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tenfourfox/dom/canvas/WebGLElementArrayCache.cpp

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/* -*- 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 <algorithm>
#include <cstdlib>
#include <cstring>
#include <limits>
#include "mozilla/Assertions.h"
#include "mozilla/MathAlgorithms.h"
#include "mozilla/MemoryReporting.h"
namespace mozilla {
static void
UpdateUpperBound(uint32_t* const out_upperBound, uint32_t newBound)
{
MOZ_ASSERT(out_upperBound);
// Move *out_upperBound to a local variable to work around a false positive
// -Wuninitialized gcc warning about std::max() in PGO builds.
uint32_t upperBound = *out_upperBound;
*out_upperBound = std::max(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 |kSkippedBottomTreeLevels| and the
* number of element array entries that each leaf corresponds to, is
* |kElementsPerLeaf|. This being a binary tree, we have:
*
* kElementsPerLeaf = 2 ^ kSkippedBottomTreeLevels.
*
* *** 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.
*
* TreeData is always a vector of length:
*
* 2 * (number of leaves).
*
* 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 kSkippedBottomTreeLevels
* 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 updating the trees, we
* have to update all existing trees. So if trees for types uint8_t, uint16_t
* and uint32_t have ever been constructed for this buffer, every subsequent
* update will have to update all trees even if one of the types is never used
* again. That's inefficient, but content should not put indices of different
* types in the same element array buffer anyways. Different index types can
* only be consumed in separate drawElements calls, so nothing particular is
* to be achieved by lumping them in the same buffer object.
*/
template<typename T>
struct WebGLElementArrayCacheTree
{
/* A too-high kSkippedBottomTreeLevels would harm the performance of small
* drawElements calls. A too-low kSkippedBottomTreeLevels would cause undue
* memory usage. The current value has been validated by some benchmarking.
* See bug 732660.
*/
static const size_t kSkippedBottomTreeLevels = 3;
static const size_t kElementsPerLeaf = 1 << kSkippedBottomTreeLevels;
// Since kElementsPerLeaf is POT:
static const size_t kElementsPerLeafMask = kElementsPerLeaf - 1;
private:
// The WebGLElementArrayCache that owns this tree:
WebGLElementArrayCache& mParent;
// The tree's internal data storage. Its length is 2 * (number of leaves)
// because of its data layout explained in the above class comment.
FallibleTArray<T> mTreeData;
public:
// Constructor. Takes a reference to the WebGLElementArrayCache that is to be
// the parent. Does not initialize the tree. Should be followed by a call
// to Update() to attempt initializing the tree.
explicit WebGLElementArrayCacheTree(WebGLElementArrayCache& value)
: mParent(value)
{
}
T GlobalMaximum() const {
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 NumLeaves() const {
// See class comment for why we the tree storage size is 2 * numLeaves.
return mTreeData.Length() >> 1;
}
size_t LeafForElement(size_t element) const {
size_t leaf = element / kElementsPerLeaf;
MOZ_ASSERT(leaf < NumLeaves());
return leaf;
}
size_t LeafForByte(size_t byte) const {
return LeafForElement(byte / sizeof(T));
}
// Returns the index, into the tree storage, where a given leaf is stored.
size_t TreeIndexForLeaf(size_t leaf) const {
// See above class comment. The tree storage is an array of length
// 2 * numLeaves. The leaves are stored in its second half.
return leaf + NumLeaves();
}
static size_t LastElementUnderSameLeaf(size_t element) {
return element | kElementsPerLeafMask;
}
static size_t FirstElementUnderSameLeaf(size_t element) {
return element & ~kElementsPerLeafMask;
}
static size_t NextMultipleOfElementsPerLeaf(size_t numElements) {
MOZ_ASSERT(numElements >= 1);
return ((numElements - 1) | kElementsPerLeafMask) + 1;
}
bool Validate(T maxAllowed, size_t firstLeaf, size_t lastLeaf,
uint32_t* const out_upperBound)
{
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 one 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" eachother. 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 one level.
firstTreeIndex = ParentNode(firstTreeIndex);
lastTreeIndex = ParentNode(lastTreeIndex);
}
}
// Updates the tree from the parent's buffer contents. Fallible, as it
// may have to resize the tree storage.
