[PATCH] D28964: [PGO] Value profile support for value ranges

[Rong Xu via llvm-commits]

On Tue, Jan 24, 2017 at 10:58 AM, Xinliang David Li <davidxl at google.com>
wrote:

>
>
> On Tue, Jan 24, 2017 at 10:49 AM, Rong Xu <xur at google.com> wrote:
>
>>
>>
>> On Mon, Jan 23, 2017 at 10:22 AM, David Li via Phabricator <
>> reviews at reviews.llvm.org> wrote:
>>
>>> davidxl added a comment.
>>>
>>> A general comment.
>>>
>>> The range profiling should not be limited to just memory intrinsics.  It
>>> should be designed to be a more general interval profiler. Interval
>>> profiler is also useful for other value profiling transformations such as
>>> mod operation for integer values (x%y). The value profiler can do interval
>>> profiling for division x/y and track resulting values in interval [0, 1].
>>>
>>
>> why do we need to track the result? I think usually we profile the
>> divisor. Current code can track the range of divisor. The only issue is
>> that I assume the type to be unsigned.
>>
>
> There are two types of value profiling related to div/mod. The one I
> mentioned is for 'mod' operation:
>
> if the value of x/y is mostly 0,
>
> x%y can be transformed into
>
>  if (x < y)
>     return x;
>  ..
>
> if 1 is also one of the hot target value of x/y, x%y can be transformed
> like
>   if (y > x - y)
>      return x-y;
>
> In other words, we only care about values in range [0,1].
>

OK. this can be handled in current implementation.

I thought we also wanted to do power of 2 value profiling to convert
operation to bit operation.
but anyway, that's another instrumentation.

>
>
>
>>
>>
>>> In a more general form, an interval profiler API should specify the
>>> following things
>>>
>>> 1. counter index
>>> 2. interval start and end (inclusive) : [S, E]
>>> 3. optionally specify the start value of a large value range [SL, inf)
>>>
>>> 3 + (E-S+1)  values will be tracked:
>>> (-inf, S) one value
>>> [S,E] one for each in the range
>>> [E, SL) one value
>>> [SL, inf) for one.
>>>
>>> The client (instrumentor) decides the the interval boundaries depending
>>> on the value profile kind.
>>>
>>>
>> Index is obtained incrementally in the instrumentation. I don't think
>> it's necessary to expose it to the API.
>> right now, there is options to control the intervals. it's universal to
>> all range profiles.
>> I can have the client choose the intervals by including it into the
>> instrumentation intrinsic.
>>
>>
> See my later comments about eliminating this change.
>
> David
>
>>
>>
>>>
>>> Repository:
>>>   rL LLVM
>>>
>>> https://reviews.llvm.org/D28964
>>>
>>>
>>>
>>>
>>
>
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