RFR: 8267098: AArch64: C1 StubFrames end confusingly

[Andrew Haley]

On Fri, 14 May 2021 11:28:45 GMT, Alan Hayward <github.com+4146708+a74nh at openjdk.org> wrote:

> For many of the stub frames, a leave/ret is generated after the stub has
> already branched or returned. This is confusing. For these cases, replace
> the superfluous code with a should_not_reach_here
> 
> For handle excception, instead of storing return from the exception
> handler on the stack, it can be moved directly into lr, replacing a store and
> load with a single move. (If/when PAC support is implemented, then this store
> would also have to be signed).

src/hotspot/cpu/aarch64/c1_Runtime1_aarch64.cpp line 442:

> 440:   //      handler regardless of whether handler existed in the nmethod.
> 441:   //      Move it out of the way to the return register.
> 442:   __ mov(lr, r0);

I don't think that leaving LR live here is a good idea. Storing into the stack frame is fine.

src/hotspot/cpu/aarch64/c1_Runtime1_aarch64.cpp line 607:

> 605:   const bool must_gc_arguments = true;
> 606:   const bool dont_gc_arguments = false;
> 607:   const bool does_not_return = true;

I wonder if an `enum` would read better here.

Something like


enum may_return_t {
  does_not_return, may_return
};

class StubFrame {

  StubFrame(StubAssembler* sasm, const char* name, bool must_gc_arguments, may_return_t may_return);

};

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

PR: https://git.openjdk.java.net/jdk/pull/4030