;;;; This file contains the control analysis pass in the compiler.
;;;; This pass determines the order in which the IR2 blocks are to be
;;;; emitted, attempting to minimize the associated branching costs.
;;;;
;;;; At this point, we commit to generating IR2 (and ultimately
;;;; assembler) for reachable blocks. Before this phase there might be
;;;; blocks that are unreachable but still appear in the DFO, due in
;;;; inadequate optimization, etc.
;;;; This software is part of the SBCL system. See the README file for
;;;; more information.
;;;;
;;;; This software is derived from the CMU CL system, which was
;;;; written at Carnegie Mellon University and released into the
;;;; public domain. The software is in the public domain and is
;;;; provided with absolutely no warranty. See the COPYING and CREDITS
;;;; files for more information.
(in-package "SB-C")
;;; Insert BLOCK in the emission order after the block AFTER.
(defun add-to-emit-order (block after)
(declare (type block-annotation block after))
(let ((next (block-annotation-next after)))
(setf (block-annotation-next after) block)
(setf (block-annotation-prev block) after)
(setf (block-annotation-next block) next)
(setf (block-annotation-prev next) block))
(values))
;;; If BLOCK looks like the head of a loop, then attempt to rotate it.
;;; A block looks like a loop head if the number of some predecessor
;;; is less than the block's number. Since blocks are numbered in
;;; reverse DFN, this will identify loop heads in a reducible flow
;;; graph.
;;;
;;; When we find a suspected loop head, we scan back from the tail to
;;; find an alternate loop head. This substitution preserves the
;;; correctness of the walk, since the old head can be reached from
;;; the new head. We determine the new head by scanning as far back as
;;; we can find increasing block numbers. Beats me if this is in
;;; general optimal, but it works in simple cases.
;;;
;;; This optimization is inhibited in functions with NLX EPs, since it
;;; is hard to do this without possibly messing up the special-case
;;; walking from NLX EPs described in CONTROL-ANALYZE-1-FUN. We also
;;; suppress rotation of loop heads which are the start of a function
;;; (i.e. tail calls), as the debugger wants functions to start at the
;;; start.
;;;
;;; The rotation also is not done if the back edge identified in the
;;; first step originates from a block that has more than one successor.
;;; This matches loops that have their terminating condition tested at
;;; the end, for which the original block order already minimizes the
;;; number of branches: the back edge starts at a conditional branch at
;;; the loop's tail and no other branches are needed. We used not to
;;; test for this situation, rotating these loops, too, resulting in
;;; machine code that looked like this
;;; jump to L1
;;; L0: body of loop
;;; conditionally branch to L2 if the loop should terminate
;;; L1: jump to L0
;;; L2:
;;; which is ugly, and larger and often slower than what is generated
;;; when not rotating these loops.
(defun find-rotated-loop-head (block)
(declare (type cblock block))
(let* ((num (block-number block))
(env (block-physenv block))
(pred (dolist (pred (block-pred block) nil)
(when (and (not (block-flag pred))
(eq (block-physenv pred) env)
(< (block-number pred) num))
(return pred)))))
(cond
((and pred
(not (physenv-nlx-info env))
(not (eq (lambda-block (block-home-lambda block)) block))
(null (cdr (block-succ pred))))
(let ((current pred)
(current-num (block-number pred)))
(block DONE
(loop
(dolist (pred (block-pred current) (return-from DONE))
(when (eq pred block)
(return-from DONE))
(when (and (not (block-flag pred))
(eq (block-physenv pred) env)
(> (block-number pred) current-num))
(setq current pred current-num (block-number pred))
(return)))))
(aver (not (block-flag current)))
current))
(t
block))))
;;; Do a graph walk linking blocks into the emit order as we go. We
;;; call FIND-ROTATED-LOOP-HEAD to do while-loop optimization.
;;;
;;; We treat blocks ending in tail local calls to other environments
;;; specially. We can't walked the called function immediately, since
;;; it is in a different function and we must keep the code for a
;;; function contiguous. Instead, we return the function that we want
;;; to call so that it can be walked as soon as possible, which is
;;; hopefully immediately.
;;;
;;; If any of the recursive calls ends in a tail local call, then we
;;; return the last such function, since it is the only one we can
;;; possibly drop through to. (But it doesn't have to be from the last
;;; block walked, since that call might not have added anything.)
