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
