--- a/ChangeLog	Sun Jan 20 22:28:37 2019 +0100
+++ b/ChangeLog	Sun Jan 20 22:57:41 2019 +0100
@@ -1,3 +1,10 @@
+2019-01-20  Bruno Haible  <bruno@clisp.org>
+
+	logl: Work around inaccurate implementation on NetBSD.
+	* m4/logl.m4 (gl_FUNC_LOGL_WORKS): Add test for a certain accuracy.
+	* lib/logl.c: Comment out unused code.
+	* doc/posix-functions/logl.texi: Mention the NetBSD bug.
+
 2019-01-20  Bruno Haible  <bruno@clisp.org>
 
 	expm1l: Work around inaccurate implementation on NetBSD.

--- a/doc/posix-functions/logl.texi	Sun Jan 20 22:28:37 2019 +0100
+++ b/doc/posix-functions/logl.texi	Sun Jan 20 22:57:41 2019 +0100
@@ -23,6 +23,10 @@
 @item
 This function returns a wrong value for a minus zero argument on some platforms:
 OSF/1 5.1.
+@item
+This function produces results which are accurate to only 16 digits on some
+platforms:
+NetBSD 8.0.
 @end itemize
 
 Portability problems not fixed by Gnulib:

--- a/lib/logl.c	Sun Jan 20 22:28:37 2019 +0100
+++ b/lib/logl.c	Sun Jan 20 22:57:41 2019 +0100
@@ -26,7 +26,7 @@
   return log (x);
 }
 
-#elif HAVE_LOGL
+#elif 0 /* was: HAVE_LOGL */
 
 long double
 logl (long double x)

--- a/m4/logl.m4	Sun Jan 20 22:28:37 2019 +0100
+++ b/m4/logl.m4	Sun Jan 20 22:57:41 2019 +0100
@@ -1,4 +1,4 @@
-# logl.m4 serial 10
+# logl.m4 serial 11
 dnl Copyright (C) 2010-2019 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -103,6 +103,7 @@
 
 dnl Test whether logl() works.
 dnl On OSF/1 5.1, logl(-0.0L) is NaN.
+dnl On NetBSD 8.0, the result is accurate to only 16 digits.
 AC_DEFUN([gl_FUNC_LOGL_WORKS],
 [
   AC_REQUIRE([AC_PROG_CC])
@@ -111,25 +112,86 @@
     [
       AC_RUN_IFELSE(
         [AC_LANG_SOURCE([[
+#ifndef __NO_MATH_INLINES
+# define __NO_MATH_INLINES 1 /* for glibc */
+#endif
+#include <float.h>
 #include <math.h>
-volatile long double x;
-long double y;
-int main ()
+/* Override the values of <float.h>, like done in float.in.h.  */
+#if defined __i386__ && (defined __BEOS__ || defined __OpenBSD__)
+# undef LDBL_MANT_DIG
+# define LDBL_MANT_DIG   64
+# undef LDBL_MIN_EXP
+# define LDBL_MIN_EXP    (-16381)
+# undef LDBL_MAX_EXP
+# define LDBL_MAX_EXP    16384
+#endif
+#if defined __i386__ && (defined __FreeBSD__ || defined __DragonFly__)
+# undef LDBL_MANT_DIG
+# define LDBL_MANT_DIG   64
+# undef LDBL_MIN_EXP
+# define LDBL_MIN_EXP    (-16381)
+# undef LDBL_MAX_EXP
+# define LDBL_MAX_EXP    16384
+#endif
+#if (defined _ARCH_PPC || defined _POWER) && defined _AIX && (LDBL_MANT_DIG == 106) && defined __GNUC__
+# undef LDBL_MIN_EXP
+# define LDBL_MIN_EXP DBL_MIN_EXP
+#endif
+#if defined __sgi && (LDBL_MANT_DIG >= 106)
+# undef LDBL_MANT_DIG
+# define LDBL_MANT_DIG 106
+# if defined __GNUC__
+#  undef LDBL_MIN_EXP
+#  define LDBL_MIN_EXP DBL_MIN_EXP
+# endif
+#endif
+#undef logl
+extern
+#ifdef __cplusplus
+"C"
+#endif
+long double logl (long double);
+static long double dummy (long double x) { return 0; }
+volatile long double gx;
+long double gy;
+int main (int argc, char *argv[])
 {
-  x = -0.0L;
-  y = logl (x);
-  if (!(y + y == y))
-    return 1;
-  return 0;
+  long double (* volatile my_logl) (long double) = argc ? logl : dummy;
+  int result = 0;
+  /* This test fails on OSF/1 5.1.  */
+  {
+    gx = -0.0L;
+    gy = logl (gx);
+    if (!(gy + gy == gy))
+      result |= 1;
+  }
+  /* This test fails on NetBSD 8.0.  */
+  {
+    const long double TWO_LDBL_MANT_DIG = /* 2^LDBL_MANT_DIG */
+      (long double) (1U << ((LDBL_MANT_DIG - 1) / 5))
+      * (long double) (1U << ((LDBL_MANT_DIG - 1 + 1) / 5))
+      * (long double) (1U << ((LDBL_MANT_DIG - 1 + 2) / 5))
+      * (long double) (1U << ((LDBL_MANT_DIG - 1 + 3) / 5))
+      * (long double) (1U << ((LDBL_MANT_DIG - 1 + 4) / 5));
+    long double x = 16.981137113807045L;
+    long double err = (my_logl (x) + my_logl (1.0L / x)) * TWO_LDBL_MANT_DIG;
+    if (!(err >= -100.0L && err <= 100.0L))
+      result |= 2;
+  }
+
+  return result;
 }
 ]])],
         [gl_cv_func_logl_works=yes],
         [gl_cv_func_logl_works=no],
         [case "$host_os" in
-           osf*)   gl_cv_func_logl_works="guessing no" ;;
-                   # Guess yes on native Windows.
-           mingw*) gl_cv_func_logl_works="guessing yes" ;;
-           *)      gl_cv_func_logl_works="guessing yes" ;;
+                          # Guess yes on glibc systems.
+           *-gnu* | gnu*) gl_cv_func_logl_works="guessing yes" ;;
+                          # Guess yes on native Windows.
+           mingw*)        gl_cv_func_logl_works="guessing yes" ;;
+                          # If we don't know, assume the worst.
+           *)             gl_cv_func_logl_works="guessing no" ;;
          esac
         ])
     ])