Re: Comparing lowtags: slightly inefficient assembly code

Yes, you can. Check nibbles for an example.

чт, 1 мая 2025 г., 00:18 David Scherfgen <[email protected]>:

> Thanks for the hint!
> Can I define VOPs in my application's source code, or do I have to modify
> SBCL itself?
> The article you linked mentions it's possible to do it at "runtime", but
> then says it's a bit "hairy".
>
> Regarding EQ/EQL/...: The comparison that's being done, unfortunately,
> cannot rely on any of these. The comparison logic is a bit intricate.
>
> Am Mi., 30. Apr. 2025 um 22:48 Uhr schrieb Vasily Postnicov <
> [email protected]>:
>
>> It's adding the fixnum's tag by shifting left.
>>
>> My guess would be to write a function which translates directly to a VOP.
>> You can write any assembly code you wish there. See here for an example:
>> https://pvk.ca/Blog/2014/08/16/how-to-define-new-intrinsics-in-sbcl/
>>
>> On the other hand, I'd recommend EQ/EQL/EQUAL/EQUALP for comparing
>> objects whose types aren't known to be anything more narrow than T.
>>
>> ср, 30 апр. 2025 г., 23:03 David Scherfgen via Sbcl-help <
>> [email protected]>:
>>
>>> 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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>>

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