Re: [PATCH v6 2/7] perf/x86/intel: Record branch type

[Jin, Yao]

On 5/9/2017 8:39 PM, Jiri Olsa wrote:
> On Tue, May 09, 2017 at 07:57:11PM +0800, Jin, Yao wrote:
>
> SNIP
>
> > > > > > +
> > > > > > +	type >>= 2; /* skip X86_BR_USER and X86_BR_KERNEL */
> > > > > > +	mask = ~(~0 << 1);
> > > > > is that a fancy way to get 1 into the mask? what do I miss?
> > > you did not comment on this one
> > Sorry, I misunderstood that this comment and the next comment had the same
> > meaning.
> >
> > In the previous version, I used the switch/case to convert from X86_BR to
> > PERF_BR. I got a comment from community that it'd better use a lookup table
> > for conversion.
> >
> > Since each bit in type represents a X86_BR type so I use a mask (0x1) to
> > filter the bit. Yes, it looks I can also directly set 0x1 to mask.
> >
> > I write the code "mask = ~(~0 << 1)" according to my coding habits. If you
> > think I should change the code to "mask = 0x1", that's OK  :)
> im ok with that.. was just wondering for the reason
> I guess compiler will make it single constant assignment anyway

I think so.  The compiler should be clever enough for this optimization.
> > > > > > +
> > > > > > +	for (i = 0; i < X86_BR_TYPE_MAP_MAX; i++) {
> > > > > > +		if (type & mask)
> > > > > > +			return branch_map[i];
> > > > > I wonder some bit search would be faster in here, but maybe not big deal
> > > > >
> > > > > jirka
> > > > I just think the branch_map[] doesn't contain many entries (16 entries
> > > > here), so maybe checking 1 bit one time should be acceptable. I just want to
> > > > keep the code simple.
> > > >
> > > > But if the number of entries is more (e.g. 64), maybe it'd better check 2 or
> > > > 4 bits one time.
> > > ook
> > >
> > > jirka
> > Sorry, what's the meaning of ook? Does it mean "OK"?
> just means ok ;-)
>
> thanks,
> jirka

Thanks so much!

Jin Yao