[PATCH] D59928: [MCA] Add an experimental MicroOpQueue stage.

[Andrea Di Biagio via Phabricator via llvm-commits]

andreadb created this revision.
andreadb added reviewers: courbet, RKSimon, mattd, lebedev.ri.
Herald added subscribers: jdoerfert, gbedwell, mgorny.

This patch adds an experimental stage named MicroOpQueueStage.
A MicroOpQueueStage can be used to simulate a hardware micro-op queue (basically, a decoupling queue between 'decode' and 'dispatch').
Users can specify a queue size, as well as a optional `MaxIPC` (which - in the absence of a "Decoders" stage - can be used to simulate a different throughput from the decoders).

This stage is added to the default pipeline between the EntryStage and the DispatchStage only if `PipelineOption::MicroOpQueue` is different than zero. By default, llvm-mca sets `PipelineOption::MicroOpQueue` to the value of hidden flag `-micro-op-queue-size`.

Throughput from the decoder can be simulated via another hidden flag named `-decoder-throughput`.
That flag allows us to quickly experiment with different frontend throughputs. For targets that declare a loop buffer, flag  `-decoder-throughput` allows users to do multiple runs, each time simulating a different throughput from the decoders.

This stage can/will be extended in future. For example, we could add a "buffer full" events to identify bottlenecks caused by backpressure. flag `-decoder-throughput` would probably go away if in future we delegate to another stage (DecoderStage?) the simulation of a (potentially variable) throughput from the decoders.
For now, flag `-decoder-throughput` is "good enough" to run some simple experiments.

Let me know if okay to commit.

-Andrea


https://reviews.llvm.org/D59928

Files:
  include/llvm/MCA/Context.h
  include/llvm/MCA/Stages/MicroOpQueueStage.h
  lib/MCA/CMakeLists.txt
  lib/MCA/Context.cpp
  lib/MCA/Stages/MicroOpQueueStage.cpp
  test/tools/llvm-mca/X86/uop-queue.s
  tools/llvm-mca/llvm-mca.cpp

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