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

[Max Reitz <mreitz@redhat.com>]

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On 2017-07-05 18:22, Max Reitz wrote:
> 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

Here comes iteration number 4: Forgot the y >= 0 check.

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

Also, I should check that the fractional part of y is 0 (through modf(y,
&_) == 0.0).

Floating point numbers are so much fun!

(And all of this gives me such great ideas for tests to add to patch 5!)

Max

> I hope. :-/
> 
> (But finally a chance to use binary exponents! Yay!)
> 
> Max
> 



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