What version of Jikes RVM are you using?  I'm assuming it is not the cvs head ppc64 because we stop using the explicit stack pointer and store with update sequence in the PPC baseline compiler almost a year ago.  This looks to me like an old code sequence for pushing null on the stack.

--dave

0x0000000060226a68:     li      r3,0
0x0000000060226a6c:     stdu    r3,-8(r12)