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

src/mips/full-codegen-mips.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1295 matching lines @@
         //
         // 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();
+        bool skip_init_check;
+        if (var->scope()->DeclarationScope() != scope()->DeclarationScope()) {
+          skip_init_check = false;
+        } else {
+          // 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.
           GetVar(v0, var);
           __ LoadRoot(at, Heap::kTheHoleValueRootIndex);
           __ subu(at, v0, at);  // Sub as compare: at == 0 on eq.
           if (var->mode() == LET || var->mode() == CONST_HARMONY) {
             // Throw a reference error when using an uninitialized let/const
             // binding in harmony mode.
             Label done;
             __ Branch(&done, ne, at, Operand(zero_reg));
@@ skipping 3083 matching lines @@
   *context_length = 0;
   return previous_;
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_MIPS