[regalloc] Optimize FindFreeRegistersForRange

Sort inactive live ranges by their assigned register and by their next
start. This allows {FindFreeRegistersForRange} to stop the search earlier
and significantly reduces compile time for some test cases.

R=sigurds@chromium.org
CC=neis@chromium.org

Bug: chromium:974804, v8:9529
Change-Id: I85e2ff8acf2c02ea0539c89daae5a427da775c2c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1795350
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63722}

src/compiler/backend/register-allocator.cc

@@ -796,12 +796,13 @@ LifetimePosition LiveRange::NextEndAfter(LifetimePosition position) const {
   return start_search->end();
 }
 
-LifetimePosition LiveRange::NextStartAfter(LifetimePosition position) const {
+LifetimePosition LiveRange::NextStartAfter(LifetimePosition position) {
   UseInterval* start_search = FirstSearchIntervalForPosition(position);
   while (start_search->start() < position) {
     start_search = start_search->next();
   }
-  return start_search->start();
+  next_start_ = start_search->start();
+  return next_start_;
 }
 
 LifetimePosition LiveRange::FirstIntersection(LiveRange* other) const {
@@ -3094,11 +3095,11 @@ LinearScanAllocator::LinearScanAllocator(RegisterAllocationData* data,
     : RegisterAllocator(data, kind),
       unhandled_live_ranges_(local_zone),
       active_live_ranges_(local_zone),
-      inactive_live_ranges_(local_zone),
+      inactive_live_ranges_(num_registers(), InactiveLiveRangeQueue(local_zone),
+                            local_zone),
       next_active_ranges_change_(LifetimePosition::Invalid()),
       next_inactive_ranges_change_(LifetimePosition::Invalid()) {
   active_live_ranges().reserve(8);
-  inactive_live_ranges().reserve(8);
 }
 
 void LinearScanAllocator::MaybeSpillPreviousRanges(LiveRange* begin_range,
@@ -3237,31 +3238,22 @@ LiveRange* LinearScanAllocator::AssignRegisterOnReload(LiveRange* range,
   // give reloading registers pecedence. That way we would compute the
   // intersection for the entire future.
   LifetimePosition new_end = range->End();
-  for (const auto inactive : inactive_live_ranges()) {
-    if (kSimpleFPAliasing || !check_fp_aliasing()) {
-      if (inactive->assigned_register() != reg) continue;
-    } else {
-      bool conflict = inactive->assigned_register() == reg;
-      if (!conflict) {
-        int alias_base_index = -1;
-        int aliases = data()->config()->GetAliases(range->representation(), reg,
-                                                   inactive->representation(),
-                                                   &alias_base_index);
-        DCHECK(aliases > 0 || (aliases == 0 && alias_base_index == -1));
-        while (aliases-- && !conflict) {
-          int aliased_reg = alias_base_index + aliases;
-          if (aliased_reg == reg) {
-            conflict = true;
-          }
-        }
-      }
-      if (!conflict) continue;
+  for (int cur_reg = 0; cur_reg < num_registers(); ++cur_reg) {
+    if ((kSimpleFPAliasing || !check_fp_aliasing()) && cur_reg != reg) {
+      continue;
     }
-    for (auto interval = inactive->first_interval(); interval != nullptr;
-         interval = interval->next()) {
-      if (interval->start() > new_end) break;
-      if (interval->end() <= range->Start()) continue;
-      if (new_end > interval->start()) new_end = interval->start();
+    for (const auto cur_inactive : inactive_live_ranges(cur_reg)) {
+      if (!kSimpleFPAliasing && check_fp_aliasing() &&
+          !data()->config()->AreAliases(cur_inactive->representation(), cur_reg,
+                                        range->representation(), reg)) {
+        continue;
+      }
+      for (auto interval = cur_inactive->first_interval(); interval != nullptr;
+           interval = interval->next()) {
+        if (interval->start() > new_end) break;
+        if (interval->end() <= range->Start()) continue;
+        if (new_end > interval->start()) new_end = interval->start();
+      }
     }
   }
   if (new_end != range->End()) {
@@ -3557,11 +3549,17 @@ void LinearScanAllocator::UpdateDeferredFixedRanges(SpillMode spill_mode,
               Min(updated->End(), next_active_ranges_change_);
         });
       }
-      for (auto inactive : inactive_live_ranges()) {
-        split_conflicting(range, inactive, [this](LiveRange* updated) {
-          next_inactive_ranges_change_ =
-              Min(updated->End(), next_inactive_ranges_change_);
-        });
+      for (int reg = 0; reg < num_registers(); ++reg) {
+        if ((kSimpleFPAliasing || !check_fp_aliasing()) &&
+            reg != range->assigned_register()) {
+          continue;
+        }
+        for (auto inactive : inactive_live_ranges(reg)) {
+          split_conflicting(range, inactive, [this](LiveRange* updated) {
+            next_inactive_ranges_change_ =
+                Min(updated->End(), next_inactive_ranges_change_);
+          });
+        }
       }
     };
     if (mode() == GENERAL_REGISTERS) {
@@ -3599,12 +3597,14 @@ void LinearScanAllocator::UpdateDeferredFixedRanges(SpillMode spill_mode,
     }
   } else {
     // Remove all ranges.
-    for (auto it = inactive_live_ranges().begin();
-         it != inactive_live_ranges().end();) {
-      if ((*it)->TopLevel()->IsDeferredFixed()) {
-        it = inactive_live_ranges().erase(it);
-      } else {
-        ++it;
+    for (int reg = 0; reg < num_registers(); ++reg) {
+      for (auto it = inactive_live_ranges(reg).begin();
+           it != inactive_live_ranges(reg).end();) {
+        if ((*it)->TopLevel()->IsDeferredFixed()) {
+          it = inactive_live_ranges(reg).erase(it);
+        } else {
+          ++it;
+        }
       }
     }
   }
@@ -3635,7 +3635,9 @@ bool LinearScanAllocator::HasNonDeferredPredecessor(InstructionBlock* block) {
 void LinearScanAllocator::AllocateRegisters() {
   DCHECK(unhandled_live_ranges().empty());
   DCHECK(active_live_ranges().empty());
-  DCHECK(inactive_live_ranges().empty());
+  for (int reg = 0; reg < num_registers(); ++reg) {
+    DCHECK(inactive_live_ranges(reg).empty());
+  }
 
