The Tor Browser: comparison content/canvas/src/WebGLElementArrayCache.cpp

--1:000000000000
+:4b3130eec5b3
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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/. */
+#include "WebGLElementArrayCache.h"
+#include "nsTArray.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/MemoryReporting.h"
+#include <cstdlib>
+#include <cstring>
+#include <limits>
+#include <algorithm>
+namespace mozilla {
+static void
+SetUpperBound(uint32_t* out_upperBound, uint32_t newBound)
+{
+if (!out_upperBound)
+return;
+*out_upperBound = newBound;
+}
+static void
+UpdateUpperBound(uint32_t* out_upperBound, uint32_t newBound)
+{
+if (!out_upperBound)
+return;
+*out_upperBound = std::max(*out_upperBound, newBound);
+}
+/*
+* WebGLElementArrayCacheTree contains most of the implementation of WebGLElementArrayCache,
+* which performs WebGL element array buffer validation for drawElements.
+*
+* Attention: Here lie nontrivial data structures, bug-prone algorithms, and non-canonical tweaks!
+* Whence the explanatory comments, and compiled unit test.
+*
+* *** What problem are we solving here? ***
+*
+* WebGL::DrawElements has to validate that the elements are in range wrt the current vertex attribs.
+* This boils down to the problem, given an array of integers, of computing the maximum in an arbitrary
+* sub-array. The naive algorithm has linear complexity; this has been a major performance problem,
+* see bug 569431. In that bug, we took the approach of caching the max for the whole array, which
+* does cover most cases (DrawElements typically consumes the whole element array buffer) but doesn't
+* help in other use cases:
+*  - when doing "partial DrawElements" i.e. consuming only part of the element array buffer
+*  - when doing frequent "partial buffer updates" i.e. bufferSubData calls updating parts of the
+*    element array buffer
+*
+* *** The solution: a binary tree ***
+*
+* The solution implemented here is to use a binary tree as the cache data structure. Each tree node
+* contains the max of its two children nodes. In this way, finding the maximum in any contiguous sub-array
+* has log complexity instead of linear complexity.
+*
+* Simplistically, if the element array is
+*
+*     1   4   3   2
+*
+* then the corresponding tree is
+*
+*           4
+*         _/ \_
+*       4       3
+*      / \     / \
+*     1   4   3   2
+*
+* In practice, the bottom-most levels of the tree are both the largest to store (because they
+* have more nodes), and the least useful performance-wise (because each node in the bottom
+* levels concerns only few entries in the elements array buffer, it is cheap to compute).
+*
+* For this reason, we stop the tree a few levels above, so that each tree leaf actually corresponds
+* to more than one element array entry.
+*
+* The number of levels that we "drop" is |sSkippedBottomTreeLevels| and the number of element array entries
+* that each leaf corresponds to, is |sElementsPerLeaf|. This being a binary tree, we have
+*
+*   sElementsPerLeaf = 2 ^ sSkippedBottomTreeLevels.
+*
+* *** Storage layout of the binary tree ***
+*
+* We take advantage of the specifics of the situation to avoid generalist tree storage and instead
+* store the tree entries in a vector, mTreeData.
+*
+* The number of leaves is given by mNumLeaves, and mTreeData is always a vector of length
+*
+*    2 * mNumLeaves.
+*
+* Its data layout is as follows: mTreeData[0] is unused, mTreeData[1] is the root node,
+* then at offsets 2..3 is the tree level immediately below the root node, then at offsets 4..7
+* is the tree level below that, etc.
+*
+* The figure below illustrates this by writing at each tree node the offset into mTreeData at
+* which it is stored:
+*
+*           1
+*         _/ \_
+*       2       3
+*      / \     / \
+*     4   5   6   7
+*    ...
+*
+* Thus, under the convention that the root level is level 0, we see that level N is stored at offsets
+*
+*    [ 2^n .. 2^(n+1) - 1 ]
+*
+* in mTreeData. Likewise, all the usual tree operations have simple mathematical expressions in
+* terms of mTreeData offsets, see all the methods such as ParentNode, LeftChildNode, etc.
