layout: Rewrite the block formatting context/float inline-size

speculation code.

The old code tried to do the speculation as a single bottom-up pass
after intrinsic inline-size calculation, which was unable to handle
cases like this:

    <div>
        <div style="float: left">Foo</div>
    </div>
    <div>
        <div style="overflow: hidden">Bar</div>
    </div>

No single bottom-up pass could possibly handle this case, because the
inline-size of the float flowing out of the "Foo" block could never make
it down to the "Bar" block, where it is needed for speculation.

On the pages I tried, this regresses layout performance by 1%-2%.

I first noticed this breaking some pages, like the Google SERPs, several
months ago.

1 files changed, 6 insertions, 0 deletions

diff --git a/components/layout/layout_thread.rs b/components/layout/layout_thread.rs
index df555b1d485..e6412348e6c 100644
--- a/components/layout/layout_thread.rs
+++ b/components/layout/layout_thread.rs
@@ -1345,6 +1345,12 @@ impl LayoutThread {
                     self.time_profiler_chan.clone(),
                     || sequential::resolve_generated_content(&mut root_flow, &layout_context));
 
+            // Guess float placement.
+            profile(time::ProfilerCategory::LayoutFloatPlacementSpeculation,
+                    self.profiler_metadata(),
+                    self.time_profiler_chan.clone(),
+                    || sequential::guess_float_placement(flow_ref::deref_mut(&mut root_flow)));
+
             // Perform the primary layout passes over the flow tree to compute the locations of all
             // the boxes.
             if flow::base(&*root_flow).restyle_damage.intersects(REFLOW | REFLOW_OUT_OF_FLOW) {