Check instruction misalignment for RV32C in the op_branch function

[dougpuob]

I encountered a signal SIGABRT error.

000104b2
00027798  _dl_aux_init
0002779a
0002779e
00000000
rv32emu: io.c:76: memory_read_ifetch: Assertion `c' failed.
fish: Job 1, 'build/rv32emu ~/petzone/llvm/da…' terminated by signal SIGABRT (Abort)

Why:
The conditions are equal making the beqz takes action to jump to the PC, which points to 0x2798a. Then the instruction misalignment checking mechanism makes the PC was reset to zero in the op_branch function. This makes next instruction fetch got problem in memory_read_ifetch() function.

00027798 <_dl_aux_init>:
   27798:	411c         lw	    a5,0(a0)
   2779a:	eca1a223     sw     a0,-316(gp) # 6f218 <_dl_auxv>
   2779e:	1e078663     beqz   a5,2798a <_dl_aux_init+0x1f2> -----+
   ...                                                                 |
   2798a:	8082         ret    <----------------------------------+ 
                                     The `beqz` should jump to here (0x2798a), but the program counter PC
                                     was reset to zero, making the `beqz` instruction cannot be executed completely.
   ^^^^^

How:
According to the chapter, "1.2 Instruction Length Encoding" in the specification (riscv-spec-v2.2.pdf), the least significant two bits of PC is possible to be 00b, 01b, and 10b with RV32C. The gcc and clang implemented the CB format with 01b and other parts in this project did so, so I add a corresponding check too.

[jserv]

Improve the git commit message to reflect what you found.

[dougpuob]

Improve the git commit message to reflect what you found.

Hi @jserv,
Thank you for reminding me. I have updated the commit message.

[jserv]

Hi @jserv, Thank you for reminding me. I have updated the commit message.

Please check this carefully: https://cbea.ms/git-commit/

• Do not end the subject line with a period

In addition, avoid using markdown syntax within git commit message(s) since the later is intended to be read by human rather than machines.

[dougpuob]

Hi @jserv, Thank you for reminding me. I have updated the commit message.

Please check this carefully: https://cbea.ms/git-commit/

• Do not end the subject line with a period

In addition, avoid using markdown syntax within git commit message(s) since the later is intended to be read by human rather than machines.

@jserv, Thank you for the suggestions. I have updated to improve the following:

• Wrapped body less than 80 chars,
• Removed the period from the subject line,
• Removed the markdown syntax.

[jserv]

Thank @dougpuob for contributing!