[regalloc] Skip hint search when there are no hints

R=sigurds@chromium.org

Bug: v8:10533
Change-Id: I81756299c286801423fd0f5170046235c8e3dede
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2235117Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68230}

src/compiler/backend/register-allocator.cc

@@ -391,8 +391,8 @@ LiveRange::LiveRange(int relative_id, MachineRepresentation rep,
       next_(nullptr),
       current_interval_(nullptr),
       last_processed_use_(nullptr),
-      current_hint_position_(nullptr),
-      splitting_pointer_(nullptr) {
+      splitting_pointer_(nullptr),
+      current_hint_position_(nullptr) {
   DCHECK(AllocatedOperand::IsSupportedRepresentation(rep));
   bits_ = AssignedRegisterField::encode(kUnassignedRegister) |
           RepresentationField::encode(rep) |
@@ -474,6 +474,7 @@ RegisterKind LiveRange::kind() const {
 }
 
 UsePosition* LiveRange::FirstHintPosition(int* register_index) const {
+  if (!TopLevel()->current_hint_position()) return nullptr;
   for (UsePosition* pos = first_pos_; pos != nullptr; pos = pos->next()) {
     if (pos->HintRegister(register_index)) return pos;
   }
@@ -2407,6 +2408,8 @@ void LiveRangeBuilder::ProcessInstructions(const InstructionBlock* block,
           if (to_range->is_phi()) {
             phi_vreg = to_vreg;
             if (to_range->is_non_loop_phi()) {
+              DCHECK_EQ(to_range->current_hint_position(),
+                        to_range->FirstHintPosition());
               hint = to_range->current_hint_position();
               hint_type = hint == nullptr ? UsePositionHintType::kNone
                                           : UsePositionHintType::kUsePos;

src/compiler/backend/register-allocator.h

@@ -625,7 +625,6 @@ class V8_EXPORT_PRIVATE LiveRange : public NON_EXPORTED_BASE(ZoneObject) {
   }
 
   UsePosition* current_hint_position() const {
-    DCHECK(current_hint_position_ == FirstHintPosition());
     return current_hint_position_;
   }
 
@@ -701,10 +700,11 @@ class V8_EXPORT_PRIVATE LiveRange : public NON_EXPORTED_BASE(ZoneObject) {
   mutable UseInterval* current_interval_;
   // This is used as a cache, it doesn't affect correctness.
   mutable UsePosition* last_processed_use_;
-  // This is used as a cache, it's invalid outside of BuildLiveRanges.
-  mutable UsePosition* current_hint_position_;
   // Cache the last position splintering stopped at.
   mutable UsePosition* splitting_pointer_;
+  // This is used as a cache in BuildLiveRanges, and to quickly check for the
+  // absence of hints during register allocation.
+  UsePosition* current_hint_position_;
   LiveRangeBundle* bundle_ = nullptr;
   // Next interval start, relative to the current linear scan position.
   LifetimePosition next_start_;