linux-kernel - RE: [PATCH 3/3] perf: Use 64-bit value when comparing sample

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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