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

[Malhar Thakkar via llvm-dev]

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.

Any suggestions on why this may be happening?

>
>
>>
>>    - 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?

Thank you.

Regards,
Malhar

>
> David
>
>
>
>> Thank you.
>>
>>
>> Regards,
>> Malhar
>>
>>
>>
>>
>> ᐧ
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>> 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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