Using Types to Analyze and Optimize Object-Oriented Programs By: Amer Diwan Presented By: Jess Martin, Noah Wallace, and Will von Rosenberg

Type-Based Alias Analysis (TBAA) Assumes type safe programming language, i.e. Modula-3. Comparative Analysis of Types and Subtypes Four Functions (Versions) of TBAA –TypeDecl –FieldTypeDecl –SMTypeRefs –SMFieldTypeRefs

TypeDecl Examine the declared type of AP and assume the AP may reference any object with the same declared type or subtype.

TypeDecl Example Class X{} Class Y{} : X Class Z{} : X Class D{} : Y Class F{} : Z

FieldTypeDecl Given AP1 and AP2 it returns true if AP1 and AP2 are Aliases Modula-3 programs may take the addresses of memory locations in only two ways: via the pass-by-reference parameter passing mechanism and the WITH statement.

FieldTypeDecl (cont’d) Identical APs always alias each other. Two qualified expressions may be aliases if they access the same field in potentially the same object. A pointer dereference may refer to the same location as a qualified subscripted expression only if their types are compatible and the program may take the address of the qualified or subscripted expression. In Modula-3, a subscripted expression cannot alias a qualified expression. Two subscripted expressions are aliases if they may subscript the same array. FieldTypeDecl ignores the actual subscripts. For all other cases of APs, including two pointer dereferences, FieldTypeDecl uses TypeDecl to determine aliases.

FieldTypeDecl Class P{ int a; float b; char c;} Class D{ int e; float g; char f;} : P Int ra[]; P and D? P.a and D.e? P.a and ra[i]? *P.f and ra[i]? D and D?

SMTypeRefs Selective Type Merging Produces a TypeRefsTable

SMTypeRefs (cont’d) Initializes Group Examines all assignment statements, merges and if types are different. Filters infeasible aliases from Group, creating asymmetry.

SMTypeRefs (cont’d)

Applause

SMTypeRefs (cont’d)

SMTypeRefs Class Blah{} Class Piglet{}:Blah Class Pooh{}:Blah Class Pinky{}:Piglet Class Brain{}:Piglet Class Scooby{}:Pooh Class Yogi{}:Pooh T New Blah; P New Piglet; F New Pooh; T = P; P = F; T = F; What are the typeref tables for the three assignments?

SMFieldTypeRefs Fields + Selectively Merge Type References We use SMTypeRefs instead of TypeDecl in the FieldTypeDecl algorithm Final version of TBAA

Using TBAA Redundant Load Elimination –Loop invariant code motion –Common subexpression elimination Resolving method invocation –Type hierarchy analysis –Intraprocedural type propagation –Intraprocedural type propagation using TBAA –Interprocedural type propagation –Interprocedural type propagation using TBAA

Polymorphism through subtyping Polymorphic – method invocation site calls more than one user procedure at run time. Monomorphic – method invocation site calls only one user procedure at run time. A method is resolved if it is identified as being monomorphic. We wish to resolve all calls and replace them with direct calls.

Type Hierarchy Analysis Simply evaluates all possible overrides (overloads) for a particular function when passed a specific variable type.

Intraprocedural Type Propogation Breaks down types into type events –Allocation –Implicit and explicit type discrimination –Assignment Complexity O(n*v) where n is number of statements and v is the number of variables in the procedure

Intraprocedural Type Propogation using TBAA When it encounters a pointer dereference, it invokes SMFieldTypeRefs to get the set of locations referenced by the pointer deref Analysis discovers monomorphic uses of general data structures Complexity – O(n*(v+NT*NF)), where n is number of statements, v is number of variables, NT is number of types, and NF is number of fields

Interprocedural Type Propogation Call graph w/edges for each Builds work list Puts functions on work list if new information Merges parameter types Interprocedural Type Propogation using TBAA is the same implementation

Summary of analysis Type Hierarchy Intraprocedural Type Propagation Intraprocedural Type Propagation using TBAA Interprocedural Type Propagation Interprocedural Type Propagation using TBAA

Results Evaluates TBAA and Method resolution analyses. Static evaluation Dynamic evaluation Limit evaluation

TBAA Evaluation

Static Evaluation of TBAA

Static (cont’d) FieldType declare is more precise than TypeDecl. SMFieldTypeRefs offers little added precision. FieldTypeDecl finds more redundant loads than TypeDecl and SMFieldTypeRefs does not change by any significant amount

Static (cont’d)

Dynamic Evaluation of TBAA

Dynamic (cont’d) Two important points –1) a more precise alias analyses is not necessarily better it depends on how the analyses is used. –2) Static metrics are insufficient by themselves for evaluation

Limit Evaluation on TBAA Optimizations eliminate between 35% - 88% of the RL. Only 5% or fewer are redundant. Reveals that TBAA performs as well as any alias analysis could with the RLE and their benchmark.

Limit evaluation (cont’d)

Method Invocation

Static and Dynamic Evaluation Type Hierarchy analysis resolves many method invocations Intraprocedural type propagations resolves very few additional method invocations but removes NIL. Type propagation is useful for languages that have defined semantics such as Modula-3 or Java TBAA along with type propagation resolves most of the method invocation in the benchmarks.

Limit Evaluation (cont’d)

Explicit type tests and cloning combined with aggressive alias analyses may be able to resolve these method invocations. 4 sites are polymorphic but they comprise more than 80% of the total method invocations.