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https://github.com/mozilla/gecko-dev.git
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643 lines
22 KiB
C++
643 lines
22 KiB
C++
/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
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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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#include "WebGLElementArrayCache.h"
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#include <algorithm>
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#include <cstdlib>
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#include <cstring>
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#include <limits>
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#include "mozilla/Assertions.h"
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#include "mozilla/MathAlgorithms.h"
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#include "mozilla/MemoryReporting.h"
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namespace mozilla {
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static void
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UpdateUpperBound(uint32_t* const out_upperBound, uint32_t newBound)
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{
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MOZ_ASSERT(out_upperBound);
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*out_upperBound = std::max(*out_upperBound, newBound);
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}
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/* WebGLElementArrayCacheTree contains most of the implementation of
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* WebGLElementArrayCache, which performs WebGL element array buffer validation
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* for drawElements.
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*
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* Attention: Here lie nontrivial data structures, bug-prone algorithms, and
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* non-canonical tweaks! Whence the explanatory comments, and compiled unit
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* test.
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*
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* *** What problem are we solving here? ***
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*
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* WebGL::DrawElements has to validate that the elements are in range wrt the
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* current vertex attribs. This boils down to the problem, given an array of
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* integers, of computing the maximum in an arbitrary sub-array. The naive
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* algorithm has linear complexity; this has been a major performance problem,
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* see bug 569431. In that bug, we took the approach of caching the max for the
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* whole array, which does cover most cases (DrawElements typically consumes the
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* whole element array buffer) but doesn't help in other use cases:
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* - when doing "partial DrawElements" i.e. consuming only part of the element
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* array buffer
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* - when doing frequent "partial buffer updates" i.e. bufferSubData calls
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* updating parts of the element array buffer
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*
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* *** The solution: A binary tree ***
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*
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* The solution implemented here is to use a binary tree as the cache data
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* structure. Each tree node contains the max of its two children nodes. In this
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* way, finding the maximum in any contiguous sub-array has log complexity
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* instead of linear complexity.
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*
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* Simplistically, if the element array is:
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*
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* [1 4 3 2]
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*
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* then the corresponding tree is:
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*
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* 4
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* _/ \_
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* 4 3
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* / \ / \
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* 1 4 3 2
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*
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* In practice, the bottom-most levels of the tree are both the largest to store
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* (because they have more nodes), and the least useful performance-wise
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* (because each node in the bottom levels concerns only few entries in the
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* elements array buffer, it is cheap to compute).
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*
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* For this reason, we stop the tree a few levels above, so that each tree leaf
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* actually corresponds to more than one element array entry.
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*
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* The number of levels that we "drop" is |kSkippedBottomTreeLevels| and the
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* number of element array entries that each leaf corresponds to, is
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* |kElementsPerLeaf|. This being a binary tree, we have:
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*
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* kElementsPerLeaf = 2 ^ kSkippedBottomTreeLevels.
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*
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* *** Storage layout of the binary tree ***
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*
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* We take advantage of the specifics of the situation to avoid generalist tree
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* storage and instead store the tree entries in a vector, mTreeData.
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*
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* TreeData is always a vector of length:
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*
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* 2 * (number of leaves).
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*
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* Its data layout is as follows: mTreeData[0] is unused, mTreeData[1] is the
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* root node, then at offsets 2..3 is the tree level immediately below the root
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* node, then at offsets 4..7 is the tree level below that, etc.
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*
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* The figure below illustrates this by writing at each tree node the offset
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* into mTreeData at which it is stored:
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*
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* 1
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* _/ \_
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* 2 3
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* / \ / \
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* 4 5 6 7
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* ...
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*
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* Thus, under the convention that the root level is level 0, we see that level
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* N is stored at offsets:
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*
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* [ 2^n .. 2^(n+1) - 1 ]
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*
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* in mTreeData. Likewise, all the usual tree operations have simple
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* mathematical expressions in terms of mTreeData offsets, see all the methods
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* such as ParentNode, LeftChildNode, etc.
