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

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Message-ID: <29d920f3-3491-2be2-f9f7-795e23217621@linux.intel.com>
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