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

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Date:   Tue, 9 May 2017 14:39:13 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     "Jin, Yao" <yao.jin@...ux.intel.com>
Cc:     acme@...nel.org, jolsa@...nel.org, peterz@...radead.org,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com,
        Linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kan.liang@...el.com, yao.jin@...el.com,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v6 2/7] perf/x86/intel: Record branch type

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

> 
> > > > > +
> > > > > +	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