Geometry Nodes: use virtual arrays in internal attribute api

A virtual array is a data structure that is similar to a normal array
in that its elements can be accessed by an index. However, a virtual
array does not have to be a contiguous array internally. Instead, its
elements can be layed out arbitrarily while element access happens
through a virtual function call. However, the virtual array data
structures are designed so that the virtual function call can be avoided
in cases where it could become a bottleneck.

Most commonly, a virtual array is backed by an actual array/span or
is a single value internally, that is the same for every index.
Besides those, there are many more specialized virtual arrays like the
ones that provides vertex positions based on the `MVert` struct or
vertex group weights.

Not all attributes used by geometry nodes are stored in simple contiguous
arrays. To provide uniform access to all kinds of attributes, the attribute
API has to provide virtual array functionality that hides the implementation
details of attributes.

Before this refactor, the attribute API provided its own virtual array
implementation as part of the `ReadAttribute` and `WriteAttribute` types.
That resulted in unnecessary code duplication with the virtual array system.
Even worse, it bound many algorithms used by geometry nodes to the specifics
of the attribute API, even though they could also use different data sources
(such as data from sockets, default values, later results of expressions, ...).

This refactor removes the `ReadAttribute` and `WriteAttribute` types and
replaces them with `GVArray` and `GVMutableArray` respectively. The `GV`
stands for "generic virtual". The "generic" means that the data type contained
in those virtual arrays is only known at run-time. There are the corresponding
statically typed types `VArray<T>` and `VMutableArray<T>` as well.

No regressions are expected from this refactor. It does come with one
improvement for users. The attribute API can convert the data type
on write now. This is especially useful when writing to builtin attributes
like `material_index` with e.g. the Attribute Math node (which usually
just writes to float attributes, while `material_index` is an integer attribute).

Differential Revision: https://developer.blender.org/D10994

1 files changed, 32 insertions, 187 deletions

diff --git a/source/blender/blenkernel/intern/attribute_access_intern.hh b/source/blender/blenkernel/intern/attribute_access_intern.hh
index 806d10e9e89..7cf585dfbfc 100644
--- a/source/blender/blenkernel/intern/attribute_access_intern.hh
+++ b/source/blender/blenkernel/intern/attribute_access_intern.hh
@@ -24,166 +24,8 @@
 