   SplitAndSpillRangesDefinedByMemoryOperand();
   data()->ResetSpillState();
@@ -3940,9 +3942,10 @@ void LinearScanAllocator::AddToActive(LiveRange* range) {
 void LinearScanAllocator::AddToInactive(LiveRange* range) {
   TRACE("Add live range %d:%d to inactive\n", range->TopLevel()->vreg(),
         range->relative_id());
-  inactive_live_ranges().push_back(range);
   next_inactive_ranges_change_ = std::min(
       next_inactive_ranges_change_, range->NextStartAfter(range->Start()));
+  DCHECK(range->HasRegisterAssigned());
+  inactive_live_ranges(range->assigned_register()).insert(range);
 }
 
 void LinearScanAllocator::AddToUnhandled(LiveRange* range) {
@@ -3965,30 +3968,36 @@ ZoneVector<LiveRange*>::iterator LinearScanAllocator::ActiveToHandled(
 ZoneVector<LiveRange*>::iterator LinearScanAllocator::ActiveToInactive(
     const ZoneVector<LiveRange*>::iterator it, LifetimePosition position) {
   LiveRange* range = *it;
-  inactive_live_ranges().push_back(range);
   TRACE("Moving live range %d:%d from active to inactive\n",
         (range)->TopLevel()->vreg(), range->relative_id());
+  LifetimePosition next_active = range->NextStartAfter(position);
   next_inactive_ranges_change_ =
-      std::min(next_inactive_ranges_change_, range->NextStartAfter(position));
+      std::min(next_inactive_ranges_change_, next_active);
+  DCHECK(range->HasRegisterAssigned());
+  inactive_live_ranges(range->assigned_register()).insert(range);
   return active_live_ranges().erase(it);
 }
 
-ZoneVector<LiveRange*>::iterator LinearScanAllocator::InactiveToHandled(
-    ZoneVector<LiveRange*>::iterator it) {
+LinearScanAllocator::InactiveLiveRangeQueue::iterator
+LinearScanAllocator::InactiveToHandled(InactiveLiveRangeQueue::iterator it) {
+  LiveRange* range = *it;
   TRACE("Moving live range %d:%d from inactive to handled\n",
-        (*it)->TopLevel()->vreg(), (*it)->relative_id());
-  return inactive_live_ranges().erase(it);
+        range->TopLevel()->vreg(), range->relative_id());
+  int reg = range->assigned_register();
+  return inactive_live_ranges(reg).erase(it);
 }
 
