Question regards to the design of GpuInstructionFusion pass

[t kevin]

hi dear XLA folks,

I'm trying to understand some GPU optimzation passes. And this piece of code is a myth to me.

https://github.com/tensorflow/tensorflow/blob/master/tensorflow/compiler/xla/service/gpu/gpu_compiler.cc#L519

fusion.AddPass<GpuInstructionFusion>(/*may_duplicate=*/false);

fusion.AddPass<GpuInstructionFusion>(/*may_duplicate=*/true);

I kinda understand what the `may_duplicate` parameter does and how it affect the fusion pass.

What I can't get is the calling sequence.

Why is GpuInstructionFusion called twice with may_duplicate=false first and then may_duplicate=true.

what is the design philosophy behind this? This code is like almost 5 years old from the initial commit.

I can't extract much information from the git commit log.

Your help is much appreciated.

Thanks a lot.

Kevin

[George Karpenkov]

+Sanjoy Das

Hi Kevin,

I'm not the expert on why the code was designed in this way, but my understanding is that it tries hard to do "cheaper" fusions first which do not require rematerializations.

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[Sanjoy Das]

The commit that introduced this landed before XLA was open sourced so you probably won't find it in the git log, the commit message says:

Before this CL, we do a single fusion pass and duplicate operations deemed non-costly, which readily reaches a local optimum on some graphs. After this CL, we do fixpoint of { a non-duplicative pass, the original fusion pass }, which produces better fusions on some such graphs.

[t kevin]

Hi George, Sanjoy

Thanks for the clarification. Much appreciated.
Have a nice day.

Kevin

Sanjoy Das <san...@google.com> 于2021年11月20日周六 下午12:14写道：