> -----Original Message----- > From: gcc-ow...@gcc.gnu.org [mailto:gcc-ow...@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? > 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? 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.
