✏ EDIT: fpu_control.h

/* FPU control word bits.  x86 version.
   Copyright (C) 1993-2018 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Olaf Flebbe.

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
   <http://www.gnu.org/licenses/>.  */

#ifndef _FPU_CONTROL_H
#define _FPU_CONTROL_H	1

/* Note that this file sets on x86-64 only the x87 FPU, it does not
   touch the SSE unit.  */

/* Here is the dirty part. Set up your 387 through the control word
 * (cw) register.
 *
 *     15-13    12  11-10  9-8     7-6     5    4    3    2    1    0
 * | reserved | IC | RC  | PC | reserved | PM | UM | OM | ZM | DM | IM
 *
 * IM: Invalid operation mask
 * DM: Denormalized operand mask
 * ZM: Zero-divide mask
 * OM: Overflow mask
 * UM: Underflow mask
 * PM: Precision (inexact result) mask
 *
 * Mask bit is 1 means no interrupt.
 *
 * PC: Precision control
 * 11 - round to extended precision
 * 10 - round to double precision
 * 00 - round to single precision
 *
 * RC: Rounding control
 * 00 - rounding to nearest
 * 01 - rounding down (toward - infinity)
 * 10 - rounding up (toward + infinity)
 * 11 - rounding toward zero
 *
 * IC: Infinity control
 * That is for 8087 and 80287 only.
 *
 * The hardware default is 0x037f which we use.
 */

#include <features.h>

/* masking of interrupts */
#define _FPU_MASK_IM  0x01
#define _FPU_MASK_DM  0x02
#define _FPU_MASK_ZM  0x04
#define _FPU_MASK_OM  0x08
#define _FPU_MASK_UM  0x10
#define _FPU_MASK_PM  0x20

/* precision control */
#define _FPU_EXTENDED 0x300	/* libm requires double extended precision.  */
#define _FPU_DOUBLE   0x200
#define _FPU_SINGLE   0x0

/* rounding control */
#define _FPU_RC_NEAREST 0x0    /* RECOMMENDED */
#define _FPU_RC_DOWN    0x400
#define _FPU_RC_UP      0x800
#define _FPU_RC_ZERO    0xC00

#define _FPU_RESERVED 0xF0C0  /* Reserved bits in cw */

/* The fdlibm code requires strict IEEE double precision arithmetic,
   and no interrupts for exceptions, rounding to nearest.  */

#define _FPU_DEFAULT  0x037f

/* IEEE:  same as above.  */
#define _FPU_IEEE     0x037f

/* Type of the control word.  */
typedef unsigned int fpu_control_t __attribute__ ((__mode__ (__HI__)));

/* Macros for accessing the hardware control word.  "*&" is used to
   work around a bug in older versions of GCC.  __volatile__ is used
   to support combination of writing the control register and reading
   it back.  Without __volatile__, the old value may be used for reading
   back under compiler optimization.

Note that the use of these macros is not sufficient anymore with
   recent hardware nor on x86-64.  Some floating point operations are
   executed in the SSE/SSE2 engines which have their own control and
   status register.  */
#define _FPU_GETCW(cw) __asm__ __volatile__ ("fnstcw %0" : "=m" (*&cw))
#define _FPU_SETCW(cw) __asm__ __volatile__ ("fldcw %0" : : "m" (*&cw))

/* Default control word set at startup.  */
extern fpu_control_t __fpu_control;

#endif	/* fpu_control.h */

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