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LibWeb: Make Layout::FormattingState copies shallow

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Previously, each NodeState in a FormattingState was shared with the
parent FormattingState, but the HashMap of NodeState had to be copied
when making FormattingState copies.

This patch makes copying instant by keeping a pointer to the parent
FormattingState instead. When fetching immutable state via get(), we may
now return a reference to a NodeState owned by a parent FormattingState.

get_mutable() will copy any NodeState found in the ancestor chain before
making a brand new one.
This commit is contained in:
Andreas Kling 2022-03-12 16:33:11 +01:00
parent b6097cf724
commit 515db5fc1b
4 changed files with 40 additions and 30 deletions
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32
Userland/Libraries/LibWeb/Layout/FormattingState.cpp
View file

@ -12,23 +12,39 @@ namespace Web::Layout {
FormattingState::NodeState& FormattingState::get_mutable(NodeWithStyleAndBoxModelMetrics const& box)
{
auto state = nodes.ensure(&box, [] { return adopt_ref(*new NodeState); });
// CoW if ref_count > 2 (1 for the entry in `this->nodes`, 1 for the `state` local in this function)
if (state->ref_count > 2) {
state = adopt_ref(*new NodeState { *state });
state->ref_count = 1;
nodes.set(&box, state);
if (auto it = nodes.find(&box); it != nodes.end())
return *it->value;
for (auto* ancestor = m_parent; ancestor; ancestor = ancestor->m_parent) {
if (auto it = ancestor->nodes.find(&box); it != ancestor->nodes.end()) {
auto cow_node_state = adopt_own(*new NodeState(*it->value));
auto* cow_node_state_ptr = cow_node_state.ptr();
nodes.set(&box, move(cow_node_state));
return *cow_node_state_ptr;
}
}
return state;
return *nodes.ensure(&box, [] { return adopt_own(*new NodeState); });
}
FormattingState::NodeState const& FormattingState::get(NodeWithStyleAndBoxModelMetrics const& box) const
{
return *const_cast<FormattingState&>(*this).nodes.ensure(&box, [] { return adopt_ref(*new NodeState); });
if (auto it = nodes.find(&box); it != nodes.end())
return *it->value;
for (auto* ancestor = m_parent; ancestor; ancestor = ancestor->m_parent) {
if (auto it = ancestor->nodes.find(&box); it != ancestor->nodes.end())
return *it->value;
}
return *const_cast<FormattingState&>(*this).nodes.ensure(&box, [] { return adopt_own(*new NodeState); });
}
void FormattingState::commit()
{
// Only the top-level FormattingState should ever be committed.
VERIFY(!m_parent);
HashTable<Layout::TextNode*> text_nodes;
for (auto& it : nodes) {