text: /*
   * Copyright 2020 Google LLC
   *
   * Use of this source code is governed by a BSD-style license that can be
   * found in the LICENSE file.
   */
  
  #ifndef GrBlockAllocator_DEFINED
  #define GrBlockAllocator_DEFINED
  
  #include "include/private/GrTypesPriv.h"
  #include "include/private/SkNoncopyable.h"
  #include "src/core/SkASAN.h"
  
  #include <memory>  // std::unique_ptr
  #include <cstddef> // max_align_t
  
  /**
   * GrBlockAllocator provides low-level support for a block allocated arena with a dynamic tail that
   * tracks space reservations within each block. Its APIs provide the ability to reserve space,
   * resize reservations, and release reservations. It will automatically create new blocks if needed
   * and destroy all remaining blocks when it is destructed. It assumes that anything allocated within
   * its blocks has its destructors called externally. It is recommended that GrBlockAllocator is
   * wrapped by a higher-level allocator that uses the low-level APIs to implement a simpler,
   * purpose-focused API w/o having to worry as much about byte-level concerns.
   *
   * GrBlockAllocator has no limit to its total size, but each allocation is limited to 512MB (which
   * should be sufficient for Ganesh's use cases). This upper allocation limit allows all internal
   * operations to be performed using 'int' and avoid many overflow checks. Static asserts are used
   * to ensure that those operations would not overflow when using the largest possible values.
   *
   * Possible use modes:
   * 1. No upfront allocation, either on the stack or as a field
   *    GrBlockAllocator allocator(policy, heapAllocSize);
   *
   * 2. In-place new'd
   *    void* mem = operator new(totalSize);
   *    GrBlockAllocator* allocator = new (mem) GrBlockAllocator(policy, heapAllocSize,
   *                                                             totalSize- sizeof(GrBlockAllocator));
   *    delete allocator;
   *
   * 3. Use GrSBlockAllocator to increase the preallocation size
   *    GrSBlockAllocator<1024> allocator(policy, heapAllocSize);
   *    sizeof(allocator) == 1024;
   */
  class GrBlockAllocator final : SkNoncopyable {
  public:
      // Largest size that can be requested from allocate(), chosen because it's the largest pow-2
      // that is less than int32_t::max()/2.
      static constexpr int kMaxAllocationSize = 1 << 29;
  
      enum class GrowthPolicy : int {
          kFixed,       // Next block size = N
          kLinear,      //   = #blocks * N
          kFibonacci,   //   = fibonacci(#blocks) * N
          kExponential, //   = 2^#blocks * N
          kLast = kExponential
      };
      static constexpr int kGrowthPolicyCount = static_cast<int>(GrowthPolicy::kLast) + 1;
  
      class Block;
  
      // Tuple representing a range of bytes, marking the unaligned start, the first aligned point
      // after any padding, and the upper limit depending on requested size.
      struct ByteRange {
          Block* fBlock;         // Owning block
          int    fStart;         // Inclusive byte lower limit of byte range
          int    fAlignedOffset; // >= start, matching alignment requirement (i.e. first real byte)
          int    fEnd;           // Exclusive upper limit of byte range
      };
  
      class Block final {
      public:
          ~Block();
          void operator delete(void* p) { ::operator delete(p); }
  
          // Return the maximum allocation size with the given alignment that can fit in this block.
          template <size_t Align = 1, size_t Padding = 0>
          int avail() const { return std::max(0, fSize - this->cursor<Align, Padding>()); }
  
          // Return the aligned offset of the first allocation, assuming it was made with the
          // specified Align, and Padding. The returned offset does not mean a valid allocation
          // starts at that offset, this is a utility function for classes built on top to manage
          // indexing into a block effectively.
          template <size_t Align = 1, size_t Padding = 0>
          int firstAlignedOffset() const { return this->alignedOffset<Align, Padding>(kDataStart); }
  
          // Convert an offset into this block's storage into a usable pointer.
          void* ptr(int offset) {
              SkASSERT(offset >= kDataStart && offset < fSize);
              return reinterpret_cast<char*>(this) + offset;
          }
          const void* ptr(int offset) const { return const_cast<Block*>(this)->ptr(offset); }
  