;;;
;;; We defer walking successors whose successor is the component tail
;;; (end in an error, NLX or tail full call.) This is to discourage
;;; making error code the drop-through.
(defun control-analyze-block (block tail)
(declare (type cblock block)
(type block-annotation tail))
(unless (block-flag block)
(let ((block (find-rotated-loop-head block)))
(setf (block-flag block) t)
(aver (and (block-component block) (not (block-delete-p block))))
(add-to-emit-order (or (block-info block)
(setf (block-info block)
(make-ir2-block block)))
(block-annotation-prev tail))
(let ((last (block-last block)))
(cond ((and (combination-p last) (node-tail-p last)
(eq (basic-combination-kind last) :local)
(not (eq (node-physenv last)
(lambda-physenv (combination-lambda last)))))
(combination-lambda last))
(t
(let ((component-tail (component-tail (block-component block)))
(block-succ (block-succ block))
(fun nil))
(dolist (succ block-succ)
(unless (eq (first (block-succ succ)) component-tail)
(let ((res (control-analyze-block succ tail)))
(when res (setq fun res)))))
(dolist (succ block-succ)
(control-analyze-block succ tail))
fun)))))))
;;; Analyze all of the NLX EPs first to ensure that code reachable
;;; only from a NLX is emitted contiguously with the code reachable
;;; from the BIND. Code reachable from the BIND is inserted *before*
;;; the NLX code so that the BIND marks the beginning of the code for
;;; the function. If the walks from NLX EPs reach the BIND block, then
;;; we just move it to the beginning.
;;;
;;; If the walk from the BIND node encountered a tail local call, then
;;; we start over again there to help the call drop through. Of
;;; course, it will never get a drop-through if either function has
;;; NLX code.
(defun control-analyze-1-fun (fun component)
(declare (type clambda fun)
(type component component))
(let* ((tail-block (block-info (component-tail component)))
(prev-block (block-annotation-prev tail-block))
(bind-block (node-block (lambda-bind fun))))
(unless (block-flag bind-block)
(dolist (nlx (physenv-nlx-info (lambda-physenv fun)))
(control-analyze-block (nlx-info-target nlx) tail-block))
(cond
((block-flag bind-block)
(let* ((block-note (block-info bind-block))
(prev (block-annotation-prev block-note))
(next (block-annotation-next block-note)))
(setf (block-annotation-prev next) prev)
(setf (block-annotation-next prev) next)
(add-to-emit-order block-note prev-block)))
(t
(let ((new-fun (control-analyze-block bind-block
(block-annotation-next
prev-block))))
(when new-fun
(control-analyze-1-fun new-fun component)))))))
(values))
;;; Do control analysis on COMPONENT, finding the emit order. Our only
;;; cleverness here is that we walk XEP's first to increase the
;;; probability that the tail call will be a drop-through.
;;;
;;; When we are done, we delete blocks that weren't reached by the
;;; walk. Some return blocks are made unreachable by LTN without
;;; setting COMPONENT-REANALYZE. We remove all deleted blocks from the
;;; IR2-COMPONENT VALUES-RECEIVERS to keep stack analysis from getting
;;; confused.
(defevent control-deleted-block "control analysis deleted dead block")
(defun control-analyze (component)
(declare (type component component))
(let* ((head (component-head component))
(head-block (make-ir2-block head))
(tail (component-tail component))
(tail-block (make-ir2-block tail)))
(setf (block-info head) head-block)
(setf (block-info tail) tail-block)
(setf (block-annotation-prev tail-block) head-block)
(setf (block-annotation-next head-block) tail-block)
(clear-flags component)
(dolist (fun (component-lambdas component))
(when (xep-p fun)
(control-analyze-1-fun fun component)))
(dolist (fun (component-lambdas component))
(control-analyze-1-fun fun component))
(do-blocks (block component)
(unless (block-flag block)
(event control-deleted-block (block-start-node block))
(delete-block block))))
(let ((2comp (component-info component)))
(when (ir2-component-p 2comp)
;; If it's not an IR2-COMPONENT, don't worry about it.
(setf (ir2-component-values-receivers 2comp)
(delete-if-not #'block-component
(ir2-component-values-receivers 2comp)))))
(values))