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In non-debug mode the read_null test fails with clang-3.0:
======================================================================
FAIL: test_disable (test.test_faulthandler.FaultHandlerTests)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/usr/home/stefan/hg/cpython/Lib/test/test_faulthandler.py", line 235, in test_disable
 self.assertNotEqual(exitcode, 0)
AssertionError: 0 == 0
clang "optimizes" the undefined behavior into a simple assignment:
$ ~/usr/bin/clang --version
clang version 3.0 (tags/RELEASE_30/final)
Target: x86_64-unknown-freebsd9.0
Thread model: posix
$ 
$ cat read_null.c
#include <stdio.h>
int
main(void)
{
 int *x = NULL, y;
 y = *x;
 printf("%d\n", y);
 return 0;
}
$ 
$ ~/usr/bin/clang -Wall -O0 -g -o read_null read_null.c 
$ ./read_null 
Segmentation fault: 11 (core dumped)
$ ~/usr/bin/clang -Wall -O3 -g -o read_null read_null.c 
$ ./read_null 
0