[regalloc] Sort ranges for PopulateReferenceMaps

PopulateReferenceMaps caches an iterator for the reference maps list,
which allows skipping many iterations when the live ranges are mostly
sorted.

When they are not sorted however, the complexity increases significantly
since we have to iterate from the beginning of the reference maps list
for each live range.

We observed a real-world wasm GC function where this phase takes up 90%
of the compilation time because of this, which drops to below 2% when
the ranges are sorted first.

R=sigurds@chromium.org
CC=​​jkummerow@chromium.org

Change-Id: I829c909c06e5a1daee0f8d10c156d0a382af9ed0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2985339
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75420}

src/compiler/backend/register-allocator.cc

@@ -135,6 +135,7 @@ LiveRangeBoundArray* LiveRangeFinder::ArrayFor(int operand_index) {
   DCHECK(operand_index < bounds_length_);
   TopLevelLiveRange* range = data_->live_ranges()[operand_index];
   DCHECK(range != nullptr && !range->IsEmpty());
+  DCHECK_EQ(range->vreg(), operand_index);
   LiveRangeBoundArray* array = &bounds_[operand_index];
   if (array->ShouldInitialize()) {
     array->Initialize(zone_, range);
@@ -1365,6 +1366,7 @@ TopLevelLiveRange* TopTierRegisterAllocationData::GetOrCreateLiveRangeFor(
     result = NewLiveRange(index, RepresentationFor(index));
     live_ranges()[index] = result;
   }
+  DCHECK_EQ(live_ranges()[index]->vreg(), index);
   return result;
 }
 
@@ -4628,6 +4630,15 @@ void ReferenceMapPopulator::PopulateReferenceMaps() {
   const ReferenceMapDeque* reference_maps = data()->code()->reference_maps();
   ReferenceMapDeque::const_iterator first_it = reference_maps->begin();
   const size_t live_ranges_size = data()->live_ranges().size();
+  // We break the invariant that live ranges are indexed by their vregs here.
+  // This is ok because we don't use that invariant here, and this is the last
+  // phase.
+  std::sort(data()->live_ranges().begin(), data()->live_ranges().end(),
+            [](TopLevelLiveRange* a, TopLevelLiveRange* b) {
+              if (!a || a->IsEmpty()) return false;
+              if (!b || b->IsEmpty()) return true;
+              return a->Start() < b->Start();
+            });
   for (TopLevelLiveRange* range : data()->live_ranges()) {
     CHECK_EQ(live_ranges_size,
              data()->live_ranges().size());  // TODO(neis): crbug.com/831822
@@ -4648,10 +4659,11 @@ void ReferenceMapPopulator::PopulateReferenceMaps() {
       DCHECK(cur->Start().ToInstructionIndex() >= start);
     }
 
-    // Most of the ranges are in order, but not all.  Keep an eye on when they
-    // step backwards and reset the first_it so we don't miss any safe points.
-    if (start < last_range_start) first_it = reference_maps->begin();
+    // Ranges should be sorted, so that the first reference map in the current
+    // live range has to be after {first_it}.
+    DCHECK_LE(last_range_start, start);
     last_range_start = start;
+    USE(last_range_start);
 
     // Step across all the safe points that are before the start of this range,
     // recording how far we step in order to save doing this for the next range.