RFR(XXS): 8227338: templateInterpreter.cpp: copy_table() needs to be safer

[Kim Barrett]

> On Jul 7, 2019, at 8:08 PM, David Holmes <david.holmes at oracle.com> wrote:
> 
> On 7/07/2019 6:48 pm, Erik Osterlund wrote:
>> The real danger is SPARC though and its BIS instructions. I don’t have the code in front of me, but I really hope not to see that switch statement and non-volatile loop in that pd_disjoint_words_atomic() function.
> 
> sparc uses the same loop.
> 
> Let's face it, almost no body expects the compiler to do these kinds of transformations. :(

See JDK-8131330 and JDK-8142368, where we saw exactly this sort of transformation from a fill-loop
to memset (which may use BIS, and indeed empirically does in some cases).  The loops in question
seem trivially convertible to memcpy/memmove.

Also see JDK-8142349.

>> And I agree that the atomic copying API should be used when we need atomic copying. And if it turns out the implementation of that API is not atomic, it should be fixed in that atomic copying API.
> 
> I agree to some extent, but we assume atomic load/stores of words all over the place - and rightly so. The issue here is that we need to hide the loop inside an API that we can somehow prevent the C++ compiler from screwing up. It's hardly intuitive or obvious when this is needed e.g if I simply copy three adjacent words without a loop could the compiler convert that to a block move that is non-atomic?
> 
>> So I think this change looks good. But it looks like we are not done yet. :c
> 
> I agree that changing the current code to use the atomic copy API to convey intent is fine.

I’ve been reserving Atomic::load/store for cases where the location “ought” to be declared std::atomic<T> if
we were using C++11 atomics (or alternatively some homebrew equivalent).  Not all places where we do
stuff “atomically” is appropriate for that though (consider card tables, being arrays of bytes, where using an
atomic<T> type might impose alignment constraints that are undesirable here).  I *think* just using volatile
here would likely be sufficient, e.g. we should have

    Copy::disjoint_words_atomic(const HeapWord* from,volatile HeapWord* to, size_t count)