d0de9309d6
Before this change, the nsSegmentedBuffer would automatically dispatch the `free` call for each individual segment to another thread, aiming to avoid jank when freeing a large number of buffers on the main thread. This changes that behaviour such that the responsibility to deal with the underlying buffer is now on the caller, which has more context about usage patterns and whether or not to batch the free calls. Differential Revision: https://phabricator.services.mozilla.com/D235246
98 lines
3.0 KiB
C++
98 lines
3.0 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef nsSegmentedBuffer_h__
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#define nsSegmentedBuffer_h__
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#include <stddef.h>
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#include <functional>
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#include "nsCOMPtr.h"
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#include "nsDebug.h"
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#include "nsError.h"
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#include "nsTArray.h"
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#include "mozilla/DataMutex.h"
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#include "mozilla/UniquePtrExtensions.h"
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class nsIEventTarget;
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class nsSegmentedBuffer {
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public:
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nsSegmentedBuffer()
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: mSegmentSize(0),
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mSegmentArray(nullptr),
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mSegmentArrayCount(0),
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mFirstSegmentIndex(0),
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mLastSegmentIndex(0) {}
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~nsSegmentedBuffer() { Clear(); }
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nsresult Init(uint32_t aSegmentSize);
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// pushes at end
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// aSegment must either be null or at least mSegmentSize bytes large.
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char* AppendNewSegment(mozilla::UniqueFreePtr<char> aSegment = nullptr);
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// pops from beginning, and returns the segment
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mozilla::UniqueFreePtr<char> PopFirstSegment();
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// pops from end, and returns the segment
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mozilla::UniqueFreePtr<char> PopLastSegment();
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// Call Realloc() on last segment. This is used to reduce memory
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// consumption when data is not an exact multiple of segment size.
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bool ReallocLastSegment(size_t aNewSize);
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void Clear(); // frees all segments
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inline uint32_t GetSegmentCount() {
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if (mFirstSegmentIndex <= mLastSegmentIndex) {
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return mLastSegmentIndex - mFirstSegmentIndex;
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} else {
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return mSegmentArrayCount + mLastSegmentIndex - mFirstSegmentIndex;
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}
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}
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inline uint32_t GetSegmentSize() { return mSegmentSize; }
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inline char* GetSegment(uint32_t aIndex) {
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NS_ASSERTION(aIndex < GetSegmentCount(), "index out of bounds");
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int32_t i = ModSegArraySize(mFirstSegmentIndex + (int32_t)aIndex);
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return mSegmentArray[i];
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}
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protected:
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inline int32_t ModSegArraySize(int32_t aIndex) {
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uint32_t result = aIndex & (mSegmentArrayCount - 1);
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NS_ASSERTION(result == aIndex % mSegmentArrayCount,
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"non-power-of-2 mSegmentArrayCount");
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return result;
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}
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inline bool IsFull() {
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return ModSegArraySize(mLastSegmentIndex + 1) == mFirstSegmentIndex;
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}
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protected:
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uint32_t mSegmentSize;
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char** mSegmentArray;
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uint32_t mSegmentArrayCount;
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int32_t mFirstSegmentIndex;
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int32_t mLastSegmentIndex;
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};
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// NS_SEGMENTARRAY_INITIAL_SIZE: This number needs to start out as a
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// power of 2 given how it gets used. We double the segment array
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// when we overflow it, and use that fact that it's a power of 2
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// to compute a fast modulus operation in IsFull.
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//
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// 32 segment array entries can accommodate 128k of data if segments
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// are 4k in size. That seems like a reasonable amount that will avoid
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// needing to grow the segment array.
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#define NS_SEGMENTARRAY_INITIAL_COUNT 32
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#endif // nsSegmentedBuffer_h__
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