From: Paul Khuong Date: Sun, 17 Jun 2012 08:53:47 +0000 (+0200) Subject: Improved merge-sort implementation for lists X-Git-Url: http://repo.macrolet.net/gitweb/?a=commitdiff_plain;h=41c307979e17a33e8700c1ca92ed8b3400a301b3;p=sbcl.git Improved merge-sort implementation for lists The new implementation is simpler and more efficient than the previous bottom-up sort. It only differs from the original patch aesthetically. STABLE-SORT-LIST is also now MAYBE-INLINE, to enable selective inlining. Thanks to Takeru Ohta for the code and for his patience. --- diff --git a/NEWS b/NEWS index 2e0dba0..f9085fb 100644 --- a/NEWS +++ b/NEWS @@ -19,6 +19,9 @@ changes relative to sbcl-1.0.57: function cannot escape. * optimization: SB-SEQUENCE:DOSEQUENCE is faster on vectors of unknown element type, and vectors that aren't SIMPLE-ARRAYs. + * optimization: CL:SORT and CL:STABLE-SORT are more efficient in execution + speed (around 1/3 the time in some cases), and a little better in terms of + comparison calls. (Thanks to Takeru Ohta) * bug fix: On SPARC, a limitation on the number of code constants emittable by the compiler has been lifted, allowing certain long functions to compiled and assembled which had previously been unsupported; fixes diff --git a/src/code/sort.lisp b/src/code/sort.lisp index 06cd2c6..504e178 100644 --- a/src/code/sort.lisp +++ b/src/code/sort.lisp @@ -18,7 +18,7 @@ ;;; application where sorting is a major bottleneck, and inlining it ;;; allows the compiler to make enough optimizations that it might be ;;; worth the (large) cost in space. -(declaim (maybe-inline sort)) +(declaim (maybe-inline sort stable-sort)) (defun sort (sequence predicate &rest args &key key) #!+sb-doc "Destructively sort SEQUENCE. PREDICATE should return non-NIL if @@ -68,109 +68,72 @@ ) ; EVAL-WHEN ;;;; stable sort of lists - -(defun last-cons-of (list) - (loop (let ((rest (rest list))) - (if rest - (setf list rest) - (return list))))) +(declaim (maybe-inline merge-lists* stable-sort-list)) ;;; Destructively merge LIST-1 with LIST-2 (given that they're already ;;; sorted w.r.t. PRED-FUN on KEY-FUN, giving output sorted the same ;;; way). In the resulting list, elements of LIST-1 are guaranteed to ;;; come before equal elements of LIST-2. ;;; -;;; Return (VALUES HEAD TAILTAIL), where HEAD is the same value you'd -;;; expect from MERGE, and TAILTAIL is the last cons in the list (i.e. -;;; the last cons in the list which NRECONC calls TAIL). -(defun merge-lists* (list-1 list-2 pred-fun key-fun) - (declare (type list list-1 list-2)) - (declare (type function pred-fun key-fun)) - (cond ((null list-1) (values list-2 (last-cons-of list-2))) - ((null list-2) (values list-1 (last-cons-of list-1))) - (t (let* ((reversed-result-so-far nil) - (key-1 (funcall key-fun (car list-1))) - (key-2 (funcall key-fun (car list-2)))) - (loop - (macrolet ((frob (list-i key-i other-list) - `(progn - ;; basically - ;; (PUSH (POP ,LIST-I) REVERSED-RESULT-SO-FAR), - ;; except doing some fancy footwork to - ;; reuse the cons cell: - (psetf (cdr ,list-i) reversed-result-so-far - reversed-result-so-far ,list-i - ,list-i (cdr ,list-i)) - ;; Now maybe we're done. - (if (endp ,list-i) - (return (values (nreconc - reversed-result-so-far - ,other-list) - (last-cons-of - ,other-list))) - (setf ,key-i - (funcall key-fun (car ,list-i))))))) - ;; Note that by making KEY-2 the first arg to - ;; PRED-FUN, we arrange that if PRED-FUN is a function - ;; in the #'< style, the outcome is stably sorted. - (if (funcall pred-fun key-2 key-1) - (frob