Re: [PATCH 2/2] RISC-V: Better support for long instructions

[Jan Beulich <jbeulich@suse.com>]

On 23.11.2022 09:40, Tsukasa OI wrote:
> c.f. PATCH v2 2/2
> <https://sourceware.org/pipermail/binutils/2022-November/124598.html>
> 
> On 2022/11/21 16:37, Jan Beulich wrote:
>> On 19.11.2022 08:10, Tsukasa OI wrote:
>>> From: Tsukasa OI <research_trasio@irq.a4lg.com>
>>>
>>> Commit bb996692bd96 ("RISC-V/gas: allow generating up to 176-bit
>>> instructions with .insn") tried to start supporting long instructions but
>>> it was insufficient.
>>>
>>> 1.  It heavily depended on the bignum internals (radix of 2^16),
>>> 2.  It generates "value conflicts with instruction length" even if a big
>>>     number instruction encoding does not exceed its expected length,
>>> 3.  Because long opcode was handled separately (from struct riscv_cl_insn),
>>>     some information like DWARF line number correspondence was missing and
>>> 4.  On the disassembler, disassembler dump was limited up to 64-bit.
>>>     For long (unknown) instructions, instruction bits are incorrectly
>>>     zeroed out.
>>
>> Just FTR - of these 1 and 4 were deliberate (as in: deemed acceptable), the
>> former the keep the code reasonably simple and the latter because focus was
>> solely on the assembler.
> 
> I thought it's possible that 1 was a deliberate choice.  I don't want to
> depend on some internal structure that could change easily (as long as
> this is a reasonable choice).  To be honest, I didn't like my
> "extracting prefix of an instruction" logic in PATCH v1 but I found a
> good function: generic_bignum_to_int32 and decided use it on PATCH v2
> (as a result, PATCH v2 2/2 is a bit simpler than PATCH v1).

FAOD by saying "deliberate" I have by no means meant to say that I'm not
happy to see you improve the state of things. I was merely trying to give
some background.

Jan

> For 4, resolving from the start would be better but since my current
> focus is the RISC-V disassembler, I'm happy to resolve it (fortunately,
> it didn't require large changes).
> 
> Nelson assigned you as the person who makes the final judgement for this
> series and I want to hear your thoughts/decision about PATCH v2 2/2.
> 
> Thanks,
> Tsukasa