>From 9003e0a643aef003c8858db4936c3f57607d450b Mon Sep 17 00:00:00 2001
From: "A. Gordon" <address@hidden>
Date: Tue, 15 Apr 2014 11:58:11 -0400
Subject: [PATCH] m4: tests for buggy expl() implementations

On some systems (OpenBSD 5.4), expl() incorrectly returns 'nan' for
small values. Compare the result of exp() with expl(), and
use Gnulib's expl if needed.
---
 m4/expl.m4 | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/m4/expl.m4 b/m4/expl.m4
index 247bc6e..1d78534 100644
--- a/m4/expl.m4
+++ b/m4/expl.m4
@@ -87,5 +87,36 @@ AC_DEFUN([gl_FUNC_EXPL],
       esac
     fi
   fi
+  dnl On some systems (OpenBSD5.4) the system's native expl() is buggy:
+  dnl it returns 'nan' for small values. This test compares expl() to exp() .
+  if test $gl_cv_func_expl_no_libm = yes \
+     || test $gl_cv_func_expl_in_libm = yes; then
+    AC_CACHE_CHECK([checks whether expl is buggy],
+        [gl_cv_func_expl_buggy],
+        [
+          save_LIBS="$LIBS"
+          LIBS="$EXPL_LIBM $FABS_LIBM"
+          AC_RUN_IFELSE(
+           [AC_LANG_PROGRAM(
+             [[#include <math.h>
+]],          [[ double l ;
+                int expl_ok = 1 ;
+                 for (l=-3 ; l < 3 ; l+=0.001)
+                   expl_ok &= (fabs(exp(l)-expl(l))<1e-10);
+                 return expl_ok?0:1 ; ]])],
+             [gl_cv_func_func_expl_buggy=no], [gl_cv_func_expl_buggy=yes],
+             [case $host_os in
+                mingw*) gl_cv_func_expl_buggy="guessing no";;
+                *) gl_cv_func_expl_buggy="guessing yes";;
+              esac])
+          LIBS="$save_LIBS"
+        ])
+    if test $gl_cv_func_expl_buggy = yes; then
+      dnl TODO: Use gnulib's expl instead of the system's expl
+      AC_MSG_NOTICE([Detected buggy expl(), using gnulib's implementatin])
+      gl_cv_func_expl_in_libm=no
+      gl_cv_func_expl_no_libm=no
+    fi
+  fi
   AC_SUBST([EXPL_LIBM])
 ])
-- 
1.9.1