Re: [PATCH] utils: Use fma in qemu_strtosz

[Philippe Mathieu-Daudé]

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? ^^^^^^^^^^^^^^^^^^^^^^^^^^

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