Re: [Sbcl-commits] master: Recognize (integer-length (ldb (byte 64 0) (1- (logand n (- n))))) as ctz

I see what you're trying to do to support older CPUs that don't implement
TZCNT, however this MOV is not technically correct.
If the processor treats TZCNT just like BSF, then the destination register
is *undefined (*https://www.felixcloutier.com/x86/bsf)*,* not *unaffected*
when the input is 0.  So the output could be zero or random bits.  I
realize that clang uses this technique, but my guess is they simply haven't
gotten complaints. gcc takes the manufacturer's spec at face value, and
uses a pedantically correct solution to fixup the result if the input was
zero
 testq %rdi, %rdi
 movl $64, %eax
 cmovne %edx, %eax


>
> +(define-vop ()
> +  (:translate count-trailing-zeros)
> +  (:policy :fast-safe)
> +  (:args (arg :scs (signed-reg unsigned-reg) :to :save))
> +  (:arg-types (:or signed-num unsigned-num))
> +  (:results (res :scs (unsigned-reg)))
> +  (:result-types unsigned-num)
> +  (:generator 5
> +    (inst mov res 64)
> +    (inst tzcnt res arg)))
>

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