From 95bd2f0873bb00ed2a9a21ede4d3220c28829a7d Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <campbell@mumble.net>
Date: Thu, 13 Nov 2014 22:59:36 +0000
Subject: [PATCH] Add thread barrier abstraction.

---
 src/runtime/runtime.pkg        |  8 ++++
 src/runtime/thread-barrier.scm | 69 ++++++++++++++++++++++++++++++++++
 2 files changed, 77 insertions(+)
 create mode 100644 src/runtime/thread-barrier.scm

diff --git a/src/runtime/runtime.pkg b/src/runtime/runtime.pkg
index 715eef596..98107c5ac 100644
--- a/src/runtime/runtime.pkg
+++ b/src/runtime/runtime.pkg
@@ -5058,6 +5058,14 @@ USA.
 	  thread-float-environment)
   (initialization (initialize-package!)))
 
+(define-package (runtime thread barrier)
+  (files "thread-barrier")
+  (parent (runtime thread))
+  (export ()
+	  make-thread-barrier
+	  thread-barrier-wait
+	  thread-barrier?))
+
 (define-package (runtime thread condition-variable)
   (files "condvar")
   (parent (runtime thread))
diff --git a/src/runtime/thread-barrier.scm b/src/runtime/thread-barrier.scm
new file mode 100644
index 000000000..48a75d92a
--- /dev/null
+++ b/src/runtime/thread-barrier.scm
@@ -0,0 +1,69 @@
+#| -*-Scheme-*-
+
+Copyright (C) 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994,
+    1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
+    2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014 Massachusetts
+    Institute of Technology
+
+This file is part of MIT/GNU Scheme.
+
+MIT/GNU Scheme is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or (at
+your option) any later version.
+
+MIT/GNU Scheme is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with MIT/GNU Scheme; if not, write to the Free Software
+Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301,
+USA.
+
+|#
+
+;;;; Thread barriers
+;;; package: (runtime thread barrier)
+
+(declare (usual-integrations))
+
+(define-structure (thread-barrier
+		   (constructor %make-thread-barrier (count current))
+		   (conc-name thread-barrier.))
+  (lock (make-thread-mutex) read-only #t)
+  (condvar (make-condition-variable) read-only #t)
+  (count #f read-only #t)
+  current
+  (generation 0))
+
+(define-guarantee thread-barrier "thread barrier")
+
+(define (make-thread-barrier count)
+  (guarantee-exact-positive-integer count 'MAKE-THREAD-BARRIER)
+  (%make-thread-barrier count count))
+
+(define (thread-barrier-wait barrier)
+  (guarantee-thread-barrier barrier 'THREAD-BARRIER-WAIT)
+  (let ((lock (thread-barrier.lock barrier))
+	(condvar (thread-barrier.condvar barrier)))
+    (with-thread-mutex-locked lock
+      (lambda ()
+	(let ((count (thread-barrier.count barrier))
+	      (current (thread-barrier.current barrier))
+	      (generation (thread-barrier.generation barrier)))
+	  (assert (< 0 current))
+	  (assert (<= current count))
+	  (let ((next (- current 1)))
+	    (if (zero? next)
+		(begin
+		  (set-thread-barrier.current! barrier count)
+		  (set-thread-barrier.generation! barrier (+ 1 generation))
+		  (condition-variable-broadcast! condvar)
+		  #t)
+		(begin
+		  (set-thread-barrier.current! barrier next)
+		  (do () ((< generation (thread-barrier.generation barrier)))
+		    (condition-variable-wait! condvar lock))
+		  #f))))))))
\ No newline at end of file
-- 
2.25.1