RFR: 8286897: Loom: Cleanup x86_64 StubGenerator [v2]

[Ron Pressler]

On Tue, 17 May 2022 17:21:10 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> During x86_32 porting, I ended up rewriting the copy of x86_64 StubGenerator significantly to make it more streamlined. The original x86_64 StubGenerator should enjoy the similar streamlining, since it would likely to be the template for many other architectures. 
>> 
>> There should be no behavior changes. The changes themselves should speak for themselves, but here is a brief list:
>>  - no need for `assert_asm` macros, instead do the inline checks, like the rest of Hotspot and Loom code
>>  - `_masm` and `masm` shadowing can just use whatever is currently defined as `__`
>>  - more strong cohesion between `post_call_nop`, its associated PC and its oopmap
>>  - consistently use `call_VM_leaf` that only takes the number of arguments, as we are doing argument pushes ourselves anyway;
>>  - for many moves, it is easier to use `ptr` flavors (e.g. `testq` -> `testptr`) to ease copy-pasting to other arches;
>>  - `generate_cont_thaw` should just do the `enabled()` check and carry the `StubCodeMark`, like other stubs do;
>>  - inline comments are reflowed to block-like comments, matching the usual style of stub code;
>>  - labels are prefixed with `L_` for clarity (used in some places in Hotspot);
>>  - `jfr_prolog` and `jfr_epilog` are used only once and do not carry their weight;
>>  - `continuation_*` are documented a bit better;
>> 
>> Additional testing:
>>  - [x] Linux x86_64 fastdebug, `java/lang/Thread/virtual`
>>  - [x] Linux x86_64 fastdebug, `serviceability/jvmti`
>>  - [x] Linux x86_64 fastdebug, `runtime/vthread`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   More changes from x86_32 PR

Marked as reviewed by rpressler (Author).

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PR: https://git.openjdk.java.net/jdk/pull/8755