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

If you look at the new intel documentation they've changed that it
does indeed not touch the original result, even going back to old cpus
(unless maybe something from i386), except some not quite old cpus do
touch 32-bit registers.
clang does that as well.

On Fri, Jan 9, 2026 at 8:22 PM Douglas Katzman via Sbcl-devel
<[email protected]> wrote:
>
> 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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