May-Alias Analysis for L3

1
May-Alias Analysis for L3

• David Abraham, Elisabeth Crawford, Sue Ann Hong,
  and
• Virginia Vassilevska

2
Introduction

• What does alias mean?
• Two pointers alias each other if both point to
  the same memory location
• What is alias analysis?
• Determining what each pointer references, so we
  can tell if two pointers alias each other
• Why is alias analysis useful?
• Many code optimizations are overly conservative
  without alias information

memory
p
q
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Use of Alias-Analysis

• BEFORE
• a mem(p)
• mem(q) b
• c mem(p)
• d a c
• Is q an alias of p?
• Yes or Dont Know
• mem(q) store kills availability of mem(p)
• No
• apply common sub-expression elimination,
• and then copy propagation

• AFTER
• a mem(p)
• mem(q) b
• d a a

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May-Alias Analysis

• Cant statically track what each pointer refers
  to exactly
• So be conservative - track what each pointer may
  refer to
• p, q may alias each other if they may refer to
  same memory location

5
Some May-Alias Rules

• Let alias(q) set of memory locations that q may
  alias

statement alias(p)
p q alias(q)
p q c, where c is a const loc c loc in alias(q)
p function() all memory locations
s p alloc(), where s is an internal label of this statement alias(p) s
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May-Alias Dataflow Analysis

• For each statement s
• IN(s) alias information for each pointer on
  entry to s
• OUT(s) alias information for each pointer on
  exit from s
• IN(s)unionpOUT(p), where p is predecessor of s
• OUT IN with statement rule applied

7
Experimental Setup

• L3 Test Suite
• Correctness tests (not reported here)
• Big directory performance tests
• Tested combinations of optimizations to identify
  performance gains and hits

Key Translation
REF Reference
3ADR Three-Address Code
CF Constant Folding
CTP Const Propagation
CSE Common Subexpression Elimination
CPP Copy Propagation
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Runtime Results
9
More Runtime Results
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Code Transformation Results
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Conclusions

• Alias analysis can greatly increase the
  effectiveness of optimizations, in terms of
• Runtime performance
• Temp usage
• Alias analysis works best in tandem with copy
  propagation
• Alias analysis is expensive performance payoffs
  may not be worthwhile in a JIT compiler

12
Bibliography

1. Modern Compiler Implementation in ML, Appel
2. On the importance of points-to-analysis and other
   memory disambiguation methods for C programs,
   Ghiya, Lavery, and Sehr
3. An improved intra-procedural may-alias analysis
   algorithm, Goyal
4. Advanced Compiler Design and Implementation,
   Muchnick
5. Survey of Alias Analysis, Wu