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| -rw-r--r-- | lib/fpucw.h | 108 |
1 files changed, 108 insertions, 0 deletions
diff --git a/lib/fpucw.h b/lib/fpucw.h new file mode 100644 index 0000000..1c6edf6 --- /dev/null +++ b/lib/fpucw.h @@ -0,0 +1,108 @@ +/* Manipulating the FPU control word. -*- coding: utf-8 -*- + Copyright (C) 2007-2020 Free Software Foundation, Inc. + Written by Bruno Haible <bruno@clisp.org>, 2007. + + This program is free software: you can redistribute it and/or modify + it under the terms of the GNU General Public License as published by + the Free Software Foundation; either version 3 of the License, or + (at your option) any later version. + + This program 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 General Public License for more details. + + You should have received a copy of the GNU 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. */ +#if (defined __i386__ || defined __x86_64__) && defined __GNUC__ + +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 */ |