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

[Roman Kennke]

No I haven't. Have you posted this somewhere and I missed it? :-)

I will try it out. Thanks!

Roman


> We can do that in a followup RFE.  For now, did you see how I did it for 
> PTR only?
> 
> _STACK_OR_PTR_REG_mask = _PTR_REG_mask;
> _STACK_OR_PTR_REG_mask.OR(STACK_OR_STACK_SLOTS_mask());
> 
> 
> dl
> 
> On 2/18/19 3:43 AM, Roman Kennke wrote:
>> I don't know how to do that, though. How should I proceed? The issue is
>> a nice-to-have for me, but not high-enough priority to dive into the
>> guts of adlc.
>>
>> Roman
>>
>>
>>> Yes it does, but in this case, only for the PTR class, right? However,
>>> it would be nice if
>>> adlc would generate it for us.
>>>
>>> dl
>>>
>>> On 2/14/19 2:47 AM, Roman Kennke wrote:
>>>> It is not so easy.
>>>>
>>>> We'd also need to generate STACK_OR_$NAME_mask() accessors for spillable
>>>> registers. See RegClass::build_register_masks().
>>>>
>>>> Roman
>>>>
>>>>> How about something like this:
>>>>>
>>>>> http://cr.openjdk.java.net/~dlong//8217909/webrev/
>>>>>
>>>>> dl
>>>>>
>>>>> On 2/13/19 5:05 AM, Roman Kennke wrote:
>>>>>> Apparently, this is a limitation of CodeSnippetRegClass, in
>>>>>> formsopt.hpp:
>>>>>>
>>>>>>      void set_stack_version(bool flag) {
>>>>>>        assert(false, "User defined register classes are not allowed to
>>>>>> spill to the stack.");
>>>>>>      }
>>>>>>
>>>>>> I believe this means I cannot use this approach.
>>>>>>
>>>>>> I also don't feel like extending the AD syntax, parser, etc at this
>>>>>> point.
>>>>>>
>>>>>> Do you have any other suggestions? Or should we go with my original
>>>>>> patch?
>>>>>>
>>>>>> Roman
>>>>>>
>>>>>>> I started a version based on Dean's approach:
>>>>>>>
>>>>>>> http://cr.openjdk.java.net/~rkennke/JDK-8217909/webrev.01/
>>>>>>>
>>>>>>> Unfortunately it barks on ptr_reg class:
>>>>>>>
>>>>>>> assert fails
>>>>>>> /home/rkennke/src/openjdk/jdk-jdk/src/hotspot/share/adlc/formsopt.hpp
>>>>>>> 246: User defined register classes are not allowed to spill to the
>>>>>>> stack.
>>>>>>>
>>>>>>> Interestingly, it doesn't do that when I only handle any_reg class.
>>>>>>> However, I am not sure what that means or how to fix it. Any hints?
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Roman
>>>>>>>
>>>>>>>> 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
>