| 1 | /* Manipulating the FPU control word. -*- coding: utf-8 -*- |
| 2 | Copyright (C) 2007-2020 Free Software Foundation, Inc. |
| 3 | Written by Bruno Haible <bruno@clisp.org>, 2007. |
| 4 | |
| 5 | This program is free software: you can redistribute it and/or modify |
| 6 | it under the terms of the GNU General Public License as published by |
| 7 | the Free Software Foundation; either version 3 of the License, or |
| 8 | (at your option) any later version. |
| 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 |
| 16 | along with this program. If not, see <https://www.gnu.org/licenses/>. */ |
| 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 |
| 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. |
| 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 |
| 38 | https://www.linux-mips.org/pub/linux/mips/doc/ABI/abi386-4.pdf page 3-12 = page 38 |
| 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 | |
| 44 | See Vincent Lefèvre's page https://www.vinc17.net/research/extended.en.html |
| 45 | for a good explanation. |
| 46 | See https://web.archive.org/web/20060905133417/http://www.uwsg.iu.edu/hypermail/linux/kernel/0103.0/0453.html |
| 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. */ |
| 65 | #if (defined __i386__ || defined __x86_64__) && defined __GNUC__ |
| 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() __extension__ \ |
| 74 | ({ fpucw_t _cw; \ |
| 75 | __asm__ __volatile__ ("fnstcw %0" : "=m" (*&_cw)); \ |
| 76 | _cw; \ |
| 77 | }) |
| 78 | # define SET_FPUCW(word) __extension__ \ |
| 79 | (void)({ fpucw_t _ncw = (word); \ |
| 80 | __asm__ __volatile__ ("fldcw %0" : : "m" (*&_ncw)); \ |
| 81 | }) |
| 82 | |
| 83 | # define DECL_LONG_DOUBLE_ROUNDING \ |
| 84 | fpucw_t oldcw; |
| 85 | # define BEGIN_LONG_DOUBLE_ROUNDING() \ |
| 86 | (void)(oldcw = GET_FPUCW (), \ |
| 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 */ |