# HG changeset patch
# User Bruno Haible <bruno@clisp.org>
# Date 1194790464 -3600
# Node ID 756069cfb5f5252b0985741d0503b63366da619b
# Parent  6eda531c067c0660073aeee157af3f832f878161
Detect round() bug on NetBSD 3.0/x86.

diff --git a/ChangeLog b/ChangeLog
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2007-11-11  Bruno Haible  <bruno@clisp.org>
+
+	* m4/round.m4 (gl_FUNC_ROUND): Test against NetBSD 3.0 bug.
+	* doc/functions/round.texi: Mention the NetBSD 3.0 bug.
+
 2007-11-11  Bruno Haible  <bruno@clisp.org>
 
 	* tests/test-argmatch.c (ARGMATCH_DIE): Undefine.
diff --git a/doc/functions/round.texi b/doc/functions/round.texi
--- a/doc/functions/round.texi
+++ b/doc/functions/round.texi
@@ -11,6 +11,9 @@
 @item
 This function is missing on some platforms:
 FreeBSD 5.2.1, OpenBSD 3.8, AIX 5.1, IRIX 6.5, OSF/1 4.0, Solaris 9, Interix 3.5.
+@item
+This functions returns a wrong result for x = 1/2 - 2^-54 on some platforms:
+NetBSD 3.0.
 @end itemize
 
 Portability problems not fixed by Gnulib:
diff --git a/m4/round.m4 b/m4/round.m4
--- a/m4/round.m4
+++ b/m4/round.m4
@@ -1,4 +1,4 @@
-# round.m4 serial 4
+# round.m4 serial 5
 dnl Copyright (C) 2007 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -13,6 +13,44 @@
   if test "$ac_cv_have_decl_round" = yes; then
     gl_CHECK_MATH_LIB([ROUND_LIBM], [x = round (x);])
   fi
+  if test "$ac_cv_have_decl_round" = yes && test "$ROUND_LIBM" != missing; then
+    dnl Test whether round() produces correct results. On NetBSD 3.0, for
+    dnl x = 1/2 - 2^-54, the system's round() returns a wrong result.
+    AC_REQUIRE([AC_PROG_CC])
+    AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
+    AC_CACHE_CHECK([whether round works], [gl_cv_func_round_works],
+      [
+        save_LIBS="$LIBS"
+        LIBS="$LIBS $ROUND_LIBM"
+        AC_TRY_RUN([
+#include <float.h>
+#include <math.h>
+int main()
+{
+  /* 2^DBL_MANT_DIG.  */
+  static const double TWO_MANT_DIG =
+    /* Assume DBL_MANT_DIG <= 5 * 31.
+       Use the identity
+       n = floor(n/5) + floor((n+1)/5) + ... + floor((n+4)/5).  */
+    (double) (1U << (DBL_MANT_DIG / 5))
+    * (double) (1U << ((DBL_MANT_DIG + 1) / 5))
+    * (double) (1U << ((DBL_MANT_DIG + 2) / 5))
+    * (double) (1U << ((DBL_MANT_DIG + 3) / 5))
+    * (double) (1U << ((DBL_MANT_DIG + 4) / 5));
+  volatile double x = 0.5 - 0.5 / TWO_MANT_DIG;
+  exit (x < 0.5 && round (x) != 0.0);
+}], [gl_cv_func_round_works=yes], [gl_cv_func_round_works=no],
+        [case "$host_os" in
+           netbsd*) gl_cv_func_round_works="guessing no";;
+           *)       gl_cv_func_round_works="guessing yes";;
+         esac
+        ])
+        LIBS="$save_LIBS"
+      ])
+    case "$gl_cv_func_round_works" in
+      *no) ROUND_LIBM=missing ;;
+    esac
+  fi
   if test "$ac_cv_have_decl_round" != yes || test "$ROUND_LIBM" = missing; then
     REPLACE_ROUND=1
     AC_LIBOBJ([round])
@@ -20,4 +58,5 @@
     gl_FUNC_CEIL_LIBS
     ROUND_LIBM="$FLOOR_LIBM $CEIL_LIBM"
   fi
-  AC_SUBST([ROUND_LIBM])])
+  AC_SUBST([ROUND_LIBM])
+])