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Re: Null pointer dereference and side effects
- From: Alexandre Oliva <aoliva at redhat dot com>
- To: Archie Cobbs <archie at dellroad dot org>
- Cc: gcc-help at gcc dot gnu dot org
- Date: 21 Jul 2004 03:44:00 -0300
- Subject: Re: Null pointer dereference and side effects
- Organization: Red Hat Global Engineering Services Compiler Team
- References: <200407201512.i6KFCXrx001516@arch20m.dellroad.org>
On Jul 20, 2004, Archie Cobbs <archie@dellroad.org> wrote:
> It appears gcc is completely ignoring the possibility of
> "side effects" when it optimizes away the comparison.
Oh, rats. That's because the reasoning about dereferencing NULL
invoking undefined behavior still applies. Since both arms of the if
return the same value, the compare is optimized away, and the
dereference of the pointer can be removed because, if it was NULL or
any other bad pointer, it would invoke undefined behavior, and
otherwise, the dereference would be safe and unused.
I think you really need to make the pointer volatile to prevent the
dereference from being optimized out.
--
Alexandre Oliva http://www.ic.unicamp.br/~oliva/
Red Hat Compiler Engineer aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist oliva@{lsd.ic.unicamp.br, gnu.org}