1  /* Manipulating the FPU control word.  -*- coding: utf-8 -*-
       2     Copyright (C) 2007-2023 Free Software Foundation, Inc.
       3     Written by Bruno Haible <bruno@clisp.org>, 2007.
       4  
       5     This file is free software: you can redistribute it and/or modify
       6     it under the terms of the GNU Lesser General Public License as
       7     published by the Free Software Foundation; either version 2.1 of the
       8     License, or (at your option) any later version.
       9  
      10     This file 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 Lesser General Public License for more details.
      14  
      15     You should have received a copy of the GNU Lesser 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 and clang.  */
      65  #if (defined __i386__ || defined __x86_64__) && (defined __GNUC__ || defined __clang__)
      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 */