G1BarrierSet::write_ref_array_pre_work roughly looks as follows:

if marking active -> exit

for all references in array:
  if array[i] != NULL
    enqueue(array[i])

enqueue every time checks again whether marking is active. In reality, the generated assembly code seems to load Thread::current() and does the check for every loop iteration which is really unnecessary.

Also some other code can be hoisted out of the loop which results in more compact generated code.