Re: Serious code generation/optimisation bug (I think)

[Chris Lattner <clattner at apple dot com>]

On Jan 27, 2009, at 5:10 PM, Ian Lance Taylor wrote:

> Chris Lattner <clattner@apple.com> writes:
>
> > On Jan 27, 2009, at 1:10 PM, Ian Lance Taylor wrote:
> >
> > > Laurent GUERBY <laurent@guerby.net> writes:
> > >
> > > > Just curious: is there a "portable" way to read from memory
> > > > address zero in C code? "portable" here means likely to work
> > > > on most compilers without exotic compile flags in 2009.
> > >
> > > char *my_null_pointer;
> > > char fn() { return *my_null_pointer; }
> > >
> > > It will be quite a while before whole program optimization is good
> > > enough to prove that my_null_pointer is not referenced by anything
> > > else.
> >
> > Perhaps in GCC, but this is not a good solution if you care about
> > other compilers.
>
> Is LLVM smart enough to optimize that away, even when using shared
> libraries?

Yes absolutely.  Just build with -fvisibility-hidden or use an export  
map to say that my_null_pointer is not exported.  If it is static, it  
will also do it at -O2.  This is not even a particularly aggressive  
optimization.

-Chris