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*
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* *** Design constraint: Element types aren't known at buffer-update time ***
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*
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* Note that a key constraint that we're operating under, is that we don't know
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* the types of the elements by the time WebGL bufferData/bufferSubData methods
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* are called. The type of elements is only specified in the drawElements call.
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* This means that we may potentially have to store caches for multiple element
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* types, for the same element array buffer. Since we don't know yet how many
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* element types we'll eventually support (extensions add more), the concern
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* about memory usage is serious. This is addressed by kSkippedBottomTreeLevels
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* as explained above. Of course, in the typical case where each element array
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* buffer is only ever used with one type, this is also addressed by having
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* WebGLElementArrayCache lazily create trees for each type only upon first use.
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*
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* Another consequence of this constraint is that when updating the trees, we
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* have to update all existing trees. So if trees for types uint8_t, uint16_t
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* and uint32_t have ever been constructed for this buffer, every subsequent
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* update will have to update all trees even if one of the types is never used
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* again. That's inefficient, but content should not put indices of different
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* types in the same element array buffer anyways. Different index types can
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* only be consumed in separate drawElements calls, so nothing particular is
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* to be achieved by lumping them in the same buffer object.
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*/
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template<typename T>
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struct WebGLElementArrayCacheTree
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{
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/* A too-high kSkippedBottomTreeLevels would harm the performance of small
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* drawElements calls. A too-low kSkippedBottomTreeLevels would cause undue
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* memory usage. The current value has been validated by some benchmarking.
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* See bug 732660.
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*/
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static const size_t kSkippedBottomTreeLevels = 3;
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static const size_t kElementsPerLeaf = 1 << kSkippedBottomTreeLevels;
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// Since kElementsPerLeaf is POT:
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static const size_t kElementsPerLeafMask = kElementsPerLeaf - 1;
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private:
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// The WebGLElementArrayCache that owns this tree:
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WebGLElementArrayCache& mParent;
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// The tree's internal data storage. Its length is 2 * (number of leaves)
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// because of its data layout explained in the above class comment.
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FallibleTArray<T> mTreeData;
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public:
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// Constructor. Takes a reference to the WebGLElementArrayCache that is to be
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// the parent. Does not initialize the tree. Should be followed by a call
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// to Update() to attempt initializing the tree.
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explicit WebGLElementArrayCacheTree(WebGLElementArrayCache& value)
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: mParent(value)
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{
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}
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T GlobalMaximum() const {
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return mTreeData[1];
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}
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// returns the index of the parent node; if treeIndex=1 (the root node),
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// the return value is 0.
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static size_t ParentNode(size_t treeIndex) {
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MOZ_ASSERT(treeIndex > 1);
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return treeIndex >> 1;
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}
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static bool IsRightNode(size_t treeIndex) {
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MOZ_ASSERT(treeIndex > 1);
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return treeIndex & 1;
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}
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static bool IsLeftNode(size_t treeIndex) {
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MOZ_ASSERT(treeIndex > 1);
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return !IsRightNode(treeIndex);
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}
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static size_t SiblingNode(size_t treeIndex) {
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MOZ_ASSERT(treeIndex > 1);
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return treeIndex ^ 1;
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}
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static size_t LeftChildNode(size_t treeIndex) {
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MOZ_ASSERT(treeIndex);
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return treeIndex << 1;
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}
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static size_t RightChildNode(size_t treeIndex) {
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MOZ_ASSERT(treeIndex);
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return SiblingNode(LeftChildNode(treeIndex));
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}
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static size_t LeftNeighborNode(size_t treeIndex, size_t distance = 1) {
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MOZ_ASSERT(treeIndex > 1);
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return treeIndex - distance;
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}
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static size_t RightNeighborNode(size_t treeIndex, size_t distance = 1) {
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MOZ_ASSERT(treeIndex > 1);
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return treeIndex + distance;
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}
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size_t NumLeaves() const {
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// See class comment for why we the tree storage size is 2 * numLeaves.