+*
+* *** Design constraint: element types aren't known at buffer-update time ***
+*
+* Note that a key constraint that we're operating under, is that we don't know the types of the elements
+* by the time WebGL bufferData/bufferSubData methods are called. The type of elements is only
+* specified in the drawElements call. This means that we may potentially have to store caches for
+* multiple element types, for the same element array buffer. Since we don't know yet how many
+* element types we'll eventually support (extensions add more), the concern about memory usage is serious.
+* This is addressed by sSkippedBottomTreeLevels as explained above. Of course, in the typical
+* case where each element array buffer is only ever used with one type, this is also addressed
+* by having WebGLElementArrayCache lazily create trees for each type only upon first use.
+*
+* Another consequence of this constraint is that when invalidating the trees, we have to invalidate
+* all existing trees. So if trees for types uint8_t, uint16_t and uint32_t have ever been constructed for this buffer,
+* every subsequent invalidation will have to invalidate all trees even if one of the types is never
+* used again. This implies that it is important to minimize the cost of invalidation i.e.
+* do lazy updates upon use as opposed to immediately updating invalidated trees. This poses a problem:
+* it is nontrivial to keep track of the part of the tree that's invalidated. The current solution
+* can only keep track of an invalidated interval, from |mFirstInvalidatedLeaf| to |mLastInvalidatedLeaf|.
+* The problem is that if one does two small, far-apart partial buffer updates, the resulting invalidated
+* area is very large even though only a small part of the array really needed to be invalidated.
+* The real solution to this problem would be to use a smarter data structure to keep track of the
+* invalidated area, probably an interval tree. Meanwhile, we can probably live with the current situation
+* as the unfavorable case seems to be a small corner case: in order to run into performance issues,
+* the number of bufferSubData in between two consecutive draws must be small but greater than 1, and
+* the partial buffer updates must be small and far apart. Anything else than this corner case
+* should run fast in the current setting.
+*/
+template<typename T>
+struct WebGLElementArrayCacheTree
+{
+// A too-high sSkippedBottomTreeLevels would harm the performance of small drawElements calls
+// A too-low sSkippedBottomTreeLevels would cause undue memory usage.
+// The current value has been validated by some benchmarking. See bug 732660.
+static const size_t sSkippedBottomTreeLevels = 3;
+static const size_t sElementsPerLeaf = 1 << sSkippedBottomTreeLevels;
+static const size_t sElementsPerLeafMask = sElementsPerLeaf - 1; // sElementsPerLeaf is POT
+private:
+WebGLElementArrayCache& mParent;
+FallibleTArray<T> mTreeData;
+size_t mNumLeaves;
+bool mInvalidated;
+size_t mFirstInvalidatedLeaf;
+size_t mLastInvalidatedLeaf;
+public:
+WebGLElementArrayCacheTree(WebGLElementArrayCache& p)
+: mParent(p)
+, mNumLeaves(0)
+, mInvalidated(false)
+, mFirstInvalidatedLeaf(0)
+, mLastInvalidatedLeaf(0)
+{
+ResizeToParentSize();
+}
+T GlobalMaximum() const {
+MOZ_ASSERT(!mInvalidated);
+return mTreeData[1];
+}
+// returns the index of the parent node; if treeIndex=1 (the root node),
+// the return value is 0.
+static size_t ParentNode(size_t treeIndex) {
+MOZ_ASSERT(treeIndex > 1);
+return treeIndex >> 1;
+}
+static bool IsRightNode(size_t treeIndex) {
+MOZ_ASSERT(treeIndex > 1);
+return treeIndex & 1;
+}
+static bool IsLeftNode(size_t treeIndex) {
+MOZ_ASSERT(treeIndex > 1);
+return !IsRightNode(treeIndex);
+}
+static size_t SiblingNode(size_t treeIndex) {
+MOZ_ASSERT(treeIndex > 1);
+return treeIndex ^ 1;
+}
+static size_t LeftChildNode(size_t treeIndex) {
+MOZ_ASSERT(treeIndex);
+return treeIndex << 1;
+}
+static size_t RightChildNode(size_t treeIndex) {
+MOZ_ASSERT(treeIndex);
+return SiblingNode(LeftChildNode(treeIndex));
+}
+static size_t LeftNeighborNode(size_t treeIndex, size_t distance = 1) {
+MOZ_ASSERT(treeIndex > 1);
+return treeIndex - distance;
+}
+static size_t RightNeighborNode(size_t treeIndex, size_t distance = 1) {
+MOZ_ASSERT(treeIndex > 1);
+return treeIndex + distance;
+}
+size_t LeafForElement(size_t element) {
+size_t leaf = element / sElementsPerLeaf;
+MOZ_ASSERT(leaf < mNumLeaves);
+return leaf;
+}
+size_t LeafForByte(size_t byte) {
+return LeafForElement(byte / sizeof(T));
+}
+// Returns the index, into the tree storage, where a given leaf is stored
+size_t TreeIndexForLeaf(size_t leaf) {
+// See above class comment. The tree storage is an array of length 2*mNumLeaves.