 namespace blender::bke {
 
-class ConstantReadAttribute final : public ReadAttribute {
- private:
-  void *value_;
-
- public:
-  ConstantReadAttribute(AttributeDomain domain,
-                        const int64_t size,
-                        const CPPType &type,
-                        const void *value)
-      : ReadAttribute(domain, type, size)
-  {
-    value_ = MEM_mallocN_aligned(type.size(), type.alignment(), __func__);
-    type.copy_to_uninitialized(value, value_);
-  }
-
-  ~ConstantReadAttribute() override
-  {
-    this->cpp_type_.destruct(value_);
-    MEM_freeN(value_);
-  }
-
-  void get_internal(const int64_t UNUSED(index), void *r_value) const override
-  {
-    this->cpp_type_.copy_to_uninitialized(value_, r_value);
-  }
-
-  void initialize_span() const override
-  {
-    const int element_size = cpp_type_.size();
-    array_buffer_ = MEM_mallocN_aligned(size_ * element_size, cpp_type_.alignment(), __func__);
-    array_is_temporary_ = true;
-    cpp_type_.fill_uninitialized(value_, array_buffer_, size_);
-  }
-};
-
-template<typename T> class ArrayReadAttribute final : public ReadAttribute {
- private:
-  Span<T> data_;
-
- public:
-  ArrayReadAttribute(AttributeDomain domain, Span<T> data)
-      : ReadAttribute(domain, CPPType::get<T>(), data.size()), data_(data)
-  {
-  }
-
-  void get_internal(const int64_t index, void *r_value) const override
-  {
-    new (r_value) T(data_[index]);
-  }
-
-  void initialize_span() const override
-  {
-    /* The data will not be modified, so this const_cast is fine. */
-    array_buffer_ = const_cast<T *>(data_.data());
-    array_is_temporary_ = false;
-  }
-};
-
-template<typename T> class OwnedArrayReadAttribute final : public ReadAttribute {
- private:
-  Array<T> data_;
-
- public:
-  OwnedArrayReadAttribute(AttributeDomain domain, Array<T> data)
-      : ReadAttribute(domain, CPPType::get<T>(), data.size()), data_(std::move(data))
-  {
-  }
-
-  void get_internal(const int64_t index, void *r_value) const override
-  {
-    new (r_value) T(data_[index]);
-  }
-
-  void initialize_span() const override
-  {
-    /* The data will not be modified, so this const_cast is fine. */
-    array_buffer_ = const_cast<T *>(data_.data());
-    array_is_temporary_ = false;
-  }
-};
-
-template<typename StructT, typename ElemT, ElemT (*GetFunc)(const StructT &)>
-class DerivedArrayReadAttribute final : public ReadAttribute {
- private:
-  Span<StructT> data_;
-
- public:
-  DerivedArrayReadAttribute(AttributeDomain domain, Span<StructT> data)
-      : ReadAttribute(domain, CPPType::get<ElemT>(), data.size()), data_(data)
-  {
-  }
-
-  void get_internal(const int64_t index, void *r_value) const override
-  {
-    const StructT &struct_value = data_[index];
-    const ElemT value = GetFunc(struct_value);
-    new (r_value) ElemT(value);
-  }
-};
-
-template<typename T> class ArrayWriteAttribute final : public WriteAttribute {
- private:
-  MutableSpan<T> data_;
-
- public:
-  ArrayWriteAttribute(AttributeDomain domain, MutableSpan<T> data)
-      : WriteAttribute(domain, CPPType::get<T>(), data.size()), data_(data)
-  {
-  }
-
-  void get_internal(const int64_t index, void *r_value) const override
-  {
-    new (r_value) T(data_[index]);
-  }
-
-  void set_internal(const int64_t index, const void *value) override
-  {
-    data_[index] = *reinterpret_cast<const T *>(value);
-  }
-
-  void initialize_span(const bool UNUSED(write_only)) override
-  {
-    array_buffer_ = data_.data();
-    array_is_temporary_ = false;
-  }
-
-  void apply_span_if_necessary() override
-  {
-    /* Do nothing, because the span contains the attribute itself already. */
-  }
-};
-
-template<typename StructT,
-         typename ElemT,
-         ElemT (*GetFunc)(const StructT &),
-         void (*SetFunc)(StructT &, const ElemT &)>
-class DerivedArrayWriteAttribute final : public WriteAttribute {
- private:
-  MutableSpan<StructT> data_;
-
- public:
-  DerivedArrayWriteAttribute(AttributeDomain domain, MutableSpan<StructT> data)
-      : WriteAttribute(domain, CPPType::get<ElemT>(), data.size()), data_(data)
-  {
-  }
-
-  void get_internal(const int64_t index, void *r_value) const override
-  {
-    const StructT &struct_value = data_[index];
-    const ElemT value = GetFunc(struct_value);
-    new (r_value) ElemT(value);
-  }
-
-  void set_internal(const int64_t index, const void *value) override
-  {
-    StructT &struct_value = data_[index];
-    const ElemT &typed_value = *reinterpret_cast<const ElemT *>(value);
-    SetFunc(struct_value, typed_value);
-  }
-};
+using fn::GVArrayPtr;
+using fn::GVMutableArrayPtr;
 
 /**
  * Utility to group together multiple functions that are used to access custom data on geometry
@@ -244,8 +86,8 @@ class BuiltinAttributeProvider {
   {
   }
 
-  virtual ReadAttributePtr try_get_for_read(const GeometryComponent &component) const = 0;
-  virtual WriteAttributePtr try_get_for_write(GeometryComponent &component) const = 0;
+  virtual GVArrayPtr try_get_for_read(const GeometryComponent &component) const = 0;
+  virtual GVMutableArrayPtr try_get_for_write(GeometryComponent &component) const = 0;
   virtual bool try_delete(GeometryComponent &component) const = 0;
   virtual bool try_create(GeometryComponent &UNUSED(component)) const = 0;
   virtual bool exists(const GeometryComponent &component) const = 0;
@@ -272,10 +114,10 @@ class BuiltinAttributeProvider {
  */
 class DynamicAttributesProvider {
  public:
-  virtual ReadAttributePtr try_get_for_read(const GeometryComponent &component,
-                                            const StringRef attribute_name) const = 0;
-  virtual WriteAttributePtr try_get_for_write(GeometryComponent &component,
-                                              const StringRef attribute_name) const = 0;
+  virtual ReadAttributeLookup try_get_for_read(const GeometryComponent &component,
+                                               const StringRef attribute_name) const = 0;
+  virtual WriteAttributeLookup try_get_for_write(GeometryComponent &component,
+                                                 const StringRef attribute_name) const = 0;
   virtual bool try_delete(GeometryComponent &component, const StringRef attribute_name) const = 0;
   virtual bool try_create(GeometryComponent &UNUSED(component),
                           const StringRef UNUSED(attribute_name),
@@ -309,11 +151,11 @@ class CustomDataAttributeProvider final : public DynamicAttributesProvider {
   {
   }
 
-  ReadAttributePtr try_get_for_read(const GeometryComponent &component,
-                                    const StringRef attribute_name) const final;
+  ReadAttributeLookup try_get_for_read(const GeometryComponent &component,
+                                       const StringRef attribute_name) const final;
 
