Re: [PATCH] utils: Use fma in qemu_strtosz

[Eric Blake]

On 3/14/21 6:48 PM, 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);

Don't you need to include <float.h> to get DBL_EPSILON?

More importantly, this patch seems wrong.  fma(a, b, c) performs (a * b)
+ c without intermediate rounding errors, but given our values for a and
b, where mul > 1 in any situation we care about, adding 2^-53 is so much
smaller than a*b that it not going to round the result up to the next
integer.  Don't you want to do fma(fraction, mul, 0.5) instead?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org