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