          // Every block has an extra 'int' for clients to use however they want. It will start
          // at 0 when a new block is made, or when the head block is reset.
          int metadata() const { return fMetadata; }
          void setMetadata(int value) { fMetadata = value; }
  
          /**
           * Release the byte range between offset 'start' (inclusive) and 'end' (exclusive). This
           * will return true if those bytes were successfully reclaimed, i.e. a subsequent allocation
           * request could occupy the space. Regardless of return value, the provided byte range that
           * [start, end) represents should not be used until it's re-allocated with allocate<...>().
           */
          inline bool release(int start, int end);
  
          /**
           * Resize a previously reserved byte range of offset 'start' (inclusive) to 'end'
           * (exclusive). 'deltaBytes' is the SIGNED change to length of the reservation.
           *
           * When negative this means the reservation is shrunk and the new length is (end - start -
           * |deltaBytes|). If this new length would be 0, the byte range can no longer be used (as if
           * it were released instead). Asserts that it would not shrink the reservation below 0.
           *
           * If 'deltaBytes' is positive, the allocator attempts to increase the length of the
           * reservation. If 'deltaBytes' is less than or equal to avail() and it was the last
           * allocation in the block, it can be resized. If there is not enough available bytes to
           * accommodate the increase in size, or another allocation is blocking the increase in size,
           * then false will be returned and the reserved byte range is unmodified.
           */
          inline bool resize(int start, int end, int deltaBytes);
  
      private:
          friend class GrBlockAllocator;
  
          Block(Block* prev, int allocationSize);
  
          // We poison the unallocated space in a Block to allow ASAN to catch invalid writes.
          void poisonRange(int start, int end) {
              sk_asan_poison_memory_region(reinterpret_cast<char*>(this) + start, end - start);
          }
          void unpoisonRange(int start, int end) {
              sk_asan_unpoison_memory_region(reinterpret_cast<char*>(this) + start, end - start);
          }
  
          // Get fCursor, but aligned such that ptr(rval) satisfies Align.
          template <size_t Align, size_t Padding>
          int cursor() const { return this->alignedOffset<Align, Padding>(fCursor); }
  
          template <size_t Align, size_t Padding>
          int alignedOffset(int offset) const;
  
          bool isScratch() const { return fCursor < 0; }
          void markAsScratch() {
              fCursor = -1;
              this->poisonRange(kDataStart, fSize);
          }
  
          SkDEBUGCODE(int fSentinel;) // known value to check for bad back pointers to blocks
  
          Block*          fNext;      // doubly-linked list of blocks
          Block*          fPrev;
  
          // Each block tracks its own cursor because as later blocks are released, an older block
          // may become the active tail again.
          int             fSize;      // includes the size of the BlockHeader and requested metadata
          int             fCursor;    // (this + fCursor) points to next available allocation
          int             fMetadata;
  
          // On release builds, a Block's other 2 pointers and 3 int fields leaves 4 bytes of padding
          // for 8 and 16 aligned systems. Currently this is only manipulated in the head block for
          // an allocator-level metadata and is explicitly not reset when the head block is "released"
          // Down the road we could instead choose to offer multiple metadata slots per block.
          int             fAllocatorMetadata;
      };
  
      // The size of the head block is determined by 'additionalPreallocBytes'. Subsequent heap blocks
      // are determined by 'policy' and 'blockIncrementBytes', although 'blockIncrementBytes' will be
      // aligned to std::max_align_t.
      //
      // When 'additionalPreallocBytes' > 0, the allocator assumes that many extra bytes immediately
      // after the allocator can be used by its inline head block. This is useful when the allocator
      // is in-place new'ed into a larger block of memory, but it should remain set to 0 if stack
      // allocated or if the class layout does not guarantee that space is present.
      GrBlockAllocator(GrowthPolicy policy, size_t blockIncrementBytes,
                       size_t additionalPreallocBytes = 0);
  
      ~GrBlockAllocator() { this->reset(); }
      void operator delete(void* p) { ::operator delete(p); }
  