+// The leaves are stored in its second half.
+return leaf + mNumLeaves;
+}
+static size_t LastElementUnderSameLeaf(size_t element) {
+return element | sElementsPerLeafMask;
+}
+static size_t FirstElementUnderSameLeaf(size_t element) {
+return element & ~sElementsPerLeafMask;
+}
+static size_t NextMultipleOfElementsPerLeaf(size_t numElements) {
+return ((numElements - 1) | sElementsPerLeafMask) + 1;
+}
+bool Validate(T maxAllowed, size_t firstLeaf, size_t lastLeaf,
+uint32_t* out_upperBound)
+{
+MOZ_ASSERT(!mInvalidated);
+size_t firstTreeIndex = TreeIndexForLeaf(firstLeaf);
+size_t lastTreeIndex  = TreeIndexForLeaf(lastLeaf);
+while (true) {
+// given that we tweak these values in nontrivial ways, it doesn't hurt to do
+// this sanity check
+MOZ_ASSERT(firstTreeIndex <= lastTreeIndex);
+// final case where there is only 1 node to validate at the current tree level
+if (lastTreeIndex == firstTreeIndex) {
+const T& curData = mTreeData[firstTreeIndex];
+UpdateUpperBound(out_upperBound, curData);
+return curData <= maxAllowed;
+}
+// if the first node at current tree level is a right node, handle it individually
+// and replace it with its right neighbor, which is a left node
+if (IsRightNode(firstTreeIndex)) {
+const T& curData = mTreeData[firstTreeIndex];
+UpdateUpperBound(out_upperBound, curData);
+if (curData > maxAllowed)
+return false;
+firstTreeIndex = RightNeighborNode(firstTreeIndex);
+}
+// if the last node at current tree level is a left node, handle it individually
+// and replace it with its left neighbor, which is a right node
+if (IsLeftNode(lastTreeIndex)) {
+const T& curData = mTreeData[lastTreeIndex];
+UpdateUpperBound(out_upperBound, curData);
+if (curData > maxAllowed)
+return false;
+lastTreeIndex = LeftNeighborNode(lastTreeIndex);
+}
+// at this point it can happen that firstTreeIndex and lastTreeIndex "crossed" each
+// other. That happens if firstTreeIndex was a right node and lastTreeIndex was its
+// right neighor: in that case, both above tweaks happened and as a result, they ended
+// up being swapped: lastTreeIndex is now the _left_ neighbor of firstTreeIndex.
+// When that happens, there is nothing left to validate.
+if (lastTreeIndex == LeftNeighborNode(firstTreeIndex)) {
+return true;
+}
+// walk up 1 level
+firstTreeIndex = ParentNode(firstTreeIndex);
+lastTreeIndex = ParentNode(lastTreeIndex);
+}
+}
+template<typename U>
+static U NextPowerOfTwo(U x) {
+U result = 1;
+while (result < x)
+result <<= 1;
+MOZ_ASSERT(result >= x);
+MOZ_ASSERT((result & (result - 1)) == 0);
+return result;
+}
+bool ResizeToParentSize()
+{
+size_t numberOfElements = mParent.ByteSize() / sizeof(T);
+size_t requiredNumLeaves = (numberOfElements + sElementsPerLeaf - 1) / sElementsPerLeaf;
+size_t oldNumLeaves = mNumLeaves;
+mNumLeaves = NextPowerOfTwo(requiredNumLeaves);
+Invalidate(0, mParent.ByteSize() - 1);
+// see class comment for why we the tree storage size is 2 * mNumLeaves
+if (!mTreeData.SetLength(2 * mNumLeaves)) {
+return false;
+}
+if (mNumLeaves != oldNumLeaves) {
+memset(mTreeData.Elements(), 0, mTreeData.Length() * sizeof(mTreeData[0]));
+}
+return true;
+}
+void Invalidate(size_t firstByte, size_t lastByte);
+void Update();
+size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
+{
+return aMallocSizeOf(this) + mTreeData.SizeOfExcludingThis(aMallocSizeOf);
+}
+};
+// TreeForType: just a template helper to select the right tree object for a given
+// element type.
