109 lines
4.7 KiB
C
109 lines
4.7 KiB
C
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/* Manipulating the FPU control word. -*- coding: utf-8 -*-
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Copyright (C) 2007-2023 Free Software Foundation, Inc.
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Written by Bruno Haible <bruno@clisp.org>, 2007.
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This file is free software: you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as
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published by the Free Software Foundation; either version 2.1 of the
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License, or (at your option) any later version.
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This file is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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#ifndef _FPUCW_H
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#define _FPUCW_H
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/* The i386 floating point hardware (the 387 compatible FPU, not the modern
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SSE/SSE2 hardware) has a controllable rounding precision. It is specified
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through the 'PC' bits in the FPU control word ('fctrl' register). (See
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the GNU libc i386 <fpu_control.h> header for details.)
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On some platforms, such as Linux or Solaris, the default precision setting
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is set to "extended precision". This means that 'long double' instructions
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operate correctly, but 'double' computations often produce slightly
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different results as on strictly IEEE 754 conforming systems.
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On some platforms, such as NetBSD, the default precision is set to
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"double precision". This means that 'long double' instructions will operate
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only as 'double', i.e. lead to wrong results. Similarly on FreeBSD 6.4, at
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least for the division of 'long double' numbers.
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The FPU control word is under control of the application, i.e. it is
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not required to be set either way by the ABI. (In fact, the i386 ABI
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https://www.linux-mips.org/pub/linux/mips/doc/ABI/abi386-4.pdf page 3-12 = page 38
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is not clear about it. But in any case, gcc treats the control word
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like a "preserved" register: it emits code that assumes that the control
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word is preserved across calls, and it restores the control word at the
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end of functions that modify it.)
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See Vincent Lefèvre's page https://www.vinc17.net/research/extended.en.html
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for a good explanation.
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See https://web.archive.org/web/20060905133417/http://www.uwsg.iu.edu/hypermail/linux/kernel/0103.0/0453.html
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some argumentation which setting should be the default. */
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/* This header file provides the following facilities:
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fpucw_t integral type holding the value of 'fctrl'
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FPU_PC_MASK bit mask denoting the precision control
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FPU_PC_DOUBLE precision control for 53 bits mantissa
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FPU_PC_EXTENDED precision control for 64 bits mantissa
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GET_FPUCW () yields the current FPU control word
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SET_FPUCW (word) sets the FPU control word
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DECL_LONG_DOUBLE_ROUNDING variable declaration for
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BEGIN/END_LONG_DOUBLE_ROUNDING
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BEGIN_LONG_DOUBLE_ROUNDING () starts a sequence of instructions with
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'long double' safe operation precision
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END_LONG_DOUBLE_ROUNDING () ends a sequence of instructions with
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'long double' safe operation precision
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*/
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/* Inline assembler like this works only with GNU C and clang. */
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#if (defined __i386__ || defined __x86_64__) && (defined __GNUC__ || defined __clang__)
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typedef unsigned short fpucw_t; /* glibc calls this fpu_control_t */
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# define FPU_PC_MASK 0x0300
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# define FPU_PC_DOUBLE 0x200 /* glibc calls this _FPU_DOUBLE */
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# define FPU_PC_EXTENDED 0x300 /* glibc calls this _FPU_EXTENDED */
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# define GET_FPUCW() __extension__ \
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({ fpucw_t _cw; \
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__asm__ __volatile__ ("fnstcw %0" : "=m" (*&_cw)); \
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_cw; \
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})
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# define SET_FPUCW(word) __extension__ \
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(void)({ fpucw_t _ncw = (word); \
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__asm__ __volatile__ ("fldcw %0" : : "m" (*&_ncw)); \
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})
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# define DECL_LONG_DOUBLE_ROUNDING \
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fpucw_t oldcw;
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# define BEGIN_LONG_DOUBLE_ROUNDING() \
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(void)(oldcw = GET_FPUCW (), \
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SET_FPUCW ((oldcw & ~FPU_PC_MASK) | FPU_PC_EXTENDED))
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# define END_LONG_DOUBLE_ROUNDING() \
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SET_FPUCW (oldcw)
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#else
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typedef unsigned int fpucw_t;
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# define FPU_PC_MASK 0
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# define FPU_PC_DOUBLE 0
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# define FPU_PC_EXTENDED 0
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# define GET_FPUCW() 0
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# define SET_FPUCW(word) (void)(word)
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# define DECL_LONG_DOUBLE_ROUNDING
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# define BEGIN_LONG_DOUBLE_ROUNDING()
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# define END_LONG_DOUBLE_ROUNDING()
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#endif
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#endif /* _FPUCW_H */
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