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LibWeb: Use copy-on-write to make cloning a FormattingState much cheaper

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Instead of making a full copy of every NodeState when cloning a
FormattingState, we make NodeState ref-counted and implement a basic
copy-on-write mechanism.

FormattingState::get_mutable() now makes a deep copy of the NodeState
when first accessed *if* it is shared with other FormattingStates.
This commit is contained in:
Andreas Kling 2022-02-28 12:32:23 +01:00
parent 0a533da22f
commit aa72ebf7aa
2 changed files with 36 additions and 18 deletions
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17
Userland/Libraries/LibWeb/Layout/FormattingState.cpp
View file

@ -9,6 +9,23 @@
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);
}
return state;
}
FormattingState::NodeState const& FormattingState::get(NodeWithStyleAndBoxModelMetrics const& box) const
{
return *const_cast<FormattingState&>(*this).nodes.ensure(&box, [] { return adopt_ref(*new NodeState); });
}
void FormattingState::commit()
{
for (auto& it : nodes) {