For the next code:

  void test (A new_next) {
    A prev_next = a.next;
    a.next = new_next;
    if (prev_next == null) {
      a.n = a.get_n();
    }
  }

C2 prodes next incorrect assembler code (64 bit x86) with Zero based COOP produces:

02c     movl    RBP, [RSI + #12 (8-bit)]        # compressed ptr ! Field Test.a
030     cmpl    R12, [R12 + RBP << 3 + #16] (compressed oop addressing) # compressed ptr (R12_heapbase==0)
035     NullCheck RBP
035
035   B2: #     B4 B3 <- B1  Freq: 0.999999
035     decode_heap_oop_not_null RSI,RBP
039     encode_heap_oop R11,RDX
040     movl    [R12 + RBP << 3 + #16] (compressed oop addressing), R11 # compressed ptr ! Field A.next
045     movq    R10, RSI        # ptr -> long
048     shrq    R10, #9
04c     movq    R11, 0xfffffd7ff4a1c000 # ptr
056     movb    [R11 + R10], R12        # short/char (R12_heapbase==0)
05a     cmpl    R12, [R12 + RBP << 3 + #16] (compressed oop addressing) # compressed ptr (R12_heapbase==0)
05f     je,s   B4  P=0.100000 C=-1.000000
05f

The test in 05a checks wrong (new_next) value.

% /java/re/jdk/6u21/latest/binaries/solaris-amd64/bin/java -d64 -XX:+UseCompressedOops -XX:+UnlockDiagnosticVMOptions -XX:+PrintCompressedOopsMode -Xbatch -XX:+PrintCompilation -Xcomp -XX:CompileOnly=Test Test

heap address: 0x00000003fc800000, zero based Compressed Oops

  1   b   Test::main (91 bytes)
  1   made not entrant  Test::main (91 bytes)
  2   b   Test::<init> (5 bytes)
  3   b   Test::test (35 bytes)
Wrong value: 1 expected: 2

I will include the test into the C2 regression tests.