Cycles: OpenCL kernel split

This commit contains all the work related on the AMD megakernel split work
which was mainly done by Varun Sundar, George Kyriazis and Lenny Wang, plus
some help from Sergey Sharybin, Martijn Berger, Thomas Dinges and likely
someone else which we're forgetting to mention.

Currently only AMD cards are enabled for the new split kernel, but it is
possible to force split opencl kernel to be used by setting the following
environment variable: CYCLES_OPENCL_SPLIT_KERNEL_TEST=1.

Not all the features are supported yet, and that being said no motion blur,
camera blur, SSS and volumetrics for now. Also transparent shadows are
disabled on AMD device because of some compiler bug.

This kernel is also only implements regular path tracing and supporting
branched one will take a bit. Branched path tracing is exposed to the
interface still, which is a bit misleading and will be hidden there soon.

More feature will be enabled once they're ported to the split kernel and
tested.

Neither regular CPU nor CUDA has any difference, they're generating the
same exact code, which means no regressions/improvements there.

Based on the research paper:

  https://research.nvidia.com/sites/default/files/publications/laine2013hpg_paper.pdf

Here's the documentation:

  https://docs.google.com/document/d/1LuXW-CV-sVJkQaEGZlMJ86jZ8FmoPfecaMdR-oiWbUY/edit

Design discussion of the patch:

