[turbofan] Splinter into one range.

Before this CL, we created one live range per successive set of
deferred blocks. For scenarios with many such blocks, this creates
an upfront pressure for the register allocator to deal with many ranges.
Linear sorts ranges, which is a super-linear operation.

The change places all deferred intervals into one range, meaning that,
at most, there will be twice as many live ranges as the original set. In
pathological cases (benchmarks/Compile/slow_nbody1.js), this change
halves the compilation time. We see some improvements elsewhere,
notably SQLite at ~4-5%.

We may be able to avoid the subsequent merge. Its cost is the
additional ranges it may need to create. The sole reason for the merge
phase is to provide an unchanged view of the world to the subsequent
phases. With the at-most-one splinter model, we may be able to teach
the other phases about splintering - should we find perf hindrances
due to merging.

Review URL: https://codereview.chromium.org/1391023007

Cr-Commit-Position: refs/heads/master@{#31224}

src/compiler/live-range-separator.cc

@@ -78,12 +78,14 @@ void CreateSplinter(TopLevelLiveRange *range, RegisterAllocationData *data,
     if (range->MayRequireSpillRange()) {
       data->CreateSpillRangeForLiveRange(range);
     }
-    TopLevelLiveRange *result = data->NextLiveRange(range->machine_type());
-    DCHECK_NULL(data->live_ranges()[result->vreg()]);
-    data->live_ranges()[result->vreg()] = result;
-
+    if (range->splinter() == nullptr) {
+      TopLevelLiveRange *splinter = data->NextLiveRange(range->machine_type());
+      DCHECK_NULL(data->live_ranges()[splinter->vreg()]);
+      data->live_ranges()[splinter->vreg()] = splinter;
+      range->SetSplinter(splinter);
+    }
     Zone *zone = data->allocation_zone();
-    range->Splinter(start, end, result, zone);
+    range->Splinter(start, end, zone);
   }
 }
 

src/compiler/register-allocator.cc

@@ -698,7 +698,8 @@ TopLevelLiveRange::TopLevelLiveRange(int vreg, MachineType machine_type)
       spilled_in_deferred_blocks_(false),
       spill_start_index_(kMaxInt),
       last_child_(this),
-      last_insertion_point_(this) {
+      last_pos_(nullptr),
+      splinter_(nullptr) {
   bits_ |= SpillTypeField::encode(SpillType::kNoSpillType);
 }
 
@@ -845,12 +846,12 @@ AllocatedOperand TopLevelLiveRange::GetSpillRangeOperand() const {
 
 
 void TopLevelLiveRange::Splinter(LifetimePosition start, LifetimePosition end,
-                                 TopLevelLiveRange* result, Zone* zone) {
+                                 Zone* zone) {
   DCHECK(start != Start() || end != End());
   DCHECK(start < end);
 
-  result->set_spill_type(spill_type());
-
+  TopLevelLiveRange splinter_temp(-1, machine_type());
+  UsePosition* last_in_splinter = nullptr;
   if (start <= Start()) {
     // TODO(mtrofin): here, the TopLevel part is in the deferred range, so we
     // may want to continue processing the splinter. However, if the value is
@@ -859,21 +860,21 @@ void TopLevelLiveRange::Splinter(LifetimePosition start, LifetimePosition end,
     // should check this, however, this may not be the place, because we don't
     // have access to the instruction sequence.
     DCHECK(end < End());
-    DetachAt(end, result, zone);
+    DetachAt(end, &splinter_temp, zone);
     next_ = nullptr;
   } else if (end >= End()) {
     DCHECK(start > Start());
-    DetachAt(start, result, zone);
+    DetachAt(start, &splinter_temp, zone);
     next_ = nullptr;
   } else {
     DCHECK(start < End() && Start() < end);
 
     const int kInvalidId = std::numeric_limits<int>::max();
 
-    UsePosition* last = DetachAt(start, result, zone);
+    UsePosition* last = DetachAt(start, &splinter_temp, zone);
 
     LiveRange end_part(kInvalidId, this->machine_type(), nullptr);
-    result->DetachAt(end, &end_part, zone);
+    last_in_splinter = splinter_temp.DetachAt(end, &end_part, zone);
 
     next_ = end_part.next_;
     last_interval_->set_next(end_part.first_interval_);
@@ -890,20 +891,39 @@ void TopLevelLiveRange::Splinter(LifetimePosition start, LifetimePosition end,
       if (last != nullptr) last->set_next(end_part.first_pos_);
     }
   }
-  result->next_ = nullptr;
-  result->top_level_ = result;
 
