[builtins] Copy TypedArray elements with the elements accessor in Set.

Performance regressed for this with the I+TF switch. This speeds up
the simple case by using optimizations in the elements accessor.

Bug: chromium:700835
Change-Id: Iaba30951b93daefa0fb32acd6656ac705cdc73ed
Reviewed-on: https://chromium-review.googlesource.com/483341
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Franziska Hinkelmann <franzih@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44913}

src/elements.cc

@@ -3131,7 +3131,7 @@ class TypedElementsAccessor
     Handle<BackingStore> destination_elements(
         BackingStore::cast(destination->elements()));
 
-    DCHECK_EQ(source->length(), destination->length());
+    DCHECK_GE(destination->length(), source->length());
     DCHECK(source->length()->IsSmi());
     DCHECK_EQ(Smi::FromInt(static_cast<int>(length)), source->length());
 
@@ -3284,6 +3284,9 @@ class TypedElementsAccessor
     return Smi::kZero;
   }
 
+  // This doesn't guarantee that the destination array will be completely
+  // filled. The caller must do this by passing a source with equal length, if
+  // that is required.
   static Object* CopyElementsHandleImpl(Handle<JSReceiver> source,
                                         Handle<JSObject> destination,
                                         size_t length) {

src/js/typedarray.js

@@ -255,9 +255,7 @@ function TypedArraySetFromArrayLike(target, source, sourceLength, offset) {
     }
   }
   else {
-    for (var i = 0; i < sourceLength; i++) {
-      target[i] = source[i];
-    }
+    %TypedArrayCopyElements(target, source, sourceLength);
   }
 }
 
@@ -338,7 +336,7 @@ function TypedArraySet(obj, offset) {
         if (IS_NUMBER(obj)) {
             // For number as a first argument, throw TypeError
             // instead of silently ignoring the call, so that
-            // the user knows (s)he did something wrong.
+            // users know they did something wrong.
             // (Consistent with Firefox and Blink/WebKit)
             throw %make_type_error(kInvalidArgument);
         }