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

[Max Reitz <mreitz@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 4785 bytes --]

On 2017-07-05 18:05, Max Reitz wrote:
> On 2017-07-05 15:48, Max Reitz wrote:
>> On 2017-07-05 09:07, Markus Armbruster wrote:
>>> Max Reitz <mreitz@redhat.com> writes:
>>>
>>>> This generic function (along with its implementations for different
>>>> types) determines whether two QObjects are equal.
>>>>
>>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>> [...]
>>>> diff --git a/qobject/qnum.c b/qobject/qnum.c
>>>> index 476e81c..784d061 100644
>>>> --- a/qobject/qnum.c
>>>> +++ b/qobject/qnum.c
>>>> @@ -213,6 +213,59 @@ QNum *qobject_to_qnum(const QObject *obj)
>>>>  }
>>>>  
>>>>  /**
>>>> + * 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 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?
>>
>>>> +        }
>>>> +        abort();
>>>> +    case QNUM_U64:
>>>> +        switch (num_y->kind) {
>>>> +        case QNUM_I64:
>>>> +            return qnum_is_equal(y, x);
>>>> +        case QNUM_U64:
>>>> +            /* Comparison in native uint64_t type */
>>>> +            return num_x->u.u64 == num_y->u.u64;
>>>> +        case QNUM_DOUBLE:
>>>> +            /* Implicit conversion of x to double; no overflow
>>>> +             * possible */
>>>> +            return num_x->u.u64 == num_y->u.dbl;
>>>
>>> Similar loss of precision.
>>>
>>>> +        }
>>>> +        abort();
>>>> +    case QNUM_DOUBLE:
>>>> +        switch (num_y->kind) {
>>>> +        case QNUM_I64:
>>>> +            return qnum_is_equal(y, x);
>>>> +        case QNUM_U64:
>>>> +            return qnum_is_equal(y, x);
>>>> +        case QNUM_DOUBLE:
>>>> +            /* Comparison in native double type */
>>>> +            return num_x->u.dbl == num_y->u.dbl;
>>>> +        }
>>>> +        abort();
>>>> +    }
>>>> +
>>>> +    abort();
>>>> +}
>>>
>>> I think there's more than one sane interpretations of "is equal",
>>> including:
>>>
>>> * The mathematical numbers represented by @x and @y are equal.
>>>
>>> * @x and @y have the same contents, i.e. same kind and u.
>>>
>>> * @x and @y are the same object (listed for completeness; we don't need
>>>   a function to compare pointers).
>>>
>>> Your patch implements yet another one.  Which one do we want, and why?
>>
>> Mine is the first one, except that I think that a floating point value
>> does not represent a single number but just some number in a range.
>>
>>> The second is easier to implement than the first.
>>
>> It seems much less useful, though.
>>
>>> If we really want the first, you need to fix the loss of precision bugs.
>>
>> I'm not sure, but I don't mind either, so...
>>
>>> 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. :-)
> 
> On second thought, this won't do, because (double)x == x is always true
> if x is an integer (because this will implicitly cast the second x to a
> double, too). However, (uint64_t)(double)x == x should work.

Hm, well, the nice thing with this is that (double)UINT64_MAX is
actually UINT64_MAX + 1, and now (uint64_t)(UINT64_MAX + 1) is
undefined... Urgs.

So I guess one thing that isn't very obvious but that should *always*
work (and is always well-defined) is this:

For uint64_t: y < 0x1p64 && (uint64_t)y == x

For int64_t: y >= -0x1p63 && y < 0x1p63 && (int64_t)y == x

I hope. :-/

(But finally a chance to use binary exponents! Yay!)

Max


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 498 bytes --]