-  result->SetSplinteredFrom(this);
-  // Ensure the result's relative ID is unique within the IDs used for this
-  // virtual register's children and splinters.
-  result->relative_id_ = GetNextChildId();
+  if (splinter()->IsEmpty()) {
+    splinter()->first_interval_ = splinter_temp.first_interval_;
+    splinter()->last_interval_ = splinter_temp.last_interval_;
+  } else {
+    splinter()->last_interval_->set_next(splinter_temp.first_interval_);
+    splinter()->last_interval_ = splinter_temp.last_interval_;
+  }
+  if (splinter()->first_pos() == nullptr) {
+    splinter()->first_pos_ = splinter_temp.first_pos_;
+  } else {
+    splinter()->last_pos_->set_next(splinter_temp.first_pos_);
+  }
+  if (last_in_splinter != nullptr) {
+    splinter()->last_pos_ = last_in_splinter;
+  } else {
+    if (splinter()->first_pos() != nullptr &&
+        splinter()->last_pos_ == nullptr) {
+      splinter()->last_pos_ = splinter()->first_pos();
+      for (UsePosition* pos = splinter()->first_pos(); pos != nullptr;
+           pos = pos->next()) {
+        splinter()->last_pos_ = pos;
+      }
+    }
+  }
+#if DEBUG
+  Verify();
+  splinter()->Verify();
+#endif
 }
 
 
 void TopLevelLiveRange::SetSplinteredFrom(TopLevelLiveRange* splinter_parent) {
-  // The splinter parent is always the original "Top".
-  DCHECK(splinter_parent->Start() < Start());
-
   splintered_from_ = splinter_parent;
   if (!HasSpillOperand() && splinter_parent->spill_range_ != nullptr) {
     SetSpillRange(splinter_parent->spill_range_);
@@ -928,43 +948,55 @@ void TopLevelLiveRange::Merge(TopLevelLiveRange* other, Zone* zone) {
   DCHECK(Start() < other->Start());
   DCHECK(other->splintered_from() == this);
 
-  LiveRange* last_other = other->last_child();
-  LiveRange* last_me = last_child();
-
-  // Simple case: we just append at the end.
-  if (last_me->End() <= other->Start()) return last_me->AppendAsChild(other);
+  LiveRange* first = this;
+  LiveRange* second = other;
+  DCHECK(first->Start() < second->Start());
+  while (first != nullptr && second != nullptr) {
+    DCHECK(first != second);
+    // Make sure the ranges are in order each time we iterate.
+    if (second->Start() < first->Start()) {
+      LiveRange* tmp = second;
+      second = first;
+      first = tmp;
+      continue;
+    }
 
-  DCHECK(last_me->End() > last_other->End());
+    if (first->End() <= second->Start()) {
+      if (first->next() == nullptr ||
+          first->next()->Start() > second->Start()) {
+        // First is in order before second.
+        LiveRange* temp = first->next();
+        first->next_ = second;
+        first = temp;
+      } else {
+        // First is in order before its successor (or second), so advance first.
+        first = first->next();
+      }
+      continue;
+    }
 
-  // In the more general case, we need to find the ranges between which to
-  // insert.
-  if (other->Start() < last_insertion_point_->Start()) {
-    last_insertion_point_ = this;
-  }
+    DCHECK(first->Start() < second->Start());
+    // If first and second intersect, split first.
+    if (first->Start() < second->End() && second->Start() < first->End()) {
+      LiveRange* temp = first->SplitAt(second->Start(), zone);
 
-  for (; last_insertion_point_->next() != nullptr &&
-         last_insertion_point_->next()->Start() <= other->Start();
-       last_insertion_point_ = last_insertion_point_->next()) {
-  }
+      temp->set_spilled(first->spilled());
+      if (!temp->spilled())
+        temp->set_assigned_register(first->assigned_register());
 
-  // When we splintered the original range, we reconstituted the original range
-  // into one range without children, but with discontinuities. To merge the
-  // splinter back in, we need to split the range - or a child obtained after
-  // register allocation splitting.
-  LiveRange* after = last_insertion_point_->next();
-  if (last_insertion_point_->End() > other->Start()) {
-    LiveRange* new_after = last_insertion_point_->SplitAt(other->Start(), zone);
-    new_after->set_spilled(last_insertion_point_->spilled());
-    if (!new_after->spilled())
-      new_after->set_assigned_register(
-          last_insertion_point_->assigned_register());
-    after = new_after;
+      first->next_ = second;
+      first = temp;
+      continue;
+    }
+    DCHECK(first->End() <= second->Start());
   }
 
-  last_other->next_ = after;
-  last_insertion_point_->next_ = other;
-  other->UpdateParentForAllChildren(TopLevel());
+  TopLevel()->UpdateParentForAllChildren(TopLevel());
   TopLevel()->UpdateSpillRangePostMerge(other);
+
+#if DEBUG
+  Verify();
+#endif
 }
 
 

src/compiler/register-allocator.h

@@ -493,8 +493,7 @@ class TopLevelLiveRange final : public LiveRange {
   // result.
   // The current range is pointed to as "splintered_from". No parent/child
   // relationship is established between this and result.
-  void Splinter(LifetimePosition start, LifetimePosition end,
-                TopLevelLiveRange* result, Zone* zone);
+  void Splinter(LifetimePosition start, LifetimePosition end, Zone* zone);
 
