not-a-number's

[Mischa Baars]

On 01/16/2013 07:23 PM, David Paterson wrote:
>> -----Original Message-----
>> From: gcc-owner@gcc.gnu.org [mailto:gcc-owner@gcc.gnu.org] On Behalf
>> Of Mischa Baars
>> Sent: 16 January 2013 12:53
>> To: Robert Dewar; gcc@gcc.gnu.org
>> Subject: Re: not-a-number's
>>
>> On 01/16/2013 01:28 PM, Robert Dewar wrote:
>>> On 1/16/2013 7:10 AM, Mischa Baars wrote:
>>>
>>>> And as I have said before: if you are satisfied with the answer
>>>> '2', then so be it and you keep the compiler the way it is,
>>>> personally I'm am not able to accept changes to the sources anyway.
>>>> I don't think it is the right answer though.
>>> The fact that you don't think that gcc shoudl follow the C standard
>>> is hardly convincing unless it is backed up by convincing technical
>>> argument. I see nothing surprising about the 2 here, indeed any
>>> other answer *would* be surprising. I still don't understand the
>>> basis for your non-stnadard views.
>>>> Mischa.
>>>>
>> Let me explain again for you. Every 'if' statement in C is translated into a 'fucom'
>> or similar instruction, which sets a number of conditions flags in the co-processor.
>> Some instructions need you to load these into the eflags register
>> manually, others don't.
>>
>> Now, as soon as the 'fucom' or similar instruction encounters a
>> 'signalling not-a- number' or 'quiet not-a-number' as one or both of
>> it's arguments, the condition code is set to 'not comparable'. This
>> has nothing to do with the C specification, but purely with the Intel/AMD hardware.
> Intel and AMD aren't the only manufacturers or developers of floating
> point hardware.  Plus there are software implementations to consider
> as well.
>
> In order for different hardware and software implementations to work
> correctly with each other, there have to be standards defining all
> aspects of their operation.  The same applies to the C language (and
> other languages!).
>
> Without standards we'd have no way to ensure programs would run in the
> same way on different hardware, or when using different compilers...
>
>> If you ask me, it would be counter-intuitive to change the value of
>> the condition code to some other value and call that the new standard.
>> Instead it would seem logical to use the 'not comparable' and terminate the 'if'
>> statement.
> What exactly do you mean by "terminate the if"??  Do you mean skip the
> whole compound statement, including any "else" clause?
Yes, exactly. Skip it, including the 'else'.
>
>> Does that make sense to you?
>>
>> In our example, a 'not-a-number' would be returned to the main
>> program, without the need for an extra 'isnan()'.
> "...returned to the main program" doesn't make sense.  How is the
> compiler going to figure out what to return and when?  What does it
> return from a void, or int function?
There is an 'r = -NAN' at the beginning which will then be the value 
returned.
>
> I'm sorry you don't like the way it's implemented, but we have
> standards for very good reasons, and  although this particular one may
> seem a bit counter- intuitive, it's well known, and many programs rely
> on it operating as it does.
>
> But, if you want to change the standards, by all means try...
>
> David P.