src/main/java/org/plumelib/bcelutil/StackVer.java
private void circulationPump(
      final MethodGen m,
      final ControlFlowGraph cfg,
      final InstructionContext start,
      final Frame vanillaFrame,
      final InstConstraintVisitor icv,
      final ExecutionVisitor ev) {
    final Random random = new Random();
    final InstructionContextQueue icq = new InstructionContextQueue();

    execute(start, vanillaFrame, new ArrayList<InstructionContext>(), icv, ev);
    // new ArrayList() <=>    no Instruction was executed before
    //                                    => Top-Level routine (no jsr call before)
    icq.add(start, new ArrayList<InstructionContext>());

    // LOOP!
    while (!icq.isEmpty()) {
      InstructionContext u;
      ArrayList<InstructionContext> ec;
      if (!DEBUG) {
        final int r = random.nextInt(icq.size());
        u = icq.getIC(r);
        ec = icq.getEC(r);
        icq.remove(r);
      } else {
        u = icq.getIC(0);
        ec = icq.getEC(0);
        icq.remove(0);
      }

      @SuppressWarnings("unchecked") // ec is of type ArrayList<InstructionContext>
      final ArrayList<InstructionContext> oldchain = (ArrayList<InstructionContext>) (ec.clone());
      @SuppressWarnings("unchecked") // ec is of type ArrayList<InstructionContext>
      final ArrayList<InstructionContext> newchain = (ArrayList<InstructionContext>) (ec.clone());
      newchain.add(u);

      if ((u.getInstruction().getInstruction()) instanceof RET) {
        // System.err.println(u);
        // We can only follow _one_ successor, the one after the
        // JSR that was recently executed.
        final RET ret = (RET) (u.getInstruction().getInstruction());
        final ReturnaddressType t =
            (ReturnaddressType) u.getOutFrame(oldchain).getLocals().get(ret.getIndex());
        final InstructionContext theSuccessor = cfg.contextOf(t.getTarget());

        // Sanity check
        InstructionContext lastJSR = null;
        int skip_jsr = 0;
        for (int ss = oldchain.size() - 1; ss >= 0; ss--) {
          if (skip_jsr < 0) {
            throw new AssertionViolatedException("More RET than JSR in execution chain?!");
          }
          // System.err.println("+"+oldchain.get(ss));
          if ((oldchain.get(ss)).getInstruction().getInstruction() instanceof JsrInstruction) {
            if (skip_jsr == 0) {
              lastJSR = oldchain.get(ss);
              break;
            }
            skip_jsr--;
          }
          if ((oldchain.get(ss)).getInstruction().getInstruction() instanceof RET) {
            skip_jsr++;
          }
        }
        if (lastJSR == null) {
          throw new AssertionViolatedException(
              "RET without a JSR before in ExecutionChain?! EC: '" + oldchain + "'.");
        }
        final JsrInstruction jsr = (JsrInstruction) (lastJSR.getInstruction().getInstruction());
        if (theSuccessor != (cfg.contextOf(jsr.physicalSuccessor()))) {
          throw new AssertionViolatedException(
              "RET '"
                  + u.getInstruction()
                  + "' info inconsistent: jump back to '"
                  + theSuccessor
                  + "' or '"
                  + cfg.contextOf(jsr.physicalSuccessor())
                  + "'?");
        }

        if (execute(theSuccessor, u.getOutFrame(oldchain), newchain, icv, ev)) {
          @SuppressWarnings(
              "unchecked") // newchain is already of type ArrayList<InstructionContext>
          final ArrayList<InstructionContext> newchainClone =
              (ArrayList<InstructionContext>) newchain.clone();
          icq.add(theSuccessor, newchainClone);
        }
      } else { // "not a ret"

        // Normal successors. Add them to the queue of successors.
        final InstructionContext[] succs = u.getSuccessors();
        for (final InstructionContext v : succs) {
          if (execute(v, u.getOutFrame(oldchain), newchain, icv, ev)) {
            @SuppressWarnings(
                "unchecked") // newchain is already of type ArrayList<InstructionContext>
            final ArrayList<InstructionContext> newchainClone =
                (ArrayList<InstructionContext>) newchain.clone();
            icq.add(v, newchainClone);
          }
        }
      } // end "not a ret"

