[jboss-user] [Javassist user questions] - Problem with branches and ldc/ldc_w

[tumbleweed]

Hello,

i'm working with javassist for quite some time now and I must say it is really impressive what this thing can do, and with what ease!

Anyways, I'm confronted with one problem:
My tool inserts an arbitrary series of conditional branches and some code afterwards. This means I have to precalculate the number of bytes inserted for all following conditions and the code in respect to set the branch offset. All works well unless I load a string constant using addLdc(String). In this case I am unable to precalculate whether there are 3 bytes (ldc) or 4 bytes (ldc_w) inserted, since this depends on the ConstTables state at insertion time.

I've developed a workaround for my case by extending the Bytecode class and altering its behaviour with

  | private ArrayList<Integer> m_branches;
  | ...
  | 
  | public void addBranch(int code, int branchOffset) {
  | 	// branch opcode is inserted at offset getSize()
  | 	m_branches.add(getSize());
  | 	addOpcode(code);
  | 	add16bit(branchOffset);
  | }
  | 	
  | private void adjustBranches(int end, int shift) {
  | 	for (Integer pos : m_branches) {
  | 		int branchOffset = read16bit(pos + 1);
  | 		if ((pos + branchOffset) <= end) continue;
  | 		write16bit(pos + 1, branchOffset + shift);
  | 	}
  | }
  | 	
  | @Override
  | public void addLdc(int i) {
  | 	if (i > 0xFF) {
  | 		adjustBranches(getSize(), 1);
  | 		addOpcode(LDC_W);
  | 		addIndex(i);
  | 	} else {
  | 		addOpcode(LDC);
  | 		add(i);
  |         }
  | }
  | 

I think this is could be extended to a general solution, if it would be well defined how many bytes one should asume to be inserted.

Is there a way around this problem that I've overseen or is this really a "bug"?

---

By the way, is there any reason there is no

  | public void add16bit(int value) {
  | 	value = value % 65536;
  | 	add(value >> 8);
  | 	add(value % 256);
  | }
  | 
defined in Bytecode? Since it is required for inserting branch offsets...

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