[Bug c/31236] incorrect output on external symbol address cast as integer used in conditional statement

["bangerth at dealii dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #6 from bangerth at dealii dot org  2007-03-21 03:59 -------
(In reply to comment #4)
> so in other words you're saying the solution to this is 
> "if you want it to work with GCC, make your code non-portable"
> 
> afaik "__attribute__ (( weak ))" is not part of the standard, its not a
> portable solution.
> 
> the fact that
>         #define SOMEVALUE ((int)&SomeLinkerScriptDefinedSymbol)
>         int i = SOMEVALUE; 
>         if(i == 0){
> and
>         if(SOMEVALUE == 0){
> 
> give a different result makes gcc act inconsistently on code that should be
> equivalent.

I would argue that it is a missed optimization that it doesn't optimize
away both cases.

That said, a compiler is entirely within the standard to assume that the
address of a symbol is non-null. That's simply the case in a well-formed
program. If you specifically want something different, then you need to
let the compiler know this.

As a second point, you are surely aware of the fact that casting a
pointer (in general) to int is not portable, and a function pointer in
particular (since function pointers may not be represented as pointers,
but potentially as structures). The latter is where the difference in
optimization comes from.

W.


-- 

bangerth at dealii dot org changed:

           What    |Removed                     |Added
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                 CC|                            |bangerth at dealii dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31236