RFR: 8200113: Make Access load proxys smarter

[Per Liden]

Looks good!

/Per

On 03/22/2018 05:03 PM, Erik Österlund wrote:
> 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
>>
>