fpucw setting: Test and Question

[Andreas Jaeger <aj at suse dot de>]

I've written a small program to test if the fpu control word is set
correctly.

On i686 the program outputs:

$ cat math/test-fpucw.out 
control word is 0x37f but should be 0x137f.

What's the right value?  The kernel uses 0x37f but glibc also sets
0x1000 which seems to be a reserved bit.  Why do we set this?

What do you think of the appended patches?

Andreas

2000-07-04  Andreas Jaeger  <aj@suse.de>

	* sysdeps/i386/fpu_control.h (_FPU_DEFAULT): Correct value.
	(_FPU_IEEE): Likewise.

	* math/Makefile (tests): Add test-fpucw.

	* math/test-fpucw.c (main): New file.

============================================================
Index: math/Makefile
--- math/Makefile	2000/05/25 06:26:11	1.96
+++ math/Makefile	2000/07/04 08:07:58
@@ -79,7 +79,7 @@
 
 # Rules for the test suite.
 tests = test-matherr test-fenv atest-exp atest-sincos atest-exp2 basic-test \
-	test-misc
+	test-misc test-fpucw
 # We do the `long double' tests only if this data type is available and
 # distinct from `double'.
 test-longdouble-yes = test-ldouble test-ildoubl
============================================================
Index: sysdeps/i386/fpu_control.h
--- sysdeps/i386/fpu_control.h	1998/11/26 12:00:07	1.7
+++ sysdeps/i386/fpu_control.h	2000/07/04 08:07:58
@@ -1,5 +1,5 @@
 /* FPU control word bits.  i387 version.
-   Copyright (C) 1993, 1995, 1996, 1997, 1998 Free Software Foundation, Inc.
+   Copyright (C) 1993, 1995, 1996, 1997, 1998, 2000 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by Olaf Flebbe.
 
@@ -50,7 +50,7 @@
  * IC: Infinity control
  * That is for 8087 and 80287 only.
  *
- * The hardware default is 0x037f. I choose 0x1372.
+ * The hardware default is 0x037f which we use.
  */
 
 #include <features.h>
@@ -80,10 +80,10 @@
 /* The fdlibm code requires strict IEEE double precision arithmetic,
    and no interrupts for exceptions, rounding to nearest.  */
 
-#define _FPU_DEFAULT  0x137f
+#define _FPU_DEFAULT  0x037f
 
 /* IEEE:  same as above.  */
-#define _FPU_IEEE     0x137f
+#define _FPU_IEEE     0x037f
 
 /* Type of the control word.  */
 typedef unsigned int fpu_control_t __attribute__ ((__mode__ (__HI__)));
============================================================
Index: math/test-fpucw.c
--- math/test-fpucw.c	created
+++ math/test-fpucw.c	Tue Jul  4 09:59:49 2000	1.1
@@ -0,0 +1,40 @@
+/* Copyright (C) 2000 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Andreas Jaeger <aj@suse.de>, 2000.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Library General Public License as
+   published by the Free Software Foundation; either version 2 of the
+   License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public
+   License along with the GNU C Library; see the file COPYING.LIB.  If not,
+   write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+   Boston, MA 02111-1307, USA.  */
+
+#include <fpu_control.h>
+#include <stdlib.h>
+
+int
+main (void)
+{
+#ifdef _FPU_GETCW
+/* Some architectures don't have _FPU_GETCW (e.g. Linux/Alpha).  */
+  fpu_control_t cw;
+
+  _FPU_GETCW (cw);
+
+  if (cw != _FPU_DEFAULT)
+    printf ("control word is 0x%x but should be 0x%x.\n", cw, _FPU_DEFAULT);
+
+  return (cw != _FPU_DEFAULT);
+  
+#else
+  return 0;
+#endif
+}

-- 
 Andreas Jaeger
  SuSE Labs aj@suse.de
   private aj@arthur.inka.de