[9] RFR (M): VM should constant fold Unsafe.get*() loads from final fields

[Paul Sandoz]

On Jun 24, 2015, at 2:59 PM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:

> Thanks, Paul!
> 
>> I like the test, you have almost hand rolled your own specializer :-)
> I tried hard to avoid bytecode generation, but having 8 almost identical copies of the same logic is too much :-)
> 

I agree, i like it and will likely use a similar approach in some tests i need to write.


>> A minor point. Since you have created a ClassWriter with "ClassWriter.COMPUTE_MAXS | ClassWriter.COMPUTE_FRAMES" can you remove the "mv.visitMax(0, 0)" calls?
>> 
>> 
>> I was a little confused by the code that checked the expected result against the actual result.
>> 
>> I am guessing the white box methods return -1 if the value is not a constant and 1 if it is. (Perhaps that can be documented, if even those methods may eventually reside somewhere else.) Whereas, Generator.expected returns 0 or 1.
>> 
>>  118         if (direct != unsafe || // difference between testDirect & testUnsafe
>>  119             (unsafe != -1 && expected != unsafe)) // differs from expected, but ignore "unknown"(-1) result
>>  120         {
>>  121             throw new AssertionError(String.format("%s: e=%d; d=%d; u=%d", t.name(), expected, direct, unsafe));
>>  122         }
>> 
>> I don't quite understand why  "unknown"(-1)  can be ignored.
>> 
>> Can that be changed to the following if Generator.expected returned the same values as the WB methods?
>> 
>>   if (direct != unsafe || unsafe != expected) { ... }
>> 
>> ?
> Though I've removed WB.isCompileConstat() in the updated version,

Ok, i definitely understand the updated version :-)



> I'll elaborate on that. WB.isCompileConstat() returns 3 values:
>  * 0: the argument is not a compile-time constant;
>  * 1: the argument is a compile-time constant;
>  * -1: it's not known whether it is constant or not
> 
> "-1" signals that there's no data from JIT. WB.isCompileConstat() is intrinsified by a JIT-compiler. I implemented the intrinsics only in C2, so if you run the test in -Xint mode or with Client VM, WB.isCompileConstant() will always return -1.
> 
> In order to be resilient, the test ignores "-1" when it checks the results.
> 

Ah, thanks, that makes sense now,
Paul.
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