      // Exception Handlers. Add them to the queue of successors.
      // [subroutines are never protected; mandated by JustIce]
      final ExceptionHandler[] exc_hds = u.getExceptionHandlers();
      for (final ExceptionHandler exc_hd : exc_hds) {
        final InstructionContext v = cfg.contextOf(exc_hd.getHandlerStart());
        // TODO: the "oldchain" and "newchain" is used to determine the subroutine
        // we're in (by searching for the last JSR) by the InstructionContext
        // implementation. Therefore, we should not use this chain mechanism
        // when dealing with exception handlers.
        // Example: a JSR with an exception handler as its successor does not
        // mean we're in a subroutine if we go to the exception handler.
        // We should address this problem later; by now we simply "cut" the chain
        // by using an empty chain for the exception handlers.
        // if (execute(v, new Frame(u.getOutFrame(oldchain).getLocals(),
        // new OperandStack (u.getOutFrame().getStack().maxStack(),
        // (exc_hds[s].getExceptionType()==null? Type.THROWABLE : exc_hds[s].getExceptionType())) ),
        // newchain), icv, ev) {
        // icq.add(v, (ArrayList) newchain.clone());
        if (execute(
            v,
            new Frame(
                u.getOutFrame(oldchain).getLocals(),
                new OperandStack(
                    u.getOutFrame(oldchain).getStack().maxStack(),
                    exc_hd.getExceptionType() == null
                        ? Type.THROWABLE
                        : exc_hd.getExceptionType())),
            new ArrayList<InstructionContext>(),
            icv,
            ev)) {
          icq.add(v, new ArrayList<InstructionContext>());
        }
      }
    } // while (!icq.isEmpty()) END

    InstructionHandle ih = start.getInstruction();
    do {
      if ((ih.getInstruction() instanceof ReturnInstruction) && (!(cfg.isDead(ih)))) {
        final InstructionContext ic = cfg.contextOf(ih);
        // TODO: This is buggy, we check only the top-level return instructions this way.
        // Maybe some maniac returns from a method when in a subroutine?
        final Frame f = ic.getOutFrame(new ArrayList<InstructionContext>());
        final LocalVariables lvs = f.getLocals();
        for (int i = 0; i < lvs.maxLocals(); i++) {
          if (lvs.get(i) instanceof UninitializedObjectType) {
            this.addMessage(
                "Warning: ReturnInstruction '"
                    + ic
                    + "' may leave method with an uninitialized object in the local variables array '"
                    + lvs
                    + "'.");
          }
        }
        final OperandStack os = f.getStack();
        for (int i = 0; i < os.size(); i++) {
          if (os.peek(i) instanceof UninitializedObjectType) {
            this.addMessage(
                "Warning: ReturnInstruction '"
                    + ic
                    + "' may leave method with an uninitialized object on the operand stack '"
                    + os
                    + "'.");
          }
        }
        /* This code from Pass3bVerifier incorrectly fails on some valid class files.
        // see JVM $4.8.2
        Type returnedType = null;
        final OperandStack inStack = ic.getInFrame().getStack();
        if (inStack.size() >= 1) {
          returnedType = inStack.peek();
        } else {
          returnedType = Type.VOID;
        }

        if (returnedType != null) {
          if (returnedType instanceof ReferenceType) {
            try {
              if (!((ReferenceType) returnedType).isCastableTo(m.getReturnType())) {
                invalidReturnTypeError(returnedType, m);
              }
            } catch (final ClassNotFoundException e) {
              // Don't know what do do now, so raise RuntimeException
              throw new RuntimeException(e);
            }
          } else if (!returnedType.equals(m.getReturnType().normalizeForStackOrLocal())) {
            invalidReturnTypeError(returnedType, m);
          }
        }
        */
      }
    } while ((ih = ih.getNext()) != null);
  }
private void circulationPump(
      final MethodGen m,
      final ControlFlowGraph cfg,
      final InstructionContext start,
      final Frame vanillaFrame,
      final InstConstraintVisitor icv,
      final ExecutionVisitor ev) {
    final Random random = new Random();
    final InstructionContextQueue icq = new InstructionContextQueue();

    execute(start, vanillaFrame, new ArrayList<InstructionContext>(), icv, ev);
    // new ArrayList() <=>    no Instruction was executed before
    //                                    => Top-Level routine (no jsr call before)
    icq.add(start, new ArrayList<InstructionContext>());

