RFR: 8290840: Refactor the "os" class [v2]

[Gerard Ziemski]

On Fri, 29 Jul 2022 20:25:52 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> src/hotspot/share/runtime/os.inline.hpp line 60:
>> 
>>> 58: inline void* os::resolve_function_descriptor(void* p) {
>>> 59:   return NULL;
>>> 60: }
>> 
>> This is unnecessary and could be removed if we did:
>> 
>> 
>> +#ifdef HAVE_FUNCTION_DESCRIPTORS
>>    // Used only on PPC.
>>    inline static void* resolve_function_descriptor(void* p);
>> +#endif
>> 
>> 
>> in `os.hpp`
>
> After this PR, os.hpp no longer includes any os- or cpu-specific os_xxx.hpp files. I did this to avoid including large amount of os-specific declarations (such as os::Linux, which is about 400 lines) into generic source files, which aren't interested in these declarations.
> 
> As a result of this change, there's no way to let os.hpp know whether `HAVE_FUNCTION_DESCRIPTORS` is defined for the current OS+CPU. If we want to do that, we need to either:
> 
> - include os_xxx.hpp back in os.hpp (which I want to avoid)
> - add new header files such as `os_<os>.defs.hpp`  `os_<os>_<cpu>.defs.hpp` to define  `HAVE_FUNCTION_DESCRIPTORS`  (this will result in lots of header files that usually have nothing inside)
> 
> Since there are only 4 functions that fall under this pattern (functions that are used in generic code but most platforms have dummy implementations), I think what I have is a good compromise

Could we do:


#if defined(LINUX) && defined(PPC) && !defined(ABI_ELFv2)
  #define HAVE_FUNCTION_DESCRIPTORS
#endif


in `os.hpp` and whatever the equivalent for AIX/PPC is?

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PR: https://git.openjdk.org/jdk/pull/9600