initial ballot this
summer (the April Florence meeting being the last chance to submit new
material). The best expert I know on the type based aliasing stuff is
Clark Nelson at Intel (clark.nelson@intel.com). We did spend a
considerable amount of time at the recent Santa Cruz meeting discussing
this subject ... see N1409 and N1422 (the minutes including a summary of
the discussion on N1409).
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1409.htm

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1422.pdf

> 
> > It basically would force us to treat *ptr vs. Obj as *ptr vs. *(Obj *)ptr2.
> >
> > ...more elision
> >> I have seen other articles that interpret the standard in this way.
> >> See section "Casting through a union (2)" from this article, which
> >> claims that casts of this sort are legal and that GCC's warnings
> >> against them are false positives:
> >>  http://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html
> >>     
> >
> > Yes, this article contains many mistakes but the author failed to listen.
> >   
> This one thing is, you say, not a mistake of the author but a mistake in 
> the standard, and it's unkind to characterize it like that <grin;>
> > ...elided a bunch--patrick...
> > Correct.  GCC follows its own documentation here, not some random
> > websites and maybe not the strict reading of the standard.  There are
> > other corner-cases where it does so, namely with the union type rule
> > (which I fail to come up with a std reference at the moment).
> >   
> I hope in all of these cases it's been brought up as an issue, defects 
> in the standard won't get fixed if there are no squeaky wheels!
> 
> Patrick


-- 
Nick