RFR: 8166188: G1 Needs pre barrier on dereference of weak JNI handles

[Kim Barrett]

> On Feb 8, 2017, at 7:00 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
>>   L571:         if (t < (size_t)os::vm_page_size()) {
>>       Again, not your issue, but that size < page size check means
>>       the oop is bad is screaming for a short comment.
> 
> Added comment.  Also removed the unnecessary intermediate variable t,
> thereby eliminating some casts.  I'm not completly convinced by the
> comment, but remaining true to the existing code.  Here's the diff:
> 
> @@ -566,9 +568,10 @@
>     uint value_state = _value_state[p];
>     if (is_value_state_indirect_oop(value_state)) {
>       intptr_t v = _value[p];
> -      if (v != 0 ) {
> -        size_t t = (size_t)v;
> -        if (t < (size_t)os::vm_page_size()) {
> +      if (v != 0) {
> +        if (v < os::vm_page_size()) {
> +          // v is a "handle" referring to an oop, cast to integral type.
> +          // There ought not be any handles in very low memory.
>           bad = true;
>         } else if (!resolve_indirect_oop(v, value_state)->is_oop_or_null(true)) {
>           bad = true;

Well, that was dumb.  v can be negative, and vm_page_size return (positive) int.
Just pretend you never saw that…