Re: Invalid compilation without -fno-strict-aliasing

[Daniel Jacobowitz <dan@debian.org>]

On Thu, Feb 27, 2003 at 07:30:03PM +0000, Linus Torvalds wrote:
> In article <20030226172213.O3910@devserv.devel.redhat.com>,
> Jakub Jelinek  <jakub@redhat.com> wrote:
> >
> >To fix that, __constant_memcpy would have to access the data through
> >union,
> 
> Which is impossible, since memcpy _fundamentally_ cannot know what the
> different types are..
> 
> > or you could as well forget about __constant_memcpy and use
> >__builtin_memcpy where gcc will take care about the constant copying.
> 
> Which is impossible because (a) historically __builtin_memcpy does a bad
> job and (b) it doesn't solve the generic case anyway, ie for other
> non-memcpy things.
> 
> The fact is, for type-based alias analysis gcc needs a way to tell it
> "this can alias", which it doesn't have.  Unions are _not_ useful,
> _regardless_ of what silly language lawyers say, since they are not a
> generic method.  Unions only work for trivial and largely uninteresting
> cases, and it doesn't _matter_ what C99 says about the issue, since that
> nasty thing called "real life" interferes.
> 
> Until we get some non-union way to say "this can alias", that
> -fno-strict-alias has to stay because gcc is too broken to allow us
> doing interesting stuff in-line without it. 
> 
> My personal opinion is (and was several years ago when this started
> coming up) that a cast (any cast) should do it. But I don't are _what_
> it is, as long as it is syntactically sane and isn't limited to special
> cases like unions.

Well, if that's all you're asking for, it's easy - I don't know if
you'll agree that the syntax is sane, but it's there.  From the GCC 3.3
manual:

`may_alias'
     Accesses to objects with types with this attribute are not
     subjected to type-based alias analysis, but are instead assumed to
     be able to alias any other type of objects, just like the `char'
     type.  See `-fstrict-aliasing' for more information on aliasing
     issues.

     Example of use:

          typedef short __attribute__((__may_alias__)) short_a;

          int
          main (void)
          { 
            int a = 0x12345678;
            short_a *b = (short_a *) &a;

            b[1] = 0;

            if (a == 0x12345678)
              abort();

            exit(0);
          }

     If you replaced `short_a' with `short' in the variable
     declaration, the above program would abort when compiled with
     `-fstrict-aliasing', which is on by default at `-O2' or above in
     recent GCC versions.


So you define a typedef for unsigned long which has the __may_alias__
attribute, and you go to town writing memcpy inline with that type
instead of a normal unsigned long.


-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer