RFR: 8253690: [PPC64] Use flag kind diagnostic for platform specific flags

[Martin Doerr]

On Wed, 30 Sep 2020 06:54:51 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Current platform implementation (globals_ppc.hpp) uses regular product flags for almost everything.
>> Most platform specific flags were never intended for official support. They are only there to diagnose issues and find
>> workarounds. So flag kind "diagnostic" fits better for them.
>> 
>> Note that I rearranged a couple of lines when looking at the diff.
>> My actual change is what is described here: https://bugs.openjdk.java.net/browse/JDK-8253692
>
> src/hotspot/cpu/ppc/globals_ppc.hpp line 116:
> 
>> 114:                                                                             \
>> 115:   /* special instructions */                                                \
>> 116:   product(bool, SuperwordUseVSX, false,                                     \
> 
> Why leave this one out?

SuperwordUseVSX switches usage of vector registers in C2. It's more fundamental than simple instruction usage switches.
It's comparable to UseAVX on x86 which is product, too. So I prefer keeping it product for now. Are you ok with it?

> src/hotspot/cpu/ppc/globals_ppc.hpp line 150:
> 
>> 148:                                                                             \
>> 149:   product(bool, ZapMemory, false, "Write 0x0101... to empty memory."        \
>> 150:           " Use this to ease debugging.")                                   \
> 
> Future cleanup: this feels like it should be in shared code. The usual way to do this is to zap in DEBUG.

Yeah, sounds like this functionality derserves some overwork. But I only want to make it diagnostic with this change.

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PR: https://git.openjdk.java.net/jdk/pull/413