list-2 key-2 list-1) - (frob list-1 key-1 list-2)))))))) - -;;; STABLE-SORT-LIST uses a bottom-up merge sort. First a pass is made -;;; over the list grabbing one element at a time and merging it with -;;; the next one to form pairs of sorted elements. Then N is doubled, -;;; and elements are taken in runs of two, merging one run with the -;;; next to form quadruples of sorted elements. This continues until N -;;; is large enough that the inner loop only runs for one iteration; -;;; that is, there are only two runs that can be merged, the first run -;;; starting at the beginning of the list, and the second being the -;;; remaining elements. -(defun stable-sort-list (list pred-fun key-fun) - (let ((head (cons :header list)) ; head holds on to everything - (n 1) ; bottom-up size of lists to be merged - unsorted ; unsorted is the remaining list to be - ; broken into n size lists and merged - list-1 ; list-1 is one length n list to be merged - last) ; last points to the last visited cell - (declare (type function pred-fun key-fun) - (type fixnum n)) +;;; Enqueues the values in the right order in HEAD's cdr, and returns +;;; the merged list. +(defun merge-lists* (head list1 list2 test key &aux (tail head)) + (declare (type cons head list1 list2) + (type function test key) + (optimize speed)) + (macrolet ((merge-one (l1 l2) + `(progn + (setf (cdr tail) ,l1 + tail ,l1) + (let ((rest (cdr ,l1))) + (cond (rest + (setf ,l1 rest)) + (t + (setf (cdr ,l1) ,l2) + (return (cdr head)))))))) (loop - ;; Start collecting runs of N at the first element. - (setf unsorted (cdr head)) - ;; Tack on the first merge of two N-runs to the head holder. - (setf last head) - (let ((n-1 (1- n))) - (declare (fixnum n-1)) - (loop - (setf list-1 unsorted) - (let ((temp (nthcdr n-1 list-1)) - list-2) - (cond (temp - ;; There are enough elements for a second run. - (setf list-2 (cdr temp)) - (setf (cdr temp) nil) - (setf temp (nthcdr n-1 list-2)) - (cond (temp - (setf unsorted (cdr temp)) - (setf (cdr temp) nil)) - ;; The second run goes off the end of the list. - (t (setf unsorted nil))) - (multiple-value-bind (merged-head merged-last) - (merge-lists* list-1 list-2 pred-fun key-fun) - (setf (cdr last) merged-head - last merged-last)) - (if (null unsorted) (return))) - ;; If there is only one run, then tack it on to the end. - (t (setf (cdr last) list-1) - (return))))) - (setf n (ash n 1)) ; (+ n n) - ;; If the inner loop only executed once, then there were only - ;; enough elements for two runs given n, so all the elements - ;; have been merged into one list. This may waste one outer - ;; iteration to realize. - (if (eq list-1 (cdr head)) - (return list-1)))))) + (if (funcall test (funcall key (car list2)) ; this way, equivalent + (funcall key (car list1))) ; values are first popped + (merge-one list2 list1) ; from list1 + (merge-one list1 list2))))) + +;;; Convenience wrapper for CL:MERGE +(declaim (inline merge-lists)) +(defun merge-lists (list1 list2 test key) + (cond ((null list1) + list2) + ((null list2) + list1) + (t + (let ((head (cons nil nil))) + (declare (dynamic-extent head)) + (merge-lists* head list1 list2 test key))))) + +;;; STABLE-SORT-LIST implements a top-down merge sort. See the closest +;;; intro to algorithms book. Benchmarks have shown significantly +;;; improved performance over the previous (hairier) bottom-up +;;; implementation, particularly on non-power-of-two sizes: bottom-up +;;; recursed on power-of-two-sized subsequences, which can result in +;;; very unbalanced recursion trees. +(defun