-  WriteAttributePtr try_get_for_write(GeometryComponent &component,
-                                      const StringRef attribute_name) const final;
+  WriteAttributeLookup try_get_for_write(GeometryComponent &component,
+                                         const StringRef attribute_name) const final;
 
   bool try_delete(GeometryComponent &component, const StringRef attribute_name) const final;
 
@@ -332,18 +174,21 @@ class CustomDataAttributeProvider final : public DynamicAttributesProvider {
 
  private:
   template<typename T>
-  ReadAttributePtr layer_to_read_attribute(const CustomDataLayer &layer,
-                                           const int domain_size) const
+  ReadAttributeLookup layer_to_read_attribute(const CustomDataLayer &layer,
+                                              const int domain_size) const
   {
-    return std::make_unique<ArrayReadAttribute<T>>(
-        domain_, Span(static_cast<const T *>(layer.data), domain_size));
+    return {std::make_unique<fn::GVArray_For_Span<T>>(
+                Span(static_cast<const T *>(layer.data), domain_size)),
+            domain_};
   }
 
   template<typename T>
-  WriteAttributePtr layer_to_write_attribute(CustomDataLayer &layer, const int domain_size) const
+  WriteAttributeLookup layer_to_write_attribute(CustomDataLayer &layer,
+                                                const int domain_size) const
   {
-    return std::make_unique<ArrayWriteAttribute<T>>(
-        domain_, MutableSpan(static_cast<T *>(layer.data), domain_size));
+    return {std::make_unique<fn::GVMutableArray_For_MutableSpan<T>>(
+                MutableSpan(static_cast<T *>(layer.data), domain_size)),
+            domain_};
   }
 
   bool type_is_supported(CustomDataType data_type) const
@@ -357,8 +202,8 @@ class CustomDataAttributeProvider final : public DynamicAttributesProvider {
  */
 class NamedLegacyCustomDataProvider final : public DynamicAttributesProvider {
  private:
-  using AsReadAttribute = ReadAttributePtr (*)(const void *data, const int domain_size);
-  using AsWriteAttribute = WriteAttributePtr (*)(void *data, const int domain_size);
+  using AsReadAttribute = GVArrayPtr (*)(const void *data, const int domain_size);
+  using AsWriteAttribute = GVMutableArrayPtr (*)(void *data, const int domain_size);
   const AttributeDomain domain_;
   const CustomDataType attribute_type_;
   const CustomDataType stored_type_;
@@ -382,10 +227,10 @@ class NamedLegacyCustomDataProvider final : public DynamicAttributesProvider {
   {
   }
 
-  ReadAttributePtr try_get_for_read(const GeometryComponent &component,
-                                    const StringRef attribute_name) const final;
-  WriteAttributePtr try_get_for_write(GeometryComponent &component,
-                                      const StringRef attribute_name) const final;
+  ReadAttributeLookup try_get_for_read(const GeometryComponent &component,
+                                       const StringRef attribute_name) const final;
+  WriteAttributeLookup try_get_for_write(GeometryComponent &component,
+                                         const StringRef attribute_name) const final;
   bool try_delete(GeometryComponent &component, const StringRef attribute_name) const final;
   bool foreach_attribute(const GeometryComponent &component,
                          const AttributeForeachCallback callback) const final;
@@ -398,8 +243,8 @@ class NamedLegacyCustomDataProvider final : public DynamicAttributesProvider {
  * the #MVert struct, but is exposed as float3 attribute.
  */
 class BuiltinCustomDataLayerProvider final : public BuiltinAttributeProvider {
-  using AsReadAttribute = ReadAttributePtr (*)(const void *data, const int domain_size);
-  using AsWriteAttribute = WriteAttributePtr (*)(void *data, const int domain_size);
+  using AsReadAttribute = GVArrayPtr (*)(const void *data, const int domain_size);
+  using AsWriteAttribute = GVMutableArrayPtr (*)(void *data, const int domain_size);
   using UpdateOnRead = void (*)(const GeometryComponent &component);
   using UpdateOnWrite = void (*)(GeometryComponent &component);
   const CustomDataType stored_type_;
@@ -430,8 +275,8 @@ class BuiltinCustomDataLayerProvider final : public BuiltinAttributeProvider {
   {
   }
 
-  ReadAttributePtr try_get_for_read(const GeometryComponent &component) const final;
-  WriteAttributePtr try_get_for_write(GeometryComponent &component) const final;
+  GVArrayPtr try_get_for_read(const GeometryComponent &component) const final;
+  GVMutableArrayPtr try_get_for_write(GeometryComponent &component) const final;
   bool try_delete(GeometryComponent &component) const final;
   bool try_create(GeometryComponent &component) const final;
   bool exists(const GeometryComponent &component) const final;