Stuart Brady a écrit : > 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! :-)
I gave a quick look at MIPS, and at least here, it is used often. >>> 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. The main problem I see for a TCG implementation is the definition of clz/ctz. Some targets define that clz(0) or ctz(0) returns 32, some other define it as being "undefined". If we go for the common denominator for the TCG op, that is clz(0) = undefined, it means that a test with brcond has to be added in the targets using clz(0) = 32, and this is likely to give more slow down than speed gain. If we go for clz(0) = 32, it means the test has to be implemented in TCG, which might be complicated for some hosts. -- Aurelien Jarno GPG: 1024D/F1BCDB73 aurel...@aurel32.net http://www.aurel32.net
