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Date: Fri, 23 Oct 2009 13:47:45 +0100
From: Stuart Brady <sdbrady@ntlworld.com>
Subject: Re: [Qemu-devel] [PATCH] target-arm: use clz32() instead of a for loop
Message-ID: <20091023124745.GA32401@miranda.arrow>
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To: Aurelien Jarno <aurelien@aurel32.net>
Cc: qemu-devel@nongnu.org

On Fri, Oct 23, 2009 at 09:04:53AM +0200, Aurelien Jarno wrote:
> Stuart Brady a écrit :
> > Just a quick note that the implementation of clz, ctz and popcnt is
> > still listed in the TCG TODO list.  The last time I looked, I noticed
> > that quite a few architectures have clz/ctz instructions:
> > 
> >    http://lkml.indiana.edu/hypermail/linux/kernel/0601.3/1683.html
> 
> OTOH, a dump shows that those instruction are not used than often, so I
> am not sure it worth implementing it.

Really?  I'm surprised, as I gather that optimised ffs/fls/hweight
functions in the kernel do give a modest gain...  I suppose I'll have
to try it on several different targets and see! :-)

> > For those that don't, I think a combination the following two hacks at
> > http://graphics.stanford.edu/~seander/bithacks.html could be used:
> 
> The best is probably to use an helper in that case, calling clz32(x).

Yes, you're right.

There are several other places that should also call clz32()/ctz32().
The ones that I can see are helper_neon_cls_s32() for ARM, helper_bsf()
and helper_bsr() for X86, helper_ff1() for M68K.  (I'm not sure about
'do_clz8' and 'do_clz16', though.)

At some point, possibly next weekend, I'll submit patches to add clz
and ctz helpers to tcg-runtime.c, and to convert Alpha, ARM, CRIS, M68K,
MIPS, PowerPC and x86 (any others I've missed?) to use those helpers.

Cheers,
-- 
Stuart Brady