[turbofan] relative_id of splinters and their children.

A LiveRange is identified by 2 integers: the vreg() of its TopLevel,
which is the virtual register (operand) ID; and a relative_id(), which has
no meaning in the program, but is valuable in debugging or tracing
scenarios.

This change ensures that relative_id is unique even in cases of splinter
ranges and their children.

Review URL: https://codereview.chromium.org/1318493005

Cr-Commit-Position: refs/heads/master@{#30665}

src/compiler/register-allocator.cc

@@ -871,6 +871,9 @@ void TopLevelLiveRange::Splinter(LifetimePosition start, LifetimePosition end,
   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();
 }
 
 

src/compiler/register-allocator.h

@@ -407,6 +407,7 @@ class LiveRange : public ZoneObject {
   typedef BitField<int32_t, 6, 6> AssignedRegisterField;
   typedef BitField<MachineType, 12, 15> MachineTypeField;
 
+  // Unique among children and splinters of the same virtual register.
   int relative_id_;
   uint32_t bits_;
   UseInterval* last_interval_;
@@ -535,7 +536,11 @@ class TopLevelLiveRange final : public LiveRange {
   void UpdateSpillRangePostMerge(TopLevelLiveRange* merged);
   int vreg() const { return vreg_; }
 
-  int GetNextChildId() { return ++last_child_id_; }
+  int GetNextChildId() {
+    return IsSplinter() ? splintered_from()->GetNextChildId()
+                        : ++last_child_id_;
+  }
+
   bool IsSpilledOnlyInDeferredBlocks() const {
     return spilled_in_deferred_blocks_;
   }

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

@@ -407,6 +407,33 @@ TEST_F(LiveRangeUnitTest, MergeAfterSplitting) {
 }
 
 
+TEST_F(LiveRangeUnitTest, IDGeneration) {
+  TopLevelLiveRange* vreg = TestRangeBuilder(zone()).Id(2).Build(0, 100);
+  EXPECT_EQ(2, vreg->vreg());
+  EXPECT_EQ(0, vreg->relative_id());
+
+  TopLevelLiveRange* splinter =
+      new (zone()) TopLevelLiveRange(101, MachineType::kRepTagged);
+  vreg->Splinter(LifetimePosition::FromInt(4), LifetimePosition::FromInt(12),
+                 splinter, zone());
+
+  EXPECT_EQ(101, splinter->vreg());
+  EXPECT_EQ(1, splinter->relative_id());
+
+  LiveRange* child = vreg->SplitAt(LifetimePosition::FromInt(50), zone());
+
+  EXPECT_EQ(2, child->relative_id());
+
+  LiveRange* splinter_child =
+      splinter->SplitAt(LifetimePosition::FromInt(8), zone());
+
+  EXPECT_EQ(1, splinter->relative_id());
+  EXPECT_EQ(3, splinter_child->relative_id());
+
+  vreg->Merge(splinter, zone());
+  EXPECT_EQ(1, splinter->relative_id());
+}
+
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8