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

3. BEFORE a = mem(p) mem(q) = b c = mem(p) d = a + c Is q an alias of p? Yes or Don’t 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 Use of Alias-Analysis

4. May-Alias Analysis • Can’t 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

6. 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

8. Runtime Results

9. More Runtime Results

10. Code Transformation Results

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