[maglev] Fix uninitialised next_use in deopts

Deopt InputLocation next_use fields are not initialised, so if a deopt
is the last use of a node we won't release it. Fix this by initialising
the input location array. Also add a DCHECK to verify that register
assignments match what registers a node thinks it's in.

Bug: v8:7700
Change-Id: I4003a027489cf8eeef7c4e60fa64f72cebd2c4e8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3657438Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80690}

src/maglev/maglev-ir.cc

@@ -386,14 +386,11 @@ DeoptInfo::DeoptInfo(Zone* zone, const MaglevCompilationUnit& compilation_unit,
       state(state),
       input_locations(zone->NewArray<InputLocation>(
           GetInputLocationsArraySize(compilation_unit, state))) {
-  // Default initialise if we're printing the graph, to avoid printing junk
-  // values.
-  if (FLAG_print_maglev_graph) {
+  // Initialise InputLocations so that they correctly don't have a next use id.
   for (size_t i = 0; i < GetInputLocationsArraySize(compilation_unit, state);
        ++i) {
     new (&input_locations[i]) InputLocation();
   }
-  }
 }
 
 // ---

src/maglev/maglev-ir.h

@@ -806,6 +806,7 @@ class ValueNode : public Node {
     end_id_ = id;
     *last_uses_next_use_id_ = id;
     last_uses_next_use_id_ = input_location->get_next_use_id_address();
+    DCHECK_EQ(*last_uses_next_use_id_, kInvalidNodeId);
   }
 
   struct LiveRange {
@@ -873,6 +874,14 @@ class ValueNode : public Node {
     }
     return registers_with_result_ != kEmptyRegList;
   }
+  bool is_in_register(Register reg) const {
+    DCHECK(!use_double_register());
+    return registers_with_result_.has(reg);
+  }
+  bool is_in_register(DoubleRegister reg) const {
+    DCHECK(use_double_register());
+    return double_registers_with_result_.has(reg);
+  }
 
   compiler::InstructionOperand allocation() const {
     if (has_register()) {

src/maglev/maglev-regalloc.cc

@@ -446,6 +446,8 @@ void StraightForwardRegisterAllocator::AllocateNode(Node* node) {
     PrintLiveRegs();
     printing_visitor_->os() << "\n";
   }
+
+  VerifyRegisterState();
 }
 
 void StraightForwardRegisterAllocator::AllocateNodeResult(ValueNode* node) {
@@ -637,6 +639,8 @@ void StraightForwardRegisterAllocator::AllocateControlNode(ControlNode* node,
     printing_visitor_->Process(node,
                                ProcessingState(compilation_info_, block_it_));
   }
+
+  VerifyRegisterState();
 }
 
 void StraightForwardRegisterAllocator::TryAllocateToInput(Phi* phi) {
@@ -806,6 +810,22 @@ void StraightForwardRegisterAllocator::VerifyInputs(NodeBase* node) {
 #endif
 }
 
+void StraightForwardRegisterAllocator::VerifyRegisterState() {
+#ifdef DEBUG
+  for (Register reg : general_registers_.used()) {
+    ValueNode* node = general_registers_.GetValue(reg);
+    DCHECK(node->is_in_register(reg));
+  }
+  for (DoubleRegister reg : double_registers_.used()) {
+    ValueNode* node = double_registers_.GetValue(reg);
+    if (!node->is_in_register(reg)) {
+      FATAL("Node n%d doesn't think it is in register %s",
+            graph_labeller()->NodeId(node), RegisterName(reg));
+    }
+  }
+#endif
+}
+
 void StraightForwardRegisterAllocator::SpillRegisters() {
   auto spill = [&](auto reg, ValueNode* node) { Spill(node); };
   general_registers_.ForEachUsedRegister(spill);

src/maglev/maglev-regalloc.h

@@ -136,6 +136,7 @@ class StraightForwardRegisterAllocator {
   void TryAllocateToInput(Phi* phi);
 
   void VerifyInputs(NodeBase* node);
+  void VerifyRegisterState();
 
   void AddMoveBeforeCurrentNode(ValueNode* node,
                                 compiler::InstructionOperand source,