[PATCH] Fix schedule(guided)

[PATCH] Fix schedule(guided)

[Jakub Jelinek]

Hi!

The attached testcase loops forever.  The problem is that during the last
iteration, ws->next and *pend is set to
ws->next + (ws->end - ws->next) / ws->incr * ws->incr
rather than
ws->end.  This is last chunk, the only chunk with < chunksize iterations.
The termination condition where gomp_iter_guided_next* returns false is
ws->next == ws->end, so if (end - start) % incr != 0, then after the last
real chunk each following gomp_iter_guided_next* will return true and an
empty range.

Fixed thusly, regtested on x86_64-linux.  Trunk/4.3.

2008-03-18  Jakub Jelinek  <jakub@redhat.com>

	PR libgomp/35625
	* iter.c (gomp_iter_guided_next_locked): If q > n, set end to ws->end.
	(gomp_iter_guided_next): Likewise.
	* testsuite/libgomp.c/pr35625.c: New test.

--- libgomp/iter.c.jj	2008-02-18 23:42:14.000000000 +0100
+++ libgomp/iter.c	2008-03-18 10:26:32.000000000 +0100
@@ -242,16 +242,16 @@ gomp_iter_guided_next_locked (long *psta
   if (ws->next == ws->end)
     return false;
 
-  n = (ws->end - ws->next) / ws->incr;
+  start = ws->next;
+  n = (ws->end - start) / ws->incr;
   q = (n + nthreads - 1) / nthreads;
 
   if (q < ws->chunk_size)
     q = ws->chunk_size;
-  if (q > n)
-    q = n;
-
-  start = ws->next;
-  end = start + q * ws->incr;
+  if (q <= n)
+    end = start + q * ws->incr;
+  else
+    end = ws->end;
 
   ws->next = end;
   *pstart = start;
@@ -286,15 +286,15 @@ gomp_iter_guided_next (long *pstart, lon
       if (start == end)
 	return false;
 
-      n = (end - start) / ws->incr;
+      n = (end - start) / incr;
       q = (n + nthreads - 1) / nthreads;
 
       if (q < chunk_size)
 	q = chunk_size;
-      if (q > n)
-	q = n;
-
-      nend = start + q * incr;
+      if (__builtin_expect (q <= n, 1))
+	nend = start + q * incr;
+      else
+	nend = end;
 
       tmp = __sync_val_compare_and_swap (&ws->next, start, nend);
       if (__builtin_expect (tmp == start, 1))
--- libgomp/testsuite/libgomp.c/pr35625.c.jj	2008-03-18 10:13:09.000000000 +0100
+++ libgomp/testsuite/libgomp.c/pr35625.c	2008-03-18 10:31:58.000000000 +0100
@@ -0,0 +1,18 @@
+/* PR libgomp/35625 */
+/* { dg-do run } */
+/* { dg-options "-std=c99" } */
+
+int
+main (void)
+{
+#pragma omp parallel
+  {
+    #pragma omp for schedule (guided, 10)
+      for (int i = 0; i < 1826; i += 10)
+	;
+    #pragma omp for schedule (guided, 10)
+      for (int i = 0; i > -1826; i -= 10)
+	;
+  }
+  return 0;
+}

	Jakub