      /**
       * Helper to calculate the minimum number of bytes needed for heap block size, under the
       * assumption that Align will be the requested alignment of the first call to allocate().
       * Ex. To store N instances of T in a heap block, the 'blockIncrementBytes' should be set to
       *   BlockOverhead<alignof(T)>() + N * sizeof(T) when making the GrBlockAllocator.
       */
      template<size_t Align = 1, size_t Padding = 0>
      static constexpr size_t BlockOverhead();
  
      /**
       * Helper to calculate the minimum number of bytes needed for a preallocation, under the
       * assumption that Align will be the requested alignment of the first call to allocate().
       * Ex. To preallocate a GrSBlockAllocator to hold N instances of T, its arge should be
       *   Overhead<alignof(T)>() + N * sizeof(T)
       */
      template<size_t Align = 1, size_t Padding = 0>
      static constexpr size_t Overhead();
  
      /**
       * Return the total number of bytes of the allocator, including its instance overhead, per-block
       * overhead and space used for allocations.
       */
      size_t totalSize() const;
      /**
       * Return the total number of bytes usable for allocations. This includes bytes that have
       * been reserved already by a call to allocate() and bytes that are still available. It is
       * totalSize() minus all allocator and block-level overhead.
       */
      size_t totalUsableSpace() const;
      /**
       * Return the total number of usable bytes that have been reserved by allocations. This will
       * be less than or equal to totalUsableSpace().
       */
      size_t totalSpaceInUse() const;
  
      /**
       * Return the total number of bytes that were pre-allocated for the GrBlockAllocator. This will
       * include 'additionalPreallocBytes' passed to the constructor, and represents what the total
       * size would become after a call to reset().
       */
      size_t preallocSize() const {
          // Don't double count fHead's Block overhead in both sizeof(GrBlockAllocator) and fSize.
          return sizeof(GrBlockAllocator) + fHead.fSize - BaseHeadBlockSize();
      }
      /**
       * Return the usable size of the inline head block; this will be equal to
       * 'additionalPreallocBytes' plus any alignment padding that the system had to add to Block.
       * The returned value represents what could be allocated before a heap block is be created.
       */
      size_t preallocUsableSpace() const {
          return fHead.fSize - kDataStart;
      }
  
      /**
       * Get the current value of the allocator-level metadata (a user-oriented slot). This is
       * separate from any block-level metadata, but can serve a similar purpose to compactly support
       * data collections on top of GrBlockAllocator.
       */
      int metadata() const { return fHead.fAllocatorMetadata; }
  
      /**
       * Set the current value of the allocator-level metadata.
       */
      void setMetadata(int value) { fHead.fAllocatorMetadata = value; }
  
      /**
       * Reserve space that will hold 'size' bytes. This will automatically allocate a new block if
       * there is not enough available space in the current block to provide 'size' bytes. The
       * returned ByteRange tuple specifies the Block owning the reserved memory, the full byte range,
       * and the aligned offset within that range to use for the user-facing pointer. The following
       * invariants hold:
       *
       *  1. block->ptr(alignedOffset) is aligned to Align
       *  2. end - alignedOffset == size
       *  3. Padding <= alignedOffset - start <= Padding + Align - 1
       *
       * Invariant #3, when Padding > 0, allows intermediate allocators to embed metadata along with
       * the allocations. If the Padding bytes are used for some 'struct Meta', then
       * ptr(alignedOffset - sizeof(Meta)) can be safely used as a Meta* if Meta's alignment
       * requirements are less than or equal to the alignment specified in allocate<>. This can be
       * easily guaranteed by using the pattern:
       *
       *    allocate<max(UserAlign, alignof(Meta)), sizeof(Meta)>(userSize);
       *
       * This ensures that ptr(alignedOffset) will always satisfy UserAlign and
       * ptr(alignedOffset - sizeof(Meta)) will always satisfy alignof(Meta).  Alternatively, memcpy
       * can be used to read and write values between start and alignedOffset without worrying about
       * alignment requirements of the metadata.
       *
       * For over-aligned allocations, the alignedOffset (as an int) may not be a multiple of Align,
       * but the result of ptr(alignedOffset) will be a multiple of Align.
       */
      template <size_t Align, size_t Padding = 0>
      ByteRange allocate(size_t size);
  
      enum ReserveFlags : unsigned {
          // If provided to reserve(), the input 'size' will be rounded up to the next size determined
          // by the growth policy of the GrBlockAllocator. If not, 'size' will be aligned to max_align
          kIgnoreGrowthPolicy_Flag  = 0b01,
          // If provided to reserve(), the number of available bytes of the current block  will not
          // be used to satisfy the reservation (assuming the contiguous range was long enough to
          // begin with).
          kIgnoreExistingBytes_Flag = 0b10,
  
          kNo_ReserveFlags          = 0b00
      };
  