   // Assuming other was splintered from this range, embeds other and its
   // children as part of the children sequence of this range.
@@ -538,7 +537,6 @@ class TopLevelLiveRange final : public LiveRange {
     spill_start_index_ = Min(start, spill_start_index_);
   }
 
-  void SetSplinteredFrom(TopLevelLiveRange* splinter_parent);
   void CommitSpillsAtDefinition(InstructionSequence* sequence,
                                 const InstructionOperand& operand,
                                 bool might_be_duplicated);
@@ -578,8 +576,20 @@ class TopLevelLiveRange final : public LiveRange {
   }
   void set_last_child(LiveRange* range) { last_child_ = range; }
   LiveRange* last_child() const { return last_child_; }
+  TopLevelLiveRange* splinter() const { return splinter_; }
+  void SetSplinter(TopLevelLiveRange* splinter) {
+    DCHECK_NULL(splinter_);
+    DCHECK_NOT_NULL(splinter);
+
+    splinter_ = splinter;
+    splinter->relative_id_ = GetNextChildId();
+    splinter->set_spill_type(spill_type());
+    splinter->SetSplinteredFrom(this);
+  }
 
  private:
+  void SetSplinteredFrom(TopLevelLiveRange* splinter_parent);
+
   typedef BitField<bool, 1, 1> HasSlotUseField;
   typedef BitField<bool, 2, 1> IsPhiField;
   typedef BitField<bool, 3, 1> IsNonLoopPhiField;
@@ -599,7 +609,8 @@ class TopLevelLiveRange final : public LiveRange {
   bool spilled_in_deferred_blocks_;
   int spill_start_index_;
   LiveRange* last_child_;
-  LiveRange* last_insertion_point_;
+  UsePosition* last_pos_;
+  TopLevelLiveRange* splinter_;
 
   DISALLOW_COPY_AND_ASSIGN(TopLevelLiveRange);
 };

test/unittests/compiler/live-range-unittest.cc

@@ -32,11 +32,14 @@ class LiveRangeUnitTest : public TestWithZone {
 
   TopLevelLiveRange* Splinter(TopLevelLiveRange* top, int start, int end,
                               int new_id = 0) {
-    TopLevelLiveRange* ret =
-        new (zone()) TopLevelLiveRange(new_id, MachineType::kRepTagged);
+    if (top->splinter() == nullptr) {
+      TopLevelLiveRange* ret =
+          new (zone()) TopLevelLiveRange(new_id, MachineType::kRepTagged);
+      top->SetSplinter(ret);
+    }
     top->Splinter(LifetimePosition::FromInt(start),
-                  LifetimePosition::FromInt(end), ret, zone());
-    return ret;
+                  LifetimePosition::FromInt(end), zone());
+    return top->splinter();
   }
 
   // Ranges first and second match structurally.
@@ -377,6 +380,25 @@ TEST_F(LiveRangeUnitTest, SplinterMultipleIntervalsRight) {
 }
 
 
+TEST_F(LiveRangeUnitTest, SplinterMergeMultipleTimes) {
+  TopLevelLiveRange* range =
+      TestRangeBuilder(zone()).Add(0, 3).Add(5, 10).Add(12, 16).Build();
+  Splinter(range, 4, 6);
+  Splinter(range, 8, 14);
+  TopLevelLiveRange* splinter = range->splinter();
+  EXPECT_EQ(nullptr, range->next());
+  EXPECT_EQ(nullptr, splinter->next());
+  EXPECT_EQ(range, splinter->splintered_from());
+
+  TopLevelLiveRange* expected_source =
+      TestRangeBuilder(zone()).Add(0, 3).Add(6, 8).Add(14, 16).Build();
+  TopLevelLiveRange* expected_splinter =
+      TestRangeBuilder(zone()).Add(5, 6).Add(8, 10).Add(12, 14).Build();
+  EXPECT_TRUE(RangesMatch(expected_source, range));
+  EXPECT_TRUE(RangesMatch(expected_splinter, splinter));
+}
+
+
 TEST_F(LiveRangeUnitTest, MergeMultipleIntervalsRight) {
   TopLevelLiveRange* original =
       TestRangeBuilder(zone()).Add(0, 3).Add(5, 8).Build();
@@ -416,8 +438,9 @@ TEST_F(LiveRangeUnitTest, IDGeneration) {
 
   TopLevelLiveRange* splinter =
       new (zone()) TopLevelLiveRange(101, MachineType::kRepTagged);
+  vreg->SetSplinter(splinter);
   vreg->Splinter(LifetimePosition::FromInt(4), LifetimePosition::FromInt(12),
-                 splinter, zone());
+                 zone());
 
   EXPECT_EQ(101, splinter->vreg());
   EXPECT_EQ(1, splinter->relative_id());