On 2018-08-15 17:26, Corinna Vinschen wrote:
On Aug 15 17:01, Houder wrote:
On 2018-08-15 16:54, Corinna Vinschen wrote:
> On Aug 15 12:59, J.H. van de Water wrote:
> > By excluding the denormal-operand exception from FE_ALL_EXCEPT, it
> > will not
> > be possible anymore to UNmask this exception by means of the API
> > defined by
> > /usr/include/fenv.h
> >
> > Note: terminology has changed since IEEE Std 854-1987; denormalized
> > numbers
> > are called subnormal numbers nowadays.
> >
> > This modification has basically been motivated by the fact that it
> > is also
> > not possible on Linux to manipulate the denormal-operand exception
> > by means
> > of the interface as defined by /usr/include/fenv.h. This has been
> > the state
> > of affairs on Linux since 2001 (Andreas Jaeger).
> >
> > The exceptions required by the standard (IEEE Std 754), in case they
> > can be
> > supported by the implementation, are:
> > FE_INEXACT, FE_UNDERFLOW, FE_OVERFLOW, FE_DIVBYZERO and FE_INVALID.
> >
> > Although it is allowed to define additional exceptions, there is no
> > reason
> > to support the "denormal-operand exception" in this case (fenv.h),
> > because
> > the subnormal numbers can be handled almost as fast the normalized
> > numbers
> > by the hardware of the x86/x86_64 architecture. Said differently, a
> > reason
> > to trap on the input of subnormal numbers does not exist. At least
> > that is
> > what William Kahan and others at Intel asserted around 2000.
> > (that is William Kahan of the K-C-S draft, the precursor to the
> > standard)
> >
> > This commit modifies winsup/cygwin/include/fenv.h as follows:
> > - redefines FE_ALL_EXCEPT from 0x3f to 0x3d
> > - removes the definition for FE_DENORMAL
> > - introduces __FE_DENORM (0x2) (enum in Linux also uses __FE_DENORM)
> > - introduces FE_ALL_EXCEPT_X86 (0x3f), i.e. ALL x86/x86_64 FP
> > exceptions
>
> Shouldn't FE_ALL_EXCEPT_X86 be defined locally in fenv.cc only?
> I don't see that Linux exports that definition.
Ah, Sorry. Do I have to resubmit my patch? Or is it easy enough for
you to
make this modification?
It's easy enough but I'm still mulling over __FE_DENORM. The glibc
fenv.h header defines it, so I guess we should stick to it. In that
case it might make sense to revert the original comment and just move
__FE_ALL_EXCEPT_X86.
... uhm, my intention was to remove FE_DENORMAL from fenv.h, because it
is no longer part of the interface.
I should have defined "a mask" in fenv.cc, that would have enabled me to
initialize the MXCSR register (i.e. mask ALL exceptions).
I defined __FE_ALL_EXCEPT_X86 (and should have defined it as 0x3f).
Basically, there was no reason to define __FE_DENORM (or __FE_DENORMAL),
now that I think it over.
(I needed __FE_DENORM(AL) for testing)
Henri