+template<typename T>
+struct TreeForType {};
+template<>
+struct TreeForType<uint8_t>
+{
+static WebGLElementArrayCacheTree<uint8_t>*& Run(WebGLElementArrayCache *b) { return b->mUint8Tree; }
+};
+template<>
+struct TreeForType<uint16_t>
+{
+static WebGLElementArrayCacheTree<uint16_t>*& Run(WebGLElementArrayCache *b) { return b->mUint16Tree; }
+};
+template<>
+struct TreeForType<uint32_t>
+{
+static WebGLElementArrayCacheTree<uint32_t>*& Run(WebGLElementArrayCache *b) { return b->mUint32Tree; }
+};
+// When the buffer gets updated from firstByte to lastByte,
+// calling this method will notify the tree accordingly
+template<typename T>
+void WebGLElementArrayCacheTree<T>::Invalidate(size_t firstByte, size_t lastByte)
+{
+lastByte = std::min(lastByte, mNumLeaves * sElementsPerLeaf * sizeof(T) - 1);
+if (firstByte > lastByte) {
+return;
+}
+size_t firstLeaf = LeafForByte(firstByte);
+size_t lastLeaf = LeafForByte(lastByte);
+if (mInvalidated) {
+mFirstInvalidatedLeaf = std::min(firstLeaf, mFirstInvalidatedLeaf);
+mLastInvalidatedLeaf = std::max(lastLeaf, mLastInvalidatedLeaf);
+} else {
+mInvalidated = true;
+mFirstInvalidatedLeaf = firstLeaf;
+mLastInvalidatedLeaf = lastLeaf;
+}
+}
+// When tree has been partially invalidated, from mFirstInvalidatedLeaf to
+// mLastInvalidatedLeaf, calling this method will 1) update the leaves in this interval
+// from the raw buffer data, and 2) propagate this update up the tree
+template<typename T>
+void WebGLElementArrayCacheTree<T>::Update()
+{
+if (!mInvalidated) {
+return;
+}
+MOZ_ASSERT(mLastInvalidatedLeaf < mNumLeaves);
+size_t firstTreeIndex = TreeIndexForLeaf(mFirstInvalidatedLeaf);
+size_t lastTreeIndex = TreeIndexForLeaf(mLastInvalidatedLeaf);
+// Step #1: initialize the tree leaves from plain buffer data.
+// That is, each tree leaf must be set to the max of the |sElementsPerLeaf| corresponding
+// buffer entries.
+// condition-less scope to prevent leaking this scope's variables into the code below
+{
+// treeIndex is the index of the tree leaf we're writing, i.e. the destination index
+size_t treeIndex = firstTreeIndex;
+// srcIndex is the index in the source buffer
+size_t srcIndex = mFirstInvalidatedLeaf * sElementsPerLeaf;
+size_t numberOfElements = mParent.ByteSize() / sizeof(T);
+while (treeIndex <= lastTreeIndex) {
+T m = 0;
+size_t a = srcIndex;
+size_t srcIndexNextLeaf = std::min(a + sElementsPerLeaf, numberOfElements);
+for (; srcIndex < srcIndexNextLeaf; srcIndex++) {
+m = std::max(m, mParent.Element<T>(srcIndex));
+}
+mTreeData[treeIndex] = m;
+treeIndex++;
+}
+}
+// Step #2: propagate the values up the tree. This is simply a matter of walking up
+// the tree and setting each node to the max of its two children.
+while (firstTreeIndex > 1) {
+// move up 1 level
+firstTreeIndex = ParentNode(firstTreeIndex);
+lastTreeIndex = ParentNode(lastTreeIndex);
+// fast-exit case where only one node is invalidated at the current level
+if (firstTreeIndex == lastTreeIndex) {
+mTreeData[firstTreeIndex] = std::max(mTreeData[LeftChildNode(firstTreeIndex)], mTreeData[RightChildNode(firstTreeIndex)]);
+continue;
+}
+// initialize local iteration variables: child and parent.
