-;;; Do a backward walk in the flow graph simulating the run-time stack
-;;; of unknown-values lvars and annotating the blocks with the result.
-;;;
-;;; BLOCK is the block that is currently being walked and STACK is the
-;;; stack of unknown-values lvars in effect immediately after
-;;; block. We simulate the stack by popping off the unknown-values
-;;; generated by this block (if any) and pushing the lvars for
-;;; values received by this block. (The role of push and pop are
-;;; interchanged because we are doing a backward walk.)
-;;;
-;;; If we run into a values generator whose lvar isn't on
-;;; stack top, then the receiver hasn't yet been reached on any walk
-;;; to this use. In this case, we ignore the push for now, counting on
-;;; Annotate-Dead-Values to clean it up if we discover that it isn't
-;;; reachable at all.
-;;;
-;;; If our final stack isn't empty, then we walk all the predecessor
-;;; blocks that don't have all the lvars that we have on our
-;;; START-STACK on their END-STACK. This is our termination condition
-;;; for the graph walk. We put the test around the recursive call so
-;;; that the initial call to this function will do something even
-;;; though there isn't initially anything on the stack.
-;;;
-;;; We can use the tailp test, since the only time we want to bottom
-;;; out with a non-empty stack is when we intersect with another path
-;;; from the same top level call to this function that has more values
-;;; receivers on that path. When we bottom out in this way, we are
-;;; counting on DISCARD-UNUSED-VALUES doing its thing.
-;;;
-;;; When we do recurse, we check that predecessor's END-STACK is a
-;;; subsequence of our START-STACK. There may be extra stuff on the
-;;; top of our stack because the last path to the predecessor may have
-;;; discarded some values that we use. There may be extra stuff on the
-;;; bottom of our stack because this walk may be from a values
-;;; receiver whose lifetime encloses that of the previous walk.
-;;;
-;;; If a predecessor block is the component head, then it must be the
-;;; case that this is a NLX entry stub. If so, we just stop our walk,
-;;; since the stack at the exit point doesn't have anything to do with
-;;; our stack.
-(defun stack-simulation-walk (block stack)
- (declare (type cblock block) (list stack))
- (let ((2block (block-info block)))
- (setf (ir2-block-end-stack 2block) stack)
- (let ((new-stack stack))
- (dolist (push (reverse (ir2-block-pushed 2block)))
- (if (eq (car new-stack) push)
- (pop new-stack)
- (aver (not (member push new-stack)))))