linux-kernel - Re: [PATCH] wifi: carl9170: micro-optimize carl9170_tx_shift

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Message-ID: <aA5McagnQw49MElr@yury>
Date: Sun, 27 Apr 2025 11:25:37 -0400
From: Yury Norov <yury.norov@...il.com>
To: Christian Lamparter <chunkeey@...il.com>
Cc: linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] wifi: carl9170: micro-optimize carl9170_tx_shift_bm()

On Wed, Mar 26, 2025 at 09:00:33PM +0100, Christian Lamparter wrote:
> Hi,
> 
> On Wed, Mar 26, 2025 at 4:52 PM Yury Norov <yury.norov@...il.com> wrote:
> >
> > The function calls bitmap_empty() just before find_first_bit(). Both
> > functions are O(N). Because find_first_bit() returns >= nbits in case of
> > empty bitmap, the bitmap_empty() test may be avoided.
> >
> 
> I looked up bitmap_empty():
> <https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/bitmap.h#n423>
> 
> apart from the small_const_nbits stuff (which carl9170 likely does not qualify
> for since from what I remember it's a 128bits bitmap) the function just does:
> 
> |   return find_first_bit(src, nbits) == nbits;
> 
> so yes, find_first_bit runs twice with same parameters... Unless the
> compiler is smart
> enough to detect this and (re-)use the intermediate result later. But
> I haven't check
> if this is the case with any current, old or future compilers. Has anyone?
> 
> Anyway, Sure.
> 
> > Signed-off-by: Yury Norov <yury.norov@...il.com>
> 
> Acked-by: Christian Lamparter <chunkeey@...il.com>

Thanks, Chrustian. So, how is that supposed to be merged?
I can move it with bitmap-for-next, unless there's no better
branch.

Thanks,
Yury