[PATCH 2/5] x86 long double: Support pseudo numbers in isnanl

[Adhemerval Zanella]

On 15/12/2020 11:13, Siddhesh Poyarekar via Libc-alpha wrote:
> Sync up with gcc behaviour.  This change splits out the core isnanl
> logic so that it can be reused.
> ---
>  sysdeps/i386/fpu/s_isnanl.c     | 10 ++--------
>  sysdeps/x86/fpu/isnanl_common.h | 32 ++++++++++++++++++++++++++++++++
>  2 files changed, 34 insertions(+), 8 deletions(-)
>  create mode 100644 sysdeps/x86/fpu/isnanl_common.h
> 
> diff --git a/sysdeps/i386/fpu/s_isnanl.c b/sysdeps/i386/fpu/s_isnanl.c
> index fb97317bc9..6824f31d96 100644
> --- a/sysdeps/i386/fpu/s_isnanl.c
> +++ b/sysdeps/i386/fpu/s_isnanl.c
> @@ -25,19 +25,13 @@ static char rcsid[] = "$NetBSD: $";
>  
>  #include <math.h>
>  #include <math_private.h>
> +#include "sysdeps/x86/fpu/isnanl_common.h"
>  
>  int __isnanl(long double x)
>  {
>  	int32_t se,hx,lx;
>  	GET_LDOUBLE_WORDS(se,hx,lx,x);
> -	se = (se & 0x7fff) << 1;
> -	/* The additional & 0x7fffffff is required because Intel's
> -	   extended format has the normally implicit 1 explicit
> -	   present.  Sigh!  */
> -	lx |= hx & 0x7fffffff;
> -	se |= (uint32_t)(lx|(-lx))>>31;
> -	se = 0xfffe - se;
> -	return (int)((uint32_t)(se))>>16;
> +	return x86_isnanl (se, hx, lx);
>  }
>  hidden_def (__isnanl)
>  weak_alias (__isnanl, isnanl)

As for fpclassify, I think a better strategy would to move this code to
sysdeps/x86/fpu.

> diff --git a/sysdeps/x86/fpu/isnanl_common.h b/sysdeps/x86/fpu/isnanl_common.h
> new file mode 100644
> index 0000000000..60a4af5b41
> --- /dev/null
> +++ b/sysdeps/x86/fpu/isnanl_common.h
> @@ -0,0 +1,32 @@
> +/* Common inline isnanl implementation for x86.
> +   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/>.  */
> +
> +static inline __always_inline int
> +x86_isnanl (int32_t se, int32_t hx, int32_t lx)
> +{
> +  se = (se & 0x7fff) << 1;
> +  /* Detect pseudo-normal numbers, i.e. exponent is non-zero and the top
> +     bit of the significand is not set.   */

This sentence sounds strange, would 'if' instead of 'of' be better?

> +  int pn = (uint32_t)((~hx & 0x80000000) & (se | (-se))) >> 31;

Ok.

> +  /* Clear the significand bit when computing mantissa.  */
> +  lx |= hx & 0x7fffffff;
> +  se |= (uint32_t)(lx | (-lx)) >> 31;
> +  se = 0xfffe - se;
> +
> +  return (int)(((uint32_t)(se)) >> 16) | pn;
> +}
> 

Not sure if this is really a gain here, is this routine really worth
to move to inline instead of just calling the __isnanl (since it seems
to be used solely on issignalingl)?