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diff --git a/gfx/layers/mlgpu/SharedBufferMLGPU.h b/gfx/layers/mlgpu/SharedBufferMLGPU.h
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+++ b/gfx/layers/mlgpu/SharedBufferMLGPU.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_gfx_layers_mlgpu_SharedBufferMLGPU_h
+#define mozilla_gfx_layers_mlgpu_SharedBufferMLGPU_h
+
+#include "ShaderDefinitionsMLGPU.h"
+#include "MLGDevice.h"
+#include "MLGDeviceTypes.h"
+#include "StagingBuffer.h"
+#include "mozilla/gfx/Logging.h"
+
+namespace mozilla {
+namespace layers {
+
+class MLGBuffer;
+
+class SharedBufferMLGPU {
+ public:
+  virtual ~SharedBufferMLGPU();
+
+  bool Init();
+
+  // Call before starting a new frame.
+  void Reset();
+
+  // Call to finish any pending uploads.
+  void PrepareForUsage();
+
+ protected:
+  SharedBufferMLGPU(MLGDevice* aDevice, MLGBufferType aType,
+                    size_t aDefaultSize);
+
+  bool EnsureMappedBuffer(size_t aBytes);
+  bool GrowBuffer(size_t aBytes);
+  void ForgetBuffer();
+  bool Map();
+  void Unmap();
+
+  uint8_t* GetBufferPointer(size_t aBytes, ptrdiff_t* aOutOffset,
+                            RefPtr<MLGBuffer>* aOutBuffer);
+
+ protected:
+  // Note: RefPtr here would cause a cycle. Only MLGDevice should own
+  // SharedBufferMLGPU objects for now.
+  MLGDevice* mDevice;
+  MLGBufferType mType;
+  size_t mDefaultSize;
+  bool mCanUseOffsetAllocation;
+
+  // When |mBuffer| is non-null, mMaxSize is the buffer size. If mapped, the
+  // position is between 0 and mMaxSize, otherwise it is always 0.
+  RefPtr<MLGBuffer> mBuffer;
+  ptrdiff_t mCurrentPosition;
+  size_t mMaxSize;
+
+  MLGMappedResource mMap;
+  bool mMapped;
+
+  // These are used to track how many frames come in under the default
+  // buffer size in a row.
+  size_t mBytesUsedThisFrame;
+  size_t mNumSmallFrames;
+};
+
+class VertexBufferSection final {
+  friend class SharedVertexBuffer;
+
+ public:
+  VertexBufferSection();
+
+  uint32_t Stride() const { return mStride; }
+  MLGBuffer* GetBuffer() const { return mBuffer; }
+  ptrdiff_t Offset() const {
+    MOZ_ASSERT(IsValid());
+    return mOffset;
+  }
+  size_t NumVertices() const { return mNumVertices; }
+  bool IsValid() const { return !!mBuffer; }
+
+ protected:
+  void Init(MLGBuffer* aBuffer, ptrdiff_t aOffset, size_t aNumVertices,
+            size_t aStride);
+
+ protected:
+  RefPtr<MLGBuffer> mBuffer;
+  ptrdiff_t mOffset;
+  size_t mNumVertices;
+  size_t mStride;
+};
+
+class ConstantBufferSection final {
+  friend class SharedConstantBuffer;
+
+ public:
+  ConstantBufferSection();
+
+  uint32_t NumConstants() const { return NumConstantsForBytes(mNumBytes); }
+  size_t NumItems() const { return mNumItems; }
+  uint32_t Offset() const {
+    MOZ_ASSERT(IsValid());
+    return mOffset / 16;
+  }
+  MLGBuffer* GetBuffer() const { return mBuffer; }
+  bool IsValid() const { return !!mBuffer; }
+  bool HasOffset() const { return mOffset != -1; }
+
+ protected:
+  static constexpr size_t NumConstantsForBytes(size_t aBytes) {
+    return (aBytes + ((256 - (aBytes % 256)) % 256)) / 16;
+  }
+
+  void Init(MLGBuffer* aBuffer, ptrdiff_t aOffset, size_t aBytes,
+            size_t aNumItems);
+
+ protected:
+  RefPtr<MLGBuffer> mBuffer;
+  ptrdiff_t mOffset;
+  size_t mNumBytes;
+  size_t mNumItems;
+};
+
+// Vertex buffers don't need special alignment.
+typedef StagingBuffer<0> VertexStagingBuffer;
+
+class SharedVertexBuffer final : public SharedBufferMLGPU {
+ public:
+  SharedVertexBuffer(MLGDevice* aDevice, size_t aDefaultSize);
+
+  // Allocate a buffer that can be uploaded immediately.
+  bool Allocate(VertexBufferSection* aHolder,
+                const VertexStagingBuffer& aStaging) {
+    return Allocate(aHolder, aStaging.NumItems(), aStaging.SizeOfItem(),
+                    aStaging.GetBufferStart());
+  }
+
+  // Allocate a buffer that can be uploaded immediately. This is the
+  // direct access version, for cases where a StagingBuffer is not
+  // needed.
