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

[Eric Blake]

On 07/05/2017 12:04 PM, Max Reitz wrote:

> Or, well, yes, it is in this case, but I meant literally UINT64_MAX + 1,
> not the uint64_t value. I meant the natural number 2^64.
> 
> Because the issue is that (double)UINT64_MAX will (or may, depending on
> the environment and such) give us 2.0^64 == 0x1p64.
> 
> And this is where the quotation I gave below comes in. When casting an
> integer to a double we may end up with a value that is not representable
> in the original integer type, so we cannot easily cast it back.

And it's more than just UINT64_MAX that rounds to the double value of
0x1p64.  Okay, I'm starting to be convinced that using a floating-point
comparison to 0x1p64 is going to be easier than an integer comparison to
the rounding point (what point is that, anyways: where the low-order 11
bits that don't fit in 53 bits of precision cause us to round up instead
of down?)

>>> Justifying the use of binary exponents is going to be harder than that,
>>> and would need a really good comment in the code, compared to just using
>>> a safe double-cast.
>>
>> It's not safe.
>>
>> Max
>>
> 
> 

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

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