RFR: 8216392: Enable cmovP_mem and cmovP_memU instructions

[Roman Kennke]

> On 1/17/19 7:51 PM, B. Blaser wrote:
>> Here it is on intel xeon with 5*10e9 iterations:
>> * mov+cmov = 10.94s
>> * cmov = 10.15s
>>
>> Thoughts?
> 
> It looks like there's not much of a performance difference, but it might
> help by freeing a register. OTOH, we'd still need to be sure we weren't
> introducing a regression. We'd have to make sure that implicit null checks
> work.

I'm pretty sure that null-checks work, in general. I used the cmov
instructions in an experiment that I did with Shenandoah barriers of
which I'm pretty sure would have blown up badly if it wouldn't. One
thing I'm not sure of is: does cmov generate a SIGSEGV on a bad address,
even if the condition is not true? I doubt it, because then we couldn't
use this for other types (long, int, etc).

I'm more worried about the bottom-type issue that is mentioned in the
comment and by Andrew Dinn, and it would be very helpful if anybody
knows about it and could clarify. Failing that we could dig deeper
and/or do extensive testing?

Roman

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