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diff --git a/intern/cycles/kernel/integrator/state.h b/intern/cycles/kernel/integrator/state.h
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+++ b/intern/cycles/kernel/integrator/state.h
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+/*
+ * Copyright 2011-2021 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.
+ */
+
+/* Integrator State
+ *
+ * This file defines the data structures that define the state of a path. Any state that is
+ * preserved and passed between kernel executions is part of this.
+ *
+ * The size of this state must be kept as small as possible, to reduce cache misses and keep memory
+ * usage under control on GPUs that may execute millions of kernels.
+ *
+ * Memory may be allocated and passed along in different ways depending on the device. There may
+ * be a scalar layout, or AoS or SoA layout for batches. The state may be passed along as a pointer
+ * to every kernel, or the pointer may exist at program scope or in constant memory. To abstract
+ * these differences between devices and experiment with different layouts, macros are used.
+ *
+ * Use IntegratorState to pass a reference to the integrator state for the current path. These are
+ * defined differently on the CPU and GPU. Use ConstIntegratorState instead of const
+ * IntegratorState for passing state as read-only, to avoid oddities in typedef behavior.
+ *
+ * INTEGRATOR_STATE(state, x, y): read nested struct member x.y of IntegratorState
+ * INTEGRATOR_STATE_WRITE(state, x, y): write to nested struct member x.y of IntegratorState
+ *
+ * INTEGRATOR_STATE_ARRAY(state, x, index, y): read x[index].y
+ * INTEGRATOR_STATE_ARRAY_WRITE(state, x, index, y): write x[index].y
+ *
+ * INTEGRATOR_STATE_NULL: use to pass empty state to other functions.
+ */
+
+#include "kernel/types.h"
+
+#include "util/types.h"
+
+#pragma once
+
+CCL_NAMESPACE_BEGIN
+
+/* Data structures */
+
+/* Integrator State
+ *
+ * CPU rendering path state with AoS layout. */
+typedef struct IntegratorShadowStateCPU {
+#define KERNEL_STRUCT_BEGIN(name) struct {
+#define KERNEL_STRUCT_MEMBER(parent_struct, type, name, feature) type name;
+#define KERNEL_STRUCT_ARRAY_MEMBER KERNEL_STRUCT_MEMBER
+#define KERNEL_STRUCT_END(name) \
+  } \
+  name;
+#define KERNEL_STRUCT_END_ARRAY(name, cpu_size, gpu_size) \
+  } \
+  name[cpu_size];
+#define KERNEL_STRUCT_VOLUME_STACK_SIZE MAX_VOLUME_STACK_SIZE
+#include "kernel/integrator/shadow_state_template.h"
+#undef KERNEL_STRUCT_BEGIN
+#undef KERNEL_STRUCT_MEMBER
+#undef KERNEL_STRUCT_ARRAY_MEMBER
+#undef KERNEL_STRUCT_END
+#undef KERNEL_STRUCT_END_ARRAY
+} IntegratorShadowStateCPU;
+
+typedef struct IntegratorStateCPU {
+#define KERNEL_STRUCT_BEGIN(name) struct {
+#define KERNEL_STRUCT_MEMBER(parent_struct, type, name, feature) type name;
+#define KERNEL_STRUCT_ARRAY_MEMBER KERNEL_STRUCT_MEMBER
+#define KERNEL_STRUCT_END(name) \
+  } \
+  name;
+#define KERNEL_STRUCT_END_ARRAY(name, cpu_size, gpu_size) \
+  } \
+  name[cpu_size];
+#define KERNEL_STRUCT_VOLUME_STACK_SIZE MAX_VOLUME_STACK_SIZE
+#include "kernel/integrator/state_template.h"
+#undef KERNEL_STRUCT_BEGIN
+#undef KERNEL_STRUCT_MEMBER
+#undef KERNEL_STRUCT_ARRAY_MEMBER
+#undef KERNEL_STRUCT_END
+#undef KERNEL_STRUCT_END_ARRAY
+#undef KERNEL_STRUCT_VOLUME_STACK_SIZE
+
+  IntegratorShadowStateCPU shadow;
+  IntegratorShadowStateCPU ao;
+} IntegratorStateCPU;
+
+/* Path Queue
+ *
+ * Keep track of which kernels are queued to be executed next in the path
