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Date: Thu, 6 Mar 2014 18:06:48 +0100 From: Jiri Olsa <jolsa@...hat.com> To: Gabriel Paubert <paubert@...m.es> Cc: David Laight <David.Laight@...LAB.COM>, "'Sukadev Bhattiprolu'" <sukadev@...ux.vnet.ibm.com>, Arnaldo Carvalho de Melo <acme@...stprotocols.net>, Michael Ellerman <michaele@....ibm.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Stephane Eranian <eranian@...gle.com>, "linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>, Paul Mackerras <paulus@...ba.org> Subject: Re: [PATCH 3/3] perf: Use 64-bit value when comparing sample_regs On Thu, Mar 06, 2014 at 12:33:32PM +0100, Gabriel Paubert wrote: > On Thu, Mar 06, 2014 at 09:44:47AM +0000, David Laight wrote: > > From: Sukadev Bhattiprolu > > > When checking whether a bit representing a register is set in > > > sample_regs, a 64-bit mask, use 64-bit value (1LL). > > > > > > Signed-off-by: Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com> > > > --- > > > tools/perf/util/unwind.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/tools/perf/util/unwind.c b/tools/perf/util/unwind.c > > > index 742f23b..2b888c6 100644 > > > --- a/tools/perf/util/unwind.c > > > +++ b/tools/perf/util/unwind.c > > > @@ -396,11 +396,11 @@ static int reg_value(unw_word_t *valp, struct regs_dump *regs, int id, > > > { > > > int i, idx = 0; > > > > > > - if (!(sample_regs & (1 << id))) > > > + if (!(sample_regs & (1LL << id))) > > > return -EINVAL; > > > > > > for (i = 0; i < id; i++) { > > > - if (sample_regs & (1 << i)) > > > + if (sample_regs & (1LL << i)) > > > idx++; > > > } > > > > There are much faster ways to count the number of set bits, especially > > if you might need to check a significant number of bits. > > There might even be a function defined somewhere to do it. > > Indeed, look for Hamming weight (hweight family of functions) > in asm/hweight.h and what is included from there. > > Besides that, many modern processors also have a machine instruction > to perform this task. In the processor manuals the instruction is > described as population count and the mnemonic starts with "popcnt" > on x86 and ppc. > > Gabriel > > > Basically you just add up the bits, for 16 bit it would be: > > val = (val & 0x5555) + (val >> 1) & 0x5555; > > val = (val & 0x3333) + (val >> 2) & 0x3333; > > val = (val & 0x0f0f) + (val >> 4) & 0x0f0f; > > val = (val & 0x00ff) + (val >> 8) & 0x00ff; > > As the size of the work increases the improvement is more significant. > > (Some of the later masking can probably be proven unnecessary.) right I think the loop could be replaced by: idx = hweight(mask & ((1 << id) - 1)) Sukadev, please also rebase against latest Arnaldo's perf/core, this code has changed just recently, it's now in: util/perf_regs.c:perf_reg_value thanks, jirka -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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