Making sure for_loop(par, 0, N, ...) is actually executed in parallel

hpx/parallel/algorithms/for_loop.hpp

@@ -232,9 +232,13 @@ namespace hpx { namespace parallel { HPX_INLINE_NAMESPACE(v2)
                     hpx::traits::is_bidirectional_iterator<E>::value);
             }
 
+            // the for_loop should be executed sequentially either if the
+            // execution policy enforces sequential execution of if the
+            // loop boundaries are input or output iterators
             typedef std::integral_constant<bool,
                     is_sequential_execution_policy<ExPolicy>::value ||
-                   !hpx::traits::is_forward_iterator<B>::value
+                    (!std::is_integral<B>::value &&
+                     !hpx::traits::is_forward_iterator<B>::value)
                 > is_seq;
 
             std::size_t size = parallel::v1::detail::distance(first, last);

tests/regressions/parallel/CMakeLists.txt

@@ -1,9 +1,10 @@
-# Copyright (c) 2014-2015 Hartmut Kaiser
+# Copyright (c) 2014-2016 Hartmut Kaiser
 #
 # Distributed under the Boost Software License, Version 1.0. (See accompanying
 # file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
 set(tests
+    for_loop_2281
     minimal_findend
     scan_different_inits
     scan_non_commutative

tests/regressions/parallel/for_loop_2281.cpp

@@ -0,0 +1,45 @@
+//  Copyright (c) 2016 Hartmut Kaiser
+//
+//  Distributed under the Boost Software License, Version 1.0. (See accompanying
+//  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <hpx/hpx_init.hpp>
+#include <hpx/hpx.hpp>
+#include <hpx/include/parallel_for_loop.hpp>
+#include <hpx/util/lightweight_test.hpp>
+
+#include <mutex>
+#include <set>
+#include <string>
+#include <vector>
+
+///////////////////////////////////////////////////////////////////////////////
+int hpx_main()
+{
+    hpx::lcos::local::spinlock mtx;
+    std::set<hpx::thread::id> thread_ids;
+
+    hpx::parallel::for_loop(
+        hpx::parallel::par, 0, 100,
+        [&](int i)
+        {
+            std::lock_guard<hpx::lcos::local::spinlock> l(mtx);
+            thread_ids.insert(hpx::this_thread::get_id());
+        });
+
+    HPX_TEST(thread_ids.size() > std::size_t(1));
+
+    return hpx::finalize();
+}
+
+int main(int argc, char* argv[])
+{
+    std::vector<std::string> const cfg = {
+        "hpx.os_threads=4"
+    };
+
+    HPX_TEST_EQ_MSG(hpx::init(argc, argv, cfg), 0,
+        "HPX main exited with non-zero status");
+
+    return hpx::util::report_errors();
+}

tests/regressions/parallel/minimal_findend.cpp

@@ -101,7 +101,7 @@ int main(int argc, char* argv[])
     };
 
     HPX_TEST_EQ_MSG(hpx::init(argc, argv, cfg), 0,
-        "HPX main exted with non-zero status");
+        "HPX main exited with non-zero status");
 
     return hpx::util::report_errors();
 }