-ZoneVector<LiveRange*>::iterator LinearScanAllocator::InactiveToActive(
-    ZoneVector<LiveRange*>::iterator it, LifetimePosition position) {
+LinearScanAllocator::InactiveLiveRangeQueue::iterator
+LinearScanAllocator::InactiveToActive(InactiveLiveRangeQueue::iterator it,
+                                      LifetimePosition position) {
   LiveRange* range = *it;
   active_live_ranges().push_back(range);
   TRACE("Moving live range %d:%d from inactive to active\n",
         range->TopLevel()->vreg(), range->relative_id());
   next_active_ranges_change_ =
       std::min(next_active_ranges_change_, range->NextEndAfter(position));
-  return inactive_live_ranges().erase(it);
+  int reg = range->assigned_register();
+  return inactive_live_ranges(reg).erase(it);
 }
 
 void LinearScanAllocator::ForwardStateTo(LifetimePosition position) {
@@ -4011,18 +4020,25 @@ void LinearScanAllocator::ForwardStateTo(LifetimePosition position) {
 
   if (position >= next_inactive_ranges_change_) {
     next_inactive_ranges_change_ = LifetimePosition::MaxPosition();
-    for (auto it = inactive_live_ranges().begin();
-         it != inactive_live_ranges().end();) {
-      LiveRange* cur_inactive = *it;
-      if (cur_inactive->End() <= position) {
-        it = InactiveToHandled(it);
-      } else if (cur_inactive->Covers(position)) {
-        it = InactiveToActive(it, position);
-      } else {
-        next_inactive_ranges_change_ =
-            std::min(next_inactive_ranges_change_,
-                     cur_inactive->NextStartAfter(position));
-        ++it;
+    for (int reg = 0; reg < num_registers(); ++reg) {
+      ZoneVector<LiveRange*> reorder(data()->allocation_zone());
+      for (auto it = inactive_live_ranges(reg).begin();
+           it != inactive_live_ranges(reg).end();) {
+        LiveRange* cur_inactive = *it;
+        if (cur_inactive->End() <= position) {
+          it = InactiveToHandled(it);
+        } else if (cur_inactive->Covers(position)) {
+          it = InactiveToActive(it, position);
+        } else {
+          next_inactive_ranges_change_ =
+              std::min(next_inactive_ranges_change_,
+                       cur_inactive->NextStartAfter(position));
+          it = inactive_live_ranges(reg).erase(it);
+          reorder.push_back(cur_inactive);
+        }
+      }
+      for (LiveRange* range : reorder) {
+        inactive_live_ranges(reg).insert(range);
       }
     }
   }
@@ -4093,31 +4109,34 @@ void LinearScanAllocator::FindFreeRegistersForRange(
     }
   }
 
