Re: [PATCH] mm: slab.h: use ilog2() in kmalloc_index()

[Andrew Morton]

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