      /**
       * Ensure the block allocator has 'size' contiguous available bytes. After calling this
       * function, currentBlock()->avail<Align, Padding>() may still report less than 'size' if the
       * reserved space was added as a scratch block. This is done so that anything remaining in
       * the current block can still be used if a smaller-than-size allocation is requested. If 'size'
       * is requested by a subsequent allocation, the scratch block will automatically be activated
       * and the request will not itself trigger any malloc.
       *
       * The optional 'flags' controls how the input size is allocated; by default it will attempt
       * to use available contiguous bytes in the current block and will respect the growth policy
       * of the allocator.
       */
      template <size_t Align = 1, size_t Padding = 0>
      void reserve(size_t size, ReserveFlags flags = kNo_ReserveFlags);
  
      /**
       * Return a pointer to the start of the current block. This will never be null.
       */
      const Block* currentBlock() const { return fTail; }
      Block* currentBlock() { return fTail; }
  
      const Block* headBlock() const { return &fHead; }
      Block* headBlock() { return &fHead; }
  
      /**
       * Return the block that owns the allocated 'ptr'. Assuming that earlier, an allocation was
       * returned as {b, start, alignedOffset, end}, and 'p = b->ptr(alignedOffset)', then a call
       * to 'owningBlock<Align, Padding>(p, start) == b'.
       *
       * If calling code has already made a pointer to their metadata, i.e. 'm = p - Padding', then
       * 'owningBlock<Align, 0>(m, start)' will also return b, allowing you to recover the block from
       * the metadata pointer.
       *
       * If calling code has access to the original alignedOffset, this function should not be used
       * since the owning block is just 'p - alignedOffset', regardless of original Align or Padding.
       */
      template <size_t Align, size_t Padding = 0>
      Block* owningBlock(const void* ptr, int start);
  
      template <size_t Align, size_t Padding = 0>
      const Block* owningBlock(const void* ptr, int start) const {
          return const_cast<GrBlockAllocator*>(this)->owningBlock<Align, Padding>(ptr, start);
      }
  
      /**
       * Find the owning block of the allocated pointer, 'p'. Without any additional information this
       * is O(N) on the number of allocated blocks.
       */
      Block* findOwningBlock(const void* ptr);
      const Block* findOwningBlock(const void* ptr) const {
          return const_cast<GrBlockAllocator*>(this)->findOwningBlock(ptr);
      }
  
      /**
       * Explicitly free an entire block, invalidating any remaining allocations from the block.
       * GrBlockAllocator will release all alive blocks automatically when it is destroyed, but this
       * function can be used to reclaim memory over the lifetime of the allocator. The provided
       * 'block' pointer must have previously come from a call to currentBlock() or allocate().
       *
       * If 'block' represents the inline-allocated head block, its cursor and metadata are instead
       * reset to their defaults.
       *
       * If the block is not the head block, it may be kept as a scratch block to be reused for
       * subsequent allocation requests, instead of making an entirely new block. A scratch block is
       * not visible when iterating over blocks but is reported in the total size of the allocator.
       */
      void releaseBlock(Block* block);
  
      /**
       * Detach every heap-allocated block owned by 'other' and concatenate them to this allocator's
       * list of blocks. This memory is now managed by this allocator. Since this only transfers
       * ownership of a Block, and a Block itself does not move, any previous allocations remain
       * valid and associated with their original Block instances. GrBlockAllocator-level functions
       * that accept allocated pointers (e.g. findOwningBlock), must now use this allocator and not
       * 'other' for these allocations.
       *
       * The head block of 'other' cannot be stolen, so higher-level allocators and memory structures
       * must handle that data differently.
       */
      void stealHeapBlocks(GrBlockAllocator* other);
  
