[PATCH] D117912: [IR] document and update ctlz/cttz intrinsics to optionally return poison rather than undef

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D117912#3262368 <https://reviews.llvm.org/D117912#3262368>, @craig.topper wrote:

> In D117912#3262280 <https://reviews.llvm.org/D117912#3262280>, @spatel wrote:
>
>> In D117912#3262125 <https://reviews.llvm.org/D117912#3262125>, @efriedma wrote:
>>
>>> You mean, we might reach a point where we could make clang emit is_zero_poison=false without anyone caring?  Don't think we're quite there yet.  x86-64-v2 doesn't include lzcnt.  And not sure about the current state of the compiler-rt/libgcc intrinsics.
>>
>> Yes, I was wondering what level of default x86 tuning was needed to flip the setting...I suppose that's x86-64-v3 ("Close to Haswell").
>
> Is updating isCheapToSpeculateCTTZ/CTLZ to check CMOV instead of BMI/LZCNT enough?

That would let this:

  int clipped2(std::uint64_t a) { return (a == 0) ? 64 : __builtin_ctzll(a); }

Become:

  bsfq	%rdi, %rcx
  movl	$64, %eax
  cmovneq	%rcx, %rax

Instead of a 'test + je' around the bsf, so that's probably good. But I can't remember all of the corner-case patterns that we're trying to account for with these builtins.


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