-  for (LiveRange* cur_inactive : inactive_live_ranges()) {
-    DCHECK(cur_inactive->End() > range->Start());
-    int cur_reg = cur_inactive->assigned_register();
-    // No need to carry out intersections, when this register won't be
-    // interesting to this range anyway.
-    // TODO(mtrofin): extend to aliased ranges, too.
-    if ((kSimpleFPAliasing || !check_fp_aliasing()) &&
-        positions[cur_reg] < range->Start()) {
-      continue;
-    }
-
-    LifetimePosition next_intersection = cur_inactive->FirstIntersection(range);
-    if (!next_intersection.IsValid()) continue;
-    if (kSimpleFPAliasing || !check_fp_aliasing()) {
-      positions[cur_reg] = Min(positions[cur_reg], next_intersection);
-      TRACE("Register %s is free until pos %d (2)\n", RegisterName(cur_reg),
-            Min(positions[cur_reg], next_intersection).value());
-    } else {
-      int alias_base_index = -1;
-      int aliases = data()->config()->GetAliases(
-          cur_inactive->representation(), cur_reg, rep, &alias_base_index);
-      DCHECK(aliases > 0 || (aliases == 0 && alias_base_index == -1));
-      while (aliases--) {
-        int aliased_reg = alias_base_index + aliases;
-        positions[aliased_reg] = Min(positions[aliased_reg], next_intersection);
+  for (int cur_reg = 0; cur_reg < num_regs; ++cur_reg) {
+    for (LiveRange* cur_inactive : inactive_live_ranges(cur_reg)) {
+      DCHECK_GT(cur_inactive->End(), range->Start());
+      CHECK_EQ(cur_inactive->assigned_register(), cur_reg);
+      // No need to carry out intersections, when this register won't be
+      // interesting to this range anyway.
+      // TODO(mtrofin): extend to aliased ranges, too.
+      if ((kSimpleFPAliasing || !check_fp_aliasing()) &&
+          positions[cur_reg] <= cur_inactive->NextStart()) {
+        break;
+      }
+      LifetimePosition next_intersection =
+          cur_inactive->FirstIntersection(range);
+      if (!next_intersection.IsValid()) continue;
+      if (kSimpleFPAliasing || !check_fp_aliasing()) {
+        positions[cur_reg] = std::min(positions[cur_reg], next_intersection);
+        TRACE("Register %s is free until pos %d (2)\n", RegisterName(cur_reg),
+              positions[cur_reg].value());
+      } else {
+        int alias_base_index = -1;
+        int aliases = data()->config()->GetAliases(
+            cur_inactive->representation(), cur_reg, rep, &alias_base_index);
+        DCHECK(aliases > 0 || (aliases == 0 && alias_base_index == -1));
+        while (aliases--) {
+          int aliased_reg = alias_base_index + aliases;
+          positions[aliased_reg] =
+              std::min(positions[aliased_reg], next_intersection);
+        }
       }
     }
   }
@@ -4336,46 +4355,46 @@ void LinearScanAllocator::AllocateBlockedReg(LiveRange* current,
     }
   }
 
-  for (LiveRange* range : inactive_live_ranges()) {
-    DCHECK(range->End() > current->Start());
-    int cur_reg = range->assigned_register();
-    bool is_fixed = range->TopLevel()->IsFixed();
-
-    // Don't perform costly intersections if they are guaranteed to not update
-    // block_pos or use_pos.
-    // TODO(mtrofin): extend to aliased ranges, too.
-    if ((kSimpleFPAliasing || !check_fp_aliasing())) {
-      if (is_fixed) {
-        if (block_pos[cur_reg] < range->Start()) continue;
-      } else {
-        if (use_pos[cur_reg] < range->Start()) continue;
+  for (int cur_reg = 0; cur_reg < num_registers(); ++cur_reg) {
+    for (LiveRange* range : inactive_live_ranges(cur_reg)) {
+      DCHECK(range->End() > current->Start());
+      DCHECK_EQ(range->assigned_register(), cur_reg);
+      bool is_fixed = range->TopLevel()->IsFixed();
+
+      // Don't perform costly intersections if they are guaranteed to not update
+      // block_pos or use_pos.
+      // TODO(mtrofin): extend to aliased ranges, too.
+      if ((kSimpleFPAliasing || !check_fp_aliasing())) {
+        DCHECK_LE(use_pos[cur_reg], block_pos[cur_reg]);
+        if (block_pos[cur_reg] <= range->NextStart()) break;
+        if (!is_fixed && use_pos[cur_reg] <= range->NextStart()) continue;
       }
-    }
 
-    LifetimePosition next_intersection = range->FirstIntersection(current);
-    if (!next_intersection.IsValid()) continue;
+      LifetimePosition next_intersection = range->FirstIntersection(current);
+      if (!next_intersection.IsValid()) continue;
 
