[12] RFR 8206895: aarch64: rework error-prone cmp instuction

[Boris Ulasevich]

And please review the following change for openjdk-12:

http://cr.openjdk.java.net/~bulasevich/8206895/webrev.00
https://bugs.openjdk.java.net/browse/JDK-8206895

Byte form of cmp instruction was added. For bigger constants we are 
supposed to state _imm12 postfix implicitly or use cmp(reg,reg,imm) macro.

thanks,
Boris

On 06.07.2018 15:34, Andrew Haley wrote:
> On 07/05/2018 06:09 PM, Boris Ulasevich wrote:
>> A new instruction cmp(rscratch, Rn, imm) alias is introduced to use it
>> when immediate value can not fit in 12 bit. I believe it's cleaner than
>> using subs.
> 
> Maybe so, yes.  So we can delete all but the byte form of cmp(reg, imm), poison
> the other forms, and add cmp(reg, reg, imm).  Works for me.
> 
>> Alternatively, if someone believes this looks risky for 11 I can propose
>> to just limit SoftwarePrefetchHintDistance. It seems unreasonable to
>> prefetch more than a few cache lines ahead anyway.
> 
> That's best for 11.
>