RFR: 8295110: RISC-V: Mark out relocations as incompressible

[Fei Yang]

On Tue, 11 Oct 2022 05:32:51 GMT, Xiaolin Zheng <xlinzheng at openjdk.org> wrote:

> This patch marks all relocations incompressible as pre-discussions at [1] and converts instructions to their 2-byte compressible counterparts as much as possible when UseRVC is enabled.
> 
> Chaining PR #10421.
> 
> 1. Code size reduction rate: about ~17% now after this patch under RVC, meaning if there's a piece of code of 1000 bytes, it may shrink to 830 bytes when RVC is enabled. [2]
> 2. Performance: conservatively no regressions observed. [3]
> 
> Several details:
> 1. The overloaded `relocate()` methods hide `IncompressibleRegion`s inside, to exclude instructions used at relocations from being compressed.
> 2. I kept two different lambda flavours because I think
> 
> __ la_patchable(t0, RuntimeAddress(dest), [&] (int32_t off) {
>     __ jalr(x1, t0, off);
> });
> 
> might make programmers overlook the `__ jalr()` inside the lambda. So I made the code with jalr aligning the previous style:
> 
> __ la_patchable(t0, RuntimeAddress(dest), [&] (int32_t off) {
> __ jalr(x1, t0, off);});
> 
> But well maybe it's up to the reviewers' flavor and I am okay with any case.
> 
> 
> Having tested several times hotspot tier1~tier4; Testing another turn on board.
> 
> 
> 
> [1] https://mail.openjdk.org/pipermail/riscv-port-dev/2022-September/000615.html
> [2] https://mail.openjdk.org/pipermail/riscv-port-dev/2022-September/000633.html
> [3] https://mail.openjdk.org/pipermail/riscv-port-dev/2022-October/000656.html

Personally, I prefer the following style:

  __ relocate(spec, [&] {
      int32_t off = 0;
      la_patchable(t0, RuntimeAddress(entry), off);
      jalr(x1, t0, off);
  });

Then the code looks more unified to me. And we don't need to extend a new la_patchable interface.

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

PR: https://git.openjdk.org/jdk/pull/10643