svn commit: r253215 - head/lib/msun/src

[David Chisnall]

On 29 Jul 2013, at 08:05, David Schultz <das at 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.

> 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.  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.  

David

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