[PATCH v3 1/2] RISC-V: Fallback for instructions longer than 64b

[Tsukasa OI]

On 2022/10/14 10:32, Nelson Chu wrote:
> In fact we don't really need this change, since so far the parameter
> length of validate_riscv_insn will only be 2 and 4,
> https://github.com/bminor/binutils-gdb/blob/master/gas/config/tc-riscv.c#L134
The argument "length" of validate_riscv_insn will be 2, 4 OR 0 but this
is not the point.

I have to agree that this change alone will not change the behavior.
Still, I don't want to surprise future developers with "instruction
length support" status (as Jan added).  To note, commit message of PATCH
1/2 is an important part of this patchset.

Could you reconsider this patchset again?

Thanks,
Tsukasa

> 
> Nelson
> 
> On Thu, Oct 6, 2022 at 5:56 PM Tsukasa OI <research_trasio@irq.a4lg.com> wrote:
>>
>> We don't support instructions longer than 64-bits yet.  Still, we can
>> modify validate_riscv_insn function to prevent unexpected behavior by
>> limiting the "length" of an instruction to 64-bit (or less).
>>
>> gas/ChangeLog:
>>
>>         * config/tc-riscv.c (validate_riscv_insn): Fix function
>>         description comment based on current spec.  Limit instruction
>>         length up to 64-bit for now.  Make sure that required_bits does
>>         not corrupt even if unsigned long long is longer than 64-bit.
>> ---
>>  gas/config/tc-riscv.c | 13 ++++++++-----
>>  1 file changed, 8 insertions(+), 5 deletions(-)
>>
>> diff --git a/gas/config/tc-riscv.c b/gas/config/tc-riscv.c
>> index 22385d1baa0..41d6dfc6062 100644
>> --- a/gas/config/tc-riscv.c
>> +++ b/gas/config/tc-riscv.c
>> @@ -1109,7 +1109,8 @@ arg_lookup (char **s, const char *const *array, size_t size, unsigned *regnop)
>>
>>  /* For consistency checking, verify that all bits are specified either
>>     by the match/mask part of the instruction definition, or by the
>> -   operand list. The `length` could be 0, 4 or 8, 0 for auto detection.  */
>> +   operand list. The `length` could be the actual instruction length or
>> +   0 for auto-detection.  */
>>
>>  static bool
>>  validate_riscv_insn (const struct riscv_opcode *opc, int length)
>> @@ -1120,11 +1121,13 @@ validate_riscv_insn (const struct riscv_opcode *opc, int length)
>>    insn_t required_bits;
>>
>>    if (length == 0)
>> -    insn_width = 8 * riscv_insn_length (opc->match);
>> -  else
>> -    insn_width = 8 * length;
>> +    length = riscv_insn_length (opc->match);
>> +  /* We don't support instructions longer than 64-bits yet.  */
>> +  if (length > 8)
>> +    length = 8;
>> +  insn_width = 8 * length;
>>
>> -  required_bits = ~0ULL >> (64 - insn_width);
>> +  required_bits = ((insn_t)~0ULL) >> (64 - insn_width);
>>
>>    if ((used_bits & opc->match) != (opc->match & required_bits))
>>      {
>> --
>> 2.34.1
>>
>