[PATCH] D139283: [llvm-exegesis] parallel snippet generator: avoid Read-After-Write pitfail for instrs w/ tied variables

Sun Dec 4 16:42:50 PST 2022

lebedev.ri created this revision.
lebedev.ri added reviewers: RKSimon, andreadb, courbet, gchatelet.
lebedev.ri added a project: LLVM.
Herald added subscribers: mstojanovic, pengfei.
Herald added a project: All.
lebedev.ri requested review of this revision.

As it is being discussed in https://github.com/llvm/llvm-project/issues/59325,
at least for the instructions with tied variables,
when trying to parallelize the instructions,
register selection is rather bad, and may either
use a register which we have used for def,
or vice versa.

That introduces serialization, and leads to
overly pessimistic inverse throughput measurement.

The new implementation avoids that,

New result:

  $ ./bin/llvm-exegesis --mode=inverse_throughput --opcode-name=VFMADD132PDr --max-configs-per-opcode=9182
  Templates 2
  Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-1b37a6.o
  ---
  mode:            inverse_throughput
  key:
    instructions:
      - 'VFMADD132PDr XMM14 XMM14 XMM7 XMM10'
      - 'VFMADD132PDr XMM12 XMM12 XMM1 XMM6'
      - 'VFMADD132PDr XMM3 XMM3 XMM8 XMM7'
      - 'VFMADD132PDr XMM4 XMM4 XMM6 XMM8'
      - 'VFMADD132PDr XMM11 XMM11 XMM9 XMM15'
      - 'VFMADD132PDr XMM2 XMM2 XMM1 XMM0'
      - 'VFMADD132PDr XMM13 XMM13 XMM6 XMM1'
      - 'VFMADD132PDr XMM5 XMM5 XMM9 XMM9'
    config:          ''
    register_initial_values:
      - 'XMM14=0x0'
      - 'XMM7=0x0'
      - 'XMM10=0x0'
      - 'MXCSR=0x0'
      - 'XMM12=0x0'
      - 'XMM1=0x0'
      - 'XMM6=0x0'
      - 'XMM3=0x0'
      - 'XMM8=0x0'
      - 'XMM4=0x0'
      - 'XMM11=0x0'
      - 'XMM9=0x0'
      - 'XMM15=0x0'
      - 'XMM2=0x0'
      - 'XMM0=0x0'
      - 'XMM13=0x0'
      - 'XMM5=0x0'
  cpu_name:        znver3
  llvm_triple:     x86_64-unknown-linux-gnu
  num_repetitions: 10000
  measurements:
    - { key: inverse_throughput, value: 0.5419, per_snippet_value: 4.3352 }
  error:           ''
  info:            instruction has tied variables, avoiding Read-After-Write issue, picking random def and use registers not aliasing each other
  assembled_snippet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
  ...
  Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-0df694.o
  ---
  mode:            inverse_throughput
  key:
    instructions:
      - 'VFMADD132PDr XMM5 XMM5 XMM14 XMM14'
      - 'VFMADD132PDr XMM13 XMM13 XMM14 XMM14'
      - 'VFMADD132PDr XMM12 XMM12 XMM14 XMM14'
      - 'VFMADD132PDr XMM11 XMM11 XMM14 XMM14'
      - 'VFMADD132PDr XMM6 XMM6 XMM14 XMM14'
      - 'VFMADD132PDr XMM10 XMM10 XMM14 XMM14'
      - 'VFMADD132PDr XMM4 XMM4 XMM14 XMM14'
      - 'VFMADD132PDr XMM9 XMM9 XMM14 XMM14'
      - 'VFMADD132PDr XMM1 XMM1 XMM14 XMM14'
      - 'VFMADD132PDr XMM8 XMM8 XMM14 XMM14'
      - 'VFMADD132PDr XMM15 XMM15 XMM14 XMM14'
      - 'VFMADD132PDr XMM3 XMM3 XMM14 XMM14'
      - 'VFMADD132PDr XMM7 XMM7 XMM14 XMM14'
      - 'VFMADD132PDr XMM0 XMM0 XMM14 XMM14'
      - 'VFMADD132PDr XMM2 XMM2 XMM14 XMM14'
    config:          ''
    register_initial_values:
      - 'XMM5=0x0'
      - 'XMM14=0x0'
      - 'MXCSR=0x0'
      - 'XMM13=0x0'
      - 'XMM12=0x0'
      - 'XMM11=0x0'
      - 'XMM6=0x0'
      - 'XMM10=0x0'
      - 'XMM4=0x0'
      - 'XMM9=0x0'
      - 'XMM1=0x0'
      - 'XMM8=0x0'
      - 'XMM15=0x0'
      - 'XMM3=0x0'
      - 'XMM7=0x0'
      - 'XMM0=0x0'
      - 'XMM2=0x0'
  cpu_name:        znver3
  llvm_triple:     x86_64-unknown-linux-gnu
  num_repetitions: 10000
  measurements:
    - { key: inverse_throughput, value: 0.5314, per_snippet_value: 7.971 }
  error:           ''
  info:            instruction has tied variables, avoiding Read-After-Write issue, picking random def registers not aliasing a single random use register
  assembled_snippet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
  ...

Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D139283

Files:
  llvm/tools/llvm-exegesis/lib/CodeTemplate.cpp
  llvm/tools/llvm-exegesis/lib/CodeTemplate.h
  llvm/tools/llvm-exegesis/lib/ParallelSnippetGenerator.cpp
  llvm/tools/llvm-exegesis/lib/SnippetGenerator.cpp
  llvm/tools/llvm-exegesis/lib/SnippetGenerator.h
  llvm/unittests/tools/llvm-exegesis/X86/SnippetGeneratorTest.cpp

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