Re: Comparing lowtags: slightly inefficient assembly code

The purpose of this code is to provide an "early exit" in a function that
compares two objects X and Y - it's the #1 function in terms of number of
calls and execution time in the application (determined by deterministic
and statistical profiling). If X and Y have different lowtags, then the
function can abort immediately. I can't tell whether the extra bit shifts
cause significant performance issues, I'd have to compare it to the
"optimal" code, but that's a bit difficult in practice.

Am Mi., 30. Apr. 2025 um 21:51 Uhr schrieb Stas Boukarev <[email protected]
>:

> 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
> > _______________________________________________
> > Sbcl-help mailing list
> > [email protected]
> > https://lists.sourceforge.net/lists/listinfo/sbcl-help
>

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