+ * for GPU rendering. */
+typedef struct IntegratorQueueCounter {
+  int num_queued[DEVICE_KERNEL_INTEGRATOR_NUM];
+} IntegratorQueueCounter;
+
+/* Integrator State GPU
+ *
+ * GPU rendering path state with SoA layout. */
+typedef struct IntegratorStateGPU {
+#define KERNEL_STRUCT_BEGIN(name) struct {
+#define KERNEL_STRUCT_MEMBER(parent_struct, type, name, feature) ccl_global type *name;
+#define KERNEL_STRUCT_ARRAY_MEMBER KERNEL_STRUCT_MEMBER
+#define KERNEL_STRUCT_END(name) \
+  } \
+  name;
+#define KERNEL_STRUCT_END_ARRAY(name, cpu_size, gpu_size) \
+  } \
+  name[gpu_size];
+#define KERNEL_STRUCT_VOLUME_STACK_SIZE MAX_VOLUME_STACK_SIZE
+
+#include "kernel/integrator/state_template.h"
+
+#include "kernel/integrator/shadow_state_template.h"
+
+#undef KERNEL_STRUCT_BEGIN
+#undef KERNEL_STRUCT_MEMBER
+#undef KERNEL_STRUCT_ARRAY_MEMBER
+#undef KERNEL_STRUCT_END
+#undef KERNEL_STRUCT_END_ARRAY
+#undef KERNEL_STRUCT_VOLUME_STACK_SIZE
+
+  /* Count number of queued kernels. */
+  ccl_global IntegratorQueueCounter *queue_counter;
+
+  /* Count number of kernels queued for specific shaders. */
+  ccl_global int *sort_key_counter[DEVICE_KERNEL_INTEGRATOR_NUM];
+
+  /* Index of shadow path which will be used by a next shadow path.  */
+  ccl_global int *next_shadow_path_index;
+
+  /* Index of main path which will be used by a next shadow catcher split.  */
+  ccl_global int *next_main_path_index;
+} IntegratorStateGPU;
+
+/* Abstraction
+ *
+ * Macros to access data structures on different devices.
+ *
+ * Note that there is a special access function for the shadow catcher state. This access is to
+ * happen from a kernel which operates on a "main" path. Attempt to use shadow catcher accessors
+ * from a kernel which operates on a shadow catcher state will cause bad memory access. */
+
+#ifdef __KERNEL_CPU__
+
+/* Scalar access on CPU. */
+
+typedef IntegratorStateCPU *ccl_restrict IntegratorState;
+typedef const IntegratorStateCPU *ccl_restrict ConstIntegratorState;
+typedef IntegratorShadowStateCPU *ccl_restrict IntegratorShadowState;
+typedef const IntegratorShadowStateCPU *ccl_restrict ConstIntegratorShadowState;
+
+#  define INTEGRATOR_STATE_NULL nullptr
+
+#  define INTEGRATOR_STATE(state, nested_struct, member) ((state)->nested_struct.member)
+#  define INTEGRATOR_STATE_WRITE(state, nested_struct, member) ((state)->nested_struct.member)
+
+#  define INTEGRATOR_STATE_ARRAY(state, nested_struct, array_index, member) \
+    ((state)->nested_struct[array_index].member)
+#  define INTEGRATOR_STATE_ARRAY_WRITE(state, nested_struct, array_index, member) \
+    ((state)->nested_struct[array_index].member)
+
+#else /* __KERNEL_CPU__ */
+
+/* Array access on GPU with Structure-of-Arrays. */
+
+typedef const int IntegratorState;
+typedef const int ConstIntegratorState;
+typedef const int IntegratorShadowState;
+typedef const int ConstIntegratorShadowState;
+
+#  define INTEGRATOR_STATE_NULL -1
+
+#  define INTEGRATOR_STATE(state, nested_struct, member) \
+    kernel_integrator_state.nested_struct.member[state]
+#  define INTEGRATOR_STATE_WRITE(state, nested_struct, member) \
+    INTEGRATOR_STATE(state, nested_struct, member)
+
+#  define INTEGRATOR_STATE_ARRAY(state, nested_struct, array_index, member) \
+    kernel_integrator_state.nested_struct[array_index].member[state]
+#  define INTEGRATOR_STATE_ARRAY_WRITE(state, nested_struct, array_index, member) \
+    INTEGRATOR_STATE_ARRAY(state, nested_struct, array_index, member)
+
+#endif /* __KERNEL_CPU__ */
+
+CCL_NAMESPACE_END