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return mTreeData.Length() >> 1;
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}
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size_t LeafForElement(size_t element) const {
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size_t leaf = element / kElementsPerLeaf;
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MOZ_ASSERT(leaf < NumLeaves());
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return leaf;
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}
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size_t LeafForByte(size_t byte) const {
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return LeafForElement(byte / sizeof(T));
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}
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// Returns the index, into the tree storage, where a given leaf is stored.
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size_t TreeIndexForLeaf(size_t leaf) const {
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// See above class comment. The tree storage is an array of length
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// 2 * numLeaves. The leaves are stored in its second half.
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return leaf + NumLeaves();
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}
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static size_t LastElementUnderSameLeaf(size_t element) {
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return element | kElementsPerLeafMask;
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}
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static size_t FirstElementUnderSameLeaf(size_t element) {
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return element & ~kElementsPerLeafMask;
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}
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static size_t NextMultipleOfElementsPerLeaf(size_t numElements) {
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MOZ_ASSERT(numElements >= 1);
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return ((numElements - 1) | kElementsPerLeafMask) + 1;
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}
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bool Validate(T maxAllowed, size_t firstLeaf, size_t lastLeaf,
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uint32_t* const out_upperBound)
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{
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size_t firstTreeIndex = TreeIndexForLeaf(firstLeaf);
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size_t lastTreeIndex = TreeIndexForLeaf(lastLeaf);
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while (true) {
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// Given that we tweak these values in nontrivial ways, it doesn't
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// hurt to do this sanity check.
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MOZ_ASSERT(firstTreeIndex <= lastTreeIndex);
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// Final case where there is only one node to validate at the
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// current tree level:
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if (lastTreeIndex == firstTreeIndex) {
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const T& curData = mTreeData[firstTreeIndex];
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UpdateUpperBound(out_upperBound, curData);
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return curData <= maxAllowed;
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}
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// If the first node at current tree level is a right node, handle
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// it individually and replace it with its right neighbor, which is
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// a left node.
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if (IsRightNode(firstTreeIndex)) {
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const T& curData = mTreeData[firstTreeIndex];
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UpdateUpperBound(out_upperBound, curData);
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if (curData > maxAllowed)
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return false;
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firstTreeIndex = RightNeighborNode(firstTreeIndex);
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}
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// If the last node at current tree level is a left node, handle it
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// individually and replace it with its left neighbor, which is a
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// right node.
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if (IsLeftNode(lastTreeIndex)) {
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const T& curData = mTreeData[lastTreeIndex];
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UpdateUpperBound(out_upperBound, curData);
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if (curData > maxAllowed)
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return false;
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lastTreeIndex = LeftNeighborNode(lastTreeIndex);
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}
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/* At this point it can happen that firstTreeIndex and lastTreeIndex
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* "crossed" eachother. That happens if firstTreeIndex was a right
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* node and lastTreeIndex was its right neighor: In that case, both
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* above tweaks happened and as a result, they ended up being
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* swapped: LastTreeIndex is now the _left_ neighbor of
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* firstTreeIndex. When that happens, there is nothing left to
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* validate.
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*/
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if (lastTreeIndex == LeftNeighborNode(firstTreeIndex))
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return true;
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// Walk up one level.
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firstTreeIndex = ParentNode(firstTreeIndex);
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lastTreeIndex = ParentNode(lastTreeIndex);
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}
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}
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// Updates the tree from the parent's buffer contents. Fallible, as it
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// may have to resize the tree storage.
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bool Update(size_t firstByte, size_t lastByte);
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size_t SizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const
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{
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return mallocSizeOf(this) + mTreeData.SizeOfExcludingThis(mallocSizeOf);
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}
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};
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// TreeForType: just a template helper to select the right tree object for a given
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// element type.
