RFR: 8277417: C1 LIR instruction for load-klass

[Igor Veresov]

On Thu, 18 Nov 2021 20:16:27 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> In C1, the load of a Klass* out of an object is currently identified by a load of type T_ADDRESS with offset oopDest::klass_offset_in_bytes(). When encountering such load, this may be decoded when +CompressedClassPointers. This is problematic and ugly: if we ever emit a T_ADDRESS load with offset 8 or 4 (== klass_offset_in_bytes) that is not a Klass*, we would attempt to decode the result. We have been lucky so far.
> 
> Also, in Lilliput, we want to do something entirely different there, and need to be able to emit more complex code, possibly including runtime call.
> 
> The change introduces a new C1 LIR opcode OpLoadKlass, and refactors the implementations in c1_LIRAssembler_xyz.cpp to emit the code there, instead of mem2reg(). Notice that I could not test anything but x86, all other platforms only received very basic testing via GHA. It would be nice if respective maintainers could give it a try.
> 
> Testing:
>  - [x] tier1 (x86_64)
>  - [ ] tier2 (x86_64)
>  - [ ] tier3 (x86_64)

Nice! Thank you!

-------------

Marked as reviewed by iveresov (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6464