[ignition] Eliminate hole checks where statically possible for loads and stores

src/ast/scopes.cc

 // Copyright 2012 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/ast/scopes.h"
 
 #include <set>
 
 #include "src/accessors.h"
 #include "src/ast/ast.h"
@@ skipping 1593 matching lines @@
           // We don't know what the (potentially lazy parsed) inner function
           // does with the variable; pessimistically assume that it's assigned.
           var->set_maybe_assigned();
         }
       }
       scope = scope->outer_scope_;
     }
   }
 }
 
+namespace {
+
+bool AccessNeedsHoleCheck(Variable* var, VariableProxy* proxy, Scope* scope) {
+  if (!var->binding_needs_init()) {
+    return false;
+  }
+
+  DCHECK(var->scope() != NULL);
+
+  if (var->is_this()) {
+    DCHECK(
+        IsSubclassConstructor(scope->GetDeclarationScope()->function_kind()));
+    // TODO(littledan): implement 'this' hole check elimination.
+    return true;
+  }
+
+  // We should always have valid source positions.
+  DCHECK(var->initializer_position() != kNoSourcePosition);
+  DCHECK(proxy->position() != kNoSourcePosition);
+
+  // Check if the binding really needs an initialization check. The check
+  // can be skipped in the following situation: we have a LET or CONST
+  // binding, both the Variable and the VariableProxy have the same
+  // declaration scope (i.e. they are both in global code, in the
+  // same function or in the same eval code), the VariableProxy is in
+  // the source physically located after the initializer of the variable,
+  // and that the initializer cannot be skipped due to a nonlinear scope.
+  //
+  // The condition on the declaration scopes is a conservative check for
+  // nested functions that access a binding and are called before the
+  // binding is initialized:
+  //   function() { f(); let x = 1; function f() { x = 2; } }
+  //
+  // The check cannot be skipped on non-linear scopes, namely switch
+  // scopes, to ensure tests are done in cases like the following:
+  //   switch (1) { case 0: let x = 2; case 1: f(x); }
+  // The scope of the variable needs to be checked, in case the use is
+  // in a sub-block which may be linear.
+  return var->scope()->GetDeclarationScope() != scope->GetDeclarationScope() ||
+         var->scope()->is_nonlinear() ||
+         var->initializer_position() >= proxy->position();
+}
+
+}  // anonymous namespace
+
 void Scope::ResolveTo(ParseInfo* info, VariableProxy* proxy, Variable* var) {
 #ifdef DEBUG
   if (info->script_is_native()) {
     // To avoid polluting the global object in native scripts
     //  - Variables must not be allocated to the global scope.
     CHECK_NOT_NULL(outer_scope());
     //  - Variables must be bound locally or unallocated.
     if (var->IsGlobalObjectProperty()) {
       // The following variable name may be minified. If so, disable
       // minification in js2c.py for better output.
       Handle<String> name = proxy->raw_name()->string();
       V8_Fatal(__FILE__, __LINE__, "Unbound variable: '%s' in native script.",
                name->ToCString().get());
     }
     VariableLocation location = var->location();
     CHECK(location == VariableLocation::LOCAL ||
           location == VariableLocation::CONTEXT ||
           location == VariableLocation::PARAMETER ||
           location == VariableLocation::UNALLOCATED);
   }
 #endif
 
   DCHECK_NOT_NULL(var);
   if (proxy->is_assigned()) var->set_maybe_assigned();
+  if (AccessNeedsHoleCheck(var, proxy, this)) proxy->set_needs_hole_check();
   proxy->BindTo(var);
 }
 
 void Scope::ResolveVariablesRecursively(ParseInfo* info) {
   DCHECK(info->script_scope()->is_script_scope());
 
   // Resolve unresolved variables for this scope.
   for (VariableProxy* proxy = unresolved_; proxy != nullptr;
        proxy = proxy->next_unresolved()) {
     ResolveVariable(info, proxy);
@@ skipping 297 matching lines @@
   Variable* function =
       is_function_scope() ? AsDeclarationScope()->function_var() : nullptr;
   bool is_function_var_in_context =
       function != nullptr && function->IsContextSlot();
   return num_heap_slots() - Context::MIN_CONTEXT_SLOTS -
          (is_function_var_in_context ? 1 : 0);
 }
 
 }  // namespace internal
 }  // namespace v8