[aarch64-port-dev ] RFR: 8191954: AArch64: disable UseCISCSpill in C2

[Andrew Dinn]

On 28/11/17 08:49, Andrew Haley wrote:
> On 28/11/17 05:54, Ningsheng Jian wrote:
>> JBS:
>> https://bugs.openjdk.java.net/browse/JDK-8191954
>>
>> Webrev:
>> http://cr.openjdk.java.net/~njian/8191954/webrev.00/
>>
>> This sets UseCISCSpill to false on AArch64, as we actually don't use
>> that. jtreg tests passed with fastdebug build.
> 
> That's too risky right now.  We'll look at it in the next cycle.
I don't think it is actually /risky/ because UseCISCSpill would only
take effect if aarch64.ad declared a cisc_spilling_operand_name in the
frame section (it doesn't).

That declaration drives adlc to generate an overriding version of a
virtual method of class Node which is used by the chaitin allocator to
determine how to spill a value. Without any override the method always
returns -1, meaning that operands are never considered for CISC spilling.

So, leaving UseCISCSpill set to true causes a small amount of extra
checking to be done but no action is ever taken. Of course that also
means the change is not going to be /critical/ to performance i.e.
rushing to squeeze it into jdk10 is not justified.

regards,


Andrew Dinn
-----------
Senior Principal Software Engineer
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