Replace CompactValue with StyleValueHandle and StyleValuePool (facebo…

…ok#42131)

Summary:

X-link: facebook/yoga#1534

Now that the storage method is a hidden implementation detail, this changes the underlying data structure used to store styles, from `CompactValue` (a customized 32-bit float with tag bits), to `StyleValuePool`.

This new structure operates on 16-bit handles, and a shared small buffer. The vast majority of real-world values can be stored directly in the handle, but we allow arbitrary 32 bit (and soon 64-bit) values to be stored, where the handle then becomes an index into the styles buffer.

This results in a real-world memory usage win, while also letting us store the 64-bit values we are wanting to use for math function support (compared to doubling the storage requirements).

| | Before | After | Δ |
| `sizeof(yoga::Style)` | 208B  | 144B | -64B/-31% |
| `sizeof(yoga::Node)` | 640B  | 576B | -64B/-10% |
| `sizeof(YogaLayoutableShadowNode) ` |  920B | 856B | -64B/-7% |
| `sizeof(YogaLayoutableShadowNode) + sizeof(YogaStylableProps)` | 1296B  | 1168B | -128B/-10% |
| `sizeof(ViewShadowNode)`  |  920B | 856B | -64B/-7% |
| `sizeof(ViewShadowNode) + sizeof(ViewShadowNodeProps)` | 2000B  | 1872B | -128B/-6% |

Differential Revision: D52223122

packages/react-native/ReactCommon/yoga/yoga/YGNodeStyle.cpp

@@ -34,15 +34,13 @@ void updateStyle(YGNodeRef node, IdxT idx, ValueT value) {
 
 } // namespace
 
-void YGNodeCopyStyle(
-    const YGNodeRef dstNodeRef,
-    const YGNodeConstRef srcNodeRef) {
-  auto dstNode = resolveRef(dstNodeRef);
-  auto srcNode = resolveRef(srcNodeRef);
-
-  if (!(dstNode->getStyle() == srcNode->getStyle())) {
-    dstNode->setStyle(srcNode->getStyle());
-    dstNode->markDirtyAndPropagate();
+void YGNodeCopyStyle(YGNodeRef dstNode, YGNodeConstRef srcNode) {
+  auto dst = resolveRef(dstNode);
+  auto src = resolveRef(srcNode);
+
+  if (dst->getStyle() != src->getStyle()) {
+    dst->setStyle(src->getStyle());
+    dst->markDirtyAndPropagate();
   }
 }
 

packages/react-native/ReactCommon/yoga/yoga/node/Node.h

@@ -375,13 +375,13 @@ class YG_EXPORT Node : public ::YGNode {
   YGBaselineFunc baselineFunc_ = {nullptr};
   YGPrintFunc printFunc_ = {nullptr};
   YGDirtiedFunc dirtiedFunc_ = nullptr;
-  Style style_ = {};
-  LayoutResults layout_ = {};
+  Style style_;
+  LayoutResults layout_;
   size_t lineIndex_ = 0;
   Node* owner_ = nullptr;
-  std::vector<Node*> children_ = {};
+  std::vector<Node*> children_;
   const Config* config_;
-  std::array<Style::Length, 2> resolvedDimensions_ = {
+  std::array<Style::Length, 2> resolvedDimensions_{
       {value::undefined(), value::undefined()}};
 };
 

packages/react-native/ReactCommon/yoga/yoga/style/SmallValueBuffer.h

@@ -0,0 +1,129 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <array>
+#include <bitset>
+#include <cstdint>
+#include <memory>
+#include <vector>
+
+namespace facebook::yoga {
+
+// Container which allows storing 32 or 64 bit integer values, whose index may
+// never change. Values are first stored in a fixed buffer of `BufferSize`
+// 32-bit chunks, before falling back to heap allocation.
+template <size_t BufferSize>
+class SmallValueBuffer {
+ public:
+  SmallValueBuffer() = default;
+  SmallValueBuffer(const SmallValueBuffer& other) {
+    *this = other;
+  }
+  SmallValueBuffer(SmallValueBuffer&& other) = default;
+
+  // Add a new element to the buffer, returning the index of the element
+  uint16_t push(uint32_t value) {
+    const auto index = count_++;
+    if (index < buffer_.size()) {
+      buffer_[index] = value;
+      return index;
+    }
+
+    if (overflow_ == nullptr) {
+      overflow_ = std::make_unique<SmallValueBuffer::Overflow>();
+    }
+
+    overflow_->buffer_.push_back(value);
+    overflow_->wideElements_.push_back(false);
+    return index;
+  }
+
+  uint16_t push(uint64_t value) {
+    const auto lsb = static_cast<uint32_t>(value & 0xFFFFFFFF);
+    const auto msb = static_cast<uint32_t>(value >> 32);
+
+    const auto lsbIndex = push(lsb);
+    push(msb);
+
+    if (lsbIndex < buffer_.size()) {
+      wideElements_[lsbIndex] = true;
+    } else {
+      overflow_->wideElements_[lsbIndex - buffer_.size()] = true;
+    }
+    return lsbIndex;
+  }
+
+  // Replace an existing element in the buffer with a new value. A new index
+  // may be returned, e.g. if a new value is wider than the previous.
+  [[nodiscard]] uint16_t replace(uint16_t index, uint32_t value) {
+    if (index < buffer_.size()) {
+      buffer_[index] = value;
+    } else {
+      overflow_->buffer_.at(index - buffer_.size()) = value;
+    }
+
+    return index;
+  }
+
+  [[nodiscard]] uint16_t replace(uint16_t index, uint64_t value) {
+    const bool isWide = index < wideElements_.size()
+        ? wideElements_[index]
+        : overflow_->wideElements_.at(index - buffer_.size());
+
+    if (isWide) {
+      const auto lsb = static_cast<uint32_t>(value & 0xFFFFFFFF);
+      const auto msb = static_cast<uint32_t>(value >> 32);
+
+      [[maybe_unused]] auto lsbIndex = replace(index, lsb);
+      [[maybe_unused]] auto msbIndex = replace(index + 1, msb);
+      return index;
+    } else {
+      return push(value);
+    }
+  }
+
+  // Get a value of a given width
+  uint32_t get32(uint16_t index) const {
+    if (index < buffer_.size()) {
+      return buffer_[index];
+    } else {
+      return overflow_->buffer_.at(index - buffer_.size());
+    }
+  }
+
+  uint64_t get64(uint16_t index) const {
+    const auto lsb = get32(index);
+    const auto msb = get32(index + 1);
+    return (static_cast<uint64_t>(msb) << 32) | lsb;
+  }
+
+  SmallValueBuffer& operator=(const SmallValueBuffer& other) {
+    count_ = other.count_;
+    buffer_ = other.buffer_;
+    wideElements_ = other.wideElements_;
+    overflow_ = other.overflow_ ? std::make_unique<Overflow>(*other.overflow_)
+                                : nullptr;
+    return *this;
+  }
+
+  SmallValueBuffer& operator=(SmallValueBuffer&& other) = default;
+
+ private:
+  struct Overflow {
+    std::vector<uint32_t> buffer_;
+    std::vector<bool> wideElements_;
+  };
+
+  uint16_t count_{0};
+  std::array<uint32_t, BufferSize> buffer_;
+  std::bitset<BufferSize> wideElements_;
+  std::unique_ptr<Overflow> overflow_;
+};
+
+} // namespace facebook::yoga