java-topology/docs/tickets/0005-inference-context-isequiv-containsall.md

0005 — InferenceContext.isEquiv(): O(B²) bound-list equality via List.containsAll()

Status: open Severity: performance — low Component: jdk.compiler / com.sun.tools.javac.comp.InferenceContext$ReachabilityVisitor

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Root Cause

InferenceContext.java:506 — isEquiv():

boolean isEquiv(UndetVar from, Type t, InferenceBound boundKind) {
    UndetVar uv = (UndetVar)asUndetVar(t);
    for (InferenceBound ib : InferenceBound.values()) {
        List<Type> b1 = from.getBounds(ib);
        ...
        List<Type> b2 = uv.getBounds(ib);
        ...
        if (!b1.containsAll(b2) || !b2.containsAll(b1)) {   // O(B²)
            return false;
        }
    }
    return true;
}

b1.containsAll(b2) on com.sun.tools.javac.util.List (javac's own linked-list) does a linear contains() check per element of b2 — O(|b1| × |b2|). Called twice.

This is a set-equality check expressed as two mutual containment checks on linear lists. Should use Set or sort-and-compare.

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Call Context

isEquiv() is called from ReachabilityVisitor.visitUndetVar() which is called during inference context minimization (InferenceContext.min()). Minimization fires when the compiler attempts to reduce the inference context to its minimal reachable set before solving.

Bounds lists are typically small (1–10 elements), so O(B²) is not severe in practice. Impact is lower than 0001–0003 but follows the same pattern.

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Fix

// Replace containsAll on linked lists with Set comparison:
if (!new LinkedHashSet<>(b1).equals(new LinkedHashSet<>(b2))) {
    return false;
}

Or, if getBounds() returned a Set instead of a List, the comparison would be direct.

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Summary

#	Location	Defect	Complexity	Module
0001	GraphUtils$Tarjan	stack.contains(n)	O(V²) Tarjan	jdk.compiler
0002	InferenceGraph.findNode() + closure()	O(N) scan + uncached DFS	O(N³) total	jdk.compiler
0003	ModuleHashesBuilder$TopoSorter	Deque.contains()	O(V²) topo sort	java.base
0004	Dependencies$Node.addDependency()	List.contains() dedup	O(N) per add	jdk.compiler
0005	InferenceContext.isEquiv()	List.containsAll()	O(B²) per call	jdk.compiler

All five share the root pattern: linear collection membership check where O(1) is achievable.