linux-kernel - Re: [PATCH] mm: slab.h: use ilog2() in kmalloc

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Message-Id: <20160621145237.dae264ea5fe6b3b7f2d2d4e6@linux-foundation.org>
Date:	Tue, 21 Jun 2016 14:52:37 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Yury Norov <yury.norov@...il.com>
Cc:	masmart@...dex.ru, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, cl@...ux.com, enberg@...nel.org,
	rientjes@...gle.com, iamjoonsoo.kim@....com,
	linux@...musvillemoes.dk, Alexey Klimov <klimov.linux@...il.com>
Subject: Re: [PATCH] mm: slab.h: use ilog2() in kmalloc_index()

On Tue, 21 Jun 2016 02:33:06 +0300 Yury Norov <yury.norov@...il.com> wrote:

> kmalloc_index() uses simple straightforward way to calculate
> bit position of nearest or equal upper power of 2.
> This effectively results in generation of 24 episodes of
> compare-branch instructions in assembler.
> 
> There is shorter way to calculate this: fls(size - 1).
> 
> The patch removes hard-coded calculation of kmalloc slab and
> uses ilog2() instead that works on top of fls(). ilog2 is used
> with intention that compiler also might optimize constant case
> during compile time if it detects that.
> 
> BUG() is moved to the beginning of function. We left it here to
> provide identical behaviour to previous version. It may be removed
> if there's no requirement in it anymore.
> 
> While we're at this, fix comment that describes return value.

kmalloc_index() is always called with a constant-valued `size' (see
__builtin_constant_p() tests) so the compiler will evaluate the switch
statement at compile-time.  This will be more efficient than calling
fls() at runtime.