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Date: Sat, 1 Nov 2008 15:34:53 +0100
From: "Laurent Desnogues" <laurent.desnogues@gmail.com>
Subject: Re: [Qemu-devel] [5592] target-ppc: optimize popcntb
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To: qemu-devel@nongnu.org

On Sat, Nov 1, 2008 at 2:57 PM, andrzej zaborowski <balrogg@gmail.com> wrote:
>
> On ppc host there might be a builtin for it, on the x86 Xeon cpus with
> SSE4 there's also a bitcounting instruction but this approach is
> actually faster than transferring the number to the MMX register,
> running the instruction and copying the value back.  In the benchmarks
> I've seen the speed is comparable with table lookup on x86 and -O3.

Bit tricks are always very sensitive.  BTW I would not trust any
benchmark that run them in loop for obvious reasons :-)

For those who like that kind of thing, here are some nice refs:

- Knuth:  http://www-cs-faculty.stanford.edu/~uno/fasc1a.ps.gz
- Anderson:  http://www-graphics.stanford.edu/~seander/bithacks.html
- Arndt:  http://www.jjj.de/bitwizardry/bitwizardrypage.html

Many of these tricks are known or obvious, but it's good reading
anyway, especially for qemu target and back-end writers.


Laurent