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
