RFR: 8294366: RISC-V: Partially mark out incompressible regions

[Fei Yang]

On Mon, 26 Sep 2022 12:19:15 GMT, Xiaolin Zheng <xlinzheng at openjdk.org> wrote:

> Shortly, the current RVC implementation in the RISC-V backend is a "whitelist mode", merely compressing instructions marked by "CompressibleRegion" that covers just part of C2 matching rules and stub code (only ~5% compression rate). Due to the originally existing large backend code base and to spread the coverage to nearly all instructions generated by the backend, we cannot modify them little by little, but should implement a "blacklist mode" (a compression rate to ~20% if complete), to exclude compressions from:
> 1. relocations
> 2. patchable instructions
> 3. fixed length code slices whose code size is calculated
> 
> Please check the discussions in the riscv-port mailing list[1] to go in for more details.
> 
> This patch contains the first half of implementations which does not change the program behavior: it just introduces an "IncompressibleRegion" to indicate a piece of code "not compressible", marking out the patchable instructions and fixed-length code slices that are not able to compress by RVC.
> 
> Besides, this patch also temporarily removes some automatic compression logic of branch instructions like "jal"s and "beq"s, for MachBranchNodes' fake labels could hamper the automatic compression as well and for making code clean. Please also check the discussions in the thread[1].
> 
> Please check the unsquashed commits to have a better review of the patch.
> 
> 
> Tested a hotspot tier1 and tier2 with the option UseRVC turning on.
> 
> [1] https://mail.openjdk.org/pipermail/riscv-port-dev/2022-September/000615.html
> 
> Thanks,
> Xiaolin

I am fine to live with this change making explict those resions invalid for compressed instruction beforehand. But I have one comment.

src/hotspot/cpu/riscv/c1_MacroAssembler_riscv.cpp line 327:

> 325:   // Make it a NOP.
> 326:   assert_alignment(pc());
> 327:   nop(false);  // 4 bytes

Normally, the parameters for the assembler functions denotes the operands of the instrunction.
It looks strange to me for the nop() assembler function to take one parameter.
With that consideration, I think it might be better to make this explicit with a incompressible region mark.
Something like:
{
  IncompressibleRegion ir(this);
  nop();
}

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

Changes requested by fyang (Reviewer).

PR: https://git.openjdk.org/jdk/pull/10421