Pattern matching

Basic pattern matching

The library has powerful support to match patterns in bytecode instruction sequences. You first define the pattern as a sequence of instructions, with wildcards. For example:

final int X = InstructionSequenceMatcher.X;
final int C = InstructionSequenceMatcher.C;

InstructionSequenceBuilder ____ =
    new InstructionSequenceBuilder();

Instruction[] pattern =
    ____.iload(X)
        .bipush(C)
        .istore(X).__();

Constant[] constants = ____.constants();

You can then find that pattern in given code:

programClassPool.classesAccept(
    new AllMethodVisitor(
    new AllAttributeVisitor(
    new AllInstructionVisitor(
    new MyMatchPrinter(
    new InstructionSequenceMatcher(constants, pattern))))));

Complete example: ApplyPeepholeOptimizations.java

Replacing patterns

Instead of just matching instruction sequences, you can also replace matched sequences by other instruction sequences, for example to optimize code or instrument code. The core classes are PeepholeEditor and InstructionSequenceReplacer. Say that you want to replace an instruction sequence "putstatic/getstatic" by an equivalent "dup/putstatic":

final int X = InstructionSequenceReplacer.X;

InstructionSequenceBuilder ____ =
    new InstructionSequenceBuilder();

Instruction[][] replacements =
{
    ____.putstatic(X)
        .getstatic(X).__(),

    ____.dup()
        .putstatic(X).__()
};

Constant[] constants = ____.constants();

BranchTargetFinder  branchTargetFinder  = new BranchTargetFinder();
CodeAttributeEditor codeAttributeEditor = new CodeAttributeEditor();

programClassPool.classesAccept(
    new AllMethodVisitor(
    new AllAttributeVisitor(
    new PeepholeEditor(branchTargetFinder, codeAttributeEditor,
    new InstructionSequenceReplacer(constants,
                                    replacements[0],
                                    constants,
                                    replacements[1],
                                    branchTargetFinder,
                                    codeAttributeEditor)))));

You can define multiple patterns and their respective replacements in one go, with the wrapper InstructionSequencesReplacer.

Complete example: ApplyPeepholeOptimizations.java