AIX/longdouble: switch the default to allow it.

Since the only really unfixable part is the inf/nan support.

Document the troublesomeness in README.aix.

2 files changed, 19 insertions, 3 deletions

diff --git a/README.aix b/README.aix
index 9a49067ff6..bdacd9dfff 100644
--- a/README.aix
+++ b/README.aix
@@ -157,6 +157,22 @@ With the default setting the size is limited to 128MB.
 The -1 removes this limit. If the "make test" fails please change
 your /etc/security/limits as stated above.
 
+=head Long doubles
+
+IBM calls its implementation of long doubles 128-bit, but it is not
+the IEEE 128-bit ("quadruple precision") which would give 116 bit of
+mantissa (nor it is implemented in hardware), instead it's a special
+software implementation called "double-double", which gives 106 bits
+of mantissa.
+
+There seem to be various problems in this long double implementation.
+If Configure detects this brokenness, it will disable the long double support.
+This can be overriden with explicit C<-Duselongdouble> (or C<-Dusemorebits>,
+which enables both long doubles and 64 bit integers).  If you decide to
+enable long doubles, for most of the broken things Perl has implemented
+workarounds, but the handling of the special values infinity and NaN
+remains badly broken: for example infinity plus zero results in NaN.
+
 =head2 Recommended Options AIX 5.1/5.2/5.3/6.1 and 7.1 (threaded/32-bit)
 
 With the following options you get a threaded Perl version which
diff --git a/hints/aix.sh b/hints/aix.sh
index 956e80666b..fce831fd12 100644
--- a/hints/aix.sh
+++ b/hints/aix.sh
@@ -584,7 +584,7 @@ fi
 # The missing math functions affect the POSIX extension math interfaces.
 
 case "$uselongdouble" in
-define)
+'')
   echo "Checking if your infinity is working with long doubles..." >&4
   cat > inf$$.c <<EOF
 #include <math.h>
@@ -598,7 +598,7 @@ EOF
   $cc -qlongdouble -o inf$$ inf$$.c -lm
   case `./inf$$` in
   INF) echo "Your infinity is working correctly with long doubles." >&4 ;;
-  *) # NaNQ
+  *) # NaNQ (or anything else than INF)
     echo " "
     echo "Your infinity is broken, I suggest disabling long doubles." >&4
     rp="Disable long doubles?"
@@ -607,7 +607,7 @@ EOF
     case "$ans" in
     [Yy]*)
       echo "Okay, disabling long doubles." >&4
-      uselongdouble=undef
+      uselongdouble="$undef"
       ccflags=`echo " $ccflags " | sed -e 's/ -qlongdouble / /'`
       libswanted=`echo " $libswanted " | sed -e 's/ c128/ /'`
       lddlflags=`echo " $lddlflags " | sed -e 's/ -lc128 / /'`