bool Update(size_t firstByte, size_t lastByte);
size_t SizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const
{
return mallocSizeOf(this) +
mTreeData.ShallowSizeOfExcludingThis(mallocSizeOf);
}
};
// 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 ScopedDeletePtr<WebGLElementArrayCacheTree<uint8_t>>&
Value(WebGLElementArrayCache* b) {
return b->mUint8Tree;
}
};
template<>
struct TreeForType<uint16_t>
{
static ScopedDeletePtr<WebGLElementArrayCacheTree<uint16_t>>&
Value(WebGLElementArrayCache* b) {
return b->mUint16Tree;
}
};
template<>
struct TreeForType<uint32_t>
{
static ScopedDeletePtr<WebGLElementArrayCacheTree<uint32_t>>&
Value(WebGLElementArrayCache* b) {
return b->mUint32Tree;
}
};
// 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>
bool
WebGLElementArrayCacheTree<T>::Update(size_t firstByte, size_t lastByte)
{
MOZ_ASSERT(firstByte <= lastByte);
MOZ_ASSERT(lastByte < mParent.mBytes.Length());
size_t numberOfElements = mParent.mBytes.Length() / sizeof(T);
size_t requiredNumLeaves = 0;
if (numberOfElements > 0) {
/* If we didn't require the number of leaves to be a power of two, then
* it would just be equal to
*
* ceil(numberOfElements / kElementsPerLeaf)
*
* The way we implement this (division+ceil) operation in integer
* arithmetic
* is as follows:
*/
size_t numLeavesNonPOT = (numberOfElements + kElementsPerLeaf - 1) / kElementsPerLeaf;
// It only remains to round that up to the next power of two:
requiredNumLeaves = RoundUpPow2(numLeavesNonPOT);
}
// Step #0: If needed, resize our tree data storage.
if (requiredNumLeaves != NumLeaves()) {
// See class comment for why we the tree storage size is 2 * numLeaves.
if (!mTreeData.SetLength(2 * requiredNumLeaves, fallible)) {
mTreeData.Clear();
return false;
}
MOZ_ASSERT(NumLeaves() == requiredNumLeaves);
if (NumLeaves()) {
// When resizing, update the whole tree, not just the subset
// corresponding to the part of the buffer being updated.
memset(mTreeData.Elements(), 0, mTreeData.Length() * sizeof(T));
firstByte = 0;
lastByte = mParent.mBytes.Length() - 1;
}
}
if (NumLeaves() == 0)
return true;
lastByte = std::min(lastByte, NumLeaves() * kElementsPerLeaf * sizeof(T) - 1);
if (firstByte > lastByte)
return true;
size_t firstLeaf = LeafForByte(firstByte);
size_t lastLeaf = LeafForByte(lastByte);
MOZ_ASSERT(firstLeaf <= lastLeaf && lastLeaf < NumLeaves());
size_t firstTreeIndex = TreeIndexForLeaf(firstLeaf);
size_t lastTreeIndex = TreeIndexForLeaf(lastLeaf);
// Step #1: Initialize the tree leaves from plain buffer data.
// That is, each tree leaf must be set to the max of the |kElementsPerLeaf|
// 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 = firstLeaf * kElementsPerLeaf;
while (treeIndex <= lastTreeIndex) {
T m = 0;
size_t a = srcIndex;
size_t srcIndexNextLeaf = std::min(a + kElementsPerLeaf, 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 one level.
firstTreeIndex = ParentNode(firstTreeIndex);
lastTreeIndex = ParentNode(lastTreeIndex);
// Fast-exit case where only one node is updated at the current level.
if (firstTreeIndex == lastTreeIndex) {
mTreeData[firstTreeIndex] = std::max(mTreeData[LeftChildNode(firstTreeIndex)], mTreeData[RightChildNode(firstTreeIndex)]);
continue;
}
size_t child = LeftChildNode(firstTreeIndex);
size_t parent = firstTreeIndex;
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);
}
}
return true;
}
WebGLElementArrayCache::WebGLElementArrayCache()
{
}
WebGLElementArrayCache::~WebGLElementArrayCache()
{
}
bool
WebGLElementArrayCache::BufferData(const void* ptr, size_t byteLength)
{
if (mBytes.Length() != byteLength) {
if (!mBytes.SetLength(byteLength, fallible)) {
mBytes.Clear();
return false;
}
}
MOZ_ASSERT(mBytes.Length() == byteLength);
return BufferSubData(0, ptr, byteLength);
}
bool
WebGLElementArrayCache::BufferSubData(size_t pos, const void* ptr,
size_t updateByteLength)
{
MOZ_ASSERT(pos + updateByteLength <= mBytes.Length());
if (!updateByteLength)
return true;
// Note, using memcpy on shared racy data is not well-defined, this
// will need to use safe-for-races operations when those become available.
// See bug 1225033.
if (ptr)
memcpy(mBytes.Elements() + pos, ptr, updateByteLength);
else
memset(mBytes.Elements() + pos, 0, updateByteLength);
return UpdateTrees(pos, pos + updateByteLength - 1);
}
bool
WebGLElementArrayCache::UpdateTrees(size_t firstByte, size_t lastByte)
{
bool result = true;
if (mUint8Tree)
result &= mUint8Tree->Update(firstByte, lastByte);
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.ShallowSizeOfExcludingThis(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
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