Mercurial > gnulib
annotate lib/fpucw.h @ 40186:8964917f9574
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| author | Karl Berry <karl@freefriends.org> |
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| date | Mon, 18 Feb 2019 08:02:49 -0800 |
| parents | b06060465f09 |
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| rev | line source |
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1 /* Manipulating the FPU control word. -*- coding: utf-8 -*- |
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2 Copyright (C) 2007-2019 Free Software Foundation, Inc. |
| 8531 | 3 Written by Bruno Haible <bruno@clisp.org>, 2007. |
| 4 | |
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5 This program is free software: you can redistribute it and/or modify |
| 8531 | 6 it under the terms of the GNU General Public License as published by |
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7 the Free Software Foundation; either version 3 of the License, or |
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8 (at your option) any later version. |
| 8531 | 9 |
| 10 This program is distributed in the hope that it will be useful, | |
| 11 but WITHOUT ANY WARRANTY; without even the implied warranty of | |
| 12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
| 13 GNU General Public License for more details. | |
| 14 | |
| 15 You should have received a copy of the GNU General Public License | |
| 19190 | 16 along with this program. If not, see <https://www.gnu.org/licenses/>. */ |
| 8531 | 17 |
| 18 #ifndef _FPUCW_H | |
| 19 #define _FPUCW_H | |
| 20 | |
| 21 /* The i386 floating point hardware (the 387 compatible FPU, not the modern | |
| 22 SSE/SSE2 hardware) has a controllable rounding precision. It is specified | |
| 23 through the 'PC' bits in the FPU control word ('fctrl' register). (See | |
| 24 the GNU libc i386 <fpu_control.h> header for details.) | |
| 25 | |
| 26 On some platforms, such as Linux or Solaris, the default precision setting | |
| 27 is set to "extended precision". This means that 'long double' instructions | |
| 28 operate correctly, but 'double' computations often produce slightly | |
| 29 different results as on strictly IEEE 754 conforming systems. | |
| 30 | |
| 31 On some platforms, such as NetBSD, the default precision is set to | |
| 32 "double precision". This means that 'long double' instructions will operate | |
| 16573 | 33 only as 'double', i.e. lead to wrong results. Similarly on FreeBSD 6.4, at |
| 34 least for the division of 'long double' numbers. | |
| 8531 | 35 |
| 36 The FPU control word is under control of the application, i.e. it is | |
| 37 not required to be set either way by the ABI. (In fact, the i386 ABI | |
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38 https://www.linux-mips.org/pub/linux/mips/doc/ABI/abi386-4.pdf page 3-12 = page 38 |
| 8531 | 39 is not clear about it. But in any case, gcc treats the control word |
| 40 like a "preserved" register: it emits code that assumes that the control | |
| 41 word is preserved across calls, and it restores the control word at the | |
| 42 end of functions that modify it.) | |
| 43 | |
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44 See Vincent Lefèvre's page https://www.vinc17.net/research/extended.en.html |
| 8531 | 45 for a good explanation. |
| 46 See http://www.uwsg.iu.edu/hypermail/linux/kernel/0103.0/0453.html for | |
| 47 some argumentation which setting should be the default. */ | |
| 48 | |
| 49 /* This header file provides the following facilities: | |
| 50 fpucw_t integral type holding the value of 'fctrl' | |
| 51 FPU_PC_MASK bit mask denoting the precision control | |
| 52 FPU_PC_DOUBLE precision control for 53 bits mantissa | |
| 53 FPU_PC_EXTENDED precision control for 64 bits mantissa | |
| 54 GET_FPUCW () yields the current FPU control word | |
| 55 SET_FPUCW (word) sets the FPU control word | |
| 56 DECL_LONG_DOUBLE_ROUNDING variable declaration for | |
| 57 BEGIN/END_LONG_DOUBLE_ROUNDING | |
| 58 BEGIN_LONG_DOUBLE_ROUNDING () starts a sequence of instructions with | |
| 59 'long double' safe operation precision | |
| 60 END_LONG_DOUBLE_ROUNDING () ends a sequence of instructions with | |
| 61 'long double' safe operation precision | |
| 62 */ | |
| 63 | |
| 64 /* Inline assembler like this works only with GNU C. */ | |
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65 #if (defined __i386__ || defined __x86_64__) && defined __GNUC__ |
| 8531 | 66 |
| 67 typedef unsigned short fpucw_t; /* glibc calls this fpu_control_t */ | |
| 68 | |
| 69 # define FPU_PC_MASK 0x0300 | |
| 70 # define FPU_PC_DOUBLE 0x200 /* glibc calls this _FPU_DOUBLE */ | |
| 71 # define FPU_PC_EXTENDED 0x300 /* glibc calls this _FPU_EXTENDED */ | |
| 72 | |
| 73 # define GET_FPUCW() \ | |
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74 ({ fpucw_t _cw; \ |
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75 __asm__ __volatile__ ("fnstcw %0" : "=m" (*&_cw)); \ |
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76 _cw; \ |
| 8531 | 77 }) |
| 78 # define SET_FPUCW(word) \ | |
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79 (void)({ fpucw_t _ncw = (word); \ |
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80 __asm__ __volatile__ ("fldcw %0" : : "m" (*&_ncw)); \ |
| 8531 | 81 }) |
| 82 | |
| 83 # define DECL_LONG_DOUBLE_ROUNDING \ | |
| 84 fpucw_t oldcw; | |
| 85 # define BEGIN_LONG_DOUBLE_ROUNDING() \ | |
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86 (void)(oldcw = GET_FPUCW (), \ |
| 8531 | 87 SET_FPUCW ((oldcw & ~FPU_PC_MASK) | FPU_PC_EXTENDED)) |
| 88 # define END_LONG_DOUBLE_ROUNDING() \ | |
| 89 SET_FPUCW (oldcw) | |
| 90 | |
| 91 #else | |
| 92 | |
| 93 typedef unsigned int fpucw_t; | |
| 94 | |
| 95 # define FPU_PC_MASK 0 | |
| 96 # define FPU_PC_DOUBLE 0 | |
| 97 # define FPU_PC_EXTENDED 0 | |
| 98 | |
| 99 # define GET_FPUCW() 0 | |
| 100 # define SET_FPUCW(word) (void)(word) | |
| 101 | |
| 102 # define DECL_LONG_DOUBLE_ROUNDING | |
| 103 # define BEGIN_LONG_DOUBLE_ROUNDING() | |
| 104 # define END_LONG_DOUBLE_ROUNDING() | |
| 105 | |
| 106 #endif | |
| 107 | |
| 108 #endif /* _FPUCW_H */ |