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Re: [Qemu-devel] [PATCH] target-arm: use clz32() instead of a for loop
From: |
Stuart Brady |
Subject: |
Re: [Qemu-devel] [PATCH] target-arm: use clz32() instead of a for loop |
Date: |
Fri, 23 Oct 2009 13:47:45 +0100 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
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