RFR: JDK-8217909: Make unused r12 register (without compressed oops) available to regalloc in C2

[Nils Eliasson]

+1

The reg classes is getting out of control. For rbp, it was acceptable, 
but with r12 as well, it has to be refactored.

I would prefer solution that extends the ad-format. Deans suggestion is 
better than duplicating the reg classes, but modifying the reg class 
definitions without any coupling to the ad-files might cause some 
unwanted suprises in the future.

Regards,

// Nils


On 2019-02-11 21:33, dean.long at oracle.com wrote:
> There is a way to do it without cloning all the register class variants.
> For the arm64 port we massaged the register masks in 
> Compile::pd_compiler2_init():
>
> http://hg.openjdk.java.net/jdk9/jdk9/hotspot/file/b756e7a2ec33/src/cpu/arm/vm/arm_64.ad#l638 
>
>
> It might be worth considering, as the number of variants grows.
>
> dl
>
> On 2/11/19 3:02 AM, Roman Kennke wrote:
>> When running with compressed oops, the r12 register holds the heapbase,
>> and thus is not available to register allocation in C2. However, when
>> *not* running with compressed oops, it is still not available and
>> remains unused. It should be made available to register allocation in
>> this case.
>>
>> This patch implements this by introducing with_r12 and no_r12 variants
>> of basically all register classes, and add dynamic reg classes to select
>> one or the other, based on current settings. I needed to add UseZGC in
>> those flags because ZGC asserts to not get r12 in its barriers. Not sure
>> that this is necessary.
>>
>> Note that we might want to use the r12 register in Shenandoah later to
>> keep GC state. In this case, we'd need to add UseShenandoahGC or such to
>> the test. Do we want to abstract this whole check? Not sure that this is
>> possible/feasible to do in .ad though...
>>
>> Bug:
>> https://bugs.openjdk.java.net/browse/JDK-8217909
>> Webrev:
>> http://cr.openjdk.java.net/~rkennke/JDK-8217909/webrev.00/
>>
>> Testing: tier1 no regressions locally, eyeball generated code, yes it
>> does use r12 now.
>>
>> Can I please get reviews?
>>
>> Roman
>