linux-kernel - Re: [patch 05/24] perfmon: X86 generic code (x86)

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Message-ID: <alpine.LFD.2.00.0811262355100.3325@localhost.localdomain>
Date:	Thu, 27 Nov 2008 00:16:45 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	eranian@...il.com
cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mingo@...e.hu, x86@...nel.org,
	sfr@...b.auug.org.au
Subject: Re: [patch 05/24] perfmon: X86 generic code (x86)

On Wed, 26 Nov 2008, stephane eranian wrote:
> > What a nonsense. We have a bitmask already. Why not iterate over the
> > bitmask and be done ?
> >
> 
> Bitmask can be sparsed. Num represents the number of bits we have to find.
> The idea is that we don't need to scan the entire bitmask, we stop as soon as
> we have found all the bits we care about (i.e., all the bits that are set).
> 
> Example:
>          num = 3
>      bitmask=0000000010001001
>                                     ^ we will iterate until we are
> done with that bit.

Errm.

#define for_each_bit(bit, addr, size) \
        for ((bit) = find_first_bit((addr), (size)); \
             (bit) < (size); \
             (bit) = find_next_bit((addr), (size), (bit) + 1))

find_first_bit() and find_next_bit() are single instructions on most
architectures. "size" is known upfront at setup time of the
context/set and can be cached.

This takes exactly 3 iterations, while your method needs 8. And it
gets worse with the following example:

Example:
  num = 1
  bitmask=1000 0000 0000 0000 0000 0000 0000 00000

          ^ you will iterate until we are done with that bit (32 times)
	 
  for_each_bit() will iterate exactly _once_.

Thanks,

	tglx
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