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/* Manipulating the FPU control word. -*- coding: utf-8 -*-
Copyright (C) 2007-2024 Free Software Foundation, Inc.
Written by Bruno Haible <bruno@clisp.org>, 2007.
This file 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.
This file 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 this program. If not, see <https://www.gnu.org/licenses/>. */
#ifndef _FPUCW_H
#define _FPUCW_H
/* The i386 floating point hardware (the 387 compatible FPU, not the modern
SSE/SSE2 hardware) has a controllable rounding precision. It is specified
through the 'PC' bits in the FPU control word ('fctrl' register). (See
the GNU libc i386 <fpu_control.h> header for details.)
On some platforms, such as Linux or Solaris, the default precision setting
is set to "extended precision". This means that 'long double' instructions
operate correctly, but 'double' computations often produce slightly
different results as on strictly IEEE 754 conforming systems.
On some platforms, such as NetBSD, the default precision is set to
"double precision". This means that 'long double' instructions will operate
only as 'double', i.e. lead to wrong results. Similarly on FreeBSD 6.4, at
least for the division of 'long double' numbers.
The FPU control word is under control of the application, i.e. it is
not required to be set either way by the ABI. (In fact, the i386 ABI
https://www.linux-mips.org/pub/linux/mips/doc/ABI/abi386-4.pdf page 3-12 = page 38
is not clear about it. But in any case, gcc treats the control word
like a "preserved" register: it emits code that assumes that the control
word is preserved across calls, and it restores the control word at the
end of functions that modify it.)
See Vincent Lefèvre's page https://www.vinc17.net/research/extended.en.html
for a good explanation.
See https://web.archive.org/web/20060905133417/http://www.uwsg.iu.edu/hypermail/linux/kernel/0103.0/0453.html
some argumentation which setting should be the default. */
/* This header file provides the following facilities:
fpucw_t integral type holding the value of 'fctrl'
FPU_PC_MASK bit mask denoting the precision control
FPU_PC_DOUBLE precision control for 53 bits mantissa
FPU_PC_EXTENDED precision control for 64 bits mantissa
GET_FPUCW () yields the current FPU control word
SET_FPUCW (word) sets the FPU control word
DECL_LONG_DOUBLE_ROUNDING variable declaration for
BEGIN/END_LONG_DOUBLE_ROUNDING
BEGIN_LONG_DOUBLE_ROUNDING () starts a sequence of instructions with
'long double' safe operation precision
END_LONG_DOUBLE_ROUNDING () ends a sequence of instructions with
'long double' safe operation precision
*/
/* Inline assembler like this works only with GNU C and clang. */
#if (defined __i386__ || defined __x86_64__) && (defined __GNUC__ || defined __clang__)
typedef unsigned short fpucw_t; /* glibc calls this fpu_control_t */
# define FPU_PC_MASK 0x0300
# define FPU_PC_DOUBLE 0x200 /* glibc calls this _FPU_DOUBLE */
# define FPU_PC_EXTENDED 0x300 /* glibc calls this _FPU_EXTENDED */
# define GET_FPUCW() __extension__ \
({ fpucw_t _cw; \
__asm__ __volatile__ ("fnstcw %0" : "=m" (*&_cw)); \
_cw; \
})
# define SET_FPUCW(word) __extension__ \
(void)({ fpucw_t _ncw = (word); \
__asm__ __volatile__ ("fldcw %0" : : "m" (*&_ncw)); \
})
# define DECL_LONG_DOUBLE_ROUNDING \
fpucw_t oldcw;
# define BEGIN_LONG_DOUBLE_ROUNDING() \
(void)(oldcw = GET_FPUCW (), \
SET_FPUCW ((oldcw & ~FPU_PC_MASK) | FPU_PC_EXTENDED))
# define END_LONG_DOUBLE_ROUNDING() \
SET_FPUCW (oldcw)
#else
typedef unsigned int fpucw_t;
# define FPU_PC_MASK 0
# define FPU_PC_DOUBLE 0
# define FPU_PC_EXTENDED 0
# define GET_FPUCW() 0
# define SET_FPUCW(word) (void)(word)
# define DECL_LONG_DOUBLE_ROUNDING
# define BEGIN_LONG_DOUBLE_ROUNDING()
# define END_LONG_DOUBLE_ROUNDING()
#endif
#endif /* _FPUCW_H */
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