Speculative Alias Analysis for Executable Code

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Speculative Alias Analysisfor Executable Code

• Manel Fernández and Roger Espasa
• Computer Architecture Department
• Universitat Politècnica de Catalunya
• Barcelona, Spain
• mfernand,roger_at_ac.upc.es

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Motivation

• Alias analysis
• Provides information for memory disambiguation
• Key issue in todays optimizing compilers
• Formulated as a dataflow analysis
• In terms of source language constructs
• Trade-off between cost and precision
• Executable code optimizers
• New optimization opportunities appear
• Whole program view, etc.
• Limited usefulness of traditional analyses
• High level information is lost
• Object code is larger than corresponding source
  code
• Memory disambiguation is one of the weak points
  of object code modification

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Alias analysis on executable code

• Existing techniques
• Instruction inspection
• Register use-def chains
• Residue-based alias analysis
• Debray et al. POPL 98
• Arithmetic computations mod-k
• Small displacements from a base register
• Fine grain disambiguation
• Problems of existing alias analysis
• Low precision
• Conservative assumptions
• Strong resource-usage constraints
• Widening less precision joining different
  definitions
• Context-insensitive formulation

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Talk outline

• Motivation
• Speculative alias analysis
• Evaluation
• Summary

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Speculative alias analysis

• Existing analysis are conservative
• Trade off between cost and precision
• A new variable safeness
• Analysis becomes speculative
• Increases precision at low cost
• Analysis is not always correct
• Our proposals
• Two may-alias approaches
• Region-based alias analysis
• Profile-guided alias analysis
• Well-suited for speculative optimizations
• Speculative reordering based on
  check-and-recovery schemes
• E.g. reordering memory operations

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Propagating memory regions

• Observations
• Conservative propagation of descriptors
• E.g. operating different definitions
• Loads produce conservative descriptors
• Nothing is known about the loaded value
• Key ideas
• Propagation without losing precision
• Propagating very basic information
• Guessing possible memory regions
• Memory regions are disjoint
• Global
• Stack
• Heap

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Region-based alias analysis

• Interprocedural low-level scheme
• Well-suited for executable code
• Computation of memory regions
• For each register r defines S
• S set of regions Global, Stack, Heap
• T Æ, G,S,H
• Dataflow propagation
• Speculative approach
• Assumptions are not always correct
• Coarse-grain disambiguation
• Can be applied coupled to a residue-based scheme
• Aggressive region-based analysis
• Loaded values are hardly ever aliased with other
  pointers
• E.g. linked lists
• Set load destination descriptors to T instead of

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Propagating likely paths

• Observations
• Widening operation is conservative
• E.g. several definitions reaching a use
• Context-insensitive analysis
• Context-sensitive is not feasible
• Key ideas
• Propagation without loosing precision
• Reducing the number of paths
• Cold references are not important
• Conflicts are not significant at run time
• Ignore cold paths

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Profile-guided alias analysis

• Interprocedural general scheme
• Needs profile information
• Only likely executed paths are considered
• Applied on top of any dataflow analysis
• Redefine join operation
• Speculative approach
• Cold paths are ignored
• Likely-path disambiguation

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A combined algorithm

• Alias analysis scheme
• Phase 1
• Use-def chains
• Phase 2
• Residue-based
• Region-based
• Phase 3
• Profile-guided Phase 2

• Disambiguation scheme

Input I1,I2 Output dependent, independent,
likely independent, unknown Method
if ud-chains(I1,I2) ? unknown return
ud-chains(I1,I2) if aliasing(I1,I2,safe) ?
unknown return aliasing(I1,I2,safe)
if aliasing(I1,I2,unsafe) ? unknown
return likely independent else
return unknown End Method
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Talk outline

• Motivation
• Speculative alias analysis
• Evaluation
• Summary

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Methodology

• Benchmark suite
• SPECint95
• Compiled on an AlphaServer with full
  optimizations
• Intrumented using Pixie to get profiling
  information
• Experimental framework
• Alto executable optimizer
• SimpleScalar safe simulator
• Evaluation
• Static precision
• Misspeculation rate

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Evaluating effectiveness

• Disambiguation query
• Question made to the memory disambiguator
• Relationship between two memory references
• Dependent, independent, likely independent
• Unknown
• Set of disambiguation queries
• Pair of references that belong to the same
  function
• Building the set
• Consider every load/store in a hot path (2nd
  item)
• Consider every load/store in previous paths (1st
  item)
• Typical behavior of compiler optimizations

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Static precision
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Misspeculation rate
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Talk outline

• Motivation
• Speculative alias analysis
• Evaluation
• Summary

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Summary

• Memory disambiguation is one of the weak points
  of object code modification
• Speculative alias analysis
• Compromise among cost, precision and safeness
• Region-based alias analysis
• Profile-guided alias analysis
• Well-suited for executable code
• Conclusions
• Precision increases from 16 up to 83 in average
• Misspeculation rate around 1
• In front of 2 using an always speculate scheme
• To be used for reordering memory operations with
  high recovery cost