[cfe-users] dereferencing null pointers

[Zenith432 via cfe-users]

target is  x86_64-apple-darwin15.4.0 (Apple LLVM 7.3.0)

This function
===== vec.c =====
#include <stdlib.h>

int vec(int index)
{
	return ((int*) NULL)[index];
}
===============

 [Yes, I know it's undefined behavior in ansi C].

Compiled with "clang -c -O0 -o vec.o vec.c" yields this code

=====
0000000000000000	55              	pushq	%rbp
0000000000000001	4889e5          	movq	%rsp, %rbp
0000000000000004	31c0            	xorl	%eax, %eax
0000000000000006	89c1            	movl	%eax, %ecx
0000000000000008	897dfc          	movl	%edi, -0x4(%rbp)
000000000000000b	486355fc        	movslq	-0x4(%rbp), %rdx
000000000000000f	8b0491          	movl	_vec(%rcx,%rdx,4), %eax
0000000000000012	5d              	popq	%rbp
0000000000000013	c3              	retq
=====

Compiled with "clang -c -Os -o vec.o vec.c" yields this code
=====
0000000000000000	55              	pushq	%rbp
0000000000000001	4889e5          	movq	%rsp, %rbp
0000000000000004	0f0b            	ud2
=====

Questions:
1) Is there a way to suppress the optimization that generates a trap, and have -Os yield working code like -O0?
2) Barring that, is there some way to have this code generate a compile-time diagnostic instead of emitting a run-time trap?

Thank You.