Re: [PATCH 1/1] RISC-V: Fix canonical extension order (K and J)

[Palmer Dabbelt <palmer@dabbelt.com>]

On Mon, 28 Mar 2022 17:12:55 PDT (-0700), Palmer Dabbelt wrote:
> On Mon, 28 Mar 2022 06:12:01 PDT (-0700), binutils@sourceware.org wrote:
>> This commit fixes canonical extension order to follow the RISC-V ISA
>> Manual draft-20210402-1271737 or later.
>>
>> bfd/ChangeLog:
>>
>> 	* elfxx-riscv.c (riscv_recognized_prefixed_ext): Fix "K" extension
>> 	prefix to be placed before "J".
>> ---
>>  bfd/elfxx-riscv.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/bfd/elfxx-riscv.c b/bfd/elfxx-riscv.c
>> index cb2cc146c04..1219a7b44d4 100644
>> --- a/bfd/elfxx-riscv.c
>> +++ b/bfd/elfxx-riscv.c
>> @@ -1338,7 +1338,7 @@ riscv_recognized_prefixed_ext (const char *ext)
>>  }
>>
>>  /* Canonical order for single letter extensions.  */
>> -static const char riscv_ext_canonical_order[] = "eigmafdqlcbjktpvn";
>> +static const char riscv_ext_canonical_order[] = "eigmafdqlcbkjtpvn";
>>
>>  /* Array is used to compare the orders of standard extensions quickly.  */
>>  static int riscv_ext_order[26] = {0};
>
> Looks like this was just a bug in binutils: K went from being
> unspecified to specified in 271737 ("Define canonical location of K
> extension in ISA string"), thus it was never allowed at that other bit
> position.
>
> It looks like GCC also has this wrong, which sort of doubles the
> headache: now we've got this odd coupling between the GCC version and
> binutils version.  I'm not sure what the right thing is to do here:
> certainly rejecting the valid ISA string should be fixed, but I think we
> might need to accept the invalid one for compatibility reasons.  That'll
> be a headache to implement, though, so I'm not sure it's worth it.
>
> Maybe someone has a clever solution to this one?

After seeing the GCC patch go by, I think the clever solution here is to 
just say that we never accepted any J stuff in the first place so it's 
not a compatibility break.