MIPS: Remove some initialization checks based on source positions.

Port r9870 (959ec43).

Original commit message:

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

BUG=
TEST=

Review URL: http://codereview.chromium.org/8467007
Patch from Gergely Kis <gergely@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9887 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/full-codegen-mips.cc

@@ -1289,9 +1289,38 @@ void FullCodeGenerator::EmitVariableLoad(VariableProxy* proxy) {
       Comment cmnt(masm_, var->IsContextSlot()
                               ? "Context variable"
                               : "Stack variable");
-      if (!var->binding_needs_init()) {
-        context()->Plug(var);
-      } else {
+      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(v0, var);
           __ LoadRoot(at, Heap::kTheHoleValueRootIndex);
@@ -1312,7 +1341,10 @@ void FullCodeGenerator::EmitVariableLoad(VariableProxy* proxy) {
             __ movz(v0, a0, at);  // Conditional move: Undefined if TheHole.
           }
           context()->Plug(v0);
+          break;
         }
+      }
+      context()->Plug(var);
       break;
     }