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Andrea, thanks for the advice. <br class="">
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<div class="">On Jan 2, 2020, at 8:09 AM, Andrea Di Biagio <<a href="mailto:andrea.dibiagio@gmail.com" class="">andrea.dibiagio@gmail.com</a>> wrote:</div>
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<div class="">Hi Lewis,</div>
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Basically - if I understand correctly - you want to design a pass that uses llvm-mca as a library to compute throughput indicators for your outlined functions. You would then use those indicators to classify outlined functions.<br class="">
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Yes basically, the idea is to build a performance model for that outlined function.<br class="">
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<div class="">llvm-mca doesn't know how to evaluate branches or instructions that affect the control flow. That basically restricts the analysis to single basic blocks that are assumed to be hot. I am not sure if this would be a blocker for your particular
use case.<br class="">
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That would be okay and is something we would need to work around on our end anyways since we don’t know the branch probability. <br class="">
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<div class="">llvm-mca only knows how to analyze/simulate a sequence of `mca::Instruction`. So, the expectation is that instructions in input have already been lowered into a sequence of mca::Instruction. The only way currently to obtain an `mca::Instruction`
is by calling method `mca::<span class="gmail-pl-en">InstrBuilder</span>::<span class="gmail-pl-c1">createInstruction</span>()` [1] on every instruction in input (see for example how it is done in llvm-mca.cpp [2]).<br class="">
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<div class="">Unfortunately method `createInstructions()` only works on `MCInst&`. This strongly limits the usability of llvm-mca as a library; the expectation/assumption is that instructions have already been lowered to a sequence of MCInst.<br class="">
Basically the only supported scenarios are:<br class="">
<div class=""> - We have reached code emission stage and instructions have already been lowered into a sequence of MCInst, or</div>
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<div class=""> - We obtained an MCInst sequence by parsing an assembly code sequence with the help of other llvm libraries (this is what the llvm-mca tool does).<br class="">
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<div class="">It is possible to implement a variant of `createInstruction()` that lowers directly from `MachineInstr` to`mca::Instruction`. That would make the mca library more usable. In particular, it would make it possible to use mca from a post regalloc
pass which runs before code emission. Unfortunately, that functionality doesn't exist today (we can definitely implement it though; it may unblock other interesting use cases). That being said, I am not sure if it could help your particular use case. When
would you want to run your new pass? Using llvm-mca to analyze llvm IR is unfortunately not possible.<br class="">
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I would want to run my pass as late as possible so that all optimizations have been run on the outlined function. The values passed into the capture function should never influence the outlined function so I could in principle do something with a python script
like: </div>
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<div>1. Compile to ASM with MCA enable comments on the function I care about. </div>
<div>2. Run llvm-mca on that region</div>
<div>3. Source to source the original code with the new MCA information. </div>
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<div>Since this solution is not awesome and we may have many functions in a given TU that we care about I was hoping to find a better way. <br class="">
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To compute the throughput indicators you would need to implement a logic similar to the one implemented by class SummaryView [3]. Ideally, most of that logic could be factored out into a helper class in order to help your particular use case and possibly avoid
code duplication.<br class="">
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Thanks, I’ll look into it. <br class="">
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<div class="">I hope it helps,<br class="">
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<div class="">-Andrea<br class="">
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<div class="">[1] <a href="https://github.com/llvm-mirror/llvm/blob/master/include/llvm/MCA/InstrBuilder.h" class="">
https://github.com/llvm-mirror/llvm/blob/master/include/llvm/MCA/InstrBuilder.h</a><br class="">
[2] <a href="https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-mca/llvm-mca.cpp" class="">
https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-mca/llvm-mca.cpp</a><br class="">
[3] <a href="https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-mca/Views/SummaryView.h" class="">
https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-mca/Views/SummaryView.h</a>
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<div dir="ltr" class="gmail_attr">On Tue, Dec 24, 2019 at 5:08 PM Lewis, Cannada via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:<br class="">
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Hi, <br class="">
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I am trying to generate performance models for specific pieces of code like an omp.outlined function. Lets say I have the following code:<br class="">
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start_collect_parallel_for_data(-1.0,-1.0,-1.0, size, “tag for this region”);<br class="">
#pragma omp parallel for<br class="">
for(auto i = 0; i < size; ++i){<br class="">
// … do work<br class="">
}<br class="">
stop_collecting_parallel_for_data();<br class="">
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The omp region will get outlined into a new function and what I would like to be be able to do in opt is compile just that function to assembly, for some target that I have chosen, run llvm-mca just on that function, and then replace the -1.0s with uOps Per
Cycle, IPC, and Block RThroughput so that my logging code has some estimate of the performance of that region.
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Is there any reasonable way to do this from inside opt? I already have everything in place to find the start_collect_parallel_for_data calls and find the functions called between start and stop, but I could use some help with the rest of my idea.<br class="">
<br class="">
Thanks<br class="">
-Cannada Lewis<br class="">
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