RFR: 8200113: Make Access load proxys smarter

[Erik Österlund]

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