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Date: Wed, 10 May 2017 08:18:37 +0800 From: "Jin, Yao" <yao.jin@...ux.intel.com> To: Jiri Olsa <jolsa@...hat.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 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
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