RFR: 8292203: AArch64: Represent Registers as values [v2]

[Quan Anh Mai]

On Thu, 11 Aug 2022 21:24:37 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> (It's a PR dependent on #9815.)
>> 
>> Along with x86 ([JDK-8292153](https://bugs.openjdk.org/browse/JDK-8292153)), I propose to refactor Register classes on AArch64 and migrate away from pointer-based representation.
>> 
>> The motivation is the same: improve compile-time checks and avoid ambiguities between overloads due to implicit conversions between pointers and integral types.
>> 
>> Proposed change hides pointer representation behind value class.
>> 
>> Pointer-based representation is kept to avoid massive migration of users (from `->` to `.`) and to enable incremental migration on per-platform basis (pointer-based representation is assumed in shared code).
>> 
>> Code quality improves:
>>   GCC:   https://godbolt.org/z/r6G36facj
>>   Clang: https://godbolt.org/z/x5oPdYEPM
>> 
>> (I noticed one downside: slowdebug builds become slower, because `operator->` isn't inlined there. If it becomes a problem, migrating performance-sensitive places from `->` to `.` should solve the problem.)
>> 
>> Testing: hs-tier1 - hs-tier5
>> 
>> PS: a number of cleanups are incorporated. In particular, I decided to expand all macros from `register.hpp` because IMO they add confusion rather than improve readability.
>
> Vladimir Ivanov has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:
> 
>  - Merge branch 'master' into rwrapper.aarch64
>  - 8292203: AArch64: Represent Registers as values
>  - cleanups
>  - x86: Register class wrappers

src/hotspot/cpu/aarch64/register_aarch64.hpp line 39:

> 37:   int _encoding;
> 38: 
> 39:   constexpr Register(int encoding, bool unused) : _encoding(encoding) {}

Why do we need unused here? If the intention is to prevent implicit conversion from `int` to `Register` then you can mark the constructor as `explicit`. Thanks.

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PR: https://git.openjdk.org/jdk/pull/9826