[Support] Add parallel_for support to run a loop in parallel

src/support/parallel_for.cc

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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/*!
+ * \file parallel_for.cc
+ * \brief An implementation to run loop in parallel.
+ */
+#include "parallel_for.h"
+
+#include <dmlc/logging.h>
+
+#include <future>
+#include <thread>
+#include <utility>
+#include <vector>
+
+namespace tvm {
+namespace support {
+
+std::vector<std::vector<int>> rr_partitioner(int begin, int end, int step, int num_threads) {
+  int total_task_count = (end - begin) / step;
+  CHECK_GT(total_task_count, 0) << "Infinite loop condition, check the input value of "
+                                << "`begin`, `end`, `step`.";
+
+  std::vector<std::vector<int>> ret(num_threads);
+  for (int thread = 0; begin < end; begin += step, thread = (thread + 1) % num_threads) {
+    ret[thread].push_back(begin);
+  }
+  return ret;
+}
+
+void parallel_for(int begin, int end, const std::function<void(int)>& f, int step,
+                  const PartitionerFuncType partitioner) {
+  int default_num_threads = std::thread::hardware_concurrency();
+  const auto& run_partitions = partitioner(begin, end, step, default_num_threads);
+
+  std::vector<std::thread> threads;
+  std::vector<std::future<void>> res_vec;
+  for (const auto& run_partition : run_partitions) {
+    if (!run_partition.empty()) {
+      std::packaged_task<void(const std::vector<int>&, const std::function<void(int)>&)> task(
+          [](const std::vector<int>& run_pattition, const std::function<void(int)>& f) {
+            for (const auto& i : run_pattition) {
+              f(i);
+            }
+          });
+      res_vec.emplace_back(task.get_future());
+      threads.emplace_back(std::move(task), run_partition, f);
+    }
+  }
+
+  for (auto& thread : threads) {
+    thread.join();
+  }
+  try {
+    for (auto& i : res_vec) {
+      i.get();
+    }
+  } catch (const std::exception& e) {
+    LOG(FATAL) << "Parallel_for error with " << e.what();
+  }
+}
+
+}  // namespace support
+}  // namespace tvm

src/support/parallel_for.h

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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/*!
+ * \file parallel_for.h
+ * \brief An implementation to run loop in parallel.
+ */
+#ifndef TVM_SUPPORT_PARALLEL_FOR_H_
+#define TVM_SUPPORT_PARALLEL_FOR_H_
+
+#include <tvm/runtime/c_runtime_api.h>
+
+#include <functional>
+#include <vector>
+
+namespace tvm {
+namespace support {
+
+using PartitionerFuncType = std::function<std::vector<std::vector<int>>(int, int, int, int)>;
+
+/*!
+ * \brief A partitioner to split the task to each thread in Round-robin manner.
+ * \param begin The start index of this parallel loop(inclusive).
+ * \param end The end index of this parallel loop(exclusive).
+ * \param step The traversal step to the index.
+ * \param num_threads The number of threads(the number of tasks to be partitioned to).
+ * \return A list with `num_threads` elements, and each is a list of integers indicating the loop
+ * indexes for the corresponding thread to process.
+ */
+std::vector<std::vector<int>> rr_partitioner(int begin, int end, int step, int num_threads);
+
+/*!
+ * \brief A runtime api provided to run the task function in parallel.
+ * e.g. A for loop:
+ *   for (int i = 0; i < 10; i++) {
+ *     std::cout << index << "\n";
+ *   }
+ * should work the same as:
+ *   parallel_for(0, 10, [](int index) {
+ *     std::cout << index << "\n";
+ *   });
+ * \param begin The start index of this parallel loop(inclusive).
+ * \param end The end index of this parallel loop(exclusive).
+ * \param f The task function to be excuted. Assert to take an int index as input with no output.
+ * \param step The traversal step to the index.
+ * \param partitioner A partition function to split tasks to different threads. Use Round-robin
+ * partitioner in default.
+ * \note Currently do not support nested parallel_for.
+ */
+TVM_DLL void parallel_for(int begin, int end, const std::function<void(int)>& f, int step = 1,
+                          const PartitionerFuncType partitioner = rr_partitioner);
+
+}  // namespace support
+}  // namespace tvm
+
+#endif  // TVM_SUPPORT_PARALLEL_FOR_H_

tests/cpp/parallel_for_test.cc

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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+#include "../../src/support/parallel_for.h"
+
+#include <dmlc/logging.h>
+#include <gtest/gtest.h>
+
+#include <vector>
+
+TEST(ParallelFor, Basic) {
+  using tvm::support::parallel_for;
+
+  int a[100], b[100];
+
+  // Default
+  for (int i = 0; i < 100; i++) {
+    a[i] = i;
+  }
+
+  parallel_for(0, 100, [&b](int i) { b[i] = i; });
+
+  for (int i = 0; i < 100; i++) {
+    CHECK_EQ(a[i], b[i]);
+  }
+
+  // Check for step != 1
+  for (int i = 0; i < 100; i += 2) {
+    a[i] *= 2;
+  }
+
+  parallel_for(
+      0, 100, [&b](int i) { b[i] *= 2; }, 2);
+
+  for (int i = 0; i < 100; i++) {
+    CHECK_EQ(a[i], b[i]);
+  }
+}
+
+TEST(ParallelFor, Nested) {
+  using tvm::support::parallel_for;
+
+  int a[100][100], b[100][100], c[100][100];
+
+  for (int i = 0; i < 100; i++) {
+    for (int j = 0; j < 100; j++) {
+      a[i][j] = i * j;
+    }
+  }
+
+  parallel_for(0, 100, [&b](int i) {
+    for (int j = 0; j < 100; j++) {
+      b[i][j] = i * j;
+    }
+  });
+
+  for (int i = 0; i < 100; i++) {
+    for (int j = 0; j < 100; j++) {
+      CHECK_EQ(a[i][j], b[i][j]);
+    }
+  }
+
+  for (int i = 0; i < 100; i++) {
+    parallel_for(0, 100, [&c, &i](int j) { c[i][j] = i * j; });
+  }
+
+  for (int i = 0; i < 100; i++) {
+    for (int j = 0; j < 100; j++) {
+      CHECK_EQ(a[i][j], c[i][j]);
+    }
+  }
+}
+
+TEST(ParallelFor, Exception) {
+  using tvm::support::parallel_for;
+
+  bool exception = false;
+  try {
+    parallel_for(0, 100, [](int i) { LOG(FATAL) << "error"; });
+  } catch (const std::exception& e) {
+    exception = true;
+  }
+  CHECK(exception);
+}
+
+int main(int argc, char** argv) {
+  testing::InitGoogleTest(&argc, argv);
+  testing::FLAGS_gtest_death_test_style = "threadsafe";
+  return RUN_ALL_TESTS();
+}