    // LOOP!
    while (!icq.isEmpty()) {
      InstructionContext u;
      ArrayList<InstructionContext> ec;
      if (!DEBUG) {
        final int r = random.nextInt(icq.size());
        u = icq.getIC(r);
        ec = icq.getEC(r);
        icq.remove(r);
      } else {
        u = icq.getIC(0);
        ec = icq.getEC(0);
        icq.remove(0);
      }

      @SuppressWarnings("unchecked") // ec is of type ArrayList<InstructionContext>
      final ArrayList<InstructionContext> oldchain = (ArrayList<InstructionContext>) (ec.clone());
      @SuppressWarnings("unchecked") // ec is of type ArrayList<InstructionContext>
      final ArrayList<InstructionContext> newchain = (ArrayList<InstructionContext>) (ec.clone());
      newchain.add(u);

      if ((u.getInstruction().getInstruction()) instanceof RET) {
        // System.err.println(u);
        // We can only follow _one_ successor, the one after the
        // JSR that was recently executed.
        final RET ret = (RET) (u.getInstruction().getInstruction());
        final ReturnaddressType t =
            (ReturnaddressType) u.getOutFrame(oldchain).getLocals().get(ret.getIndex());
        final InstructionContext theSuccessor = cfg.contextOf(t.getTarget());

        // Sanity check
        InstructionContext lastJSR = null;
        int skip_jsr = 0;
        for (int ss = oldchain.size() - 1; ss >= 0; ss--) {
          if (skip_jsr < 0) {
            throw new AssertionViolatedException("More RET than JSR in execution chain?!");
          }
          // System.err.println("+"+oldchain.get(ss));
          if ((oldchain.get(ss)).getInstruction().getInstruction() instanceof JsrInstruction) {
            if (skip_jsr == 0) {
              lastJSR = oldchain.get(ss);
              break;
            }
            skip_jsr--;
          }
          if ((oldchain.get(ss)).getInstruction().getInstruction() instanceof RET) {
            skip_jsr++;
          }
        }
        if (lastJSR == null) {
          throw new AssertionViolatedException(
              "RET without a JSR before in ExecutionChain?! EC: '" + oldchain + "'.");
        }
        final JsrInstruction jsr = (JsrInstruction) (lastJSR.getInstruction().getInstruction());
        if (theSuccessor != (cfg.contextOf(jsr.physicalSuccessor()))) {
          throw new AssertionViolatedException(
              "RET '"
                  + u.getInstruction()
                  + "' info inconsistent: jump back to '"
                  + theSuccessor
                  + "' or '"
                  + cfg.contextOf(jsr.physicalSuccessor())
                  + "'?");
        }

        if (execute(theSuccessor, u.getOutFrame(oldchain), newchain, icv, ev)) {
          @SuppressWarnings(
              "unchecked") // newchain is already of type ArrayList<InstructionContext>
          final ArrayList<InstructionContext> newchainClone =
              (ArrayList<InstructionContext>) newchain.clone();
          icq.add(theSuccessor, newchainClone);
        }
      } else { // "not a ret"

        // Normal successors. Add them to the queue of successors.
        final InstructionContext[] succs = u.getSuccessors();
        for (final InstructionContext v : succs) {
          if (execute(v, u.getOutFrame(oldchain), newchain, icv, ev)) {
            @SuppressWarnings(
                "unchecked") // newchain is already of type ArrayList<InstructionContext>
            final ArrayList<InstructionContext> newchainClone =
                (ArrayList<InstructionContext>) newchain.clone();
            icq.add(v, newchainClone);
          }
        }
      } // end "not a ret"

