PSOldPromotionLAB::allocate (at https://hg.openjdk.java.net/jdk/jdk/file/7cf02b2c1455/src/hotspot/share/gc/parallel/psPromotionLAB.hpp#l116) says

  HeapWord* allocate(size_t size) {
    ....
    HeapWord* obj = top();
    HeapWord* new_top = obj + size;
    // The 'new_top>obj' check is needed to detect overflow of obj+size.
    if (new_top > obj && new_top <= end()) {
      set_top(new_top);
    ....

where there is an explicit check for overflow of the addition. The usual way to write that code is

    HeapWord* obj = top();
    if (pointer_delta(end(), obj) >= size) {
      HeapWord* new_top = obj + size;
      set_top(new_top)

so that there would not have to be an explicit test for overflow.

The suggested change does not improve the correctness of the code. I don't think it will be possible to measure the performance effect of the suggested change. But the code would be cleaner, and more like the other allocations.