RFR(xxs): 8210068: Unsafe.allocateMemory() should not round up requested memory size

[Thomas Stüfe]

Hello,

may I have reviews please for this tiny fix.

bug: https://bugs.openjdk.java.net/browse/JDK-8210068
webrev: http://cr.openjdk.java.net/~stuefe/webrevs/8210068-Unsafe.allocateMemory-shall-not-round-up-user-size/webrev.00/webrev/

In Unsafe_AllocateMemory0(), we align the size requested by the caller
up like this:

  sz = align_up(sz, HeapWordSize);
  void* x = os::malloc(sz, mtOther);

This rounding up is unnecessary, since the additional space is not
needed. I tried to find the reason for that rounding, but it predates
the start of OpenJDK history.

The problem is that this rounding causes NMT to overreport the numbers
of mtOther - which contains the nio DirectByteBuffers - the more
buffers and the smaller they are the more inaccurate those numbers
become.

Note that even though the numbers are off, little real memory is
wasted by the alignment. This is probably because the libc malloc
allocators internally round up allocations to at least pointer sizes
anyway, or at least waste the trailing bytes of each allocation.

Also note that yes, theoretically we may now uncover buffer overruns -
program errors which were hidden by the rounding up of the user size.
In my opinion, if we do, we should fix them. In debug builds,
os::malloc() and friends have guard fences around the allocated user
portions, so we should notice. In any case we will run the submit
tests as well as our suite of nightlies.

Kind Regards, Thomas