-    if (kSimpleFPAliasing || !check_fp_aliasing()) {
-      if (is_fixed) {
-        block_pos[cur_reg] = Min(block_pos[cur_reg], next_intersection);
-        use_pos[cur_reg] = Min(block_pos[cur_reg], use_pos[cur_reg]);
-      } else {
-        use_pos[cur_reg] = Min(use_pos[cur_reg], next_intersection);
-      }
-    } else {
-      int alias_base_index = -1;
-      int aliases = data()->config()->GetAliases(
-          range->representation(), cur_reg, rep, &alias_base_index);
-      DCHECK(aliases > 0 || (aliases == 0 && alias_base_index == -1));
-      while (aliases--) {
-        int aliased_reg = alias_base_index + aliases;
+      if (kSimpleFPAliasing || !check_fp_aliasing()) {
         if (is_fixed) {
-          block_pos[aliased_reg] =
-              Min(block_pos[aliased_reg], next_intersection);
-          use_pos[aliased_reg] =
-              Min(block_pos[aliased_reg], use_pos[aliased_reg]);
+          block_pos[cur_reg] = Min(block_pos[cur_reg], next_intersection);
+          use_pos[cur_reg] = Min(block_pos[cur_reg], use_pos[cur_reg]);
         } else {
-          use_pos[aliased_reg] = Min(use_pos[aliased_reg], next_intersection);
+          use_pos[cur_reg] = Min(use_pos[cur_reg], next_intersection);
+        }
+      } else {
+        int alias_base_index = -1;
+        int aliases = data()->config()->GetAliases(
+            range->representation(), cur_reg, rep, &alias_base_index);
+        DCHECK(aliases > 0 || (aliases == 0 && alias_base_index == -1));
+        while (aliases--) {
+          int aliased_reg = alias_base_index + aliases;
+          if (is_fixed) {
+            block_pos[aliased_reg] =
+                Min(block_pos[aliased_reg], next_intersection);
+            use_pos[aliased_reg] =
+                Min(block_pos[aliased_reg], use_pos[aliased_reg]);
+          } else {
+            use_pos[aliased_reg] = Min(use_pos[aliased_reg], next_intersection);
+          }
         }
       }
     }
@@ -4489,40 +4508,38 @@ void LinearScanAllocator::SplitAndSpillIntersecting(LiveRange* current,
     it = ActiveToHandled(it);
   }
 
-  for (auto it = inactive_live_ranges().begin();
-       it != inactive_live_ranges().end();) {
-    LiveRange* range = *it;
-    DCHECK(range->End() > current->Start());
-    if (range->TopLevel()->IsFixed()) {
-      ++it;
-      continue;
-    }
+  for (int cur_reg = 0; cur_reg < num_registers(); ++cur_reg) {
     if (kSimpleFPAliasing || !check_fp_aliasing()) {
-      if (range->assigned_register() != reg) {
+      if (cur_reg != reg) continue;
+    }
+    for (auto it = inactive_live_ranges(cur_reg).begin();
+         it != inactive_live_ranges(cur_reg).end();) {
+      LiveRange* range = *it;
+      if (!kSimpleFPAliasing && check_fp_aliasing() &&
+          !data()->config()->AreAliases(current->representation(), reg,
+                                        range->representation(), cur_reg)) {
         ++it;
         continue;
       }
-    } else {
-      if (!data()->config()->AreAliases(current->representation(), reg,
-                                        range->representation(),
-                                        range->assigned_register())) {
+      DCHECK(range->End() > current->Start());
+      if (range->TopLevel()->IsFixed()) {
         ++it;
         continue;
       }
-    }
 
-    LifetimePosition next_intersection = range->FirstIntersection(current);
-    if (next_intersection.IsValid()) {
-      UsePosition* next_pos = range->NextRegisterPosition(current->Start());
-      if (next_pos == nullptr) {
-        SpillAfter(range, split_pos, spill_mode);
+      LifetimePosition next_intersection = range->FirstIntersection(current);
+      if (next_intersection.IsValid()) {
+        UsePosition* next_pos = range->NextRegisterPosition(current->Start());
+        if (next_pos == nullptr) {
+          SpillAfter(range, split_pos, spill_mode);
+        } else {
+          next_intersection = Min(next_intersection, next_pos->pos());
+          SpillBetween(range, split_pos, next_intersection, spill_mode);
+        }
+        it = InactiveToHandled(it);
       } else {
-        next_intersection = Min(next_intersection, next_pos->pos());
-        SpillBetween(range, split_pos, next_intersection, spill_mode);
+        ++it;
       }
-      it = InactiveToHandled(it);
-    } else {
-      ++it;
     }
   }
 }