stable-sort-list (list test key &aux (head (cons :head list))) + (declare (type list list) + (type function test key) + (dynamic-extent head)) + (labels ((recur (list size) + (declare (optimize speed) + (type cons list) + (type (and fixnum unsigned-byte) size)) + (if (= 1 size) + (values list (shiftf (cdr list) nil)) + (let ((half (ash size -1))) + (multiple-value-bind (list1 rest) + (recur list half) + (multiple-value-bind (list2 rest) + (recur rest (- size half)) + (values (merge-lists* head list1 list2 test key) + rest))))))) + (when list + (values (recur list (length list)))))) ;;;; stable sort of vectors @@ -348,11 +311,7 @@ ;; FIXME: This implementation is remarkably inefficient in various ;; ways. In decreasing order of estimated user astonishment, I note: ;; full calls to SPECIFIER-TYPE at runtime; copying input vectors - ;; to lists before doing MERGE-LISTS*; and walking input lists - ;; (because of the call to MERGE-LISTS*, which walks the list to - ;; find the last element for its second return value) even in cases - ;; like (MERGE 'LIST (LIST 1) (LIST 2 3 4 5 ... 1000)) where one list - ;; can be largely ignored. -- WHN 2003-01-05 + ;; to lists before doing MERGE-LISTS -- WHN 2003-01-05 (let ((type (specifier-type result-type))) (cond ((csubtypep type (specifier-type 'list)) @@ -367,7 +326,7 @@ (%coerce-callable-to-fun key) #'identity))) (when (type= type (specifier-type 'list)) - (return-from merge (values (merge-lists* s1 s2 pred-fun key-fun)))) + (return-from merge (merge-lists s1 s2 pred-fun key-fun))) (when (eq type *empty-type*) (bad-sequence-type-error nil)) (when (type= type (specifier-type 'null)) @@ -387,7 +346,7 @@ (sequence-type-length-mismatch-error type length)) (unless (>= length min) (sequence-type-length-mismatch-error type length))) - (values (merge-lists* s1 s2 pred-fun key-fun)))) + (merge-lists s1 s2 pred-fun key-fun))) (sequence-type-too-hairy result-type)))) ((csubtypep type (specifier-type 'vector)) (let* ((vector-1 (coerce sequence1 'vector)) diff --git a/tests/dynamic-extent.impure.lisp b/tests/dynamic-extent.impure.lisp index ff26c1d..4eebf0f 100644 --- a/tests/dynamic-extent.impure.lisp +++ b/tests/dynamic-extent.impure.lisp @@ -876,7 +876,9 @@ nil))) (assert-notes 0 `(lambda (list) (declare (optimize (space 0))) - (sort list #'<))) + (sort list (lambda (x y) ; shut unrelated notes up + (< (truly-the fixnum x) + (truly-the fixnum y)))))) (assert-notes 0 `(lambda (other) #'(lambda (s c n) (ignore-errors (funcall other s c n))))))) diff --git a/tests/seq.pure.lisp b/tests/seq.pure.lisp index ded374f..ab1c8ba 100644 --- a/tests/seq.pure.lisp +++ b/tests/seq.pure.lisp @@ -303,3 +303,36 @@ (funcall (lambda () (declare (optimize speed)) (search #() #(1 1) :from-end t)))))) + +(with-test (:name :sort-smoke-test) + (flet ((iota (n type &aux (i 0)) + (map-into (make-sequence type n) + (lambda () + (incf i)))) + (shuffle (n type) + (let ((vector (let ((i 0)) + (map-into (make-array n) + (lambda () + (incf i)))))) + (dotimes (i n (coerce vector type)) + (let ((j (+ i (random (- n i))))) + (rotatef (aref vector i) (aref vector j)))))) + (sortedp (x) + (let* ((nonce (list nil)) + (prev nonce)) + (every (lambda (x) + (prog1 (or (eql prev nonce) + (< prev x)) + (setf prev x))) + x)))) + (dolist (type '(simple-vector list)) + (dolist (size '(7 8 9 13 1023 1024 1025 1536)) + (loop for repeat below 5 do + (assert (sortedp + (sort (funcall (case repeat + (0 #'iota) + (1 (lambda (n type) + (reverse (iota n type)))) + (t #'shuffle)) + size type) + #'<))))))))