[llvm-dev] Queries Regarding Usage of PGOInstrumentation Passes instead of Deprecated ProfileInfo

Wed Aug 15 14:14:46 PDT 2018

On Wed, Aug 15, 2018 at 1:28 PM Xinliang David Li <xinliangli at gmail.com>
wrote:

>
>
> On Wed, Aug 15, 2018 at 12:46 PM Malhar Thakkar <cs13b1031 at iith.ac.in>
> wrote:
>
>> Thank you so much for your response.
>>
>> On Wed, Aug 15, 2018 at 3:08 PM, Xinliang David Li <xinliangli at gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Wed, Aug 15, 2018 at 7:36 AM Malhar Thakkar via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hey all,
>>>>
>>>> I have a piece of code (written in LLVM 2.8) which uses profiling
>>>> results produced by ProfileInfo. It essentially computes the number of
>>>> iterations performed by a loop from the profiling information available.
>>>> The code snippet in my pass looks something like this.
>>>>
>>>>   BasicBlock *header = loop->getHeader();
>>>>   ProfileInfo &pi = getAnalysis< ProfileInfo >();
>>>>   for(pred_iterator i=pred_begin(header), e=pred_end(header); i!=e; ++i)
>>>>   {
>>>>     BasicBlock *pred = *i;
>>>>
>>>>     const double edgeCount = *pi.getEdgeWeight*( ProfileInfo::Edge(pred,header) );
>>>>     */* Some code */*
>>>>   }
>>>>
>>>> Now, since ProfileInfo has been deprecated and we have
>>>> PGOInstrumentation passes along with BranchProbability and BlockFrequency,
>>>> I wish to update the above snippet, in particular, obtaining the edge
>>>> weights (highlighted in the code above) using the new tools.
>>>>
>>>> In order to do that, I'm first generating edge count profile by the
>>>> PGOInstrumentationGen pass and then interpreting the edge count profile
>>>> with PGOInstrumentationUse.
>>>>
>>>> In the original code, where I had
>>>> au.addRequired< ProfileInfo >(), I now have
>>>>
>>>>
>>>> *au.addRequired< BlockFrequencyInfoWrapperPass >();au.addRequired<
>>>> BranchProbabilityInfoWrapperPass >();*
>>>> Now, I have the following queries.
>>>>
>>>>    - How do I use the edge count and branch weights data obtained
>>>>    using PGOInstrumentationUse in my pass and replace the
>>>>    *getEdgeWeight()* from ProfileInfo?
>>>>
>>>>
>>> You can do
>>>  BPI.getEdgeProbability(Src, Dest).scale(BFI.getBlockProfileCount(Src));
>>>
>>
>> I tried doing this but there are some semantic issues that I'm facing.
>> There is a loop in my program which I know is a hot loop since it is the
>> only loop inside the function main() and essentially, all the computations
>> are performed within this function via function calls. However, when I
>> perform getBlockProfileCount(Src) where Src is the preheader of the said
>> loop, I find that it returns zero and upon further investigation, I found
>> that the entry frequency for the function was 6148904414811 and the block
>> frequency for the aforementioned preheader block was 6148904394768 and
>> after performing udiv, since it is an integer division,
>> getBlockProfileCount(Src) returned zero and hence, my code infers that it
>> is not a hot loop when in fact it clearly is.
>>
>
Easwaran, I think this is a bug that needs to be fixed.

David

>
>> Any suggestions on why this may be happening?
>>
>
> What is function entry's profile count? Note it is different from entry
> frequency (which is synthetic). You can find the function entry's count
> from entry's prof meta data. There is also an API to get it.
>
>
>>
>>>
>>>>
>>>>    - If I call *getBlockProfileCount() *from the BlockFrequency pass,
>>>>    will that give me the data read during PGOInstrumentationUse or data based
>>>>    on static analysis? In other words, does the data obtained during
>>>>    PGOInstrumentationUse "propagate" to BlockFrequency and BranchProbability?
>>>>
>>>>
>>>> Correct.
>>>
>> Just to confirm, when you say "correct", do you mean to say that the
>> information in fact does get propagated to BlockFrequency and
>> BranchProbability? If yes, how does the propagation take place?
>>
>
> Branch probability info analysis pass will use the profile meta data (read
> in from profile data and annotated in the IR) and use that to compute
> branch probability. The block frequency propagation pass uses BPI info to
> compute relative block frequencies.   The actual profile count of the
> function entry is kept, and a block's actual profile count can then be
> recomputed using Entry Frequency, block frequency, and the entry count data.
>
> David
>
>
>
>>
>>
>> Thank you.
>>
>> Regards,
>> Malhar
>>
>>>
>>> David
>>>
>>>
>>>
>>>> Thank you.
>>>>
>>>>
>>>> Regards,
>>>> Malhar
>>>>
>>>>
>>>>
>>>>
>>>> ᐧ
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
>> ᐧ
>>
>
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