Re: [PATCH v2] RISC-V: Fix disassembly of partial instructions

[Charlie Jenkins <charlie@rivosinc.com>]

On Fri, Dec 20, 2024 at 11:38:01AM +0100, Jan Beulich wrote:
> On 19.12.2024 18:37, Nelson Chu wrote:
> > Some minor GNU coding styles as follows.  Also cc Jan and Andrew, hope they
> > still have time in their busy schedules can help to see if there are some
> > side effects.
> 
> I did actually take a look, but then pretty quickly decided I must be missing
> something: There's no disassembly of anything here afaics, and I also can't
> see how a partial insn could be disassembled in the first place. Such can
> only ever be displayed as raw hex data, I think. And that's what the patch

I think this is just an issue with my terminology. I was referring to
raw hex data as the "disassembly". Should the title say "raw hex data"
instead of "disassembly"? I was thinking that "disassembly" included
any instruction data that was being output from objdump.

> appears to be doing, just that it's description says otherwise. That plus the
> lack of a testcase demonstrating the intended behavior made me simply stop
> any effort here.

I should have posted a minimal test case. The test case I was using was
the linux perf test case that I posted. Here is a better example where I
have the output on binutils 2.40, 2.41, and with this patch.

As a summary, on 2.40 the hex of the instruction is shown even if the
full instruction can't be dumped. On 2.41 it errors with "Address <addr> is
out of bounds." in this case. With this patch, it reverts to the 2.40
behavior of showing the hex, but also encodes the partial instruction
using assembler directives instead of printing the "out of bounds" error.

$ cat objdump.s
c.lw a0, 0(a0)
.option push
.option norvc
nop
.option pop

$ riscv64-linux-gnu-gcc -c objdump.s

Example 2.40:
$ ./objdump-2.40 --start-address 0 --stop-address 4 -d -z objdump.o

objdump.o:     file format elf64-littleriscv


Disassembly of section .text:

0000000000000000 <.text>:
   0:	4108                	lw	a0,0(a0)
   2:	13 00             	Address 0x2 is out of bounds.



Example 2.41
$ ./objdump-2.41 --start-address 0 --stop-address 4 -d -z objdump.o

objdump.o:     file format elf64-littleriscv


Disassembly of section .text:

0000000000000000 <.text>:
   0:	4108                	lw	a0,0(a0)
   2:	Address 0x2 is out of bounds.


Example with this patch:
$ ./binutils/objdump --start-address 0 --stop-address 4 -d -z objdump.o                                               [11:20:54]

objdump.o:     file format elf64-littleriscv


Disassembly of section .text:

0000000000000000 <.text>:
   0:   4108                    lw      a0,0(a0)
   2:   0013                    .short  0x0013


The difference here being that "Address <address> is out of bounds" is
replaced by the raw hex data.

> 
> Jan