      // Exception Handlers. Add them to the queue of successors.
      // [subroutines are never protected; mandated by JustIce]
      final ExceptionHandler[] exc_hds = u.getExceptionHandlers();
      for (final ExceptionHandler exc_hd : exc_hds) {
        final InstructionContext v = cfg.contextOf(exc_hd.getHandlerStart());
        // TODO: the "oldchain" and "newchain" is used to determine the subroutine
        // we're in (by searching for the last JSR) by the InstructionContext
        // implementation. Therefore, we should not use this chain mechanism
        // when dealing with exception handlers.
        // Example: a JSR with an exception handler as its successor does not
        // mean we're in a subroutine if we go to the exception handler.
        // We should address this problem later; by now we simply "cut" the chain
        // by using an empty chain for the exception handlers.
        // if (execute(v, new Frame(u.getOutFrame(oldchain).getLocals(),
        // new OperandStack (u.getOutFrame().getStack().maxStack(),
        // (exc_hds[s].getExceptionType()==null? Type.THROWABLE : exc_hds[s].getExceptionType())) ),
        // newchain), icv, ev) {
        // icq.add(v, (ArrayList) newchain.clone());
        if (execute(
            v,
            new Frame(
                u.getOutFrame(oldchain).getLocals(),
                new OperandStack(
                    u.getOutFrame(oldchain).getStack().maxStack(),
                    exc_hd.getExceptionType() == null
                        ? Type.THROWABLE
                        : exc_hd.getExceptionType())),
            new ArrayList<InstructionContext>(),
            icv,
            ev)) {
          icq.add(v, new ArrayList<InstructionContext>());
        }
      }
    } // while (!icq.isEmpty()) END

    InstructionHandle ih = start.getInstruction();
    do {
      if ((ih.getInstruction() instanceof ReturnInstruction) && (!(cfg.isDead(ih)))) {
        final InstructionContext ic = cfg.contextOf(ih);
        // TODO: This is buggy, we check only the top-level return instructions this way.
        // Maybe some maniac returns from a method when in a subroutine?
        final Frame f = ic.getOutFrame(new ArrayList<InstructionContext>());
        final LocalVariables lvs = f.getLocals();
        for (int i = 0; i < lvs.maxLocals(); i++) {
          if (lvs.get(i) instanceof UninitializedObjectType) {
            this.addMessage(
                "Warning: ReturnInstruction '"
                    + ic
                    + "' may leave method with an uninitialized object in the local variables array '"
                    + lvs
                    + "'.");
          }
        }
        final OperandStack os = f.getStack();
        for (int i = 0; i < os.size(); i++) {
          if (os.peek(i) instanceof UninitializedObjectType) {
            this.addMessage(
                "Warning: ReturnInstruction '"
                    + ic
                    + "' may leave method with an uninitialized object on the operand stack '"
                    + os
                    + "'.");
          }
        }
        /* This code from Pass3bVerifier incorrectly fails on some valid class files.
        // see JVM $4.8.2
        Type returnedType = null;
        final OperandStack inStack = ic.getInFrame().getStack();
        if (inStack.size() >= 1) {
          returnedType = inStack.peek();
        } else {
          returnedType = Type.VOID;
        }

        if (returnedType != null) {
          if (returnedType instanceof ReferenceType) {
            try {
              if (!((ReferenceType) returnedType).isCastableTo(m.getReturnType())) {
                invalidReturnTypeError(returnedType, m);
              }
            } catch (final ClassNotFoundException e) {
              // Don't know what do do now, so raise RuntimeException
              throw new RuntimeException(e);
            }
          } else if (!returnedType.equals(m.getReturnType().normalizeForStackOrLocal())) {
            invalidReturnTypeError(returnedType, m);
          }
        }
        */
      }
    } while ((ih = ih.getNext()) != null);
  }
Whenever the outgoing frame situation of an InstructionContext changes, all its successors are
put [back] into the queue [as if they were unvisited]. The proof of termination is about the
existence of a fix point of frame merging.