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

Roman Kennke rkennke at redhat.com
Wed Feb 13 13:05:15 UTC 2019

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?


> 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

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