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 pq memory

3 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 Don’t Know: mem(q) store kills availability of mem(p)  No: apply common sub-expression elimination, and then copy propagation AFTER 1. a = mem(p) 2. mem(q) = b 3. d = a + a

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 statementalias(p) = p = qalias(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}

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)=union p OUT(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 KeyTranslation REFReference 3ADRThree-Address Code CFConstant Folding CTPConst Propagation CSECommon Subexpression Elimination CPPCopy Propagation

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 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