Allocate typed arrays uninitialized when constructing from array-like.

src/runtime.cc

 // Copyright 2012 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 915 matching lines @@
 
   Handle<JSArrayBuffer> buffer = isolate->factory()->NewJSArrayBuffer();
   size_t length = NumberToSize(isolate, *length_obj);
   size_t byte_length = length * element_size;
   if (byte_length < length) {  // Overflow
     return isolate->Throw(*isolate->factory()->
           NewRangeError("invalid_array_buffer_length",
             HandleVector<Object>(NULL, 0)));
   }
 
+  // NOTE: not initializing backing store.
   // We assume that the caller of this function will initialize holder
   // with the loop
   //      for(i = 0; i < length; i++) { holder[i] = source[i]; }
+  // We assume that the caller of this function is always a typed array
+  // constructor.
   // If source is a typed array, this loop will always run to completion,
   // so we are sure that the backing store will be initialized.
-  // Otherwise, we do not know (the indexing operation might throw).
-  // Hence we require zero initialization unless our source is a typed array.
-  bool should_zero_initialize = !source->IsJSTypedArray();
+  // Otherwise, the indexing operation might throw, so the loop will not
+  // run to completion and the typed array might remain partly initialized.
+  // However we further assume that the caller of this function is a typed array
+  // constructor, and the exception will propagate out of the constructor,
+  // therefore uninitialized memory will not be accessible by a user program.
+  //
+  // TODO(dslomov): revise this once we support subclassing.
 
   if (!Runtime::SetupArrayBufferAllocatingData(
-        isolate, buffer, byte_length, should_zero_initialize)) {
+        isolate, buffer, byte_length, false)) {
     return isolate->Throw(*isolate->factory()->
           NewRangeError("invalid_array_buffer_length",
             HandleVector<Object>(NULL, 0)));
   }
 
   holder->set_buffer(*buffer);
   holder->set_byte_offset(Smi::FromInt(0));
   Handle<Object> byte_length_obj(
       isolate->factory()->NewNumberFromSize(byte_length));
   holder->set_byte_length(*byte_length_obj);
@@ skipping 13844 matching lines @@
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     isolate->counters()->gc_last_resort_from_js()->Increment();
     isolate->heap()->CollectAllGarbage(Heap::kNoGCFlags,
                                        "Runtime::PerformGC");
   }
 }
 
 
 } }  // namespace v8::internal