      /**
       * Explicitly free all blocks (invalidating all allocations), and resets the head block to its
       * default state. The allocator-level metadata is reset to 0 as well.
       */
      void reset();
  
      /**
       * Remove any reserved scratch space, either from calling reserve() or releaseBlock().
       */
      void resetScratchSpace();
  
      template <bool Forward, bool Const> class BlockIter;
  
      /**
       * Clients can iterate over all active Blocks in the GrBlockAllocator using for loops:
       *
       * Forward iteration from head to tail block (or non-const variant):
       *   for (const Block* b : this->blocks()) { }
       * Reverse iteration from tail to head block:
       *   for (const Block* b : this->rblocks()) { }
       *
       * It is safe to call releaseBlock() on the active block while looping.
       */
      inline BlockIter<true, false> blocks();
      inline BlockIter<true, true> blocks() const;
      inline BlockIter<false, false> rblocks();
      inline BlockIter<false, true> rblocks() const;
  
  #ifdef SK_DEBUG
      static constexpr int kAssignedMarker = 0xBEEFFACE;
      static constexpr int kFreedMarker    = 0xCAFEBABE;
  
      void validate() const;
  #endif
  
  #if GR_TEST_UTILS
      int testingOnly_scratchBlockSize() const { return this->scratchBlockSize(); }
  #endif
  
  private:
      static constexpr int kDataStart = sizeof(Block);
      #ifdef SK_FORCE_8_BYTE_ALIGNMENT
          // This is an issue for WASM builds using emscripten, which had std::max_align_t = 16, but
          // was returning pointers only aligned to 8 bytes.
          // https://github.com/emscripten-core/emscripten/issues/10072
          //
          // Setting this to 8 will let GrBlockAllocator properly correct for the pointer address if
          // a 16-byte aligned allocation is requested in wasm (unlikely since we don't use long
          // doubles).
          static constexpr size_t kAddressAlign = 8;
      #else
          // The alignment Block addresses will be at when created using operator new
          // (spec-compliant is pointers are aligned to max_align_t).
          static constexpr size_t kAddressAlign = alignof(std::max_align_t);
      #endif
  
      // Calculates the size of a new Block required to store a kMaxAllocationSize request for the
      // given alignment and padding bytes. Also represents maximum valid fCursor value in a Block.
      template<size_t Align, size_t Padding>
      static constexpr size_t MaxBlockSize();
  
      static constexpr int BaseHeadBlockSize() {
          return sizeof(GrBlockAllocator) - offsetof(GrBlockAllocator, fHead);
      }
  
      // Append a new block to the end of the block linked list, updating fTail. 'minSize' must
      // have enough room for sizeof(Block). 'maxSize' is the upper limit of fSize for the new block
      // that will preserve the static guarantees GrBlockAllocator makes.
      void addBlock(int minSize, int maxSize);
  
      int scratchBlockSize() const { return fHead.fPrev ? fHead.fPrev->fSize : 0; }
  
      Block* fTail; // All non-head blocks are heap allocated; tail will never be null.
  
      // All remaining state is packed into 64 bits to keep GrBlockAllocator at 16 bytes + head block
      // (on a 64-bit system).
  
      // Growth of the block size is controlled by four factors: BlockIncrement, N0 and N1, and a
      // policy defining how N0 is updated. When a new block is needed, we calculate N1' = N0 + N1.
      // Depending on the policy, N0' = N0 (no growth or linear growth), or N0' = N1 (Fibonacci), or
      // N0' = N1' (exponential). The size of the new block is N1' * BlockIncrement * MaxAlign,
      // after which fN0 and fN1 store N0' and N1' clamped into 23 bits. With current bit allocations,
      // N1' is limited to 2^24, and assuming MaxAlign=16, then BlockIncrement must be '2' in order to
      // eventually reach the hard 2^29 size limit of GrBlockAllocator.
  
