RFR: 8286301: Port JEP 425 to RISC-V [v4]

[Richard Reingruber]

On Fri, 4 Nov 2022 05:19:32 GMT, Fei Yang <fyang at openjdk.org> wrote:

>> OK, skipping the assert for now.  Regarding `NOT_RISCV64(+ frame::metadata_words)` @reinrich just posted the PR for the PPC64 port (see JDK-8286302), which introduces metadata_words_at_top/metadata_words_at_bottom.  I'm not sure what values RISC-V would use for these new constants, but merging with the PPC64 changes might allow a platform-independent setting of fsize here.
>
> Hi, I went through the PPC64 changes to shared code and I think for RISC-V we should define metadata_words_at_top and metadata_words_at_bottom to 0 and 2 (which equals metadata_words as defined by this PR) respectively. The PPC64 changes will help eliminate the RISC-V-specific change made in stackChunkOopDesc::is_usable_in_chunk in file src/hotspot/share/oops/stackChunkOop.inline.hpp. But we might still need a RISC-V platform-dependent change here due to the particularity of the RISC-V frame structure. Thanks.

The value for `fsize` as it is calculated is correct on PPC64 but only because errors compensate. The calculation makes assumptions about the position of fp which is not specified.

This is the assumed layout:


  :                 :
  :                 :
  |                 |
  |-----------------|
  |                 |
  | locals array    |
  |                 |<- callers_SP
  ===================
  |                 |
  | metadata at bottom |
  |                 |<- FP
  |-----------------|
  |                 |
  |                 |
  |                 |
  |                 |
  |                 |<- SP
  ===================


Here the metadata lies outside [FP, SP] and needs to be added to FP-SP just like the size of the locals array.

This coincidentally matches the layout on PPC64:


  :                 :
  :                 :
  |                 |
  |-----------------|
  |                 |
  | locals array    |
  |                 |
  |-----------------|
  |                 |
  | metadata at top    |
  |                 |<- FP / callers_SP
  ===================
  |                 |
  |                 |
  |                 |
  |                 |
  |-----------------|
  |                 |
  | metadata at top    |
  |                 |<- SP
  ===================



I assume the layout on Risc-V is like this:


  :                 :
  :                 :
  |                 |
  |-----------------|
  |                 |
  | locals array    |
  |                 |<- FP / callers_SP
  ===================
  |                 |
  | metadata at bottom |
  |                 |
  |-----------------|
  |                 |
  |                 |
  |                 |
  |                 |
  |                 |<- SP
  ===================

Here the metadata is included in [FP, SP]


We could change the line to

```c++
  const int fsize = ContinuationHelper::InterpretedFrame::callers_sp(f) + frame::metadata_words_at_top + locals - stack_frame_top;


with a platform dependent implementation of `callers_sp()`

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

PR: https://git.openjdk.org/jdk/pull/10917