On Mon, Jul 29, 2013, David Chisnall wrote: > On 29 Jul 2013, at 08:05, David Schultz <d...@freebsd.org> wrote: > > > On Thu, Jul 11, 2013, David Chisnall wrote: > >> +static __inline int > >> +__inline_isnan(double __x) > >> +{ > >> + > >> + return (__x != __x); > >> +} > >> + > >> +static __inline int > >> +__inline_isnanf(float __x) > >> +{ > >> + > >> + return (__x != __x); > >> +} > >> + > >> +static __inline int > >> +__inline_isnanl(long double __x) > >> +{ > >> + > >> + return (__x != __x); > >> +} > > > > This has already been covered at greater length, but I believe > > this part is incorrect. Relational operators can raise an invalid > > exception when one of the arguments is a NaN -- even a quiet NaN. > > Raising an exception is optional in C99 (7.12.14) and required in > > IEEE-754... in practice, it tends to be platform- and compiler- > > specific. > > > > That is the whole reason the is* macros are defined by the > > standard in the first place, and also why we didn't use the > > trivial implementation above. The is* macros are required to not > > raise an exception. > > What would you suggest replacing them with? Note that currently LLVM iR > doesn't provide any way of distinguishing the != comparison from something > that is guaranteed not to raise an exception. I don't know how this works in > GIMPLE, althouhg I'd imagine that, since gcc has a working Fortran front end, > there is some better support for it.
I'm not sure what the inlines here were supposed to achieve, but I think the ideal implementation would be a compiler intrinsic, with a fallback of the libm functions if there's no working compiler support. As I recall, gcc has a __builtin_isnan() and macros to test whether __builtin_nan() exists. Presumably it wouldn't be too hard to do the same thing in clang. > > P.S. It would be great if clang implemented the FENV_ACCESS pragma > > and provided an intrinsic that produced a fast inline isnan() when > > the pragma is off, and the full, correct one when the pragma is on. > > > I almost agree, but C is a really terrible language for mathematical work and > I'd prefer that people just used Fortran instead of trying to force C to be > Fortran. Fortran has its own problems and isn't very well supported. But for what it's worth, C++ is actually a good choice for high-performance numerics, IMO, mainly because of operator overloading and generics. I can write a function that looks like actual math, and call it with a float, a double, or even an arbitrary-precision mpfr_t, and it just works. In C, on the other hand, they added all this "type-generic arithmetic" and complex number nonsense that's of very limited interest. In a better language, that functionality could have been implemented as a third-party library instead of as a built-in part of the language. So in that sense, I agree with you that C went too far with trying to compete with Fortran... FENV_ACCESS is more reasonable, though. It basically says that sophisticated users ought to be able to take advantage of the IEEE floating-point features that nearly all hardware FPUs support, without having the compiler mess things up. Meanwhile, there's a second mode for users who don't care, where the optimizer is allowed to make a lot more assumptions. Before C99, compilers tended to have some muddled combination of the two extremes, which is bad for everyone. Unfortunately, only the commercial compilers actually implement FENV_ACCESS these days... > In particular, take a look in the C11 spec for the semantics of this: > > _Atomic(double) x = ...; > x += 1; > > It's quite astonishingly horrible. We don't implement it correctly in clang, > and I hope never to have to. I hope it does something horrible to the programmer who thought of the idea of atomic double-precision arithmetic. _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"