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.
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