c.unimp instruction problem

[fly-1011]

Before start

• I have read the RISC-V ISA Manual and this is not a RISC-V ISA question. 我已经阅读过 RISC-V 指令集手册，这不是一个指令集相关的问题。
• I have read the XiangShan Documents. 我已经阅读过香山文档。
• I have searched the previous issues and did not find anything relevant. 我已经搜索过之前的 issue，并没有找到相关的。
• I have searched the previous discussions and did not find anything relevant. 我已经搜索过之前的 discussions，并没有找到相关的。
• I have reviewed the commit messages from the relevant commit history. 我已经浏览过相关的提交历史和提交信息。

Describe the question

When testing Xiangshan using NEMU as a reference model, many of my test cases showed inconsistencies similar to the following:

At first I thought it was a NEMU-related issue. But I tested it with the following assembly instructions:

.section .text
.globl _start
_start:
   
    li     t0, 0x8000000a00100a00     
    csrw    mstatus, t0 
    .word 0x9efd

The following is a screenshot of the log information：

When I use spike for debugging, .word 0x9efd corresponds to the unknown instruction：

Version Information

XiangShan :commit dd02bc3 (HEAD -> master, origin/master, origin/HEAD)
ready-to-run: commit a449a38534ec8330842ad5e975b872686b421ebc (HEAD, origin/master, origin/HEAD)
nemu: 39f546c42275cb9bc2f74170e7ff6486c98ef4c9

[eastonman]

This a bug related to Zcb decode.

https://github.com/OpenXiangShan/rocket-chip/blob/d24ca81a61727157ae8b7768b24b9cdbb1ddc8dd/src/main/scala/rocket/RVC.scala#L222-#L227

[fly-1011]

Thank you for your timely response and for confirming the bug.