Merging Equivalent Contexts for Scalable Heap-cloning-based Points-to Analysis

Merging Equivalent Contexts for Scalable Heap-cloning-based Points-to Analysis Guoqing Xu and Atanas Rountev Ohio State University Supported by NSF under CAREER grant CCF-0546040

Precise and Scalable Points-to Analysis Analysis precision Calling context sensitivity – e.g. chain of call sites Heap cloning [Nystrom-PASTE’04, Lhotak-CC’06] The most precise analysis: refinement-based analysis [Sridharan-PLDI’06], but does not scale well Analysis scalability Millions of distinct call chains in a moderate-size Java program Option 1: sacrifice precision with k-length chain Option 2: merge equivalent relationships using BDDs BDDs incurs running time overhead, and may not scale for heap-cloning-based analysis 2

Motivation • Manyclients require the underlying pointer analysis to quickly provide a precise points-to solution for all variables (e.g., slicing; verification) • Example: • The refinement-based analysis used 1000 sec to provide a precise solution for all variables in a 30-line Java program (including variables in all relevant library code) • The solution produced under a 500-second budget made our slicer generate a slice containing the whole program Goal: relatively precise result in a more scalable manner

Merge Equivalent Contexts • Equivalence classes exists in the representation of calling contexts [Lhotak-CC’06] • Can we find and merge equivalent calling contexts? • We would be able to scale the points-to analysis without relying on the merging inside the BDD “black box” • A unique replacement context (URC) can be used to represent an equivalence class in the points-to relationships

Outline • A model of equivalent contexts • Abstraction functions for pointer variables and targets • Proposed for pointer analysis, but can be applied to other context-sensitive analysis algorithms • A whole-program points-to analysis for Java • Implements the model • Context-sensitive for both pointer variables and targets • Does not limit the length of context strings (not k-CFA) • Bottom-up, summary-based • Experimental evaluation • Much more precise and efficient than state-of-the-art 1-object-sensitive analysis with BDDs [Lhotak-CC’06] • More efficient than the refinement-based analysis

Motivating Example void main(String[] args){ A a1 = new A(); A a2 = new A(); foo(a1); //call site 1 foo(a2); //call site 2 } void foo(A a){ t = new B(); a.fld = t ; bar(a.fld); //call site 3 } B bar(B b){ p = b; return p; } p(1,3)---> new B(1,3) p(2,3)---> new B(2,3) t1 ---> new B1 t2 ---> new B2 Observation: 1. t points to new B, under all calling contexts 2. p points to new B, under calling contexts (*,3)

A Better Representation? • Can we represent the points-to relationships like this? • t new B • p3new B3 • 1 copy of t and p, 2 copies of new B • Key insights • Context-sensitivity corresponds to inlining; full context-sensitivity is achieved if all reachable methods are inlined in main • If a points-to relationship can be determined at method mduring inlining, it will not be affected by m’s callers

Two Hypothetical Inlining-based Analyses • (v, cv, o, co) represents a fully context-sensitive points-to relationship; v is local var; o is alloc site • Analysis 1: all-the-way inlining • A statement p := q is rewritten as p(e) := q(e), at call graph edge e • An intraprocedural points-to analysis is performed on the body of main • Analysis 2: a variation of analysis 1 • An intraprocedural analysis is performed immediately after a methodm inlines all its call sites • Suppose (v, cv, o, co) is produced for m • (v, (e) cv, o, (e) co) must be produced later for m’s caller n; e is the call graph edge between n andm

Calling Context Reduction • Inlining preserves context-sensitive points-to relationships • All statements in mare also in n • The lifetime of a tuple (v, cv, o, co) consists of • A single creation event in some method m: flowing point • A sequence of inlining steps that increase both cv and co in synch • URCs computed by abstraction functions for calling context; m is the flowing point • Fv(cv) = (e0, e1, …, ei) where e0.src = m • Fo(co) = (f0, f1, …, fj) where f0.src = m • Keep only the relevant suffix of the call chain

Approximation in the Presence of Recursion • Two-phase approximation: (1) map an infinite call chain to a finite one, collapsing cycles while going backwards; (2) then, use the abstraction function shown earlier • Precision loss may result from phase 1: e.g., p points to o under context eabcdabf, but does not under context eabf

