[heap] Use MemCopy in CopyElements

This CL uses MemCopy in CopyElements, because we think that the input
FixedArrays can never overlap. This CL also adds a DCHECK to ensure
this. If our assumption is wrong, it will be interesting to learn why.

Change-Id: Ib8258b934f43b82b1281481c592762e43b35938f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1549154
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60589}

src/heap/heap.cc

@@ -1535,6 +1535,8 @@ void Heap::CopyElements(FixedArray dst_array, FixedArray src_array,
   DCHECK_NE(dst_array->map(), ReadOnlyRoots(this).fixed_cow_array_map());
   ObjectSlot dst = dst_array->RawFieldOfElementAt(dst_index);
   ObjectSlot src = src_array->RawFieldOfElementAt(src_index);
+  // Ensure ranges do not overlap.
+  DCHECK(dst + len <= src || src + len <= dst);
   if (FLAG_concurrent_marking && incremental_marking()->IsMarking()) {
     if (dst < src) {
       for (int i = 0; i < len; i++) {
@@ -1553,8 +1555,7 @@ void Heap::CopyElements(FixedArray dst_array, FixedArray src_array,
       }
     }
   } else {
-    // TODO(sigurds): Figure out whether we can use MemCopy here.
-    MemMove(dst.ToVoidPtr(), src.ToVoidPtr(), len * kTaggedSize);
+    MemCopy(dst.ToVoidPtr(), src.ToVoidPtr(), len * kTaggedSize);
   }
   if (mode == SKIP_WRITE_BARRIER) return;
   FIXED_ARRAY_ELEMENTS_WRITE_BARRIER(this, dst_array, dst_index, len);