RFR: 8200113: Make Access load proxys smarter

[Erik Österlund]

Looping in hotspot-dev.

/Erik

On 2018-03-22 15:19, Erik Österlund wrote:
> Hi,
>
> Access returns the result of loads through load proxy objects that 
> implicity convert themselves to a template inferred type. This is a 
> metaprogramming technique used to infer return types in C++.
>
> However, I have heard requests that it would be great if it could be a 
> bit smarter and do more than just be assigned to a type.
>
> Example use cases that do not work today without workarounds:
>
>   oop val = ...;
>   narrowOop narrow = 0u;
>   oop *oop_val = &val;
>   narrowOop *narrow_val = &narrow;
>   HeapWord *heap_word_val = reinterpret_cast<HeapWord*>(oop_val);
>
>   if (val == HeapAccess<>::oop_load_at(val, 16)) {}
>   if (HeapAccess<>::oop_load_at(val, 16) == val) {}
>   if (val != HeapAccess<>::oop_load_at(val, 16)) {}
>   if (HeapAccess<>::oop_load_at(val, 16) != val) {}
>
>   if (HeapAccess<>::oop_load(oop_val) != val) {}
>   if (HeapAccess<>::oop_load(heap_word_val) != val) {}
>   if (RawAccess<>::oop_load(narrow_val) != narrow) {}
>
>   if (HeapAccess<>::oop_load(oop_val) == val) {}
>   if (HeapAccess<>::oop_load(heap_word_val) == val) {}
>   if (RawAccess<>::oop_load(narrow_val) == narrow) {}
>
>   if (val != HeapAccess<>::oop_load(oop_val)) {}
>   if (val != HeapAccess<>::oop_load(heap_word_val)) {}
>   if (narrow != RawAccess<>::oop_load(narrow_val)) {}
>
>   if (val == HeapAccess<>::oop_load(oop_val)) {}
>   if (val == HeapAccess<>::oop_load(heap_word_val)) {}
>   if (narrow == RawAccess<>::oop_load(narrow_val)) {}
>
>   if ((oop)HeapAccess<>::oop_load(oop_val) == NULL) {}
>
>   oop tmp = true ? HeapAccess<>::oop_load(oop_val) : val;
>
> Here is a patch that solves this:
> http://cr.openjdk.java.net/~eosterlund/8200113/webrev.00/
>
> ...and here is the bug ID:
> https://bugs.openjdk.java.net/browse/JDK-8200113
>
> Thanks,
> /Erik
>