[Bug tree-optimization/53419] loop incorrectly optimized to endless loop at -O2 for table delimited by extern addresses (x86-32)

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53419

--- Comment #8 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-05-21 09:30:52 UTC ---
(In reply to comment #6)
> (In reply to comment #5)
> > > extern func_ptr_t init_array_begin[1];
> > > extern func_ptr_t init_array_end[1];
> > 
> > The array sizes say they are size of one.  If you want to be correct and not
> > allow GCC to optimize away the check because array overflow, use [] instead of
> > [1].
> 
> Good point.  Do you happen to know if
>   extern func_ptr_t init_array_begin[];
>   extern func_ptr_t init_array_end[];
> is say, C89 compatible?  I ask, because I thought I ran into problems with some
> older compilers given the above syntax.

Yes, it's C89 compatible.

> typedef unsigned int size_t;
> 
> typedef void (*func_ptr_t) (void);
> extern func_ptr_t init_array_begin[1];
> extern func_ptr_t init_array_end[1];
> 
> void
> per_thread_init (void)
> {
>   size_t n_init = (init_array_end - init_array_begin);
>   int i;
>   for (i = 0; i < n_init; ++i)
>     {
>       func_ptr_t init_func = init_array_begin[i];
>       if (init_func)
>         (*init_func) ();
>     }
> }
> 
> Questions regarding the optimization of the above.
> 
> If the compiler concludes that n_init must be 1, then code that creates an
> endless loop is not a valid optimization?
> 
> Simplifying so that the loop executes only once might be, but I'm still having
> a little trouble adjusting to that idea.

  size_t n_init = (init_array_end - init_array_begin);

is by itself also undefined - you are taking the difference between two
pointers to two distinct objects.  I suppose you should instead declare


extern func_ptr_t init_array_begin[];
extern func_ptr_t *init_array_end;

> Is there an -f option that disables this sort of optimization?

-fno-tree-vrp will, for your case.

> 
> 
> 1