src/compiler/backend/register-allocator.h

@@ -626,9 +626,10 @@ class V8_EXPORT_PRIVATE LiveRange : public NON_EXPORTED_BASE(ZoneObject) {
   bool ShouldBeAllocatedBefore(const LiveRange* other) const;
   bool CanCover(LifetimePosition position) const;
   bool Covers(LifetimePosition position) const;
-  LifetimePosition NextStartAfter(LifetimePosition position) const;
+  LifetimePosition NextStartAfter(LifetimePosition position);
   LifetimePosition NextEndAfter(LifetimePosition position) const;
   LifetimePosition FirstIntersection(LiveRange* other) const;
+  LifetimePosition NextStart() const { return next_start_; }
 
   void VerifyChildStructure() const {
     VerifyIntervals();
@@ -689,6 +690,8 @@ class V8_EXPORT_PRIVATE LiveRange : public NON_EXPORTED_BASE(ZoneObject) {
   // Cache the last position splintering stopped at.
   mutable UsePosition* splitting_pointer_;
   LiveRangeBundle* bundle_ = nullptr;
+  // Next interval start, relative to the current linear scan position.
+  LifetimePosition next_start_;
 
   DISALLOW_COPY_AND_ASSIGN(LiveRange);
 };
@@ -1309,16 +1312,28 @@ class LinearScanAllocator final : public RegisterAllocator {
       const InstructionBlock* block);
   bool HasNonDeferredPredecessor(InstructionBlock* block);
 
-  struct LiveRangeOrdering {
+  struct UnhandledLiveRangeOrdering {
     bool operator()(const LiveRange* a, const LiveRange* b) const {
       return a->ShouldBeAllocatedBefore(b);
     }
   };
-  using LiveRangeQueue = ZoneMultiset<LiveRange*, LiveRangeOrdering>;
-  LiveRangeQueue& unhandled_live_ranges() { return unhandled_live_ranges_; }
+
+  struct InactiveLiveRangeOrdering {
+    bool operator()(const LiveRange* a, const LiveRange* b) const {
+      return a->NextStart() < b->NextStart();
+    }
+  };
+
+  using UnhandledLiveRangeQueue =
+      ZoneMultiset<LiveRange*, UnhandledLiveRangeOrdering>;
+  using InactiveLiveRangeQueue =
+      ZoneMultiset<LiveRange*, InactiveLiveRangeOrdering>;
+  UnhandledLiveRangeQueue& unhandled_live_ranges() {
+    return unhandled_live_ranges_;
+  }
   ZoneVector<LiveRange*>& active_live_ranges() { return active_live_ranges_; }
-  ZoneVector<LiveRange*>& inactive_live_ranges() {
-    return inactive_live_ranges_;
+  InactiveLiveRangeQueue& inactive_live_ranges(int reg) {
+    return inactive_live_ranges_[reg];
   }
 
   void SetLiveRangeAssignedRegister(LiveRange* range, int reg);
@@ -1331,10 +1346,10 @@ class LinearScanAllocator final : public RegisterAllocator {
       ZoneVector<LiveRange*>::iterator it);
   ZoneVector<LiveRange*>::iterator ActiveToInactive(
       ZoneVector<LiveRange*>::iterator it, LifetimePosition position);
-  ZoneVector<LiveRange*>::iterator InactiveToHandled(
-      ZoneVector<LiveRange*>::iterator it);
-  ZoneVector<LiveRange*>::iterator InactiveToActive(
-      ZoneVector<LiveRange*>::iterator it, LifetimePosition position);
+  InactiveLiveRangeQueue::iterator InactiveToHandled(
+      InactiveLiveRangeQueue::iterator it);
+  InactiveLiveRangeQueue::iterator InactiveToActive(
+      InactiveLiveRangeQueue::iterator it, LifetimePosition position);
 
   void ForwardStateTo(LifetimePosition position);
 
@@ -1384,9 +1399,9 @@ class LinearScanAllocator final : public RegisterAllocator {
 
   void PrintRangeOverview(std::ostream& os);
 
-  LiveRangeQueue unhandled_live_ranges_;
+  UnhandledLiveRangeQueue unhandled_live_ranges_;
   ZoneVector<LiveRange*> active_live_ranges_;
-  ZoneVector<LiveRange*> inactive_live_ranges_;
+  ZoneVector<InactiveLiveRangeQueue> inactive_live_ranges_;
 
   // Approximate at what position the set of ranges will change next.
   // Used to avoid scanning for updates even if none are present.