[turbofan] Fix liveness analysis for let variable in TDZ.

src/compiler/ast-graph-builder.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/ast-graph-builder.h"
 
 #include "src/compiler.h"
 #include "src/compiler/ast-loop-assignment-analyzer.h"
 #include "src/compiler/control-builders.h"
 #include "src/compiler/js-type-feedback.h"
@@ skipping 3474 matching lines @@
           value = BuildHoleCheckSilent(current, value, current);
         }
       } else if (mode == CONST_LEGACY && op != Token::INIT_CONST_LEGACY) {
         // Non-initializing assignment to legacy const is
         // - exception in strict mode.
         // - ignored in sloppy mode.
         if (is_strict(language_mode())) {
           return BuildThrowConstAssignError(bailout_id);
         }
         return value;
+      } else if (mode == LET && op == Token::INIT_LET) {
+        // No initialization check needed because scoping guarantees it. Note
+        // that we still perform a lookup to keep the variable live, because
+        // baseline code might contain debug code that inspects the variable.
+        Node* current = environment()->Lookup(variable);
+        CHECK_NOT_NULL(current);
       } else if (mode == LET && op != Token::INIT_LET) {
         // Perform an initialization check for let declared variables.
         Node* current = environment()->Lookup(variable);
         if (current->op() == the_hole->op()) {
-          value = BuildThrowReferenceError(variable, bailout_id);
+          return BuildThrowReferenceError(variable, bailout_id);
         } else if (current->opcode() == IrOpcode::kPhi) {
-          value = BuildHoleCheckThenThrow(current, variable, value, bailout_id);
+          BuildHoleCheckThenThrow(current, variable, value, bailout_id);
         }
       } else if (mode == CONST && op == Token::INIT_CONST) {
         // Perform an initialization check for const {this} variables.
         // Note that the {this} variable is the only const variable being able
         // to trigger bind operations outside the TDZ, via {super} calls.
         Node* current = environment()->Lookup(variable);
         if (current->op() != the_hole->op() && variable->is_this()) {
           value = BuildHoleCheckElseThrow(current, variable, value, bailout_id);
         }
       } else if (mode == CONST && op != Token::INIT_CONST) {
@@ skipping 776 matching lines @@
     // Phi does not exist yet, introduce one.
     value = NewPhi(inputs, value, control);
     value->ReplaceInput(inputs - 1, other);
   }
   return value;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8