Reapply r9870 "Remove some initialization checks based on source positions.".

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index a80a8b792e8dd8fe66d17c9846472bf15aad7e60..0588597bb1f29f0c5332761d12019d12c1a783b3 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -1240,27 +1240,63 @@ void FullCodeGenerator::EmitVariableLoad(VariableProxy* proxy) {
     case Variable::LOCAL:
     case Variable::CONTEXT: {
       Comment cmnt(masm_, var->IsContextSlot() ? "Context slot" : "Stack slot");
-      if (!var->binding_needs_init()) {
-        context()->Plug(var);
-      } else {
-        // Let and const need a read barrier.
-        Label done;
-        GetVar(rax, var);
-        __ CompareRoot(rax, Heap::kTheHoleValueRootIndex);
-        __ j(not_equal, &done, Label::kNear);
-        if (var->mode() == LET || var->mode() == CONST_HARMONY) {
-          // Throw a reference error when using an uninitialized let/const
-          // binding in harmony mode.
-          __ Push(var->name());
-          __ CallRuntime(Runtime::kThrowReferenceError, 1);
+      if (var->binding_needs_init()) {
+        // var->scope() may be NULL when the proxy is located in eval code and
+        // refers to a potential outside binding. Currently those bindings are
+        // always looked up dynamically, i.e. in that case
+        //     var->location() == LOOKUP.
+        // always holds.
+        ASSERT(var->scope() != NULL);
+
+        // 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 in harmony mode, 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) and the VariableProxy is in
+        // the source physically located after the initializer of the variable.
+        //
+        // We cannot skip any initialization checks for CONST in non-harmony
+        // mode because const variables may be declared but never initialized:
+        //   if (false) { const x; }; var y = x;
+        //
+        // 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; } }
+        //
+        bool skip_init_check;
+        if (var->scope()->DeclarationScope() != scope()->DeclarationScope()) {
+          skip_init_check = false;
         } else {
-          // Uninitalized const bindings outside of harmony mode are unholed.
-          ASSERT(var->mode() == CONST);
-          __ LoadRoot(rax, Heap::kUndefinedValueRootIndex);
+          // Check that we always have valid source position.
+          ASSERT(var->initializer_position() != RelocInfo::kNoPosition);
+          ASSERT(proxy->position() != RelocInfo::kNoPosition);
+          skip_init_check = var->mode() != CONST &&
+              var->initializer_position() < proxy->position();
+        }
+
+        if (!skip_init_check) {
+          // Let and const need a read barrier.
+          Label done;
+          GetVar(rax, var);
+          __ CompareRoot(rax, Heap::kTheHoleValueRootIndex);
+          __ j(not_equal, &done, Label::kNear);
+          if (var->mode() == LET || var->mode() == CONST_HARMONY) {
+            // Throw a reference error when using an uninitialized let/const
+            // binding in harmony mode.
+            __ Push(var->name());
+            __ CallRuntime(Runtime::kThrowReferenceError, 1);
+          } else {
+            // Uninitalized const bindings outside of harmony mode are unholed.
+            ASSERT(var->mode() == CONST);
+            __ LoadRoot(rax, Heap::kUndefinedValueRootIndex);
+          }
+          __ bind(&done);
+          context()->Plug(rax);
+          break;
         }
-        __ bind(&done);
-        context()->Plug(rax);
       }
+      context()->Plug(var);
       break;
     }