Running a 32bit JVM (HotSpot 8u45) on Linux 64 bit hardware jmap reports strange region information for G1. Xmx is set to 2G (which fits MaxHeapSize in the output below) but region count (almost 15k) and capacity is incorrect. Both these values are based on the length of the G1HeapRegionTable.

Heap Configuration:
   MinHeapFreeRatio = 40
   MaxHeapFreeRatio = 70
   MaxHeapSize = 2147483648 (2048.0MB)
   NewSize = 1048576 (1.0MB)
   MaxNewSize = 428867584 (409.0MB)
   OldSize = 4194304 (4.0MB)
   NewRatio = 2
   SurvivorRatio = 8
   MetaspaceSize = 16777216 (16.0MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize = 4294963200 (4095.99609375MB)
   G1HeapRegionSize = 1048576 (1.0MB)

Heap Usage:
G1 Heap:
   regions = 14336
   capacity = 15032385536 (14336.0MB)
   used = 76347888 (72.81101989746094MB)
   free = 14956037648 (14263.188980102539MB)
   0.5078893687043872% used

The bug was reported by Jakub Kubrynski on hotspot-gc-dev@openjdk.java.net: http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2015-May/013434.html