protected BitSet infer(Code.Block block, int start, int end) {
BitSet modified = new BitSet(block.numSlots());
int size = block.size();
for(int i=start;i<end;++i) {
Code.Block.Entry entry = block.get(i);
Code code = entry.code;
if (code instanceof Code.AbstractAssignable) {
Code.AbstractAssignable aa = (Code.AbstractAssignable) code;
if(aa.target() != Codes.NULL_REG) {
modified.set(aa.target());