Feedback-directed optimizations with estimated edge profiles from hardware event sampling

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Feedback-directed optimizations with estimated
edge profiles from hardware event sampling

• Open64 workshop, CGO 2008
• April 6, 2008

Vinodha Ramasamy, Robert Hundt Google Inc., Dehao
Chen, Wenguang Chen Tsinghua University
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Background

• Traditional FDO model Instrument Run
  Recompile
• Usage Model
• Difficulties in generating representative
  training datasets
• High overhead of profile collection
• Requires dual-compilation - tightly coupled
  builds
• Benefits
• Supports both value and edge profiling
• High performance potential

TRAINING DATA
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Overview
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Algorithm

• Basic block counts
• Scale samples per source line by of
  instructions
• Samples per source line stored in profile
  datafile
• Annotate IR statements in basic blocks with
  source line sample counts
• Scale basic block sample count
• BB.count (? IR.count) / num_IR_stmts

pbla.c60 iplus iplus-gtpred // 280 4
70 100 804a8b7 mov    0x10(ebp),eax30
804a8ba mov    0x8(eax),eax70 804a8bd
mov    eax,0x10(ebp)80 804a8c0 jmp   
804a94b ltprimal_iminus0x137gt
IR1 70 IR2 10 IR3 70 IR4 0 IR5 0
?IR.count 70 10 70 0 0 150 BB.count
150 5 30
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Edge frequency estimation

• Edge counts from basic block counts
• Uses higher level program structure - branch,
  loop etc.,
• Recursive algorithm used to smooth sample counts

500
ENTRY 0
ENTRY 500
BODY 0
BODY 7954
?
BR 7954
BR 7954
NT 30
T 7922
BACK 0
BACK 7454
NT 32
T 7922
JOIN 420
JOIN 7954
EXIT 0
EXIT 500
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Challenges

• Inaccuracies inherent to sampling
• Source position information issues
• Missing information due to optimization
  transformations
• Disambiguating samples per source line
• if (cond) stmt1 stmt2
• Edge estimation heuristics
• Evaluate algorithm proposed by Levin et. al.
• Inlining
• Annotate early inlined functions with scaled
  sample counts

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Results

• SPEC2006 C benchmarks
• Intel Core-2 platform using 64-bit binaries
• -O2 FDO with instrumented runs
• 45 gain over default O2 runs
• -O2 FDO with sampled profiles
• Profile collection using O2 binaries
• 60 of FDO instrumented gain

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Thank You!

• QA