Remove some initialization checks based on source positions.

This depends on
http://codereview.chromium.org/8352039 and
http://codereview.chromium.org/8423005 .

Review URL: http://codereview.chromium.org/8422010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9870 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/arm/full-codegen-arm.cc

@@ -1283,28 +1283,60 @@ void FullCodeGenerator::EmitVariableLoad(VariableProxy* proxy) {
       Comment cmnt(masm_, var->IsContextSlot()
                               ? "Context variable"
                               : "Stack variable");
-      if (!var->binding_needs_init()) {
-        context()->Plug(var);
-      } else {
-        // Let and const need a read barrier.
-        GetVar(r0, var);
-        __ CompareRoot(r0, Heap::kTheHoleValueRootIndex);
-        if (var->mode() == LET || var->mode() == CONST_HARMONY) {
-          // Throw a reference error when using an uninitialized let/const
-          // binding in harmony mode.
-          Label done;
-          __ b(ne, &done);
-          __ mov(r0, Operand(var->name()));
-          __ push(r0);
-          __ CallRuntime(Runtime::kThrowReferenceError, 1);
-          __ bind(&done);
-        } else {
-          // Uninitalized const bindings outside of harmony mode are unholed.
-          ASSERT(var->mode() == CONST);
-          __ LoadRoot(r0, Heap::kUndefinedValueRootIndex, eq);
+      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; } }
+        //
+        // Check that we always have valid source position.
+        ASSERT(var->initializer_position() != RelocInfo::kNoPosition);
+        ASSERT(proxy->position() != RelocInfo::kNoPosition);
+        bool skip_init_check =
+            var->mode() != CONST &&
+            var->scope()->DeclarationScope() == scope()->DeclarationScope() &&
+            var->initializer_position() < proxy->position();
+        if (!skip_init_check) {
+          // Let and const need a read barrier.
+          GetVar(r0, var);
+          __ CompareRoot(r0, Heap::kTheHoleValueRootIndex);
+          if (var->mode() == LET || var->mode() == CONST_HARMONY) {
+            // Throw a reference error when using an uninitialized let/const
+            // binding in harmony mode.
+            Label done;
+            __ b(ne, &done);
+            __ mov(r0, Operand(var->name()));
+            __ push(r0);
+            __ CallRuntime(Runtime::kThrowReferenceError, 1);
+            __ bind(&done);
+          } else {
+            // Uninitalized const bindings outside of harmony mode are unholed.
+            ASSERT(var->mode() == CONST);
+            __ LoadRoot(r0, Heap::kUndefinedValueRootIndex, eq);
+          }
+          context()->Plug(r0);
+          break;
         }
-        context()->Plug(r0);
       }
+      context()->Plug(var);
       break;
     }
 

src/ia32/full-codegen-ia32.cc

@@ -1234,27 +1234,59 @@ void FullCodeGenerator::EmitVariableLoad(VariableProxy* proxy) {
       Comment cmnt(masm_, var->IsContextSlot()
                               ? "Context variable"
                               : "Stack variable");
-      if (!var->binding_needs_init()) {
-        context()->Plug(var);
-      } else {
-        // Let and const need a read barrier.
-        Label done;
-        GetVar(eax, var);
-        __ cmp(eax, isolate()->factory()->the_hole_value());
-        __ 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(Immediate(var->name()));
-          __ CallRuntime(Runtime::kThrowReferenceError, 1);
-        } else {
-          // Uninitalized const bindings outside of harmony mode are unholed.
-          ASSERT(var->mode() == CONST);
-          __ mov(eax, isolate()->factory()->undefined_value());
+      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; } }
+        //
+        // Check that we always have valid source position.
+        ASSERT(var->initializer_position() != RelocInfo::kNoPosition);
+        ASSERT(proxy->position() != RelocInfo::kNoPosition);
+        bool skip_init_check =
+            var->mode() != CONST &&
+            var->scope()->DeclarationScope() == scope()->DeclarationScope() &&
+            var->initializer_position() < proxy->position();
+        if (!skip_init_check) {
+          // Let and const need a read barrier.
+          Label done;
+          GetVar(eax, var);
+          __ cmp(eax, isolate()->factory()->the_hole_value());
+          __ 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(Immediate(var->name()));
+            __ CallRuntime(Runtime::kThrowReferenceError, 1);
+          } else {
+            // Uninitalized const bindings outside of harmony mode are unholed.
+            ASSERT(var->mode() == CONST);
+            __ mov(eax, isolate()->factory()->undefined_value());
+          }
+          __ bind(&done);
+          context()->Plug(eax);
+          break;
         }
-        __ bind(&done);
-        context()->Plug(eax);
       }
+      context()->Plug(var);
       break;
     }
 

src/parser.cc

@@ -1773,7 +1773,7 @@ Block* Parser::ParseVariableDeclarations(
     // For let/const declarations in harmony mode, we can also immediately
     // pre-resolve the proxy because it resides in the same scope as the
     // declaration.
-    Declare(name, mode, NULL, mode != VAR, CHECK_OK);
+    VariableProxy* proxy = Declare(name, mode, NULL, mode != VAR, CHECK_OK);
     nvars++;
     if (declaration_scope->num_var_or_const() > kMaxNumFunctionLocals) {
       ReportMessageAt(scanner().location(), "too_many_variables",
@@ -1828,6 +1828,11 @@ Block* Parser::ParseVariableDeclarations(
       if (decl_props != NULL) *decl_props = kHasInitializers;
     }
 
+    // Record the end position of the initializer.
+    if (proxy->var() != NULL) {
+      proxy->var()->set_initializer_position(scanner().location().end_pos);
+    }
+
     // Make sure that 'const x' and 'let x' initialize 'x' to undefined.
     if (value == NULL && needs_init) {
       value = GetLiteralUndefined();

src/variables.cc

@@ -66,6 +66,7 @@ Variable::Variable(Scope* scope,
     kind_(kind),
     location_(UNALLOCATED),
     index_(-1),
+    initializer_position_(RelocInfo::kNoPosition),
     local_if_not_shadowed_(NULL),
     is_valid_LHS_(is_valid_LHS),
     is_accessed_from_inner_scope_(false),

src/variables.h

@@ -103,6 +103,9 @@ class Variable: public ZoneObject {
   bool is_used() { return is_used_; }
   void set_is_used(bool flag) { is_used_ = flag; }
 
+  int initializer_position() { return initializer_position_; }
+  void set_initializer_position(int pos) { initializer_position_ = pos; }
+
   bool IsVariable(Handle<String> n) const {
     return !is_this() && name().is_identical_to(n);
   }
@@ -163,6 +166,7 @@ class Variable: public ZoneObject {
   Kind kind_;
   Location location_;
   int index_;
+  int initializer_position_;
 
   // If this field is set, this variable references the stored locally bound
   // variable, but it might be shadowed by variable bindings introduced by

src/x64/full-codegen-x64.cc

@@ -1240,27 +1240,59 @@ 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);
-        } else {
-          // Uninitalized const bindings outside of harmony mode are unholed.
-          ASSERT(var->mode() == CONST);
-          __ LoadRoot(rax, Heap::kUndefinedValueRootIndex);
+      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; } }
+        //
+        // Check that we always have valid source position.
+        ASSERT(var->initializer_position() != RelocInfo::kNoPosition);
+        ASSERT(proxy->position() != RelocInfo::kNoPosition);
+        bool skip_init_check =
+            var->mode() != CONST &&
+            var->scope()->DeclarationScope() == scope()->DeclarationScope() &&
+            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;
     }