--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2011-10-10  Bruno Haible  <bruno@clisp.org>
+
+	acosl: Simplify for platforms where 'long double' == 'double'.
+	* lib/acosl.c (acosl) [HAVE_SAME_LONG_DOUBLE_AS_DOUBLE]: New
+	alternative implementation.
+	* m4/acosl.m4 (gl_FUNC_ACOSL): Require gl_LONG_DOUBLE_VS_DOUBLE.
+	Determine ACOSL_LIBM according to HAVE_SAME_LONG_DOUBLE_AS_DOUBLE.
+	* modules/acosl (Depends-on): Add acos. Update conditions.
+
 2011-10-10  Bruno Haible  <bruno@clisp.org>
 
 	asinl: Simplify for platforms where 'long double' == 'double'.

--- a/lib/acosl.c
+++ b/lib/acosl.c
@@ -14,6 +14,16 @@
 /* Specification.  */
 #include <math.h>
 
+#if HAVE_SAME_LONG_DOUBLE_AS_DOUBLE
+
+long double
+acosl (long double x)
+{
+  return acos (x);
+}
+
+#else
+
 /*
   Long double expansions contributed by
   Stephen L. Moshier <moshier@na-net.ornl.gov>
@@ -186,6 +196,8 @@
     return 2 * asinl (sqrtl ((1 - x) / 2));
 }
 
+#endif
+
 #if 0
 int
 main (void)

--- a/m4/acosl.m4
+++ b/m4/acosl.m4
@@ -1,4 +1,4 @@
-# acosl.m4 serial 5
+# acosl.m4 serial 6
 dnl Copyright (C) 2010-2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -7,6 +7,8 @@
 AC_DEFUN([gl_FUNC_ACOSL],
 [
   AC_REQUIRE([gl_MATH_H_DEFAULTS])
+  AC_REQUIRE([gl_LONG_DOUBLE_VS_DOUBLE])
+
   dnl Persuade glibc <math.h> to declare acosl().
   AC_REQUIRE([gl_USE_SYSTEM_EXTENSIONS])
 
@@ -56,18 +58,23 @@
     HAVE_DECL_ACOSL=0
     HAVE_ACOSL=0
     dnl Find libraries needed to link lib/acosl.c.
-    AC_REQUIRE([gl_FUNC_ASINL])
-    AC_REQUIRE([gl_FUNC_SQRTL])
-    dnl Append $ASINL_LIBM to ACOSL_LIBM, avoiding gratuitous duplicates.
-    case " $ACOSL_LIBM " in
-      *" $ASINL_LIBM "*) ;;
-      *) ACOSL_LIBM="$ACOSL_LIBM $ASINL_LIBM" ;;
-    esac
-    dnl Append $SQRTL_LIBM to ACOSL_LIBM, avoiding gratuitous duplicates.
-    case " $ACOSL_LIBM " in
-      *" $SQRTL_LIBM "*) ;;
-      *) ACOSL_LIBM="$ACOSL_LIBM $SQRTL_LIBM" ;;
-    esac
+    if test $HAVE_SAME_LONG_DOUBLE_AS_DOUBLE = 1; then
+      AC_REQUIRE([gl_FUNC_ACOS])
+      ACOSL_LIBM="$ACOS_LIBM"
+    else
+      AC_REQUIRE([gl_FUNC_ASINL])
+      AC_REQUIRE([gl_FUNC_SQRTL])
+      dnl Append $ASINL_LIBM to ACOSL_LIBM, avoiding gratuitous duplicates.
+      case " $ACOSL_LIBM " in
+        *" $ASINL_LIBM "*) ;;
+        *) ACOSL_LIBM="$ACOSL_LIBM $ASINL_LIBM" ;;
+      esac
+      dnl Append $SQRTL_LIBM to ACOSL_LIBM, avoiding gratuitous duplicates.
+      case " $ACOSL_LIBM " in
+        *" $SQRTL_LIBM "*) ;;
+        *) ACOSL_LIBM="$ACOSL_LIBM $SQRTL_LIBM" ;;
+      esac
+    fi
   fi
   AC_SUBST([ACOSL_LIBM])
 ])

--- a/modules/acosl
+++ b/modules/acosl
@@ -8,8 +8,9 @@
 Depends-on:
 math
 extensions
-asinl           [test $HAVE_ACOSL = 0]
-sqrtl           [test $HAVE_ACOSL = 0]
+acos            [test $HAVE_ACOSL = 0 && test $HAVE_SAME_LONG_DOUBLE_AS_DOUBLE = 1]
+asinl           [test $HAVE_ACOSL = 0 && test $HAVE_SAME_LONG_DOUBLE_AS_DOUBLE = 0]
+sqrtl           [test $HAVE_ACOSL = 0 && test $HAVE_SAME_LONG_DOUBLE_AS_DOUBLE = 0]
 
 configure.ac:
 gl_FUNC_ACOSL