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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D0F6D3983@AcuExch.aculab.com> Date: Thu, 6 Mar 2014 09:44:47 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'Sukadev Bhattiprolu' <sukadev@...ux.vnet.ibm.com>, "Arnaldo Carvalho de Melo" <acme@...stprotocols.net> CC: 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>, Jiri Olsa <jolsa@...hat.com> Subject: RE: [PATCH 3/3] perf: Use 64-bit value when comparing sample_regs 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. 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.) David
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