Re: [PATCH] utils: Reduce chance of rounding inaccuracy in qemu_strtosz.

[Eric Blake <eblake@redhat.com>]

On 3/13/21 3:48 PM, Richard Henderson wrote:
> On 3/11/21 2:07 PM, Eric Blake wrote:
>> +        /*
>> +         * Add in a fudge-factor (2^53 when double is IEEE format) for
>> +         * all scales less than P (2^50), so that things like
>> +         * 12.345M with unit 1000 produce 12345000 instead of
>> +         * 12344999.
>> +         */
>> +        if (mul > 1e49) {
> 
> The comment says less than, the code says greater than.

Shoot. A demonstration that I did not have an environment that actually
reproduced the bug (and my request for help in figuring out how to kick
off a gitlab CI run that would catch it).

> 
> 
>> An alternative patch might be writing (uint64_t)(fraction * mul + 0.5)
>> (that is, introduce the fudge factor after the multiplication instead
>> of before).  Preferences?
> 
> I think I would prefer this, or for further rounding error reduction,
> fma(fraction, mul, 0.5).

Indeed, fma() sounds a bit nicer at minimizing the chance for double
rounding errors.

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