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template<typename T>
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struct TreeForType {};
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template<>
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struct TreeForType<uint8_t>
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{
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static ScopedDeletePtr<WebGLElementArrayCacheTree<uint8_t>>&
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Value(WebGLElementArrayCache* b) {
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return b->mUint8Tree;
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}
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};
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template<>
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struct TreeForType<uint16_t>
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{
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static ScopedDeletePtr<WebGLElementArrayCacheTree<uint16_t>>&
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Value(WebGLElementArrayCache* b) {
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return b->mUint16Tree;
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}
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};
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template<>
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struct TreeForType<uint32_t>
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{
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static ScopedDeletePtr<WebGLElementArrayCacheTree<uint32_t>>&
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Value(WebGLElementArrayCache* b) {
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return b->mUint32Tree;
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}
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};
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// Calling this method will 1) update the leaves in this interval
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// from the raw buffer data, and 2) propagate this update up the tree.
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template<typename T>
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bool
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WebGLElementArrayCacheTree<T>::Update(size_t firstByte, size_t lastByte)
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{
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MOZ_ASSERT(firstByte <= lastByte);
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MOZ_ASSERT(lastByte < mParent.mBytes.Length());
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size_t numberOfElements = mParent.mBytes.Length() / sizeof(T);
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size_t requiredNumLeaves = 0;
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if (numberOfElements > 0) {
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/* If we didn't require the number of leaves to be a power of two, then
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* it would just be equal to
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*
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* ceil(numberOfElements / kElementsPerLeaf)
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*
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* The way we implement this (division+ceil) operation in integer
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* arithmetic
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* is as follows:
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*/
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size_t numLeavesNonPOT = (numberOfElements + kElementsPerLeaf - 1) / kElementsPerLeaf;
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// It only remains to round that up to the next power of two:
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requiredNumLeaves = RoundUpPow2(numLeavesNonPOT);
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}
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// Step #0: If needed, resize our tree data storage.
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if (requiredNumLeaves != NumLeaves()) {
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// See class comment for why we the tree storage size is 2 * numLeaves.
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if (!mTreeData.SetLength(2 * requiredNumLeaves)) {
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mTreeData.SetLength(0);
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return false;
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}
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MOZ_ASSERT(NumLeaves() == requiredNumLeaves);
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if (NumLeaves()) {
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// When resizing, update the whole tree, not just the subset
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// corresponding to the part of the buffer being updated.
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memset(mTreeData.Elements(), 0, mTreeData.Length() * sizeof(T));
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firstByte = 0;
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lastByte = mParent.mBytes.Length() - 1;
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}
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}
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if (NumLeaves() == 0)
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return true;
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lastByte = std::min(lastByte, NumLeaves() * kElementsPerLeaf * sizeof(T) - 1);
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if (firstByte > lastByte)
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return true;
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size_t firstLeaf = LeafForByte(firstByte);
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size_t lastLeaf = LeafForByte(lastByte);
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MOZ_ASSERT(firstLeaf <= lastLeaf && lastLeaf < NumLeaves());
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size_t firstTreeIndex = TreeIndexForLeaf(firstLeaf);
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size_t lastTreeIndex = TreeIndexForLeaf(lastLeaf);
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// Step #1: Initialize the tree leaves from plain buffer data.
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// That is, each tree leaf must be set to the max of the |kElementsPerLeaf|
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// corresponding buffer entries.
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// Condition-less scope to prevent leaking this scope's variables into the
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// code below:
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{
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// TreeIndex is the index of the tree leaf we're writing, i.e. the
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// destination index.
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size_t treeIndex = firstTreeIndex;
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// srcIndex is the index in the source buffer.
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size_t srcIndex = firstLeaf * kElementsPerLeaf;
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while (treeIndex <= lastTreeIndex) {
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T m = 0;
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size_t a = srcIndex;
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size_t srcIndexNextLeaf = std::min(a + kElementsPerLeaf, numberOfElements);
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for (; srcIndex < srcIndexNextLeaf; srcIndex++) {
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m = std::max(m, mParent.Element<T>(srcIndex));
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}
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mTreeData[treeIndex] = m;
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treeIndex++;
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}
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}
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// Step #2: Propagate the values up the tree. This is simply a matter of
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// walking up the tree and setting each node to the max of its two children.
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while (firstTreeIndex > 1) {
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// Move up one level.
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firstTreeIndex = ParentNode(firstTreeIndex);
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lastTreeIndex = ParentNode(lastTreeIndex);
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// Fast-exit case where only one node is updated at the current level.
