Hi J.H., thanks for the patch. Only problem, this patch is non-trivial enough to require the BSD waiver per https://cygwin.com/contrib.html, see the "Before you get started" section. I add you to the winsup/CONTRIBUTORS file then. That ok with you?
Thanks,
Corinna
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
> ---
> winsup/cygwin/fenv.cc | 8 +++++---
> winsup/cygwin/include/fenv.h | 14 ++++++++------
> 2 files changed, 13 insertions(+), 9 deletions(-)
>
> diff --git a/winsup/cygwin/fenv.cc b/winsup/cygwin/fenv.cc
> index 066704b..0067da0 100644
> --- a/winsup/cygwin/fenv.cc
> +++ b/winsup/cygwin/fenv.cc
> @@ -417,7 +417,7 @@ fesetprec (int prec)
> void
> _feinitialise (void)
> {
> - unsigned int edx, eax, mxcsr;
> + unsigned int edx, eax;
>
> /* Check for presence of SSE: invoke CPUID #1, check EDX bit 25. */
> eax = 1;
> @@ -431,11 +431,13 @@ _feinitialise (void)
> /* The default cw value, 0x37f, is rounding mode zero. The MXCSR has
> no precision control, so the only thing to do is set the exception
> mask bits. */
> - mxcsr = FE_ALL_EXCEPT << FE_SSE_EXCEPT_MASK_SHIFT;
> +
> + /* initialize the MXCSR register: mask all exceptions */
> + unsigned int mxcsr = __FE_ALL_EXCEPT_X86 << FE_SSE_EXCEPT_MASK_SHIFT;
> if (use_sse)
> __asm__ volatile ("ldmxcsr %0" :: "m" (mxcsr));
>
> - /* Setup unmasked environment. */
> + /* Setup unmasked environment, but leave __FE_DENORM masked. */
> feenableexcept (FE_ALL_EXCEPT);
> fegetenv (&fe_nomask_env);
>
> diff --git a/winsup/cygwin/include/fenv.h b/winsup/cygwin/include/fenv.h
> index 7ec5d4d..5fdbe5a 100644
> --- a/winsup/cygwin/include/fenv.h
> +++ b/winsup/cygwin/include/fenv.h
> @@ -87,16 +87,18 @@ typedef __uint32_t fexcept_t;
> #define FE_OVERFLOW (1 << 3)
> #define FE_UNDERFLOW (1 << 4)
>
> -/* This is not defined by Posix, but since x87 supports it we provide
> - a definition according to the same naming scheme used above. */
> -#define FE_DENORMAL (1 << 1)
> -
> /* The <fenv.h> header shall define the following constant, which is
> simply the bitwise-inclusive OR of all floating-point exception
> constants defined above: */
>
> -#define FE_ALL_EXCEPT (FE_DIVBYZERO | FE_INEXACT | FE_INVALID \
> - | FE_OVERFLOW | FE_UNDERFLOW | FE_DENORMAL)
> +/* in agreement w/ Linux the subnormal exception will always be masked */
> +#define FE_ALL_EXCEPT \
> + (FE_INEXACT | FE_UNDERFLOW | FE_OVERFLOW | FE_DIVBYZERO | FE_INVALID)
> +
> +#define __FE_DENORM (1 << 1)
> +
> +/* mask (= 0x3f) to disable all exceptions at initialization */
> +#define __FE_ALL_EXCEPT_X86 (FE_ALL_EXCEPT | __FE_DENORM)
>
> /* The <fenv.h> header shall define the following constants if and only
> if the implementation supports getting and setting the represented
> --
> 2.7.5
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Maintainer cygwin AT cygwin DOT com
Red Hat
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