Re: Comparing lowtags: slightly inefficient assembly code

Is that causing you some unexpected performance issues?

On Wed, Apr 30, 2025 at 10:50 PM David Scherfgen via Sbcl-help
<[email protected]> wrote:
>
> Hello,
>
> I have this SBCL-specific code to determine whether two objects X and Y have the same lowtag:
>
> (defun same-lowtag-p (x y)
>   (= (sb-kernel:lowtag-of x) (sb-kernel:lowtag-of y)))
>
> The relevant part of the disassembly:
>
> MOV RDX, R8
> SHL RDX, 1
> AND EDX, 30
> MOV RDI, RSI
> SHL RDI, 1
> AND EDI, 30
> CMP RDI, RDX
>
> It shifts the address of X and Y to the left by 1 bit, then masks with 30 (= 15 << 1), then compares.
> The bit shift is unnecessary. It could mask X and Y with 15 and directly compare without shifting.
>
> Is there any (hacky, using internals) way to modify the Lisp function so that it results in optimal assembly without bit shifts?
>
> Thank you.
>
> Best regards
> David Scherfgen
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> Sbcl-help mailing list
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