BLI_task: Add new 'BLI_task_parallel_range_finalize()'.

Together with the extended loop callback and userdata_chunk, this allows to perform
cumulative tasks (like aggregation) in a lockfree way using local userdata_chunk to store temp data,
and once all workers have finished, to merge those userdata_chunks in the finalize callback
(from calling thread, so no need to lock here either).

Note that this changes how userdata_chunk is handled (now fully from 'main' thread,
which means a given worker thread will always get the same userdata_chunk, without
being re-initialized anymore to init value at start of each iter chunk).

2 files changed, 77 insertions, 22 deletions

diff --git a/source/blender/blenlib/BLI_task.h b/source/blender/blenlib/BLI_task.h
index c511ec432ee..967e0be6d0a 100644
--- a/source/blender/blenlib/BLI_task.h
+++ b/source/blender/blenlib/BLI_task.h
@@ -119,11 +119,13 @@ size_t BLI_task_pool_tasks_done(TaskPool *pool);
 /* Parallel for routines */
 typedef void (*TaskParallelRangeFunc)(void *userdata, const int iter);
 typedef void (*TaskParallelRangeFuncEx)(void *userdata, void *userdata_chunk, const int iter, const int thread_id);
+typedef void (*TaskParallelRangeFuncFinalize)(void *userdata, void *userdata_chunk);
 void BLI_task_parallel_range_ex(
         int start, int stop,
         void *userdata,
         void *userdata_chunk,
-        const size_t userdata_chunk_size, TaskParallelRangeFuncEx func_ex,
+        const size_t userdata_chunk_size,
+        TaskParallelRangeFuncEx func_ex,
         const bool use_threading,
         const bool use_dynamic_scheduling);
 void BLI_task_parallel_range(
@@ -132,6 +134,16 @@ void BLI_task_parallel_range(
         TaskParallelRangeFunc func,
         const bool use_threading);
 
+void BLI_task_parallel_range_finalize(
+        int start, int stop,
+        void *userdata,
+        void *userdata_chunk,
+        const size_t userdata_chunk_size,
+        TaskParallelRangeFuncEx func_ex,
+        TaskParallelRangeFuncFinalize func_finalize,
+        const bool use_threading,
+        const bool use_dynamic_scheduling);
+
 typedef void (*TaskParallelListbaseFunc)(void *userdata,
                                          struct Link *iter,
                                          int index);
diff --git a/source/blender/blenlib/intern/task.c b/source/blender/blenlib/intern/task.c
index a34d9e97fd5..4ef42dcce6f 100644
--- a/source/blender/blenlib/intern/task.c
+++ b/source/blender/blenlib/intern/task.c
@@ -768,8 +768,6 @@ size_t BLI_task_pool_tasks_done(TaskPool *pool)
 typedef struct ParallelRangeState {
 	int start, stop;
 	void *userdata;
-	void *userdata_chunk;
-	size_t userdata_chunk_size;
 
 	TaskParallelRangeFunc func;
 	TaskParallelRangeFuncEx func_ex;
@@ -792,24 +790,16 @@ BLI_INLINE bool parallel_range_next_iter_get(
 
 static void parallel_range_func(
         TaskPool * __restrict pool,
-        void *UNUSED(taskdata),
+        void *userdata_chunk,
         int threadid)
 {
 	ParallelRangeState * __restrict state = BLI_task_pool_userdata(pool);
 	int iter, count;
 
-	const bool use_userdata_chunk = (state->func_ex != NULL) &&
-	                                (state->userdata_chunk_size != 0) && (state->userdata_chunk != NULL);
-	void *userdata_chunk = use_userdata_chunk ? MALLOCA(state->userdata_chunk_size) : NULL;
-
 	while (parallel_range_next_iter_get(state, &iter, &count)) {
 		int i;
 
 		if (state->func_ex) {
-			if (use_userdata_chunk) {
-				memcpy(userdata_chunk, state->userdata_chunk, state->userdata_chunk_size);
-			}
-
 			for (i = 0; i < count; ++i) {
 				state->func_ex(state->userdata, userdata_chunk, iter + i, threadid);
 			}
@@ -820,8 +810,6 @@ static void parallel_range_func(
 			}
 		}
 	}
-
-	MALLOCA_FREE(userdata_chunk, state->userdata_chunk_size);
 }
 
 /**
@@ -836,6 +824,7 @@ static void task_parallel_range_ex(
         const size_t userdata_chunk_size,
         TaskParallelRangeFunc func,
         TaskParallelRangeFuncEx func_ex,
+        TaskParallelRangeFuncFinalize func_finalize,
         const bool use_threading,
         const bool use_dynamic_scheduling)
 {
@@ -844,6 +833,10 @@ static void task_parallel_range_ex(
 	ParallelRangeState state;
 	int i, num_threads, num_tasks;
 
