java-topology/defects/jsc/patch/jsc-0001-bytecodebasicblock-switch-hashset.md

JSC-0001: BytecodeBasicBlock::computeImpl() O(B²×T) edge linking via Vector::contains

File: Source/JavaScriptCore/bytecode/BytecodeBasicBlock.cpp Lines: 181–193 (branch block linking loop) Severity: HIGH CWE: CWE-407 (Inefficient Algorithmic Complexity)

Description

BytecodeBasicBlock<OpcodeTraits>::computeImpl() links bytecode basic blocks to their successors. For branch instructions (including switch), it collects jump targets into a temporary Vector<Offset, 1> called bytecodeOffsetsJumpedTo, then walks every basic block in the function checking membership:

// line 181
Vector<typename InstructionStreamType::Offset, 1> bytecodeOffsetsJumpedTo;
findJumpTargetsForInstruction(codeBlock, instruction, bytecodeOffsetsJumpedTo);

size_t numberOfJumpTargets = bytecodeOffsetsJumpedTo.size();
for (auto& otherBlock : basicBlocks) {                    // O(B)
    if (bytecodeOffsetsJumpedTo.contains(otherBlock.leaderOffset())) { // O(T)
        linkBlocks(block, otherBlock);
        --numberOfJumpTargets;
        if (!numberOfJumpTargets)
            break;
    }
}

This entire nested scan runs for every branch block in the function. With B basic blocks and T switch targets per branch:

• Outer loop (line 137): O(B) iterations over all blocks
• For each branch block: inner loop O(B) × Vector::contains O(T)
• Total: O(B² × T)

For a large function with a 1 000-case switch (T=1000) and 200 basic blocks (B=200):

• Slow path: 200 × 200 × 1000 = 40 000 000 comparisons

Vector::contains on line 187 is a linear scan (std::find under the hood) — confirmed by WebKit's Vector.h.

Fix

Before the inner loop, insert the jump targets into a HashSet<Offset>, then replace bytecodeOffsetsJumpedTo.contains(...) with jumpTargetSet.contains(...) — O(1) per lookup.

HashSet<Offset> jumpTargetSet;
for (auto offset : bytecodeOffsetsJumpedTo)
    jumpTargetSet.add(offset);

for (auto& otherBlock : basicBlocks) {
    if (jumpTargetSet.contains(otherBlock.leaderOffset())) {
        linkBlocks(block, otherBlock);
        ...
    }
}

HashSet is WTF::HashSet, already imported in BytecodeBasicBlock.cpp.

Complexity

Scenario	Before	After
Single branch (T targets, B blocks)	O(B×T)	O(B + T)
Full function (B branches, B blocks, T targets)	O(B²×T)	O(B×(B+T))
200 blocks × 1000-case switch	40 000 000 ops	200 200 ops

Speedup: ~200× for 1000-case switch in 200-block function

Affected Callers

• BytecodeBasicBlock<JSOpcodeTraits>::compute(CodeBlock*, ...)
• BytecodeBasicBlock<JSOpcodeTraits>::compute(UnlinkedCodeBlockGenerator*, ...)
• Used by DFG JIT, liveness analysis, register allocation — called every time a function is compiled

References

• WTF/HashSet.h — already in scope via config.h
• JSC/bytecode/BytecodeBasicBlock.h — PredecessorList typedef