Cycles: Faster split branched path tracing by sharing samples with inactive threads

Unlike regular path tracing, branched path tracing is usually used with lower
sample counts, at least for primary rays. This means that are less samples for
the GPU to work on in parallel and rendering is slower. As there is less work
overall there is also more inactive threads during rendering with BPT. This
patch makes use of those inactive rays to render branched samples in parallel
with other samples.

Each thread that is preparing for a branched sample will attempt to find an
inactive thread and if one is found the state for the sample is copied to that
thread. Potentially, if there are enough inactive threads, 100s of branched
samples could be generated from the same originating thread and ran in
parallel giving large speed ups.

Gives 70% faster render for pavillion midday scene. 20-60% faster on BMW
with car paint replaced with SSS/volumes.

1 files changed, 51 insertions, 1 deletions

diff --git a/intern/cycles/kernel/split/kernel_branched.h b/intern/cycles/kernel/split/kernel_branched.h
index dc74a2ada53..e2762a85fc8 100644
--- a/intern/cycles/kernel/split/kernel_branched.h
+++ b/intern/cycles/kernel/split/kernel_branched.h
@@ -63,12 +63,49 @@ ccl_device_inline void kernel_split_branched_path_indirect_loop_end(KernelGlobal
 	REMOVE_RAY_FLAG(kernel_split_state.ray_state, ray_index, RAY_BRANCHED_INDIRECT);
 }
 
+ccl_device_inline bool kernel_split_branched_indirect_start_shared(KernelGlobals *kg, int ray_index)
+{
+	ccl_global char *ray_state = kernel_split_state.ray_state;
+
+	int inactive_ray = dequeue_ray_index(QUEUE_INACTIVE_RAYS,
+		kernel_split_state.queue_data, kernel_split_params.queue_size, kernel_split_params.queue_index);
+
+	if(!IS_STATE(ray_state, inactive_ray, RAY_INACTIVE)) {
+		return false;
+	}
+
+#define SPLIT_DATA_ENTRY(type, name, num) \
+		kernel_split_state.name[inactive_ray] = kernel_split_state.name[ray_index];
+	SPLIT_DATA_ENTRIES_BRANCHED_SHARED
+#undef SPLIT_DATA_ENTRY
+
+	kernel_split_state.branched_state[inactive_ray].shared_sample_count = 0;
+	kernel_split_state.branched_state[inactive_ray].original_ray = ray_index;
+	kernel_split_state.branched_state[inactive_ray].waiting_on_shared_samples = false;
+
+	PathRadiance *L = &kernel_split_state.path_radiance[ray_index];
+	PathRadiance *inactive_L = &kernel_split_state.path_radiance[inactive_ray];
+
+	path_radiance_init(inactive_L, kernel_data.film.use_light_pass);
+	inactive_L->direct_throughput = L->direct_throughput;
+	path_radiance_copy_indirect(inactive_L, L);
+
+	ray_state[inactive_ray] = RAY_REGENERATED;
+	ADD_RAY_FLAG(ray_state, inactive_ray, RAY_BRANCHED_INDIRECT_SHARED);
+	ADD_RAY_FLAG(ray_state, inactive_ray, IS_FLAG(ray_state, ray_index, RAY_BRANCHED_INDIRECT));
+
+	atomic_fetch_and_inc_uint32((ccl_global uint*)&kernel_split_state.branched_state[ray_index].shared_sample_count);
+
+	return true;
+}
+
 /* bounce off surface and integrate indirect light */
 ccl_device_noinline bool kernel_split_branched_path_surface_indirect_light_iter(KernelGlobals *kg,
                                                                                 int ray_index,
                                                                                 float num_samples_adjust,
                                                                                 ShaderData *saved_sd,
-                                                                                bool reset_path_state)
+                                                                                bool reset_path_state,
+                                                                                bool wait_for_shared)
 {
 	SplitBranchedState *branched_state = &kernel_split_state.branched_state[ray_index];
 
@@ -155,12 +192,25 @@ ccl_device_noinline bool kernel_split_branched_path_surface_indirect_light_iter(
 			/* start the indirect path */
 			*tp *= num_samples_inv;
 
+			if(kernel_split_branched_indirect_start_shared(kg, ray_index)) {
+				continue;
+			}
+
 			return true;
 		}
 
 		branched_state->next_sample = 0;
 	}
 
+	branched_state->next_closure = sd->num_closure;
+
+	if(wait_for_shared) {
+		branched_state->waiting_on_shared_samples = (branched_state->shared_sample_count > 0);
+		if(branched_state->waiting_on_shared_samples) {
+			return true;
+		}
+	}
+
 	return false;
 }