The Tor Browser: content/canvas/src/WebGLElementArrayCache.cpp@6474c204b198 (annotated)

content/canvas/src/WebGLElementArrayCache.cpp@6474c204b198 (annotated)

content/canvas/src/WebGLElementArrayCache.cpp

Wed, 31 Dec 2014 06:09:35 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Wed, 31 Dec 2014 06:09:35 +0100
changeset 0: 6474c204b198
permissions: -rw-r--r--

Cloned upstream origin tor-browser at tor-browser-31.3.0esr-4.5-1-build1
revision ID fc1c9ff7c1b2defdbc039f12214767608f46423f for hacking purpose.

 /* -*- 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

The Tor Browser / annotate

content/canvas/src/WebGLElementArrayCache.cpp@6474c204b198 (annotated)

content/canvas/src/WebGLElementArrayCache.cpp