When ObjectAlignmentInBytes=16 is specified we now get heap based compressed oops instead of zero based compressed oops, even when we specify a really small heap size.

In build JDK8-b76 (hs25-b17) both these command lines gave zero based compressed oops:

/localhome/java/jdk-8-ea-bin-b76/bin/java -XX:ObjectAlignmentInBytes=8 -Xmx32m -XX:ClassMetaspaceSize=32m -XX:+UnlockDiagnosticVMOptions -XX:+PrintCompressedOopsMode -version
heap address: 0x00000000fe000000, size: 32 MB, zero based Compressed Oops, 32-bits Oops

$ /localhome/java/jdk-8-ea-bin-b76/bin/java -XX:ObjectAlignmentInBytes=16 -Xmx32m -XX:ClassMetaspaceSize=32m -XX:+UnlockDiagnosticVMOptions -XX:+PrintCompressedOopsMode -version
heap address: 0x00000000fe000000, size: 32 MB, zero based Compressed Oops, 32-bits Oops

In build JDK8-b77 (hs25-b18) we get the following:

$ /localhome/java/jdk-8-ea-bin-b77/bin/java -XX:ObjectAlignmentInBytes=8 -Xmx32m -XX:ClassMetaspaceSize=32m -XX:+UnlockDiagnosticVMOptions -XX:+PrintCompressedOopsMode -version

heap address: 0x00000000fe000000, size: 32 MB, zero based Compressed Oops, 32-bits Oops

/localhome/java/jdk-8-ea-bin-b77/bin/java -XX:ObjectAlignmentInBytes=16 -Xmx32m -XX:ClassMetaspaceSize=32m -XX:+UnlockDiagnosticVMOptions -XX:+PrintCompressedOopsMode -version

heap address: 0x0000000800000000, size: 32 MB, Compressed Oops with base: 0x00000007fdfff000


The problem is that the narrow shift for oops is 4 while the narrow shift for Klasses is 3. Because of this, the following if statement because OopEncodingHeapMax is 64GB while KlassEncodingMetaspaceMax is 32 GB.

      } else if ((total_size <= OopEncodingHeapMax) && (mode != HeapBasedNarrowOop) &&
          (!UseCompressedKlassPointers ||
            (((OopEncodingHeapMax - heap_size) + Universe::class_metaspace_size()) <= KlassEncodingMetaspaceMax))) {

ILW = LHM -> P4
I: L
L: H
W: M