Re: [Patch, testsuite] Skip addr_equal-1 if target keeps null pointer checks

[Senthil Kumar Selvaraj <senthil_kumar dot selvaraj at atmel dot com>]

On Mon, Sep 28, 2015 at 01:38:18PM -0600, Jeff Law wrote:
> On 09/28/2015 02:15 AM, Senthil Kumar Selvaraj wrote:
> >Hi,
> >
> >   The below patch skips gcc.dg/addr_equal-1.c if the target keeps null
> >   pointer checks.
> >
> >   The test fails for such targets (avr, in my case) because the address
> >   comparison in the below code does not resolve to a constant, causing
> >   builtin_constant_p to return false and fail the test.
> >
> >   /* Variables and functions do not share same memory locations otherwise.  */
> >   if (!__builtin_constant_p ((void *)undef_fn0 == (void *)&undef_var0))
> >     abort ();
> >
> >   For targets that delete null pointer checks, the equality comparison expression
> >   is optimized away to 0, as the code in match.pd knows they can only be
> >   equal if they are both NULL, which cannot be true since
> >   flag-delete-null-pointer-checks is on.
> >
> >   For targets that keep null pointer checks, 0 is a valid address and the
> >	comparison expression is left as is, and that causes a later pass to
> >	fold the builtin_constant_p to a false value, resulting in the test failure.
> This sounds like a failing in the compiler itself, not a testsuite issue.
> 
> Even on a target where objects can be at address 0, you can't have a
> variable and a function at the same address.

Hmm, symtab_node::equal_address_to, which is where the address equality
check happens, has a comment that contradicts
your statement, and the function variable overlap check is done after the
NULL possibility check. The current code looks like this

   /* If both symbols may resolve to NULL, we can not really prove them different.  */                                                                                                                             
    if (!nonzero_address () && !s2->nonzero_address ())
      return 2;
  
    /* Except for NULL, functions and variables never overlap.  */
    if (TREE_CODE (decl) != TREE_CODE (s2->decl))
      return 0;

Does anyone know why?

Regards
Senthil