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if (firstTreeIndex == lastTreeIndex) {
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mTreeData[firstTreeIndex] = std::max(mTreeData[LeftChildNode(firstTreeIndex)], mTreeData[RightChildNode(firstTreeIndex)]);
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continue;
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}
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size_t child = LeftChildNode(firstTreeIndex);
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size_t parent = firstTreeIndex;
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while (parent <= lastTreeIndex) {
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T a = mTreeData[child];
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child = RightNeighborNode(child);
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T b = mTreeData[child];
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child = RightNeighborNode(child);
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mTreeData[parent] = std::max(a, b);
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parent = RightNeighborNode(parent);
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}
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}
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return true;
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}
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WebGLElementArrayCache::WebGLElementArrayCache()
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{
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}
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WebGLElementArrayCache::~WebGLElementArrayCache()
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{
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}
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bool
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WebGLElementArrayCache::BufferData(const void* ptr, size_t byteLength)
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{
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if (mBytes.Length() != byteLength) {
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if (!mBytes.SetLength(byteLength)) {
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mBytes.SetLength(0);
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return false;
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}
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}
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MOZ_ASSERT(mBytes.Length() == byteLength);
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return BufferSubData(0, ptr, byteLength);
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}
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bool
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WebGLElementArrayCache::BufferSubData(size_t pos, const void* ptr,
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size_t updateByteLength)
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{
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MOZ_ASSERT(pos + updateByteLength <= mBytes.Length());
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if (!updateByteLength)
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return true;
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if (ptr)
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memcpy(mBytes.Elements() + pos, ptr, updateByteLength);
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else
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memset(mBytes.Elements() + pos, 0, updateByteLength);
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return UpdateTrees(pos, pos + updateByteLength - 1);
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}
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bool
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WebGLElementArrayCache::UpdateTrees(size_t firstByte, size_t lastByte)
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{
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bool result = true;
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if (mUint8Tree)
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result &= mUint8Tree->Update(firstByte, lastByte);
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if (mUint16Tree)
|
|
result &= mUint16Tree->Update(firstByte, lastByte);
|
|
if (mUint32Tree)
|
|
result &= mUint32Tree->Update(firstByte, lastByte);
|
|
return result;
|
|
}
|
|
|
|
template<typename T>
|
|
bool
|
|
WebGLElementArrayCache::Validate(uint32_t maxAllowed, size_t firstElement,
|
|
size_t countElements,
|
|
uint32_t* const out_upperBound)
|
|
{
|
|
*out_upperBound = 0;
|
|
|
|
// If maxAllowed is >= the max T value, then there is no way that a T index
|
|
// could be invalid.
|
|
uint32_t maxTSize = std::numeric_limits<T>::max();
|
|
if (maxAllowed >= maxTSize) {
|
|
UpdateUpperBound(out_upperBound, maxTSize);
|
|
return true;
|
|
}
|
|
|
|
T maxAllowedT(maxAllowed);
|
|
|
|
// Integer overflow must have been handled earlier, so we assert that
|
|
// maxAllowedT is exactly the max allowed value.
|
|
MOZ_ASSERT(uint32_t(maxAllowedT) == maxAllowed);
|
|
|
|
if (!mBytes.Length() || !countElements)
|
|
return true;
|
|
|
|
ScopedDeletePtr<WebGLElementArrayCacheTree<T>>& tree = TreeForType<T>::Value(this);
|
|
if (!tree) {
|
|
tree = new WebGLElementArrayCacheTree<T>(*this);
|
|
if (mBytes.Length()) {
|
|
bool valid = tree->Update(0, mBytes.Length() - 1);
|
|
if (!valid) {
|
|
// Do not assert here. This case would happen if an allocation
|
|
// failed. We've already settled on fallible allocations around
|
|
// here.
