On Wed, Jan 16, 2013 at 1:52 PM, Mischa Baars <mjbaars1...@gmail.com> wrote: > 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. > > 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. > > 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()'.
You can enable FP exceptions via fpsetexceptflag and friends (it's disabled in the FPU by default) and if you then make sure to compile with -fsignalling-nans (that's not the default) you will get the desired behavior (program termination via an exception). Richard. > Mischa.
