| 1 | /* Manipulating the FPU control word. -*- coding: utf-8 -*-
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| 2 | Copyright (C) 2007-2021 Free Software Foundation, Inc.
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| 3 | Written by Bruno Haible <bruno@clisp.org>, 2007.
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| 4 |
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| 5 | This file is free software: you can redistribute it and/or modify
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| 6 | it under the terms of the GNU Lesser General Public License as
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| 7 | published by the Free Software Foundation; either version 2.1 of the
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| 8 | License, or (at your option) any later version.
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| 9 |
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| 10 | This file is distributed in the hope that it will be useful,
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| 11 | but WITHOUT ANY WARRANTY; without even the implied warranty of
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| 12 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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| 13 | GNU Lesser General Public License for more details.
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| 14 |
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| 15 | You should have received a copy of the GNU Lesser General Public License
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| 16 | along with this program. If not, see <https://www.gnu.org/licenses/>. */
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| 17 |
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| 18 | #ifndef _FPUCW_H
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| 19 | #define _FPUCW_H
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| 20 |
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| 21 | /* The i386 floating point hardware (the 387 compatible FPU, not the modern
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| 22 | SSE/SSE2 hardware) has a controllable rounding precision. It is specified
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| 23 | through the 'PC' bits in the FPU control word ('fctrl' register). (See
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| 24 | the GNU libc i386 <fpu_control.h> header for details.)
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| 25 |
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| 26 | On some platforms, such as Linux or Solaris, the default precision setting
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| 27 | is set to "extended precision". This means that 'long double' instructions
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| 28 | operate correctly, but 'double' computations often produce slightly
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| 29 | different results as on strictly IEEE 754 conforming systems.
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| 30 |
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| 31 | On some platforms, such as NetBSD, the default precision is set to
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| 32 | "double precision". This means that 'long double' instructions will operate
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| 33 | only as 'double', i.e. lead to wrong results. Similarly on FreeBSD 6.4, at
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| 34 | least for the division of 'long double' numbers.
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| 35 |
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| 36 | The FPU control word is under control of the application, i.e. it is
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| 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
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| 39 | is not clear about it. But in any case, gcc treats the control word
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| 40 | like a "preserved" register: it emits code that assumes that the control
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| 41 | word is preserved across calls, and it restores the control word at the
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| 42 | end of functions that modify it.)
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| 43 |
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| 44 | See Vincent LefÚvre's page https://www.vinc17.net/research/extended.en.html
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| 45 | for a good explanation.
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| 46 | See https://web.archive.org/web/20060905133417/http://www.uwsg.iu.edu/hypermail/linux/kernel/0103.0/0453.html
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| 47 | some argumentation which setting should be the default. */
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| 48 |
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| 49 | /* This header file provides the following facilities:
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| 50 | fpucw_t integral type holding the value of 'fctrl'
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| 51 | FPU_PC_MASK bit mask denoting the precision control
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| 52 | FPU_PC_DOUBLE precision control for 53 bits mantissa
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| 53 | FPU_PC_EXTENDED precision control for 64 bits mantissa
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| 54 | GET_FPUCW () yields the current FPU control word
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| 55 | SET_FPUCW (word) sets the FPU control word
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| 56 | DECL_LONG_DOUBLE_ROUNDING variable declaration for
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| 57 | BEGIN/END_LONG_DOUBLE_ROUNDING
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| 58 | BEGIN_LONG_DOUBLE_ROUNDING () starts a sequence of instructions with
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| 59 | 'long double' safe operation precision
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| 60 | END_LONG_DOUBLE_ROUNDING () ends a sequence of instructions with
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| 61 | 'long double' safe operation precision
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| 62 | */
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| 63 |
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| 64 | /* Inline assembler like this works only with GNU C and clang. */
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| 65 | #if (defined __i386__ || defined __x86_64__) && (defined __GNUC__ || defined __clang__)
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| 66 |
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| 67 | typedef unsigned short fpucw_t; /* glibc calls this fpu_control_t */
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| 68 |
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| 69 | # define FPU_PC_MASK 0x0300
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| 70 | # define FPU_PC_DOUBLE 0x200 /* glibc calls this _FPU_DOUBLE */
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| 71 | # define FPU_PC_EXTENDED 0x300 /* glibc calls this _FPU_EXTENDED */
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| 72 |
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| 73 | # define GET_FPUCW() __extension__ \
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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; \
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| 77 | })
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| 78 | # define SET_FPUCW(word) __extension__ \
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| 79 | (void)({ fpucw_t _ncw = (word); \
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| 80 | __asm__ __volatile__ ("fldcw %0" : : "m" (*&_ncw)); \
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| 81 | })
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| 82 |
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| 83 | # define DECL_LONG_DOUBLE_ROUNDING \
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| 84 | fpucw_t oldcw;
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| 85 | # define BEGIN_LONG_DOUBLE_ROUNDING() \
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| 86 | (void)(oldcw = GET_FPUCW (), \
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| 87 | SET_FPUCW ((oldcw & ~FPU_PC_MASK) | FPU_PC_EXTENDED))
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| 88 | # define END_LONG_DOUBLE_ROUNDING() \
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| 89 | SET_FPUCW (oldcw)
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| 90 |
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| 91 | #else
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| 92 |
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| 93 | typedef unsigned int fpucw_t;
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| 94 |
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| 95 | # define FPU_PC_MASK 0
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| 96 | # define FPU_PC_DOUBLE 0
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| 97 | # define FPU_PC_EXTENDED 0
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| 98 |
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| 99 | # define GET_FPUCW() 0
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| 100 | # define SET_FPUCW(word) (void)(word)
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| 101 |
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| 102 | # define DECL_LONG_DOUBLE_ROUNDING
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| 103 | # define BEGIN_LONG_DOUBLE_ROUNDING()
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| 104 | # define END_LONG_DOUBLE_ROUNDING()
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| 105 |
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| 106 | #endif
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| 107 |
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| 108 | #endif /* _FPUCW_H */
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