Cycles: decouple shadow paths from main path on GPU

The motivation for this is twofold. It improves performance (5-10% on most
benchmark scenes), and will help  to bring back transparency support for the
ambient occlusion pass.

* Duplicate some members from the main path state in the shadow path state.
* Add shadow paths incrementally to the array similar to what we do for
  the shadow catchers.
* For the scheduling, allow running shade surface and shade volume kernels
  as long as there is enough space in the shadow paths array. If not, execute
  shadow kernels until it is empty.

* Add IntegratorShadowState and ConstIntegratorShadowState typedefs that
  can be different between CPU and GPU. For GPU both main and shadow paths
  juse have an integer for SoA access. Bt with CPU it's a different pointer
  type so we get type safety checks in code shared between CPU and GPU.
* For CPU, add a separate IntegratorShadowStateCPU struct embedded in
  IntegratorShadowState.
* Update various functions to take the shadow state, and make SVM take either
  type of state using templates.

Differential Revision: https://developer.blender.org/D12889

1 files changed, 6 insertions, 3 deletions

diff --git a/intern/cycles/kernel/kernel_accumulate.h b/intern/cycles/kernel/kernel_accumulate.h
index 5f32150d33c..848aaa18aae 100644
--- a/intern/cycles/kernel/kernel_accumulate.h
+++ b/intern/cycles/kernel/kernel_accumulate.h
@@ -393,17 +393,20 @@ ccl_device_inline void kernel_accum_emission_or_background_pass(KernelGlobals kg
 
 /* Write light contribution to render buffer. */
 ccl_device_inline void kernel_accum_light(KernelGlobals kg,
-                                          ConstIntegratorState state,
+                                          ConstIntegratorShadowState state,
                                           ccl_global float *ccl_restrict render_buffer)
 {
   /* The throughput for shadow paths already contains the light shader evaluation. */
   float3 contribution = INTEGRATOR_STATE(state, shadow_path, throughput);
   kernel_accum_clamp(kg, &contribution, INTEGRATOR_STATE(state, shadow_path, bounce));
 
-  ccl_global float *buffer = kernel_accum_pixel_render_buffer(kg, state, render_buffer);
+  const uint32_t render_pixel_index = INTEGRATOR_STATE(state, shadow_path, render_pixel_index);
+  const uint64_t render_buffer_offset = (uint64_t)render_pixel_index *
+                                        kernel_data.film.pass_stride;
+  ccl_global float *buffer = render_buffer + render_buffer_offset;
 
   const uint32_t path_flag = INTEGRATOR_STATE(state, shadow_path, flag);
-  const int sample = INTEGRATOR_STATE(state, path, sample);
+  const int sample = INTEGRATOR_STATE(state, shadow_path, sample);
 
   kernel_accum_combined_pass(kg, path_flag, sample, contribution, buffer);