RFR: 8286301: Port JEP 425 to RISC-V

[Dean Long]

On Mon, 31 Oct 2022 12:41:28 GMT, Fei Yang <fyang at openjdk.org> wrote:

> Hi,
> 
> Please review this PR porting JEP 425 (Virtual Threads) to RISC-V.
> 
> This is mainly adapted from the work of AArch64 port. Most of the changes lie in RISC-V scope.
> Changes to HotSpot shared code are trivial and are always guarded by RISCV64 macro. So this won't
> affect the rest of the world in theory.
> 
> There exists some differences in frame structure between AArch64 and RISC-V.
> For AArch64, we have:
> 
> enum {
>   link_offset           = 0,
>   return_addr_offset    = 1,
>   sender_sp_offset      = 2
> };
> 
> While for RISC-V, we have:
> 
> enum {
>   link_offset           = -2,
>   return_addr_offset    = -1,
>   sender_sp_offset      =  0
> };
> 
> So we need adapations in some places where the code relies on value of sender_sp_offset to work.
> Note that implementation for Post-call NOPs optimization is not incorporated in this PR as we plan to
> evaluate more on its impact on performance.
> 
> Testing on Linux-riscv64 HiFive Unmatched board:
>   - Minimal, Client and Server release & fastdebug build OK.
>   - Passed tier1-tier4 tests (release build).
>   - Passed jtreg tests under test/jdk/java/lang/Thread/virtual with extra JVM options: -XX:+VerifyContinuations -XX:+VerifyStack (fastdebug build).
>   - Performed benchmark tests like Dacapo, SPECjvm2008, SPECjbb2015, etc. to make sure no performance regression are introduced (release build).

src/hotspot/share/runtime/continuationFreezeThaw.cpp line 1053:

> 1051: 
> 1052:   intptr_t* const stack_frame_bottom = ContinuationHelper::InterpretedFrame::frame_bottom(f);
> 1053:   assert(stack_frame_bottom - stack_frame_top >= fsize, ""); // == on x86

Are there any ports where (stack_frame_bottom - stack_frame_top) != fsize?  It would be nice if we could use (stack_frame_bottom - stack_frame_top) for fsize and remove the platform-specific computation using frame::metadata_words above.

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