ASM: Stateful Transformation

Question

I want to write a MethodVisitor that transforms LDC instructions that are for multiplication.

Example bytecode:

ldc #26
imul

This basically pushes a constant and then multiplies it.

It has to be a stateful transformation because I first have to check that it is for multiply and, if it is, I need to go back to the ldc instruction and modify the constant. I'm not entirely sure how I would go about this, and I don't know how to modify the constant (when I tried to pass a different value, the old value still remained in the constant pool).

Edit:

public class AdditionTransformer extends MethodAdapter {
    boolean replace = false;
    int operand = 0;

    AdditionTransformer(MethodVisitor mv) {
        super(mv);
    }

    @Override
    public void visitInsn(int opcode) {
        if (opcode == IMUL && replace) {
            operand *= 2;
            visitLdcInsn(operand);
            replace = false;
        }
        mv.visitInsn(opcode);
    }

    @Override
    public void visitLdcInsn(Object cst) {
        if (cst instanceof Integer && !replace) {
            operand = (Integer) cst;
            replace = true;
        } else {
            mv.visitLdcInsn(cst);
        }
    }
}

This is what I have, but it doesn't remove the old value in the constant pool, and it may have bugs.

Answer 1

If you are interested in modifying bytecode in such a manner, you may want to look into the ASM tree API. You can easily replace LdcInsnNode.cst through a more comfortable DOM-style tree interface as opposed to the SAX-style visitor interface you are attempting to use.