Geometry Nodes: Improve point instance node performance

This commit uses two changes to improve the performance of the point
instance node.

**Prevent Reallocations**
At 64 bytes, the transform matrix for every instance is rather large,
so reallocating the vector as it grows can become a performance bottle-
neck. This commit reserves memory for the instances that will be added
to prevent unecessary reallocations as the instance vector grows.

In a test with 4 million instances of 3 objects in a collection, the
node was about 40% faster, from 370ms to 270ms for the node.

**Parallelization**
Currently the instances are added by appending to a vector. By changing
this slightly to fill indices instead, we can parallelize the operation
so that multiple threads can fill data at the same time. Tested on a
Ryzen 3700x, this reduced the runtime from the above 270ms to 44ms
average, bringing the total speedup to ~8x.

Note that displaying the instances in the viewport is still much slower
than the calculations in node, this change doesn't affect that.

2 files changed, 20 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_geometry_set.hh b/source/blender/blenkernel/BKE_geometry_set.hh
index 106af8172d1..bd2b062152d 100644
--- a/source/blender/blenkernel/BKE_geometry_set.hh
+++ b/source/blender/blenkernel/BKE_geometry_set.hh
@@ -597,6 +597,7 @@ class InstancesComponent : public GeometryComponent {
   void clear();
 
   void reserve(int min_capacity);
+  void resize(int capacity);
 
   int add_reference(InstanceReference reference);
   void add_instance(int instance_handle, const blender::float4x4 &transform, const int id = -1);
@@ -604,6 +605,7 @@ class InstancesComponent : public GeometryComponent {
   blender::Span<InstanceReference> references() const;
 
   blender::Span<int> instance_reference_handles() const;
+  blender::MutableSpan<int> instance_reference_handles();
   blender::MutableSpan<blender::float4x4> instance_transforms();
   blender::Span<blender::float4x4> instance_transforms() const;
   blender::MutableSpan<int> instance_ids();
diff --git a/source/blender/blenkernel/intern/geometry_component_instances.cc b/source/blender/blenkernel/intern/geometry_component_instances.cc
index 55025ed5ac9..f1294341464 100644
--- a/source/blender/blenkernel/intern/geometry_component_instances.cc
+++ b/source/blender/blenkernel/intern/geometry_component_instances.cc
@@ -56,6 +56,19 @@ void InstancesComponent::reserve(int min_capacity)
   instance_ids_.reserve(min_capacity);
 }
 
+/**
+ * Resize the transform, handles, and ID vectors to the specified capacity.
+ *
+ * \note This function should be used carefully, only when it's guarenteed
+ * that the data will be filled.
+ */
+void InstancesComponent::resize(int capacity)
+{
+  instance_reference_handles_.resize(capacity);
+  instance_transforms_.resize(capacity);
+  instance_ids_.resize(capacity);
+}
+
 void InstancesComponent::clear()
 {
   instance_reference_handles_.clear();
@@ -81,6 +94,11 @@ blender::Span<int> InstancesComponent::instance_reference_handles() const
   return instance_reference_handles_;
 }
 
+blender::MutableSpan<int> InstancesComponent::instance_reference_handles()
+{
+  return instance_reference_handles_;
+}
+
 blender::MutableSpan<blender::float4x4> InstancesComponent::instance_transforms()
 {
   return instance_transforms_;