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

[Richard Smith]

On Sun, Oct 26, 2014 at 10:24 PM, Chandler Carruth <chandlerc at google.com>
wrote:

> Sorry for the late replies, but as the last person to work on the CLZ /
> CTZ stuff, I have some small opinions here. =]
>
> On Sun, Oct 26, 2014 at 9:52 PM, David Majnemer <david.majnemer at gmail.com>
> wrote:
>
>> I think Paul's patch is fine.
>>
>
> I firmly disagree.
>
>
>>
>> Having `__builtin_ctz` asking the target if `isCLZForZeroUndef` seems
>> busted and this patch seems to fix that just fine.
>>
>
> Yes, that is busted and we should fix that aspect regardless. But changing
> the behavior on x86 seems actively harmful to portability.
>
>
> So, let's look at the history.
>
> I went and taught LLVM to produce optimal code in cases where
> undef-at-zero was acceptable and only instructions with undef-at-zero were
> available. In order to generate reasonable code for these scenarios, Clang
> and LLVM had to respect the contract specified for the GCC __builtin_clz
> and __builtin_ctz. That spec[1] *very* clearly states that if the input is
> zero the result is undefined.
>
> 1: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
>
> However, after this Bob changed ARM to not follow this spec citing a radar
> only in r149086. I objected at the time, and continue to feel this is the
> *wrong* approach for ARM. We should not be deviating from the clear,
> documented, and widely followed spec here. However, I had no real basis for
> arguing about the behavior of ARM at the time, and I doubt that I have much
> say in the behavior there even now. =] That said, on x86, I am firmly
> opposed to deviating from the clear and unambiguous spec.
>
> If folks want a builtin to access unambiguously defined-at-zero CLZ or CTZ
> behavior, we should define *new* builtins which have a clear spec here.
> However, I'm unpersuaded that this is better or more portable. The
> alternative code seems quite palatable: (x == 0 ? 32 : __builtin_clz(x))
>
> If LLVM fails to turn this into the maximally efficient code on some
> architecture, then much as others have suggested, we should enhance LLVM.
>

I agree with Chandler on all points here.

__builtin_clz and __builtin_ctz is clearly and unambiguously defined by
GCC's specification for them, and we should not be providing additional
definedness  guarantees. If we want a defined-for-zero-input form of these
intrinsics, they should have a different name (or an additional argument,
or be written as "x == 0 ? 32 : __builtin_clz(x)" or similar).

It is a major feature of Clang that we try not to lock people in by
providing incompatible extensions.
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