summaryrefslogtreecommitdiffstats
path: root/lib/fpucw.h
diff options
context:
space:
mode:
Diffstat (limited to 'lib/fpucw.h')
-rw-r--r--lib/fpucw.h108
1 files changed, 108 insertions, 0 deletions
diff --git a/lib/fpucw.h b/lib/fpucw.h
new file mode 100644
index 0000000..7dcb310
--- /dev/null
+++ b/lib/fpucw.h
@@ -0,0 +1,108 @@
+/* Manipulating the FPU control word. -*- coding: utf-8 -*-
+ Copyright (C) 2007-2023 Free Software Foundation, Inc.
+ Written by Bruno Haible <bruno@clisp.org>, 2007.
+
+ This file is free software: you can redistribute it and/or modify
+ it under the terms of the GNU Lesser General Public License as
+ published by the Free Software Foundation; either version 2.1 of the
+ License, or (at your option) any later version.
+
+ This file is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public License
+ along with this program. If not, see <https://www.gnu.org/licenses/>. */
+
+#ifndef _FPUCW_H
+#define _FPUCW_H
+
+/* The i386 floating point hardware (the 387 compatible FPU, not the modern
+ SSE/SSE2 hardware) has a controllable rounding precision. It is specified
+ through the 'PC' bits in the FPU control word ('fctrl' register). (See
+ the GNU libc i386 <fpu_control.h> header for details.)
+
+ On some platforms, such as Linux or Solaris, the default precision setting
+ is set to "extended precision". This means that 'long double' instructions
+ operate correctly, but 'double' computations often produce slightly
+ different results as on strictly IEEE 754 conforming systems.
+
+ On some platforms, such as NetBSD, the default precision is set to
+ "double precision". This means that 'long double' instructions will operate
+ only as 'double', i.e. lead to wrong results. Similarly on FreeBSD 6.4, at
+ least for the division of 'long double' numbers.
+
+ The FPU control word is under control of the application, i.e. it is
+ not required to be set either way by the ABI. (In fact, the i386 ABI
+ https://www.linux-mips.org/pub/linux/mips/doc/ABI/abi386-4.pdf page 3-12 = page 38
+ is not clear about it. But in any case, gcc treats the control word
+ like a "preserved" register: it emits code that assumes that the control
+ word is preserved across calls, and it restores the control word at the
+ end of functions that modify it.)
+
+ See Vincent Lefèvre's page https://www.vinc17.net/research/extended.en.html
+ for a good explanation.
+ See https://web.archive.org/web/20060905133417/http://www.uwsg.iu.edu/hypermail/linux/kernel/0103.0/0453.html
+ some argumentation which setting should be the default. */
+
+/* This header file provides the following facilities:
+ fpucw_t integral type holding the value of 'fctrl'
+ FPU_PC_MASK bit mask denoting the precision control
+ FPU_PC_DOUBLE precision control for 53 bits mantissa
+ FPU_PC_EXTENDED precision control for 64 bits mantissa
+ GET_FPUCW () yields the current FPU control word
+ SET_FPUCW (word) sets the FPU control word
+ DECL_LONG_DOUBLE_ROUNDING variable declaration for
+ BEGIN/END_LONG_DOUBLE_ROUNDING
+ BEGIN_LONG_DOUBLE_ROUNDING () starts a sequence of instructions with
+ 'long double' safe operation precision
+ END_LONG_DOUBLE_ROUNDING () ends a sequence of instructions with
+ 'long double' safe operation precision
+ */
+
+/* Inline assembler like this works only with GNU C and clang. */
+#if (defined __i386__ || defined __x86_64__) && (defined __GNUC__ || defined __clang__)
+
+typedef unsigned short fpucw_t; /* glibc calls this fpu_control_t */
+
+# define FPU_PC_MASK 0x0300
+# define FPU_PC_DOUBLE 0x200 /* glibc calls this _FPU_DOUBLE */
+# define FPU_PC_EXTENDED 0x300 /* glibc calls this _FPU_EXTENDED */
+
+# define GET_FPUCW() __extension__ \
+ ({ fpucw_t _cw; \
+ __asm__ __volatile__ ("fnstcw %0" : "=m" (*&_cw)); \
+ _cw; \
+ })
+# define SET_FPUCW(word) __extension__ \
+ (void)({ fpucw_t _ncw = (word); \
+ __asm__ __volatile__ ("fldcw %0" : : "m" (*&_ncw)); \
+ })
+
+# define DECL_LONG_DOUBLE_ROUNDING \
+ fpucw_t oldcw;
+# define BEGIN_LONG_DOUBLE_ROUNDING() \
+ (void)(oldcw = GET_FPUCW (), \
+ SET_FPUCW ((oldcw & ~FPU_PC_MASK) | FPU_PC_EXTENDED))
+# define END_LONG_DOUBLE_ROUNDING() \
+ SET_FPUCW (oldcw)
+
+#else
+
+typedef unsigned int fpucw_t;
+
+# define FPU_PC_MASK 0
+# define FPU_PC_DOUBLE 0
+# define FPU_PC_EXTENDED 0
+
+# define GET_FPUCW() 0
+# define SET_FPUCW(word) (void)(word)
+
+# define DECL_LONG_DOUBLE_ROUNDING
+# define BEGIN_LONG_DOUBLE_ROUNDING()
+# define END_LONG_DOUBLE_ROUNDING()
+
+#endif
+
+#endif /* _FPUCW_H */