The sparcv9 uses the generic sparc64 sysdeps/sparc/sparc64/memmove.S implementation and the initial size test is done with a signed comparison:

ENTRY(memmove)
        mov     %o0, %g2        /* Save pointer to destination  */
        cmp     %o1, %o0        /* if from address is >= to use forward copy  */
        bgeu,a  %XCC, 2f        /* else use backward if ...  */
         cmp    %o2, 17         /* delay slot, for small counts copy bytes  */

        sub     %o0, %o1, %o4   /* get difference of two addresses  */
        cmp     %o2, %o4        /* compare size and difference of addresses  */
        bgu     %XCC, .Lovbc    /* if size is bigger, have to do overlapped copy  */
         cmp    %o2, 17         /* delay slot, for small counts copy bytes  */
/*
 * normal, copy forwards
 */
2:      ble    %XCC, .Ldbytecp
         andcc  %o1, 3, %o5     /* is src word aligned  */

Where it should be a 'bleu' due the buffer length being a size_t.