linux-kernel - Re: [PATCH 01/12] perf_events: add generic taken branch sampling support

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Date:	Fri, 7 Oct 2011 12:28:47 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, acme@...hat.com,
	ming.m.lin@...el.com, andi@...stfloor.org, robert.richter@....com,
	ravitillo@....gov
Subject: Re: [PATCH 01/12] perf_events: add generic taken branch sampling support

On Thu, Oct 6, 2011 at 6:57 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2011-10-06 at 16:49 +0200, Stephane Eranian wrote:
>>  struct perf_branch_entry {
>>         __u64                           from;
>>         __u64                           to;
>> +       struct {
>> +               __u64                   mispred:1,  /* target mispredicted */
>> +                                       predicted:1,/* target predicted */
>> +                                       reserved:62;
>> +       };
>>  };
>
> Why that anonymous structure?
>
The interface can return more than source/destination, it can also
return prediction infos.
Prediction is boolean, thus we only need a couple of bits. The reason
there are two bits
and not just one is to disambiguate between: predicted correctly and
'prediction reporting
unsupported'. For instance, prediction is also supported since
Nehalem, Core2/Atom do
not have it.
But maybe you're just commenting of the anonymous vs. named struct for that?
It is just for convenience. Isn't that the same argument for the
anonymous bitfield
in struct perf_event_attr?
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