Geometry Nodes: refactor virtual array system

Goals of this refactor:
* Simplify creating virtual arrays.
* Simplify passing virtual arrays around.
* Simplify converting between typed and generic virtual arrays.
* Reduce memory allocations.

As a quick reminder, a virtual arrays is a data structure that behaves like an
array (i.e. it can be accessed using an index). However, it may not actually
be stored as array internally. The two most important implementations
of virtual arrays are those that correspond to an actual plain array and those
that have the same value for every index. However, many more
implementations exist for various reasons (interfacing with legacy attributes,
unified iterator over all points in multiple splines, ...).

With this refactor the core types (`VArray`, `GVArray`, `VMutableArray` and
`GVMutableArray`) can be used like "normal values". They typically live
on the stack. Before, they were usually inside a `std::unique_ptr`. This makes
passing them around much easier. Creation of new virtual arrays is also
much simpler now due to some constructors. Memory allocations are
reduced by making use of small object optimization inside the core types.

Previously, `VArray` was a class with virtual methods that had to be overridden
to change the behavior of a the virtual array. Now,`VArray` has a fixed size
and has no virtual methods. Instead it contains a `VArrayImpl` that is
similar to the old `VArray`. `VArrayImpl` should rarely ever be used directly,
unless a new virtual array implementation is added.

To support the small object optimization for many `VArrayImpl` classes,
a new `blender::Any` type is added. It is similar to `std::any` with two
additional features. It has an adjustable inline buffer size and alignment.
The inline buffer size of `std::any` can't be relied on and is usually too
small for our use case here. Furthermore, `blender::Any` can store
additional user-defined type information without increasing the
stack size.

