RFR: 8285711: riscv: RVC: Support disassembler show-bytes option [v2]

[Xiaolin Zheng]

> When RVC (under which instruction size could become 2-byte) is enabled currently, the disassembler 'show-bytes' output is not right, for `Assembler::instr_len` doesn't get adjusted.
> 
> Using `-XX:+UnlockExperimentalVMOptions -XX:+UseRVC -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly -XX:PrintAssemblyOptions=no-aliases,numeric,show-bytes -XX:+PrintAssembly`
> 
> Before: (wrong)
> 
> ...
> 0x000000401354f9bc: 9722 4107  | auipc  x5,0x7412                               ;   {runtime_call handle_exception_from_callee Runtime1 stub}
>   0x000000401354f9c0: e780 4210  | jalr  x1,260(x5) # 0x000000401a961ac0
>   0x000000401354f9c4: 1571 06e0  | c.addi16sp  x2,-224
>   0x000000401354f9c6: 06e0 16e4  | c.sdsp  x1,0(x2)
>   0x000000401354f9c8: 16e4 1ae8  | c.sdsp  x5,8(x2)
>   0x000000401354f9ca: 1ae8 1eec  | c.sdsp  x6,16(x2)
>   0x000000401354f9cc: 1eec 22f0  | c.sdsp  x7,24(x2)
>   0x000000401354f9ce: 22f0 26f4  | c.sdsp  x8,32(x2)
>   0x000000401354f9d0: 26f4 2af8  | c.sdsp  x9,40(x2)
> ...
> 
> 
> After: (right)
> 
> 0x0000004013546c2c: 97a2 4107  | auipc  x5,0x741a                               ;   {runtime_call handle_exception_from_callee Runtime1 stub}
>   0x0000004013546c30: e780 42b9  | jalr  x1,-1132(x5) # 0x000000401a9607c0
>   0x0000004013546c34: 1571    | c.addi16sp  x2,-224
>   0x0000004013546c36: 06e0    | c.sdsp  x1,0(x2)
>   0x0000004013546c38: 16e4    | c.sdsp  x5,8(x2)
>   0x0000004013546c3a: 1ae8    | c.sdsp  x6,16(x2)
>   0x0000004013546c3c: 1eec    | c.sdsp  x7,24(x2)
>   0x0000004013546c3e: 22f0    | c.sdsp  x8,32(x2)
>   0x0000004013546c40: 26f4    | c.sdsp  x9,40(x2)
> 
> 
> The [RISC-V ISA manual](https://github.com/riscv/riscv-isa-manual/blob/04cc07bccea63f6587371b6c75b228af3e5ebb02/src/intro.tex#L630-L635) reads, 
>> We have to fix the order in which instruction parcels are stored in memory, independent of memory system endianness, to ensure that **the length-encoding bits always appear first in halfword address order**. This allows the length of a variable-length instruction to be quickly determined by an instruction-fetch unit by examining only the first few bits of the first 16-bit instruction parcel.
> 
> Instructions are stored in little-endian order, and in that only the first 16-bit matters, we could just check if the lowest 2 bits of it to detect the instruction length. 
> (48-bit and 64-bit instructions are not supported yet in the current backend)
> (extracting an `is_compressed_instr`, for this might get used in the future)
> 
> Thanks,
> Xiaolin

Xiaolin Zheng has updated the pull request incrementally with one additional commit since the last revision:

  Polish comments

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8421/files
  - new: https://git.openjdk.java.net/jdk/pull/8421/files/17c90be8..20d8f09b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8421&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8421&range=00-01

  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8421.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8421/head:pull/8421

PR: https://git.openjdk.java.net/jdk/pull/8421