Observations from a simple JMH benchmark

[Paul Sandoz]

> On Feb 16, 2018, at 4:38 AM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
> 
>> Better! still some artifacts:
> ...
>> I think you can drop the first index check for case 2. IIUC you are assuming constraints vlen > 0 and length >= 0, so for:
>>   Objects.checkIndex(ix, length - (vlen - 1));
>> the check will fail if length - (vlen - 1) < 0.
> 
> From correctness perspective, yes. But I was mostly concerned about C2 and having 2 checks which mimicing ordinary range checks looked like a safer bet. Preconditions.checkIndex() is intrinsified as CmpI(length,0) + CmpU(index,length) and strength-reduction to single CmpU works only if (length >= 0) which is the case for array length. So, the difference is CmpU + CmpU vs CmpI + CmpU.
> 
> I hoped that C2 could statically prove that (length - (vlen - 1)) >= 0 after the first check and optimize it accordingly. But something goes wrong.
> 
> Briefly looking into generated IR, it seems compiler does prove (length - (vlen - 1)) >= 0, but (length - constant) shape confuses some other transformation, so hoisting isn't performed.
> 

Ok, i don’t recall this kind of thing happening with say the access of ints from a ByteBuffer, implying this might be something vector specific (i would need to go back and double check buffer accesses, but i am sure i would have spotted this when doing the VarHandles work).

Paul.