Using the Reduced Contexts • A URC is used to represent a set of calling contexts in the points-to relationships • A query (v, c) can be answered as follows: • Find all (v, urcv, o, urco) such that substring(urcv, c) holds • Return all (o, co) such that substring(urco, co) holds • A BDD may not be as effective for an analysis with heap cloning • Without heap cloning, an equivalence class is defined by a single string urcv • For an analysis with heap cloning, an equivalence class is defined by a pair (urcv , urco) • More classes = fewer opportunities for merging

Points-to Analysis • A specific algorithm that implements this model • Using URCs to represent calling contexts • The use of URCs could be applicable to other categories of points-to analysis • Resembles bottom-up inlining • Heap-cloning-based • Context-sensitively treat both pointer variables and targets • Partial unification (bi-directional flow of values)

Intraprocedural Analysis • Symbolic points-to graph (SPG) • A symbolic object node is introduced for each (1) formal parameter, (2) base variable v of load a = v.f, and (3) lhs vof a call site v = a.b(…) • Standard points-to analysis algorithm [Lhotak-CC’03] is used for SPG construction • SPG contains much fewer nodes and edges than the original program • Example void add (Integer t) { this.names = t; }

Escape Analysis • An allocation node new C or symbolic node SO directly escapes a method, if • It is pointed to by a formal parameter • It is pointed to by a returned variable • It is pointed to by a static field • A node indirectly escapes a method, if • It is reachable from nodes that directly escape • Compute a set of allocation/symbolic nodes that escape the method where they are defined

Interprocedural Analysis • Summary-based • Bottom-up traverse the call graph SCC-DAG • Summary function definition: • {[Of, Gf]} • Gf : the subgraph of all escaping objects (reachable from Of) and their points-to edges • Clone a summary function for each incoming call graph edge e • If o1c1 o2c2where o1ando2escape: in the caller, create o1(e) c1  o2(e) c2 • If v c1 s c2where s is an escaping symbolic node: create v(e) c1 s(e) c2 f f

Interprocedural Analysis • Composition of summary functions • For each [Oactual , Gactual ] • Find [O’formal , G’formal ] • Merge Gactual and G’formal • Subgraph merging • Simultaneously traverse Gactual and G’formal from Oactual and O’formal • Merge b and c, if • where d and e have been merged

Merging Two Nodes

Interprocedural Analysis • The points-to solution is built on the fly • Once v c1 o c2 is formed, (v, c1, o, c2) is added to the points-to solution • The edge is removed from the SPG: we have found the flowing point and no further propagation is necessary • (c1, c2) defines an equivalence class for contexts • Node merging is essentially a dynamic transitive closure computation for identifying memory aliases

K-Last-Substring-Merging • A scalability-precision tradeoff • When composing summary functions, do not clone node oc1in the callee if there already exists a node oc2in the caller such that suffix(c1, k) == suffix(c2, k) • Does not limit the context length • Can be used to tune the amount of context merging throughout the program • Example • k =3 • O(defg) and O(hefg) are distinct nodes if d.src != h.src • O(defg) and O(hefg) are merged if d.src == h.src

Experiments • Benchmark set contains 19 Java programs, from SPECJVM, Ashes, and DaCapo • Experimentally compared our analysis with • Refinement: the refinement-based analysis from [Sridharan-PLDI’06] with its default budget for refinement • the most precise publicly-available analysis for computing an on-demand solution • queried the points-to sets for all possible (v, c) • 1H: 1-object-sensitive analysis with heap cloning, using BDDs [Lhotak-CC’06]; the most precise publicly-available analysis for computing a whole-program solution • Our analysis: computes a whole-program solution • 1Equiv: 1-last-substring-merging • 2Equiv: 2-last-substring-merging

Precision: #downcasts proven to be safe 2Equiv proves 59% more safe downcasts than 1H But less than Refinement

Cost: time to get a whole-program solution 2Equiv is 13 times faster than 1H 5.3 times faster than Refinement

Conclusions Context equivalence class identification An equivalence class is represented by a URC Only URCs need to be explicitly represented in the data structures of the analysis A points-to analysis for Java Uses URCs to represent contexts Summary-based, bottom-up, with partial unification Heap-cloning Experimental evaluation Precision approaches that of the refinement-based analysis, and is much higher than that of 1H Faster than both refinement-based and 1H

Merging Equivalent Contexts for Scalable Heap-cloning-based Points-to Analysis