[Bug tree-optimization/21305] New: flag_delete_null_pointer_checks is target specific

["roger at eyesopen dot com" <gcc-bugzilla at gcc dot gnu dot org>]

The current -fdelete_null_pointer_checks implementation makes assumptions that
the target processor will trap on reads or writes to address zero.  Unfortunately,
these assumptions are target (often operating system) specific.

Here's a test case for AIX:

void abort();

int test(char *ptr)
{
  int x = *ptr;
  return ptr ? x+1: 2;
}

int main()
{
  if(test(0) != 2)
    abort();
}

When compiled with mainline this aborts when compiled at -O2 or higher, and
passes when compiled at -O1 or lower.  AIX allows reads from but not writes
to address zero.  Other targets, specifically those without virtual memory
often allow both reading and writing to address zero.

I propose we should add target macros, TARGET_NULL_POINTER_READABLE and
TARGET_NULL_POINTER_WRITABLE and control the effects of the
-fdelete_null_pointer_checks with that.  The current situation is that
these transformations are enabled automatically at -O2 without regard to
the target environment.

-- 
           Summary: flag_delete_null_pointer_checks is target specific
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: roger at eyesopen dot com
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: powerpc-ibm-aix5.2.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21305