Loop Fusion in the JikesRVM

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Loop Fusion in the JikesRVM
Kit Barton CMPUT 605 Project Proposal February
25, 2003
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Introduction

• Little work has been done on loop optimizations
  for Java JIT compilers
• Loop optimizations have the potential to improve
  program performance
• Many loop optimizations are expensive in both
  time and space
• Investigate tradeoffs (benefits vs costs)

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Loop Fusion

• Combine the bodies of two loops, creating a
  single loop
• Advantages
• Decrease loop overhead
• Increase data reuse
• Disadvantages
• Analysis can be expensive
• Requires fairly specific loop structure

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Objectives

• Implement basic loop fusion in the JikesRVM
• Analyze impact on performance
• Enhance loop fusion
• Create cost function

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Timeline

• February 28
• Understand loop structure in JikesRVM
• Modification of JikesRVM
• Addition of new compilation phase to JikesRVM
• March 7
• Basic loop fusion instrumentation
• Begin loop fusion implementation

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Timeline

• March 14
• Basic loop fusion implementation complete
• Detailed loop fusion instrumentation complete
• Initial performance measurements
• March 21
• Investigation of additional code transformations
• Initial planning of cost function

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Timeline

• March 28
• Decision on additional code transformations
• Description of cost function
• Project progress report
• April 4
• Implementation of code transformations
• Implementation of cost function

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Timeline

• April 11
• Implementation of code transformations
• Integration of cost function and loop fusion into
  JikesRVM
• April 18
• Performance measurements

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Timeline

• April 25
• Performance measurements
• Final report
• Project presentation

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Potential Roadblock (1)

• Potential Exception-throwing Instructions
• Jikes uses a Factored Control Flow Graph
• Loops containing PEIs cannot be fused because
  exceptions in Java are precise
• "When the transfer of control takes place,
  all effects of the statements executed and
  expressions evaluated before the point from which
  the exception is thrown must appear to have taken
  place. No expressions, statements, or parts
  thereof that occur after the point from which the
  exception is thrown may appear to have been
  evaluated."1

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Possible PEI Solutions

• Hoist PEIs out of loop
• Create a Lfused loop and two Lorig loops
• Lfused is executed normally
• If an exception occurs, execute Lorig
• Ignore exceptions for the purpose of this study

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Potential Roadblock (2)

• Loop structure not sufficient for loop fusion
• All information necessary for loop fusion may not
  be available
• eg. exit test may not be easily obtained
• Loop structure may not be in a format that is
  typical for loop fusion
• eg. loops may not be normalized

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Loop Structure Solution

• Modify loop structure
• Add necessary information
• Transform loop structure into one that is easier
  to use during fusion
• Modify algorithms

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Conclusions

• Results will show the relationship between the
  benefits and costs of loop fusion
• Results may help predict the potential impact on
  performance for other loop optimizations

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References

• Christopher Barton. Code Transformations to
  Augment the Scope of Loop Fusion in a Production
  Compiler. Master's thesis, University of Alberta,
  January 2003.
• Jong-Deok Choi, David Grove, Michael Hind and
  Vivek Sarkar. Efficient and precise modeling of
  exceptions for the analysis of Java programs. In
  Proceedings of the 1999 ACM SIGPLAN-SIGSOFT
  workshop on Program analysis for software tools
  and engineering, pages 21-31, Toulouse, France,
  1999.
• James Gosling, Bill Joy, Guy Steele and Gilad
  Bracha. The Java language specification, second
  edition. http//java.sun.com/docs/books/jls/second
  \_edition/html/j.title.doc.html