Re: [PATCH] utils: Use fma in qemu_strtosz

[Thomas Huth]

On 15/03/2021 00.48, Richard Henderson wrote:
> Use fma to simulatneously scale and round up fraction.
>
> 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,
>           retval = -ERANGE;
>           goto out;
>       }
> -    *result = val * mul + (uint64_t) (fraction * mul);
> +    *result = val * mul + (uint64_t)fma(fraction, mul, DBL_EPSILON);
>       retval = 0;
>   
>   out:

Will this fix the failure that we're currently seeing with 32-bit builds?

( https://gitlab.com/qemu-project/qemu/-/jobs/1096980112#L3258 for example )

Reviewed-by: Thomas Huth <thuth@redhat.com>