  https://developer.blender.org/T44197

Differential Revision: https://developer.blender.org/D1200

1 files changed, 193 insertions, 0 deletions

diff --git a/intern/cycles/kernel/kernel_work_stealing.h b/intern/cycles/kernel/kernel_work_stealing.h
new file mode 100644
index 00000000000..9b83d972e97
--- /dev/null
+++ b/intern/cycles/kernel/kernel_work_stealing.h
@@ -0,0 +1,193 @@
+/*
+ * Copyright 2011-2015 Blender Foundation
+ *
+ * Licensed 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.
+ */
+
+#ifndef __KERNEL_WORK_STEALING_H__
+#define __KERNEL_WORK_STEALING_H__
+
+/*
+ * Utility functions for work stealing
+ */
+
+#ifdef __WORK_STEALING__
+
+#ifdef __KERNEL_OPENCL__
+#pragma OPENCL EXTENSION cl_khr_global_int32_base_atomics : enable
+#endif
+
+uint get_group_id_with_ray_index(uint ray_index,
+                                 uint tile_dim_x,
+                                 uint tile_dim_y,
+                                 uint parallel_samples,
+                                 int dim)
+{
+	if(dim == 0) {
+		uint x_span = ray_index % (tile_dim_x * parallel_samples);
+		return x_span / get_local_size(0);
+	}
+	else /*if(dim == 1)*/ {
+		kernel_assert(dim == 1);
+		uint y_span = ray_index / (tile_dim_x * parallel_samples);
+		return y_span / get_local_size(1);
+	}
+}
+
+uint get_total_work(uint tile_dim_x,
+                    uint tile_dim_y,
+                    uint grp_idx,
+                    uint grp_idy,
+                    uint num_samples)
+{
+	uint threads_within_tile_border_x =
+		(grp_idx == (get_num_groups(0) - 1)) ? tile_dim_x % get_local_size(0)
+		                                     : get_local_size(0);
+	uint threads_within_tile_border_y =
+		(grp_idy == (get_num_groups(1) - 1)) ? tile_dim_y % get_local_size(1)
+		                                     : get_local_size(1);
+
+	threads_within_tile_border_x =
+		(threads_within_tile_border_x == 0) ? get_local_size(0)
+		                                    : threads_within_tile_border_x;
+	threads_within_tile_border_y =
+		(threads_within_tile_border_y == 0) ? get_local_size(1)
+		                                    : threads_within_tile_border_y;
+
+	return threads_within_tile_border_x *
+	       threads_within_tile_border_y *
+	       num_samples;
+}
+
+/* Returns 0 in case there is no next work available */
+/* Returns 1 in case work assigned is valid */
+int get_next_work(ccl_global uint *work_pool,
+                  ccl_private uint *my_work,
+                  uint tile_dim_x,
+                  uint tile_dim_y,
+                  uint num_samples,
+                  uint parallel_samples,
+                  uint ray_index)
+{
+	uint grp_idx = get_group_id_with_ray_index(ray_index,
+	                                           tile_dim_x,
+	                                           tile_dim_y,
+	                                           parallel_samples,
+	                                           0);
+	uint grp_idy = get_group_id_with_ray_index(ray_index,
+	                                           tile_dim_x,
+	                                           tile_dim_y,
+	                                           parallel_samples,
+	                                           1);
+	uint total_work = get_total_work(tile_dim_x,
+	                                 tile_dim_y,
+	                                 grp_idx,
+	                                 grp_idy,
+	                                 num_samples);
+	uint group_index = grp_idy * get_num_groups(0) + grp_idx;
+	*my_work = atomic_inc(&work_pool[group_index]);
+	return (*my_work < total_work) ? 1 : 0;
+}
+
+/* This function assumes that the passed my_work is valid. */
+/* Decode sample number w.r.t. assigned my_work. */
+uint get_my_sample(uint my_work,
+                   uint tile_dim_x,
+                   uint tile_dim_y,
+                   uint parallel_samples,
+                   uint ray_index)
+{
+	uint grp_idx = get_group_id_with_ray_index(ray_index,
+	                                           tile_dim_x,
+	                                           tile_dim_y,
+	                                           parallel_samples,
+	                                           0);
+	uint grp_idy = get_group_id_with_ray_index(ray_index,
+	                                           tile_dim_x,
+	                                           tile_dim_y,
+	                                           parallel_samples,
+	                                           1);
+	uint threads_within_tile_border_x =
+		(grp_idx == (get_num_groups(0) - 1)) ? tile_dim_x % get_local_size(0)
+		                                     : get_local_size(0);
+	uint threads_within_tile_border_y =
+		(grp_idy == (get_num_groups(1) - 1)) ? tile_dim_y % get_local_size(1)
+		                                     : get_local_size(1);
+
+	threads_within_tile_border_x =
+		(threads_within_tile_border_x == 0) ? get_local_size(0)
+		                                    : threads_within_tile_border_x;
+	threads_within_tile_border_y =
+		(threads_within_tile_border_y == 0) ? get_local_size(1)
+		                                    : threads_within_tile_border_y;
+
+	return my_work /
+	       (threads_within_tile_border_x * threads_within_tile_border_y);
+}
+
+/* Decode pixel and tile position w.r.t. assigned my_work. */
+void get_pixel_tile_position(ccl_private uint *pixel_x,
+                             ccl_private uint *pixel_y,
+                             ccl_private uint *tile_x,
+                             ccl_private uint *tile_y,
+                             uint my_work,
+                             uint tile_dim_x,
+                             uint tile_dim_y,
+                             uint tile_offset_x,
+                             uint tile_offset_y,
+                             uint parallel_samples,
+                             uint ray_index)
+{
+	uint grp_idx = get_group_id_with_ray_index(ray_index,
+	                                           tile_dim_x,
+	                                           tile_dim_y,
+	                                           parallel_samples,
+	                                           0);
+	uint grp_idy = get_group_id_with_ray_index(ray_index,
+	                                           tile_dim_x,
+	                                           tile_dim_y,
+	                                           parallel_samples,
+	                                           1);
+	uint threads_within_tile_border_x =
+		(grp_idx == (get_num_groups(0) - 1)) ? tile_dim_x % get_local_size(0)
+		                                     : get_local_size(0);
+	uint threads_within_tile_border_y =
+		(grp_idy == (get_num_groups(1) - 1)) ? tile_dim_y % get_local_size(1)
+		                                     : get_local_size(1);
+
+	threads_within_tile_border_x =
+		(threads_within_tile_border_x == 0) ? get_local_size(0)
+		                                    : threads_within_tile_border_x;
+	threads_within_tile_border_y =
+		(threads_within_tile_border_y == 0) ? get_local_size(1)
+		                                    : threads_within_tile_border_y;
+
+	uint total_associated_pixels =
+		threads_within_tile_border_x * threads_within_tile_border_y;
+	uint work_group_pixel_index = my_work % total_associated_pixels;
+	uint work_group_pixel_x =
+		work_group_pixel_index % threads_within_tile_border_x;
+	uint work_group_pixel_y =
+		work_group_pixel_index / threads_within_tile_border_x;
+
+	*pixel_x =
+		tile_offset_x + (grp_idx * get_local_size(0)) + work_group_pixel_x;
+	*pixel_y =
+		tile_offset_y + (grp_idy * get_local_size(1)) + work_group_pixel_y;
+	*tile_x = *pixel_x - tile_offset_x;
+	*tile_y = *pixel_y - tile_offset_y;
+}
+
+#endif  /* __WORK_STEALING__ */
+
+#endif  /* __KERNEL_WORK_STEALING_H__ */