Re: [PATCH v2 4/5] x86 long double: Consider pseudo numbers as signaling

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 23/12/2020 08:56, Siddhesh Poyarekar wrote:
> Add support to treat pseudo-numbers specially and implement x86
> version to consider all of them as signaling.
> ---
>  sysdeps/generic/nan-pseudo-number.h      | 27 ++++++++++++++++++++
>  sysdeps/ieee754/ldbl-96/s_issignalingl.c |  7 +++++-
>  sysdeps/x86/fpu/nan-pseudo-number.h      | 32 ++++++++++++++++++++++++
>  3 files changed, 65 insertions(+), 1 deletion(-)
>  create mode 100644 sysdeps/generic/nan-pseudo-number.h
>  create mode 100644 sysdeps/x86/fpu/nan-pseudo-number.h
> 
> diff --git a/sysdeps/generic/nan-pseudo-number.h b/sysdeps/generic/nan-pseudo-number.h
> new file mode 100644
> index 0000000000..ce1176a208
> --- /dev/null
> +++ b/sysdeps/generic/nan-pseudo-number.h
> @@ -0,0 +1,27 @@
> +/* Pseudo-normal number handling.  Generic version.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#ifndef NAN_PSEUDO_NUMBER_H
> +#define NAN_PSEUDO_NUMBER_H	1
> +
> +/* Define this macro to 1 and provide an definition for IS_PSEUDO_SIGNALING if
> +   pseudo-normal numbers need special handling.  */
> +#define HANDLE_PSEUDO_NUMBERS 0
> +#define IS_PSEUDO_SIGNALING(x) 0
> +
> +#endif /* nan-pseudo-number.h */
> diff --git a/sysdeps/ieee754/ldbl-96/s_issignalingl.c b/sysdeps/ieee754/ldbl-96/s_issignalingl.c
> index ec542ad468..fff6a85932 100644
> --- a/sysdeps/ieee754/ldbl-96/s_issignalingl.c
> +++ b/sysdeps/ieee754/ldbl-96/s_issignalingl.c
> @@ -19,12 +19,14 @@
>  #include <math.h>
>  #include <math_private.h>
>  #include <nan-high-order-bit.h>
> +#include <nan-pseudo-number.h>
>  
>  int
>  __issignalingl (long double x)
>  {
>    uint32_t exi, hxi, lxi;
>    GET_LDOUBLE_WORDS (exi, hxi, lxi, x);
> +  int ret = 0;
>  #if HIGH_ORDER_BIT_IS_SET_FOR_SNAN
>  # error not implemented
>  #else
> @@ -36,9 +38,12 @@ __issignalingl (long double x)
>    hxi |= (lxi | -lxi) >> 31;
>    /* We do not recognize a pseudo NaN as sNaN; they're invalid on 80387 and
>       later.  */
> +#if HANDLE_PSEUDO_NUMBERS
> +  ret = IS_PSEUDO_SIGNALING(x);
> +#endif
>    /* We have to compare for greater (instead of greater or equal), because x's
>       significand being all-zero designates infinity not NaN.  */
> -  return ((exi & 0x7fff) == 0x7fff) && (hxi > 0xc0000000);
> +  return ret || (((exi & 0x7fff) == 0x7fff) && (hxi > 0xc0000000));
>  #endif
>  }
>  libm_hidden_def (__issignalingl)
> diff --git a/sysdeps/x86/fpu/nan-pseudo-number.h b/sysdeps/x86/fpu/nan-pseudo-number.h
> new file mode 100644
> index 0000000000..9cc37a3eee
> --- /dev/null
> +++ b/sysdeps/x86/fpu/nan-pseudo-number.h
> @@ -0,0 +1,32 @@
> +/* Pseudo-normal number handling.  x86 version.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#ifndef NAN_PSEUDO_NUMBER_H
> +#define NAN_PSEUDO_NUMBER_H	1
> +
> +#define HANDLE_PSEUDO_NUMBERS 1
> +/* A pseudo-normal number (that is not pseudo-denormal) has a non-zero exponent
> +   and the MSB of significand is clear.  All pseudo-normal numbers are deemed
> +   signaling.  */
> +#define IS_PSEUDO_SIGNALING(x) ({					      \
> +  uint32_t exi, hxi, lxi __attribute__ ((unused));			      \
> +  GET_LDOUBLE_WORDS (exi, hxi, lxi, x);					      \
> +  ((exi & 0x7fff) && ((hxi & 0x80000000) == 0));			      \
> +})
> +
> +#endif /* nan-pseudo-number.h */
> 

Maybe a static inline function would be better? Something like:

  static inline int pseudo_signaling (uint32_t exi, uint32_t hxi)
  {
    return (exi & 0x7fff) && (hxi & 0x80000000) == 0;
  }