      // Next heap block size = (fBlockIncrement * alignof(std::max_align_t) * (fN0 + fN1))
      uint64_t fBlockIncrement : 16;
      uint64_t fGrowthPolicy   : 2;  // GrowthPolicy
      uint64_t fN0             : 23; // = 1 for linear/exp.; = 0 for fixed/fibonacci, initially
      uint64_t fN1             : 23; // = 1 initially
  
      // Inline head block, must be at the end so that it can utilize any additional reserved space
      // from the initial allocation.
      // The head block's prev pointer may be non-null, which signifies a scratch block that may be
      // reused instead of allocating an entirely new block (this helps when allocate+release calls
      // bounce back and forth across the capacity of a block).
      alignas(kAddressAlign) Block fHead;
  
      static_assert(kGrowthPolicyCount <= 4);
  };
  
  // A wrapper around GrBlockAllocator that includes preallocated storage for the head block.
  // N will be the preallocSize() reported by the allocator.
  template<size_t N>
  class GrSBlockAllocator : SkNoncopyable {
  public:
      using GrowthPolicy = GrBlockAllocator::GrowthPolicy;
  
      GrSBlockAllocator() {
          new (fStorage) GrBlockAllocator(GrowthPolicy::kFixed, N, N - sizeof(GrBlockAllocator));
      }
      explicit GrSBlockAllocator(GrowthPolicy policy) {
          new (fStorage) GrBlockAllocator(policy, N, N - sizeof(GrBlockAllocator));
      }
  
      GrSBlockAllocator(GrowthPolicy policy, size_t blockIncrementBytes) {
          new (fStorage) GrBlockAllocator(policy, blockIncrementBytes, N - sizeof(GrBlockAllocator));
      }
  
      ~GrSBlockAllocator() {
          this->allocator()->~GrBlockAllocator();
      }
  
      GrBlockAllocator* operator->() { return this->allocator(); }
      const GrBlockAllocator* operator->() const { return this->allocator(); }
  
      GrBlockAllocator* allocator() { return reinterpret_cast<GrBlockAllocator*>(fStorage); }
      const GrBlockAllocator* allocator() const {
          return reinterpret_cast<const GrBlockAllocator*>(fStorage);
      }
  
  private:
      static_assert(N >= sizeof(GrBlockAllocator));
  
      // Will be used to placement new the allocator
      alignas(GrBlockAllocator) char fStorage[N];
  };
  
  ///////////////////////////////////////////////////////////////////////////////////////////////////
  // Template and inline implementations
  
  GR_MAKE_BITFIELD_OPS(GrBlockAllocator::ReserveFlags)
  
  template<size_t Align, size_t Padding>
  constexpr size_t GrBlockAllocator::BlockOverhead() {
      static_assert(GrAlignTo(kDataStart + Padding, Align) >= sizeof(Block));
      return GrAlignTo(kDataStart + Padding, Align);
  }
  
  template<size_t Align, size_t Padding>
  constexpr size_t GrBlockAllocator::Overhead() {
      // NOTE: On most platforms, GrBlockAllocator is packed; this is not the case on debug builds
      // due to extra fields, or on WASM due to 4byte pointers but 16byte max align.
      return std::max(sizeof(GrBlockAllocator),
                      offsetof(GrBlockAllocator, fHead) + BlockOverhead<Align, Padding>());
  }
  
  template<size_t Align, size_t Padding>
  constexpr size_t GrBlockAllocator::MaxBlockSize() {
      // Without loss of generality, assumes 'align' will be the largest encountered alignment for the
      // allocator (if it's not, the largest align will be encountered by the compiler and pass/fail
      // the same set of static asserts).
      return BlockOverhead<Align, Padding>() + kMaxAllocationSize;
  }
  
  template<size_t Align, size_t Padding>
  void GrBlockAllocator::reserve(size_t size, ReserveFlags flags) {
      if (size > kMaxAllocationSize) {
          SK_ABORT("Allocation too large (%zu bytes requested)", size);
      }
      int iSize = (int) size;
      if ((flags & kIgnoreExistingBytes_Flag) ||
          this->currentBlock()->avail<Align, Padding>() < iSize) {
  
          int blockSize = BlockOverhead<Align, Padding>() + iSize;
          int maxSize = (flags & kIgnoreGrowthPolicy_Flag) ? blockSize
                                                           : MaxBlockSize<Align, Padding>();
          SkASSERT((size_t) maxSize <= (MaxBlockSize<Align, Padding>()));
  