+size_t child = LeftChildNode(firstTreeIndex);
+size_t parent = firstTreeIndex;
+// the unrolling makes this look more complicated than it is; the plain non-unrolled
+// version is in the second while loop below
+const int unrollSize = 8;
+while (RightNeighborNode(parent, unrollSize - 1) <= lastTreeIndex)
+{
+for (int unroll = 0; unroll < unrollSize; unroll++)
+{
+T a = mTreeData[child];
+child = RightNeighborNode(child);
+T b = mTreeData[child];
+child = RightNeighborNode(child);
+mTreeData[parent] = std::max(a, b);
+parent = RightNeighborNode(parent);
+}
+}
+// plain non-unrolled version, used to terminate the job after the last unrolled iteration
+while (parent <= lastTreeIndex)
+{
+T a = mTreeData[child];
+child = RightNeighborNode(child);
+T b = mTreeData[child];
+child = RightNeighborNode(child);
+mTreeData[parent] = std::max(a, b);
+parent = RightNeighborNode(parent);
+}
+}
+mInvalidated = false;
+}
+WebGLElementArrayCache::~WebGLElementArrayCache() {
+delete mUint8Tree;
+delete mUint16Tree;
+delete mUint32Tree;
+free(mUntypedData);
+}
+bool WebGLElementArrayCache::BufferData(const void* ptr, size_t byteSize) {
+mByteSize = byteSize;
+if (mUint8Tree)
+if (!mUint8Tree->ResizeToParentSize())
+return false;
+if (mUint16Tree)
+if (!mUint16Tree->ResizeToParentSize())
+return false;
+if (mUint32Tree)
+if (!mUint32Tree->ResizeToParentSize())
+return false;
+mUntypedData = realloc(mUntypedData, byteSize);
+if (!mUntypedData)
+return false;
+BufferSubData(0, ptr, byteSize);
+return true;
+}
+void WebGLElementArrayCache::BufferSubData(size_t pos, const void* ptr, size_t updateByteSize) {
+if (!updateByteSize) return;
+if (ptr)
+memcpy(static_cast<uint8_t*>(mUntypedData) + pos, ptr, updateByteSize);
+else
+memset(static_cast<uint8_t*>(mUntypedData) + pos, 0, updateByteSize);
+InvalidateTrees(pos, pos + updateByteSize - 1);
+}
+void WebGLElementArrayCache::InvalidateTrees(size_t firstByte, size_t lastByte)
+{
+if (mUint8Tree)
+mUint8Tree->Invalidate(firstByte, lastByte);
+if (mUint16Tree)
+mUint16Tree->Invalidate(firstByte, lastByte);
+if (mUint32Tree)
+mUint32Tree->Invalidate(firstByte, lastByte);
+}
+template<typename T>
+bool
+WebGLElementArrayCache::Validate(uint32_t maxAllowed, size_t firstElement,
+size_t countElements, uint32_t* out_upperBound)
+{
+SetUpperBound(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) {
+SetUpperBound(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 (!mByteSize || !countElements)
+return true;
+WebGLElementArrayCacheTree<T>*& tree = TreeForType<T>::Run(this);
+if (!tree) {
+tree = new WebGLElementArrayCacheTree<T>(*this);
+}
+size_t lastElement = firstElement + countElements - 1;
+tree->Update();
+// 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)
+{
+SetUpperBound(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 sElementsPerLeaf.
+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* 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;
+}
+size_t
+WebGLElementArrayCache::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
+{
+size_t uint8TreeSize  = mUint8Tree  ? mUint8Tree->SizeOfIncludingThis(aMallocSizeOf) : 0;
+size_t uint16TreeSize = mUint16Tree ? mUint16Tree->SizeOfIncludingThis(aMallocSizeOf) : 0;
+size_t uint32TreeSize = mUint32Tree ? mUint32Tree->SizeOfIncludingThis(aMallocSizeOf) : 0;
+return aMallocSizeOf(this) +
+mByteSize +
+uint8TreeSize +
+uint16TreeSize +
+uint32TreeSize;
+}
+} // end namespace mozilla

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comparison: content/canvas/src/WebGLElementArrayCache.cpp

content/canvas/src/WebGLElementArrayCache.cpp