Re: [RFC PATCH v2 2/2] RISC-V: Add support for the Zfa extension

[Jan Beulich <jbeulich@suse.com>]

On 30.03.2023 12:30, Christoph Müllner wrote:
> On Mon, Mar 27, 2023 at 11:54 AM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 27.03.2023 10:53, Kito Cheng wrote:
>>> Wait, I mean the hex floating point format defined in C99/C++17, not
>>> the raw hex value.
>>> so something like 0x1p-16 (0.0000152587890625), 0x1p-2 (0.25) 0x1p+0,
>>> -0x1p+0 could be used for fli.* instruction.
>>>
>>> You could use printf with %a to get those values.
>>>
>>> https://gcc.gnu.org/onlinedocs/gcc/Hex-Floats.html
>>> https://developer.arm.com/documentation/dui0375/latest/Compiler-Coding-Practices/Hexadecimal-floating-point-numbers-in-C99
>>
>> Sure, my (secondary) suggestion ...
>>
>>> On Mon, Mar 27, 2023 at 4:39 PM Jan Beulich via Binutils
>>> <binutils@sourceware.org> wrote:
>>>>
>>>> On 27.03.2023 10:01, Christoph Muellner wrote:
>>>>> --- a/opcodes/riscv-opc.c
>>>>> +++ b/opcodes/riscv-opc.c
>>>>> @@ -110,6 +110,16 @@ const char * const riscv_vma[2] =
>>>>>    "mu", "ma"
>>>>>  };
>>>>>
>>>>> +/* The FLI.[HSDQ] value constants.  */
>>>>> +const char * const riscv_fli_value[32] =
>>>>> +{
>>>>> +  "-1.0", "min", "0.0000152587890625", "0.000030517578125",
>>>>> +  "0.00390625", "0.0078125", "0.0625", "0.125",
>>>>> +  "0.25", "0.3125", "0.375", "0.4375", "0.5", "0.625", "0.75", "0.875",
>>>>> +  "1.0", "1.25", "1.5", "1.75", "2.0", "2.5", "3.0", "4.0",
>>>>> +  "8.0", "16.0", "128.0", "256.0", "32768.0", "65536.0",  "inf", "nan",
>>>>> +};
>>>>
>>>> Especially for values like 1.0x2^^-n (entries 2 and onwards) I question
>>>> the spelled out numbers to be the most suitable ones usability wise. At
>>>> least some alternative spelling (e.g. 2.e-16) ought to be recognized as
>>>> well. But since there are meany reasonable spellings (leading 0 omitted
>>>> in 0.<fraction> or trailing zero omitted in <num>.0), I guess I'd prefer
>>>> if values were actually parsed as a floating point number (e.g. via
>>>> ieee_md_atof()), and then matched against values stored in the table.
>>>> One might further consider to also permit the 2nd form accepted
>>>> elsewhere, see read.c:parse_one_float().
>>
>> ... here wasn't meant to collide with yours. What you're asking for is
>> covered by my primary suggestion (to actually parse the values), extended
>> by the need to actually recognize C99 hex float in the parser then (leaving
>> aside for now whether that's feasible in the first place).
> 
> Thanks for all the suggestions!
> 
> I worked my way through this and I believe that the following would be
> a reasonable solution:
> * constants min, inf and nan must be symbols (as stated in the specification)
> * other constants are parsed by scanf("%f") as floats and compared
> (float compare in C) against the numeric constants in the table
> * output in the disassembly uses symbols for min/inf/nan and %a (hex
> FP literals) for other constants
> 
> So we support every format that '%f' accepts including hex FP literals
> (e.g. -0x1p0, 0x1p+0, ...) and normal FP constants (e.g.
> 0.0000152587890625, 25E-4).

How's this going to work with a cross-assembler run on an architecture
supporting a floating point format other than IEEE 754's? Hence why I
suggested using ieee_md_atof() instead.

Jan