Differential Revision: https://developer.blender.org/D12986

1 files changed, 319 insertions, 0 deletions

diff --git a/source/blender/blenlib/BLI_any.hh b/source/blender/blenlib/BLI_any.hh
new file mode 100644
index 00000000000..0fc5de5540f
--- /dev/null
+++ b/source/blender/blenlib/BLI_any.hh
@@ -0,0 +1,319 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#pragma once
+
+/** \file
+ * \ingroup bli
+ *
+ * A #blender::Any is a type-safe container for single values of any copy constructible type.
+ * It is similar to #std::any but provides the following two additional features:
+ * - Adjustable inline buffer capacity and alignment. #std::any has a small inline buffer in most
+ *   implementations as well, but its size is not guaranteed.
+ * - Can store additional user-defined type information without increasing the stack size of #Any.
+ */
+
+#include <algorithm>
+#include <utility>
+
+#include "BLI_memory_utils.hh"
+
+namespace blender {
+
+namespace detail {
+
+/**
+ * Contains function pointers that manage the memory in an #Any.
+ * Additional type specific #ExtraInfo can be embedded here as well.
+ */
+template<typename ExtraInfo> struct AnyTypeInfo {
+  void (*copy_construct)(void *dst, const void *src);
+  void (*move_construct)(void *dst, void *src);
+  void (*destruct)(void *src);
+  const void *(*get)(const void *src);
+  ExtraInfo extra_info;
+
+  /**
+   * Used when #T is stored directly in the inline buffer of the #Any.
+   */
+  template<typename T> static const AnyTypeInfo &get_for_inline()
+  {
+    static AnyTypeInfo funcs = {[](void *dst, const void *src) { new (dst) T(*(const T *)src); },
+                                [](void *dst, void *src) { new (dst) T(std::move(*(T *)src)); },
+                                [](void *src) { ((T *)src)->~T(); },
+                                [](const void *src) { return src; },
+                                ExtraInfo::template get<T>()};
+    return funcs;
+  }
+
+  /**
+   * Used when #T can't be stored directly in the inline buffer and is stored in a #std::unique_ptr
+   * instead. In this scenario, the #std::unique_ptr is stored in the inline buffer.
+   */
+  template<typename T> static const AnyTypeInfo &get_for_unique_ptr()
+  {
+    using Ptr = std::unique_ptr<T>;
+    static AnyTypeInfo funcs = {
+        [](void *dst, const void *src) { new (dst) Ptr(new T(**(const Ptr *)src)); },
+        [](void *dst, void *src) { new (dst) Ptr(new T(std::move(**(Ptr *)src))); },
+        [](void *src) { ((Ptr *)src)->~Ptr(); },
+        [](const void *src) -> const void * { return &**(const Ptr *)src; },
+        ExtraInfo::template get<T>()};
+    return funcs;
+  }
+
+  /**
+   * Used when the #Any does not contain any type currently.
+   */
+  static const AnyTypeInfo &get_for_empty()
+  {
+    static AnyTypeInfo funcs = {[](void *UNUSED(dst), const void *UNUSED(src)) {},
+                                [](void *UNUSED(dst), void *UNUSED(src)) {},
+                                [](void *UNUSED(src)) {},
+                                [](const void *UNUSED(src)) -> const void * { return nullptr; },
+                                ExtraInfo{}};
+    return funcs;
+  }
+};
+
+/**
+ * Dummy extra info that is used when no additional type information should be stored in the #Any.
+ */
+struct NoExtraInfo {
+  template<typename T> static NoExtraInfo get()
+  {
+    return {};
+  }
+};
+
+}  // namespace detail
+
+template<
+    /**
+     * Either void or a struct that contains data members for additional type information.
+     * The struct has to have a static `ExtraInfo get<T>()` method that initializes the struct
+     * based on a type.
+     */
+    typename ExtraInfo = void,
+    /**
+     * Size of the inline buffer. This allows types that are small enough to be stored directly
+     * inside the #Any without an additional allocation.
+     */
+    size_t InlineBufferCapacity = 8,
+    /**
+     * Required minimum alignment of the inline buffer. If this is smaller than the alignment
+     * requirement of a used type, a separate allocation is necessary.
+     */
+    size_t Alignment = 8>
+class Any {
+ private:
+  /* Makes it possible to use void in the template parameters. */
+  using RealExtraInfo =
+      std::conditional_t<std::is_void_v<ExtraInfo>, detail::NoExtraInfo, ExtraInfo>;
+  using Info = detail::AnyTypeInfo<RealExtraInfo>;
+
+  /**
+   * Inline buffer that either contains nothing, the stored value directly, or a #std::unique_ptr
+   * to the value.
+   */
+  AlignedBuffer<std::max(InlineBufferCapacity, sizeof(std::unique_ptr<int>)), Alignment> buffer_{};
+
+  /**
+   * Information about the type that is currently stored.
+   */
+  const Info *info_ = &Info::get_for_empty();
+
+ public:
+  /** Only copy constructible types can be stored in #Any. */
