Re: [PATCH 3/3] perf: Use 64-bit value when comparing sample_regs

[Gabriel Paubert <paubert@iram.es>]

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@linux.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.)
> 
> 	David
> 
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