4 files changed, 82 insertions, 0 deletions

diff --git a/source/blender/gpu/CMakeLists.txt b/source/blender/gpu/CMakeLists.txt
index cf6009c2881..8468985309f 100644
--- a/source/blender/gpu/CMakeLists.txt
+++ b/source/blender/gpu/CMakeLists.txt
@@ -394,6 +394,8 @@ if(WITH_GTESTS)
   if(WITH_OPENGL_DRAW_TESTS)
     set(TEST_SRC
       tests/gpu_testing.cc
+
+      tests/gpu_index_buffer_test.cc
       tests/gpu_shader_test.cc
 
       tests/gpu_testing.hh
diff --git a/source/blender/gpu/GPU_index_buffer.h b/source/blender/gpu/GPU_index_buffer.h
index 4e8d854c7ce..03d60c60b4b 100644
--- a/source/blender/gpu/GPU_index_buffer.h
+++ b/source/blender/gpu/GPU_index_buffer.h
@@ -44,6 +44,7 @@ typedef struct GPUIndexBufBuilder {
   uint index_max;
   GPUPrimType prim_type;
   uint32_t *data;
+  const struct GPUIndexBufBuilder *parent;
 } GPUIndexBufBuilder;
 
 /* supports all primitive types. */
@@ -53,6 +54,19 @@ void GPU_indexbuf_init_ex(GPUIndexBufBuilder *, GPUPrimType, uint index_len, uin
 void GPU_indexbuf_init(GPUIndexBufBuilder *, GPUPrimType, uint prim_len, uint vertex_len);
 GPUIndexBuf *GPU_indexbuf_build_on_device(uint index_len);
 
+/*
+ * Thread safe sub builders.
+ *
+ * Note that `GPU_indexbuf_subbuilder_init` and `GPU_indexbuf_subbuilder_finish` are not thread
+ * safe and should be called when no threads are active. The pattern is to create a subbuilder for
+ * each thread/task. Each thread/task would update their sub builder. When all thread are completed
+ * the sub-builders can then be merged back to the parent builder.
+ */
+void GPU_indexbuf_subbuilder_init(const GPUIndexBufBuilder *parent_builder,
+                                  GPUIndexBufBuilder *sub_builder);
+void GPU_indexbuf_subbuilder_finish(GPUIndexBufBuilder *builder,
+                                    const GPUIndexBufBuilder *parent_builder);
+
 void GPU_indexbuf_add_generic_vert(GPUIndexBufBuilder *, uint v);
 void GPU_indexbuf_add_primitive_restart(GPUIndexBufBuilder *);
 
diff --git a/source/blender/gpu/intern/gpu_index_buffer.cc b/source/blender/gpu/intern/gpu_index_buffer.cc
index 9f283a3a944..3cdcaac5544 100644
--- a/source/blender/gpu/intern/gpu_index_buffer.cc
+++ b/source/blender/gpu/intern/gpu_index_buffer.cc
@@ -25,6 +25,7 @@
 
 #include "MEM_guardedalloc.h"
 
+#include "BLI_math_base.h"
 #include "BLI_utildefines.h"
 
 #include "gpu_backend.hh"
@@ -56,6 +57,7 @@ void GPU_indexbuf_init_ex(GPUIndexBufBuilder *builder,
   builder->index_max = 0;
   builder->prim_type = prim_type;
   builder->data = (uint *)MEM_callocN(builder->max_index_len * sizeof(uint), "GPUIndexBuf data");
+  builder->parent = nullptr;
 }
 
 void GPU_indexbuf_init(GPUIndexBufBuilder *builder,
@@ -78,6 +80,23 @@ GPUIndexBuf *GPU_indexbuf_build_on_device(uint index_len)
   return elem_;
 }
 
+void GPU_indexbuf_subbuilder_init(const GPUIndexBufBuilder *parent_builder,
+                                  GPUIndexBufBuilder *sub_builder)
+{
+  BLI_assert(parent_builder->parent == nullptr);
+  memcpy(sub_builder, parent_builder, sizeof(GPUIndexBufBuilder));
+  sub_builder->parent = parent_builder;
+}
+
+void GPU_indexbuf_subbuilder_finish(GPUIndexBufBuilder *parent_builder,
+                                    const GPUIndexBufBuilder *sub_builder)
+{
+  BLI_assert(parent_builder == sub_builder->parent);
+  parent_builder->index_len = max_uu(parent_builder->index_len, sub_builder->index_len);
+  parent_builder->index_min = min_uu(parent_builder->index_min, sub_builder->index_min);
+  parent_builder->index_max = max_uu(parent_builder->index_max, sub_builder->index_max);
+}
+
 void GPU_indexbuf_add_generic_vert(GPUIndexBufBuilder *builder, uint v)
 {
 #if TRUST_NO_ONE
diff --git a/source/blender/gpu/tests/gpu_index_buffer_test.cc b/source/blender/gpu/tests/gpu_index_buffer_test.cc
new file mode 100644
index 00000000000..ebc110056e3
--- /dev/null
+++ b/source/blender/gpu/tests/gpu_index_buffer_test.cc
@@ -0,0 +1,47 @@
+/* Apache License, Version 2.0 */
+
+#include "testing/testing.h"
+
+#include "MEM_guardedalloc.h"
+
+#include "GPU_index_buffer.h"
+
+#include "gpu_testing.hh"
+
+namespace blender::gpu::tests {
+
+TEST_F(GPUTest, gpu_index_buffer_subbuilders)
+{
+  const uint num_subbuilders = 10;
+  const uint verts_per_subbuilders = 100;
+  const uint vertex_len = num_subbuilders * verts_per_subbuilders;
+
+  GPUIndexBufBuilder builder;
+  GPU_indexbuf_init(&builder, GPU_PRIM_POINTS, vertex_len, vertex_len);
+
+  GPUIndexBufBuilder subbuilders[num_subbuilders];
+  for (int subbuilder_index = 0; subbuilder_index < num_subbuilders; subbuilder_index++) {
+    GPU_indexbuf_subbuilder_init(&builder, &subbuilders[subbuilder_index]);
+  }
+
+  for (int subbuilder_index = 0; subbuilder_index < num_subbuilders; subbuilder_index++) {
+    GPUIndexBufBuilder &subbuilder = subbuilders[subbuilder_index];
+    for (int subbuilder_vert_index = 0; subbuilder_vert_index < verts_per_subbuilders;
+         subbuilder_vert_index++) {
+      int vert_index_to_update = subbuilder_index * verts_per_subbuilders + subbuilder_vert_index;
+      GPU_indexbuf_set_point_vert(&subbuilder, vert_index_to_update, vert_index_to_update);
+    }
+  }
+
+  for (int subbuilder_index = 0; subbuilder_index < num_subbuilders; subbuilder_index++) {
+    EXPECT_EQ(builder.index_len, subbuilder_index * verts_per_subbuilders);
+    GPU_indexbuf_subbuilder_finish(&builder, &subbuilders[subbuilder_index]);
+    EXPECT_EQ(builder.index_len, (subbuilder_index + 1) * verts_per_subbuilders);
+  }
+
+  GPUIndexBuf *index_buffer = GPU_indexbuf_build(&builder);
+  EXPECT_NE(index_buffer, nullptr);
+  GPU_INDEXBUF_DISCARD_SAFE(index_buffer);
+}
+
+}  // namespace blender::gpu::tests