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Date: Thu, 25 May 2023 11:16:53 +0200
From: Jₑₙₛ Gustedt <jens.gustedt@...ia.fr>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: [C23 new stdlib 2/4] C23: add the memalignment function
Rich,
on Wed, 24 May 2023 17:28:35 -0400 you (Rich Felker <dalias@...c.org>)
wrote:
> There's a more efficient implementation of this function which does
> not depend on __builtin_ctzll (which we don't): p^(p-1)&p
yes, nice
If you have other such nice formulas for some of the interfaces in
<stdbit.h>, please let me know. They are hard to find in search
engines.
For `__builtin_ctzll`, hm, I counted also to use these for the
<stdbit.h> interfaces. I think the intent of those is really to
provide instruction level support of these bit functionalities that
avoids going through function calls.
I see that we use other quite specific builtins, such as
`__builtin_fma`. Is there a particular reason not to have the bit
level ones?
So you want me to use the ctz and clz interfaces from the internal
atomics for the implementation of <stdbit.h>? Are they not overkill
for this simple purpose? (I mean they are meant to be atomic, arent't
they?)
> I'm not clear that there's a really good motivation for having it
> implemented in the header. It kinda falls under the "maybe do it
> because it's easy" case, but once you switch to the non-GNUC-specific
> version, it needs a static inline function to avoid multiple
> evaluation,
I guess using `({ ... })` would also not be acceptable?
> and then it gets to the point of "this is really too much
> code to have in a header" (which is more an issue of avoiding
> "creative" content in headers than anything else).
You probably won't like what I have now for the bit operations, then
:-(
Thanks
Jₑₙₛ
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