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Date: Thu, 1 Jun 2023 16:35:04 +0300 (MSK)
From: Alexander Monakov <>
cc: Rich Felker <>
Subject: Re: [C23 new stdlib 2/4] C23: add the memalignment function

On Thu, 25 May 2023, Jā‚‘ā‚™ā‚› Gustedt wrote:

> Rich,
> on Wed, 24 May 2023 17:28:35 -0400 you (Rich Felker <>)
> 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.

It seems this was unanswered. Hacker's Delight by Henry Warren offers
a lot of material on such topics. For this particular question it also
gives the efficient implementation: 'p & -p'.

I looked over the recently posted stdbit.h patch and I don't have anything
to suggest in terms of formulas. The things that would worry me are of a
different nature anyway.


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