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

[Malhar Thakkar via llvm-dev]

Thank you so much!
ᐧ

On Tue, Aug 21, 2018 at 6:11 PM, Easwaran Raman <eraman at google.com> wrote:

> I have fixed the rounding issue recently at r339835.
>
> - Easwaran
>
> On Tue, Aug 21, 2018 at 2:20 PM, Malhar Thakkar <cs13b1031 at iith.ac.in>
> wrote:
>
>> I forgot to mention that I'm using LLVM 5.0.
>>
>> Is the problem something that was fixed post LLVM 5.0?
>>
>>
>>
>> Thank you.
>>
>> Regards,
>> Malhar
>>
>> On Wed, Aug 15, 2018 at 5:14 PM, Xinliang David Li <xinliangli at gmail.com>
>> wrote:
>>
>>>
>>>
>>> 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.getBlockProfil
>>>>>> eCount(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
>>>>>
>>>>
>> ᐧ
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180821/c83fca63/attachment.html>