<div dir="ltr"><div>Hi Lewis,</div><div><br></div>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><div><br></div><div>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></div><div><br>
<div>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></div><br></div><div>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>Basically the only supported scenarios are:<br>
<div> - We have reached code emission stage and instructions have already been lowered into a sequence of MCInst, or<br> - 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><br>
<div><div>
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></div></div><br>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><br></div><div>I hope it helps,<br></div><div>-Andrea<br></div><div>
<br></div></div><div>[1] <a href="https://github.com/llvm-mirror/llvm/blob/master/include/llvm/MCA/InstrBuilder.h">https://github.com/llvm-mirror/llvm/blob/master/include/llvm/MCA/InstrBuilder.h</a><br>[2] <a href="https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-mca/llvm-mca.cpp">https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-mca/llvm-mca.cpp</a><br>[3]
<a href="https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-mca/Views/SummaryView.h">https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-mca/Views/SummaryView.h</a>
</div></div><br><div class="gmail_quote"><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">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi, <br>
<br>
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>
<br>
start_collect_parallel_for_data(-1.0,-1.0,-1.0, size, “tag for this region”);<br>
#pragma omp parallel for<br>
for(auto i = 0; i < size; ++i){<br>
// … do work<br>
}<br>
stop_collecting_parallel_for_data();<br>
<br>
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. <br>
<br>
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>
<br>
Thanks<br>
-Cannada Lewis<br>
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