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

[David Laight]

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@xxxxxxxxxxxxxxxxxx>
> ---
>  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