[PATCH][X86] __builtin_ctz/clz sometimed defined for zero input

[Chandler Carruth]

On Mon, Oct 27, 2014 at 7:07 PM, Sean Silva <chisophugis at gmail.com> wrote:

> Curiously, it looks like GCC is depending on definedness for 0 in its own
> headers:
> https://github.com/gcc-mirror/gcc/blob/master/gcc/config/i386/lzcntintrin.h
>
> *shrug*
>

I don't think that's really a justification for much of anything though.

We should still add UBSan checks for this, and make our code correct
according to the spec for these builtins.

Currently, I continue to prefer not using a builtin and instead defining
the __lzcnt stuff with a ternary operator.
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