Re: [Qemu-devel] [PATCH v3 2/5] qapi: Add qobject_is_equal()

[Eric Blake <eblake@redhat.com>]

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On 07/05/2017 08:48 AM, Max Reitz wrote:
>>>  /**
>>> + * qnum_is_equal(): Test whether the two QNums are equal
>>> + */
>>> +bool qnum_is_equal(const QObject *x, const QObject *y)
>>> +{
>>> +    QNum *num_x = qobject_to_qnum(x);
>>> +    QNum *num_y = qobject_to_qnum(y);
>>> +
>>> +    switch (num_x->kind) {
>>> +    case QNUM_I64:
>>> +        switch (num_y->kind) {
>>> +        case QNUM_I64:
>>> +            /* Comparison in native int64_t type */
>>> +            return num_x->u.i64 == num_y->u.i64;
>>> +        case QNUM_U64:
>>> +            /* Implicit conversion of x to uin64_t, so we have to
>>> +             * check its sign before */
>>> +            return num_x->u.i64 >= 0 && num_x->u.i64 == num_y->u.u64;
>>> +        case QNUM_DOUBLE:
>>> +            /* Implicit conversion of x to double; no overflow
>>> +             * possible */
>>> +            return num_x->u.i64 == num_y->u.dbl;
>>
>> Overflow is impossible, but loss of precision is possible:
>>
>>     (double)9007199254740993ull == 9007199254740992.0
>>
>> yields true.  Is this what we want?
> 
> I'd argue that yes, because the floating point value represents
> basically all of the values which are "equal" to it.

But the problem is that we CAN represent the fully-precise number as an
integer, so having multiple distinct integers that compare differently
against each other, but equal to the same double, is awkward.

> 
> But I don't have a string opinion. I guess the alternative would be to
> convert the double to an integer instead and check for overflows before?

That's the solution Markus gave, and I'm in favor of the tighter check:

> 
>> I guess the obvious fix is
>>
>>     return (double)x == x && x == y;
> 
> Yes, that would do, too; and spares me of having to think about how well
> comparing an arbitrary double to UINT64_MAX actually works. :-)

It basically says that we are unwilling to declare an integer equivalent
to the double if the double loses precision when trying to store the
integer.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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