          SkDEBUGCODE(auto oldTail = fTail;)
          this->addBlock(blockSize, maxSize);
          SkASSERT(fTail != oldTail);
          // Releasing the just added block will move it into scratch space, allowing the original
          // tail's bytes to be used first before the scratch block is activated.
          this->releaseBlock(fTail);
      }
  }
  
  template <size_t Align, size_t Padding>
  GrBlockAllocator::ByteRange GrBlockAllocator::allocate(size_t size) {
      // Amount of extra space for a new block to make sure the allocation can succeed.
      static constexpr int kBlockOverhead = (int) BlockOverhead<Align, Padding>();
  
      // Ensures 'offset' and 'end' calculations will be valid
      static_assert((kMaxAllocationSize + GrAlignTo(MaxBlockSize<Align, Padding>(), Align))
                          <= (size_t) std::numeric_limits<int32_t>::max());
      // Ensures size + blockOverhead + addBlock's alignment operations will be valid
      static_assert(kMaxAllocationSize + kBlockOverhead + ((1 << 12) - 1) // 4K align for large blocks
                          <= std::numeric_limits<int32_t>::max());
  
      if (size > kMaxAllocationSize) {
          SK_ABORT("Allocation too large (%zu bytes requested)", size);
      }
  
      int iSize = (int) size;
      int offset = fTail->cursor<Align, Padding>();
      int end = offset + iSize;
      if (end > fTail->fSize) {
          this->addBlock(iSize + kBlockOverhead, MaxBlockSize<Align, Padding>());
          offset = fTail->cursor<Align, Padding>();
          end = offset + iSize;
      }
  
      // Check invariants
      SkASSERT(end <= fTail->fSize);
      SkASSERT(end - offset == iSize);
      SkASSERT(offset - fTail->fCursor >= (int) Padding &&
               offset - fTail->fCursor <= (int) (Padding + Align - 1));
      SkASSERT(reinterpret_cast<uintptr_t>(fTail->ptr(offset)) % Align == 0);
  
      int start = fTail->fCursor;
      fTail->fCursor = end;
  
      fTail->unpoisonRange(offset - Padding, end);
  
      return {fTail, start, offset, end};
  }
  
  template <size_t Align, size_t Padding>
  GrBlockAllocator::Block* GrBlockAllocator::owningBlock(const void* p, int start) {
      // 'p' was originally formed by aligning 'block + start + Padding', producing the inequality:
      //     block + start + Padding <= p <= block + start + Padding + Align-1
      // Rearranging this yields:
      //     block <= p - start - Padding <= block + Align-1
      // Masking these terms by ~(Align-1) reconstructs 'block' if the alignment of the block is
      // greater than or equal to Align (since block & ~(Align-1) == (block + Align-1) & ~(Align-1)
      // in that case). Overalignment does not reduce to inequality unfortunately.
      if /* constexpr */ (Align <= kAddressAlign) {
          Block* block = reinterpret_cast<Block*>(
                  (reinterpret_cast<uintptr_t>(p) - start - Padding) & ~(Align - 1));
          SkASSERT(block->fSentinel == kAssignedMarker);
          return block;
      } else {
          // There's not a constant-time expression available to reconstruct the block from 'p',
          // but this is unlikely to happen frequently.
          return this->findOwningBlock(p);
      }
  }
  
  template <size_t Align, size_t Padding>
  int GrBlockAllocator::Block::alignedOffset(int offset) const {
      static_assert(SkIsPow2(Align));
      // Aligning adds (Padding + Align - 1) as an intermediate step, so ensure that can't overflow
      static_assert(MaxBlockSize<Align, Padding>() + Padding + Align - 1
                          <= (size_t) std::numeric_limits<int32_t>::max());
  
      if /* constexpr */ (Align <= kAddressAlign) {
          // Same as GrAlignTo, but operates on ints instead of size_t
          return (offset + Padding + Align - 1) & ~(Align - 1);
      } else {
          // Must take into account that 'this' may be starting at a pointer that doesn't satisfy the
          // larger alignment request, so must align the entire pointer, not just offset
          uintptr_t blockPtr = reinterpret_cast<uintptr_t>(this);
          uintptr_t alignedPtr = (blockPtr + offset + Padding + Align - 1) & ~(Align - 1);
          SkASSERT(alignedPtr - blockPtr <= (uintptr_t) std::numeric_limits<int32_t>::max());
          return (int) (alignedPtr - blockPtr);
      }
  }
  