+  bool Allocate(VertexBufferSection* aHolder, size_t aNumItems,
+                size_t aSizeOfItem, const void* aData);
+
+  template <typename T>
+  bool Allocate(VertexBufferSection* aHolder, const T& aItem) {
+    return Allocate(aHolder, 1, sizeof(T), &aItem);
+  }
+};
+
+// To support older Direct3D versions, we need to support one-off MLGBuffers,
+// where data is uploaded immediately rather than at the end of all batch
+// preparation. We achieve this through a small helper class.
+//
+// Note: the unmap is not inline sincce we don't include MLGDevice.h.
+class MOZ_STACK_CLASS AutoBufferUploadBase {
+ public:
+  AutoBufferUploadBase();
+  ~AutoBufferUploadBase();
+
+  void Init(void* aPtr) {
+    MOZ_ASSERT(!mPtr && aPtr);
+    mPtr = aPtr;
+  }
+  void Init(void* aPtr, MLGDevice* aDevice, MLGBuffer* aBuffer);
+  void* get() { return const_cast<void*>(mPtr); }
+
+ private:
+  void UnmapBuffer();
+
+ protected:
+  RefPtr<MLGDevice> mDevice;
+  RefPtr<MLGBuffer> mBuffer;
+  void* mPtr;
+};
+
+// This is a typed helper for AutoBufferUploadBase.
+template <typename T>
+class AutoBufferUpload : public AutoBufferUploadBase {
+ public:
+  AutoBufferUpload() = default;
+
+  T* operator->() const { return reinterpret_cast<T*>(mPtr); }
+};
+
+class SharedConstantBuffer final : public SharedBufferMLGPU {
+ public:
+  SharedConstantBuffer(MLGDevice* aDevice, size_t aDefaultSize);
+
+  // Allocate a buffer that can be immediately uploaded.
+  bool Allocate(ConstantBufferSection* aHolder,
+                const ConstantStagingBuffer& aStaging) {
+    MOZ_ASSERT(aStaging.NumItems() * aStaging.SizeOfItem() ==
+               aStaging.NumBytes());
+    return Allocate(aHolder, aStaging.NumItems(), aStaging.SizeOfItem(),
+                    aStaging.GetBufferStart());
+  }
+
+  // Allocate a buffer of one item that can be immediately uploaded.
+  template <typename T>
+  bool Allocate(ConstantBufferSection* aHolder, const T& aItem) {
+    return Allocate(aHolder, 1, sizeof(aItem), &aItem);
+  }
+
+  // Allocate a buffer of N items that can be immediately uploaded.
+  template <typename T>
+  bool Allocate(ConstantBufferSection* aHolder, const T* aItems,
+                size_t aNumItems) {
+    return Allocate(aHolder, aNumItems, sizeof(T), aItems);
+  }
+
+  // Allocate a buffer that is uploaded after the caller has finished writing
+  // to it. This should method should generally not be used unless copying T
+  // is expensive, since the default immediate-upload version has an implicit
+  // extra copy to the GPU. This version exposes the mapped memory directly.
+  template <typename T>
+  bool Allocate(ConstantBufferSection* aHolder, AutoBufferUpload<T>* aPtr) {
+    MOZ_ASSERT(sizeof(T) % 16 == 0, "Items must be padded to 16 bytes");
+
+    return Allocate(aHolder, aPtr, 1, sizeof(T));
+  }
+
+ private:
+  bool Allocate(ConstantBufferSection* aHolder, size_t aNumItems,
+                size_t aSizeOfItem, const void* aData) {
+    AutoBufferUploadBase ptr;
+    if (!Allocate(aHolder, &ptr, aNumItems, aSizeOfItem)) {
+      return false;
+    }
+    memcpy(ptr.get(), aData, aNumItems * aSizeOfItem);
+    return true;
+  }
+
+  bool Allocate(ConstantBufferSection* aHolder, AutoBufferUploadBase* aPtr,
+                size_t aNumItems, size_t aSizeOfItem);
+
+  bool GetBufferPointer(AutoBufferUploadBase* aPtr, size_t aBytes,
+                        ptrdiff_t* aOutOffset, RefPtr<MLGBuffer>* aOutBuffer) {
+    if (!mCanUseOffsetAllocation) {
+      uint8_t* ptr = AllocateNewBuffer(aBytes, aOutOffset, aOutBuffer);
+      if (!ptr) {
+        return false;
+      }
+      aPtr->Init(ptr, mDevice, *aOutBuffer);
+      return true;
+    }
+
+    // Align up the allocation to 256 bytes, since D3D11 requires that
+    // constant buffers start at multiples of 16 elements.
+    size_t alignedBytes = AlignUp<256>::calc(aBytes);
+
+    uint8_t* ptr = SharedBufferMLGPU::GetBufferPointer(alignedBytes, aOutOffset,
+                                                       aOutBuffer);
+    if (!ptr) {
+      return false;
+    }
+
+    aPtr->Init(ptr);
+    return true;
+  }
+
+  uint8_t* AllocateNewBuffer(size_t aBytes, ptrdiff_t* aOutOffset,
+                             RefPtr<MLGBuffer>* aOutBuffer);
+
+ private:
+  size_t mMaxConstantBufferBindSize;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_gfx_layers_mlgpu_SharedBufferMLGPU_h