Imported Upstream version 1.3.14

[debian/gzip] / lib / fpucw.h

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+/* Manipulating the FPU control word.
+   Copyright (C) 2007-2008 Free Software Foundation, Inc.
+   Written by Bruno Haible <bruno@clisp.org>, 2007.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://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 wrong results.
+
+   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
+   http://refspecs.freestandards.org/elf/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 http://www.vinc17.org/research/extended.en.html
+   for a good explanation.
+   See http://www.uwsg.iu.edu/hypermail/linux/kernel/0103.0/0453.html for
+   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.  */
+#if (defined __i386__ || defined __x86_64__) && defined __GNUC__
+
+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() \
+  ({ fpucw_t _cw;                                              \
+     __asm__ __volatile__ ("fnstcw %0" : "=m" (*&_cw));                \
+     _cw;                                                      \
+   })
+# define SET_FPUCW(word) \
+  (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 */