EPS Enhanced Pipeline Scheduling Project

1
EPS (Enhanced Pipeline Scheduling) Project

• EPS is a software pipelining technique based on
  global DAG scheduling
• For a given loop, we just repeat DAG scheduling
• When instructions move across the loop back-edge,
  the pipelining effect takes place
  (cross-iteration code motion)
• This code-motion pipelining is different from
  code-placement pipelining in modulo scheduling
  (MS)
• MS destroys the original loop and creates a new
  loop

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Advantage of EPS compared to MS

• We can software-pipeline ANY loops
• Loops with arbitrary control flows
• Loops whose trip counts are not constants
• e.g., pointer-chasing loops
• Outer loops
• Unstructured loops
• Can achieve tight, variable II for multi-path
  loops
• Particularly useful for optimizing integer code

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Global DAG Scheduling in EPS

• Employs aggressive code motion techniques
• All-path speculation, combined with unification
• Allow useful code motion even if no profiling
  information is available
• Join code motion, renaming, and forward
  substitution
• e.g., move x1 to block A across all possible
  paths

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EPS Implementation on Open64

• We have implemented EPS on Open64-3.0
• Made be (backend) invoke EPS
• EPS does not touch any loops scheduled by MS
• EPS can be complementary to MS
• EPS and the current GIS (global instruction
  scheduler of Open64-3.0) are collaborating well
• Often getting the best results when both are
  turned on
• Initial results are promising
• 5 improvement in 164.gzip

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EPS Objectives and Goals

• Provide the first, open-source version of EPS
• Performance Goal
• 4 improvement on SPEC CINT2006 Base
• Research and evaluate various scheduling issues
• Exploit speculative loads wisely
• Since execution of recovery block can be an
  overhead
• On-demand if-conversion in the middle of code
  motion
• Rotating register allocation for EPS loops
• More parallel execution of branches and data
  instructions
• ..