JDK-8277373: Make maximum card table size dependent on ObjectAlignmentInBytes

[Vishal Chand]

Hi,

This is regarding the open issue - JDK-8277373
<https://bugs.openjdk.java.net/browse/JDK-8277373>.
By making the necessary changes to the code and running a test program with
G1GC, CardSize=2048, ObjectAlignment=16, I observed the following guarantee
in
*/home/javauser/vishal/jdk_dev/jdk/src/hotspot/share/gc/g1/g1BlockOffsetTable.cpp:175*
failing -  guarantee(_bot->offset_array(start_card) ==
BOTConstants::card_size_in_words()) failed: Wrong value in second card. 0
observed, 256 expected. This is expected as a bot entry is only 1 byte.

Digging deeper into the code made it clear that the HeapWord size puts the
restriction on max. card size to 1024 bytes on 64-bit machines and 512
bytes on 32-bit machines.

There are two options to proceed from here:

   1. Increase the bot entry size. Seems like a straightforward change with
   straightforward issues.
   2. Introduce ObjectAlignment in offset calculations. Seems like this was
   the original goal of  JDK-8277373
   <https://bugs.openjdk.java.net/browse/JDK-8277373>.

Is there any other option that might have been missed and which option
would be preferred? Looking forward to hearing from the community soon.

Thanks and Regards,
Vishal