+  template<typename T> static constexpr inline bool is_allowed_v = std::is_copy_constructible_v<T>;
+
+  /**
+   * Checks if the type will be stored in the inline buffer or if it requires a separate
+   * allocation.
+   */
+  template<typename T>
+  static constexpr inline bool is_inline_v = std::is_nothrow_move_constructible_v<T> &&
+                                             sizeof(T) <= InlineBufferCapacity &&
+                                             alignof(T) <= Alignment;
+
+  /**
+   * Checks if #T is the same type as this #Any, because in this case the behavior of e.g. the
+   * assignment operator is different.
+   */
+  template<typename T>
+  static constexpr inline bool is_same_any_v = std::is_same_v<std::decay_t<T>, Any>;
+
+ private:
+  template<typename T> const Info &get_info() const
+  {
+    using DecayT = std::decay_t<T>;
+    static_assert(is_allowed_v<DecayT>);
+    if constexpr (is_inline_v<DecayT>) {
+      return Info::template get_for_inline<DecayT>();
+    }
+    else {
+      return Info::template get_for_unique_ptr<DecayT>();
+    }
+  }
+
+ public:
+  Any() = default;
+
+  Any(const Any &other) : info_(other.info_)
+  {
+    info_->copy_construct(&buffer_, &other.buffer_);
+  }
+
+  /**
+   * \note The #other #Any will not be empty afterwards if it was not before. Just its value is in
+   * a moved-from state.
+   */
+  Any(Any &&other) noexcept : info_(other.info_)
+  {
+    info_->move_construct(&buffer_, &other.buffer_);
+  }
+
+  /**
+   * Constructs a new #Any that contains the given type #T from #args. The #std::in_place_type_t is
+   * used to disambiguate this and the copy/move constructors.
+   */
+  template<typename T, typename... Args> explicit Any(std::in_place_type_t<T>, Args &&...args)
+  {
+    using DecayT = std::decay_t<T>;
+    static_assert(is_allowed_v<DecayT>);
+    info_ = &this->template get_info<DecayT>();
+    if constexpr (is_inline_v<DecayT>) {
+      /* Construct the value directly in the inline buffer. */
+      new (&buffer_) DecayT(std::forward<Args>(args)...);
+    }
+    else {
+      /* Construct the value in a new allocation and store a #std::unique_ptr to it in the inline
+       * buffer. */
+      new (&buffer_) std::unique_ptr<DecayT>(new DecayT(std::forward<Args>(args)...));
+    }
+  }
+
+  /**
+   * Constructs a new #Any that contains the given value.
+   */
+  template<typename T, typename X = std::enable_if_t<!is_same_any_v<T>, void>>
+  Any(T &&value) : Any(std::in_place_type<T>, std::forward<T>(value))
+  {
+  }
+
+  ~Any()
+  {
+    info_->destruct(&buffer_);
+  }
+
+  /**
+   * \note: Only needed because the template below does not count as copy assignment operator.
+   */
+  Any &operator=(const Any &other)
+  {
+    if (this == &other) {
+      return *this;
+    }
+    this->~Any();
+    new (this) Any(other);
+    return *this;
+  }
+
+  /** Assign any value to the #Any. */
+  template<typename T> Any &operator=(T &&other)
+  {
+    if constexpr (is_same_any_v<T>) {
+      if (this == &other) {
+        return *this;
+      }
+    }
+    this->~Any();
+    new (this) Any(std::forward<T>(other));
+    return *this;
+  }
+
+  /** Destruct any existing value to make it empty. */
+  void reset()
+  {
+    info_->destruct(&buffer_);
+    info_ = &Info::get_for_empty();
+  }
+
+  operator bool() const
+  {
+    return this->has_value();
+  }
+
+  bool has_value() const
+  {
+    return info_ != &Info::get_for_empty();
+  }
+
+  template<typename T, typename... Args> std::decay_t<T> &emplace(Args &&...args)
+  {
+    this->~Any();
+    new (this) Any(std::in_place_type<T>, std::forward<Args>(args)...);
+    return this->get<T>();
+  }
+
+  /** Return true when the value that is currently stored is a #T. */
+  template<typename T> bool is() const
+  {
+    return info_ == &this->template get_info<T>();
+  }
+
+  /** Get a pointer to the stored value. */
+  void *get()
+  {
+    return const_cast<void *>(info_->get(&buffer_));
+  }
+
+  /** Get a pointer to the stored value. */
+  const void *get() const
+  {
+    return info_->get(&buffer_);
+  }
+
+  /**
+   * Get a reference to the stored value. This invokes undefined behavior when #T does not have the
+   * correct type.
+   */
+  template<typename T> std::decay_t<T> &get()
+  {
+    BLI_assert(this->is<T>());
+    return *static_cast<std::decay_t<T> *>(this->get());
+  }
+
+  /**
+   * Get a reference to the stored value. This invokes undefined behavior when #T does not have the
+   * correct type.
+   */
+  template<typename T> const std::decay_t<T> &get() const
+  {
+    BLI_assert(this->is<T>());
+    return *static_cast<const std::decay_t<T> *>(this->get());
+  }
+
+  /**
+   * Get extra information that has been stored for the contained type.
+   */
+  const RealExtraInfo &extra_info() const
+  {
+    return info_->extra_info;
+  }
+};
+
+}  // namespace blender