CAS code misses check for null

[Roland Westrelin]

> Do you think that it might be easier to have a macro which takes a dst
> and does this?
>
> void ShenandoahBarrierSet::interpreter_read_barrier(MacroAssembler* masm, Register src, Register dst) {
>   Label is_null;
>   __ mov(dst, src);
>   __ cbz(src, is_null);
>   __ ldr(dst, Address(src, BrooksPointer::BYTE_OFFSET));
>   __ bind(is_null);
> }
>
> It'd make all this stuff much easier to understand.  And the
> efficiency would not be very much different.

It would indeed be much easier to understand. Can I call
interpreter_read_barrier() directly from
MacroAssembler::cmpxchg_oop_shenandoah()? I see it's used from other
places that are not the interpreter.

Roiland.