  bool GrBlockAllocator::Block::resize(int start, int end, int deltaBytes) {
      SkASSERT(fSentinel == kAssignedMarker);
      SkASSERT(start >= kDataStart && end <= fSize && start < end);
  
      if (deltaBytes > kMaxAllocationSize || deltaBytes < -kMaxAllocationSize) {
          // Cannot possibly satisfy the resize and could overflow subsequent math
          return false;
      }
      if (fCursor == end) {
          int nextCursor = end + deltaBytes;
          SkASSERT(nextCursor >= start);
          // We still check nextCursor >= start for release builds that wouldn't assert.
          if (nextCursor <= fSize && nextCursor >= start) {
              if (nextCursor < fCursor) {
                  // The allocation got smaller; poison the space that can no longer be used.
                  this->poisonRange(nextCursor + 1, end);
              } else {
                  // The allocation got larger; unpoison the space that can now be used.
                  this->unpoisonRange(end, nextCursor);
              }
  
              fCursor = nextCursor;
              return true;
          }
      }
      return false;
  }
  
  // NOTE: release is equivalent to resize(start, end, start - end), and the compiler can optimize
  // most of the operations away, but it wasn't able to remove the unnecessary branch comparing the
  // new cursor to the block size or old start, so release() gets a specialization.
  bool GrBlockAllocator::Block::release(int start, int end) {
      SkASSERT(fSentinel == kAssignedMarker);
      SkASSERT(start >= kDataStart && end <= fSize && start < end);
  
      this->poisonRange(start, end);
  
      if (fCursor == end) {
          fCursor = start;
          return true;
      } else {
          return false;
      }
  }
  
  ///////// Block iteration
  template <bool Forward, bool Const>
  class GrBlockAllocator::BlockIter {
  private:
      using BlockT = typename std::conditional<Const, const Block, Block>::type;
      using AllocatorT =
              typename std::conditional<Const, const GrBlockAllocator, GrBlockAllocator>::type;
  
  public:
      BlockIter(AllocatorT* allocator) : fAllocator(allocator) {}
  
      class Item {
      public:
          bool operator!=(const Item& other) const { return fBlock != other.fBlock; }
  
          BlockT* operator*() const { return fBlock; }
  
          Item& operator++() {
              this->advance(fNext);
              return *this;
          }
  
      private:
          friend BlockIter;
  
          Item(BlockT* block) { this->advance(block); }
  
          void advance(BlockT* block) {
              fBlock = block;
              fNext = block ? (Forward ? block->fNext : block->fPrev) : nullptr;
              if (!Forward && fNext && fNext->isScratch()) {
                  // For reverse-iteration only, we need to stop at the head, not the scratch block
                  // possibly stashed in head->prev.
                  fNext = nullptr;
              }
              SkASSERT(!fNext || !fNext->isScratch());
          }
  
          BlockT* fBlock;
          // Cache this before operator++ so that fBlock can be released during iteration
          BlockT* fNext;
      };
  
      Item begin() const { return Item(Forward ? &fAllocator->fHead : fAllocator->fTail); }
      Item end() const { return Item(nullptr); }
  
  private:
      AllocatorT* fAllocator;
  };
  
  GrBlockAllocator::BlockIter<true, false> GrBlockAllocator::blocks() {
      return BlockIter<true, false>(this);
  }
  GrBlockAllocator::BlockIter<true, true> GrBlockAllocator::blocks() const {
      return BlockIter<true, true>(this);
  }
  GrBlockAllocator::BlockIter<false, false> GrBlockAllocator::rblocks() {
      return BlockIter<false, false>(this);
  }
  GrBlockAllocator::BlockIter<false, true> GrBlockAllocator::rblocks() const {
      return BlockIter<false, true>(this);
  }
  
  #endif // GrBlockAllocator_DEFINED
  