+	void *userdata_chunk_local = NULL;
+	void *userdata_chunk_array = NULL;
+	const bool use_userdata_chunk = (func_ex != NULL) && (userdata_chunk_size != 0) && (userdata_chunk != NULL);
+
 	if (start == stop) {
 		return;
 	}
@@ -859,9 +852,6 @@ static void task_parallel_range_ex(
 	 */
 	if (!use_threading) {
 		if (func_ex) {
-			const bool use_userdata_chunk = (userdata_chunk_size != 0) && (userdata_chunk != NULL);
-			void *userdata_chunk_local = NULL;
-
 			if (use_userdata_chunk) {
 				userdata_chunk_local = MALLOCA(userdata_chunk_size);
 				memcpy(userdata_chunk_local, userdata_chunk, userdata_chunk_size);
@@ -871,6 +861,10 @@ static void task_parallel_range_ex(
 				func_ex(userdata, userdata_chunk, i, 0);
 			}
 
+			if (func_finalize) {
+				func_finalize(userdata, userdata_chunk);
+			}
+
 			MALLOCA_FREE(userdata_chunk_local, userdata_chunk_size);
 		}
 		else {
@@ -895,8 +889,6 @@ static void task_parallel_range_ex(
 	state.start = start;
 	state.stop = stop;
 	state.userdata = userdata;
-	state.userdata_chunk = userdata_chunk;
-	state.userdata_chunk_size = userdata_chunk_size;
 	state.func = func;
 	state.func_ex = func_ex;
 	state.iter = start;
@@ -910,15 +902,34 @@ static void task_parallel_range_ex(
 	num_tasks = min_ii(num_tasks, (stop - start) / state.chunk_size);
 	atomic_fetch_and_add_uint32((uint32_t *)(&state.iter), 0);
 
+	if (use_userdata_chunk) {
+        userdata_chunk_array = MALLOCA(userdata_chunk_size * num_tasks);
+	}
+
 	for (i = 0; i < num_tasks; i++) {
+		if (use_userdata_chunk) {
+			userdata_chunk_local = (char *)userdata_chunk_array + (userdata_chunk_size * i);
+			memcpy(userdata_chunk_local, userdata_chunk, userdata_chunk_size);
+		}
+		/* Use this pool's pre-allocated tasks. */
 		BLI_task_pool_push_from_thread(task_pool,
 		                               parallel_range_func,
-		                               NULL, false,
+		                               userdata_chunk_local, false,
 		                               TASK_PRIORITY_HIGH, 0);
 	}
 
 	BLI_task_pool_work_and_wait(task_pool);
 	BLI_task_pool_free(task_pool);
+
+	if (use_userdata_chunk) {
+		if (func_finalize) {
+			for (i = 0; i < num_tasks; i++) {
+				userdata_chunk_local = (char *)userdata_chunk_array + (userdata_chunk_size * i);
+				func_finalize(userdata, userdata_chunk_local);
+			}
+		}
+        MALLOCA_FREE(userdata_chunk_array, userdata_chunk_size * num_tasks);
+	}
 }
 
 /**
@@ -946,7 +957,7 @@ void BLI_task_parallel_range_ex(
         const bool use_dynamic_scheduling)
 {
 	task_parallel_range_ex(
-	            start, stop, userdata, userdata_chunk, userdata_chunk_size, NULL, func_ex,
+	            start, stop, userdata, userdata_chunk, userdata_chunk_size, NULL, func_ex, NULL,
 	            use_threading, use_dynamic_scheduling);
 }
 
@@ -967,7 +978,39 @@ void BLI_task_parallel_range(
         TaskParallelRangeFunc func,
         const bool use_threading)
 {
-	task_parallel_range_ex(start, stop, userdata, NULL, 0, func, NULL, use_threading, false);
+	task_parallel_range_ex(start, stop, userdata, NULL, 0, func, NULL, NULL, use_threading, false);
+}
+
+/**
+ * This function allows to parallelize for loops in a similar way to OpenMP's 'parallel for' statement,
+ * with an additional 'finalize' func called from calling thread once whole range have been processed.
+ *
+ * \param start First index to process.
+ * \param stop Index to stop looping (excluded).
+ * \param userdata Common userdata passed to all instances of \a func.
+ * \param userdata_chunk Optional, each instance of looping chunks will get a copy of this data
+ *                       (similar to OpenMP's firstprivate).
+ * \param userdata_chunk_size Memory size of \a userdata_chunk.
+ * \param func_ex Callback function (advanced version).
+ * \param func_finalize Callback function, called after all workers have finisehd, useful to finalize accumulative tasks.
+ * \param use_threading If \a true, actually split-execute loop in threads, else just do a sequential forloop
+ *                      (allows caller to use any kind of test to switch on parallelization or not).
+ * \param use_dynamic_scheduling If \a true, the whole range is divided in a lot of small chunks (of size 32 currently),
+ *                               otherwise whole range is split in a few big chunks (num_threads * 2 chunks currently).
+ */
+void BLI_task_parallel_range_finalize(
+        int start, int stop,
+        void *userdata,
+        void *userdata_chunk,
+        const size_t userdata_chunk_size,
+        TaskParallelRangeFuncEx func_ex,
+        TaskParallelRangeFuncFinalize func_finalize,
+        const bool use_threading,
+        const bool use_dynamic_scheduling)
+{
+	task_parallel_range_ex(
+	            start, stop, userdata, userdata_chunk, userdata_chunk_size, NULL, func_ex, func_finalize,
+	            use_threading, use_dynamic_scheduling);
 }
 
 #undef MALLOCA