Re: [PATCH] utils: Use fma in qemu_strtosz

[Richard Henderson <richard.henderson@linaro.org>]

On 3/15/21 3:10 AM, Philippe Mathieu-Daudé wrote:
> On 3/15/21 12:48 AM, Richard Henderson wrote:
>> Use fma to simulatneously scale and round up fraction.
> 
> "simultaneously"
> 
>> The libm function will always return a properly rounded double precision
>> value, which will eliminate any extra precision the x87 co-processor may
>> give us, which will keep the output predictable vs other hosts.
>>
>> Adding DBL_EPSILON while scaling should help with fractions like
>> 12.345, where the closest representable number is actually 12.3449*.
>>
>> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>> ---
>>   util/cutils.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/util/cutils.c b/util/cutils.c
>> index d89a40a8c3..f7f8e48a68 100644
>> --- a/util/cutils.c
>> +++ b/util/cutils.c
>> @@ -342,7 +342,7 @@ static int do_strtosz(const char *nptr, const char **end,
>>       if (val > (UINT64_MAX - ((uint64_t) (fraction * mul))) / mul) {
> 
> Shouldn't we use fma() here too? ^^^^^^^^^^^^^^^^^^^^^^^^^^

Yep, I should have looked at the larger context.


r~

> 
>>           retval = -ERANGE;
>>           goto out;
>>       }
>> -    *result = val * mul + (uint64_t) (fraction * mul);
>> +    *result = val * mul + (uint64_t)fma(fraction, mul, DBL_EPSILON);
>>       retval = 0;
>>   
>>   out:
>>
>