|
|
tree = nullptr;
|
|
return false;
|
|
}
|
|
}
|
|
}
|
|
|
|
size_t lastElement = firstElement + countElements - 1;
|
|
|
|
// Fast-exit path when the global maximum for the whole element array buffer
|
|
// falls in the allowed range:
|
|
T globalMax = tree->GlobalMaximum();
|
|
if (globalMax <= maxAllowedT) {
|
|
UpdateUpperBound(out_upperBound, globalMax);
|
|
return true;
|
|
}
|
|
|
|
const T* elements = Elements<T>();
|
|
|
|
// Before calling tree->Validate, we have to validate ourselves the
|
|
// boundaries of the elements span, to round them to the nearest multiple of
|
|
// kElementsPerLeaf.
|
|
size_t firstElementAdjustmentEnd = std::min(lastElement,
|
|
tree->LastElementUnderSameLeaf(firstElement));
|
|
while (firstElement <= firstElementAdjustmentEnd) {
|
|
const T& curData = elements[firstElement];
|
|
UpdateUpperBound(out_upperBound, curData);
|
|
if (curData > maxAllowedT)
|
|
return false;
|
|
|
|
firstElement++;
|
|
}
|
|
size_t lastElementAdjustmentEnd = std::max(firstElement,
|
|
tree->FirstElementUnderSameLeaf(lastElement));
|
|
while (lastElement >= lastElementAdjustmentEnd) {
|
|
const T& curData = elements[lastElement];
|
|
UpdateUpperBound(out_upperBound, curData);
|
|
if (curData > maxAllowedT)
|
|
return false;
|
|
|
|
lastElement--;
|
|
}
|
|
|
|
// at this point, for many tiny validations, we're already done.
|
|
if (firstElement > lastElement)
|
|
return true;
|
|
|
|
// general case
|
|
return tree->Validate(maxAllowedT, tree->LeafForElement(firstElement),
|
|
tree->LeafForElement(lastElement), out_upperBound);
|
|
}
|
|
|
|
bool
|
|
WebGLElementArrayCache::Validate(GLenum type, uint32_t maxAllowed,
|
|
size_t firstElement, size_t countElements,
|
|
uint32_t* const out_upperBound)
|
|
{
|
|
if (type == LOCAL_GL_UNSIGNED_BYTE)
|
|
return Validate<uint8_t>(maxAllowed, firstElement, countElements,
|
|
out_upperBound);
|
|
if (type == LOCAL_GL_UNSIGNED_SHORT)
|
|
return Validate<uint16_t>(maxAllowed, firstElement, countElements,
|
|
out_upperBound);
|
|
if (type == LOCAL_GL_UNSIGNED_INT)
|
|
return Validate<uint32_t>(maxAllowed, firstElement, countElements,
|
|
out_upperBound);
|
|
|
|
MOZ_ASSERT(false, "Invalid type.");
|
|
return false;
|
|
}
|
|
|
|
template<typename T>
|
|
static size_t
|
|
SizeOfNullable(mozilla::MallocSizeOf mallocSizeOf, const T& obj)
|
|
{
|
|
if (!obj)
|
|
return 0;
|
|
return obj->SizeOfIncludingThis(mallocSizeOf);
|
|
}
|
|
|
|
size_t
|
|
WebGLElementArrayCache::SizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const
|
|
{
|
|
return mallocSizeOf(this) +
|
|
mBytes.SizeOfExcludingThis(mallocSizeOf) +
|
|
SizeOfNullable(mallocSizeOf, mUint8Tree) +
|
|
SizeOfNullable(mallocSizeOf, mUint16Tree) +
|
|
SizeOfNullable(mallocSizeOf, mUint32Tree);
|
|
}
|
|
|
|
bool
|
|
WebGLElementArrayCache::BeenUsedWithMultipleTypes() const
|
|
{
|
|
// C++ Standard ($4.7)
|
|
// "If the source type is bool, the value false is converted to zero and
|
|
// the value true is converted to one."
|
|
const int num_types_used = (mUint8Tree != nullptr) +
|
|
(mUint16Tree != nullptr) +
|
|
(mUint32Tree != nullptr);
|
|
return num_types_used > 1;
|
|
}
|
|
|
|
} // end namespace mozilla
|