diff --git a/config/powerpc-ibm-aix5.x b/config/powerpc-ibm-aix5.x
index 8b8ae02e40..18be845bd3 100644
--- a/config/powerpc-ibm-aix5.x
+++ b/config/powerpc-ibm-aix5.x
@@ -4,32 +4,51 @@
 # after configure starts and defines, among other things, flags for
 # the various compile modes.
 
+# Use AIX supplied C compiler by default, xlc for serial, mpcc_r for parallel.
+# Use -D_LARGE_FILES by default to support large file size.
+if test "X-" =  "X-$CC"; then
+  if test "X-$enable_parallel" = "X-yes"; then
+    CC='mpcc_r -D_LARGE_FILES'
+    CC_BASENAME=mpcc_r
+  else
+    CC='xlc -D_LARGE_FILES'
+    CC_BASENAME=xlc
+  fi
+fi
 
 #----------------------------------------------------------------------------
 # Compiler flags. The CPPFLAGS values should not include package debug
 # flags like `-DH5G_DEBUG' since these are added with the
 # `--enable-debug' switch of configure.
 
-# What must *always* be present for things to compile correctly?
-#CFLAGS="$CFLAGS -ansi"
-#CPPFLAGS="$CPPFLAGS -I."
+case $CC_BASENAME in
+    xlc|mpcc_r)
+	# Turn off shared lib option.  It causes some test suite to fail.
+	enable_shared="${enable_shared:-no}"
+	# CFLAGS must be set else configure set it to -g
+	CFLAGS="$CFLAGS"
+	DEBUG_CFLAGS="-g"
+	DEBUG_CPPFLAGS=
+	PROD_CFLAGS="-O"
+	PROD_CPPFLAGS=
+	PROFILE_CFLAGS="-pg"
+	PROFILE_CPPFLAGS=
+	;;
 
-# CFLAGS must be set else configure set it to -g
-CFLAGS="$CFLAGS"
-
-# What compiler flags should be used for code development?
-DEBUG_CFLAGS=
-DEBUG_CPPFLAGS=
-
-# What compiler flags should be used for building a production
-# library?
-PROD_CFLAGS=
-PROD_CPPFLAGS=
-
-# What compiler flags enable code profiling?
-PROFILE_CFLAGS=
-PROFILE_CPPFLAGS=
+    gcc)
+	. $srcdir/config/gnu-flags
+	;;
 
+    *)
+	CFLAGS="$CFLAGS -ansi"
+	DEBUG_CFLAGS="-g"
+	DEBUG_CPPFLAGS=
+	PROD_CFLAGS="-O"
+	PROD_CPPFLAGS=
+	PROFILE_CFLAGS="-pg"
+	PROFILE_CPPFLAGS=
+	;;
+esac
 
 #----------------------------------------------------------------------------
 # Values for overriding configuration tests when cross compiling.
@@ -48,7 +67,6 @@ RUNPARALLEL=${RUNPARALLEL="MP_PROCS=3 MP_TASKS_PER_NODE=3 poe"}
 ac_cv_sizeof_char=${ac_cv_sizeof_char=1}
 ac_cv_sizeof_short=${ac_cv_sizeof_short=2}
 ac_cv_sizeof_int=${ac_cv_sizeof_int=4}
-ac_cv_sizeof_long=${ac_cv_sizeof_long=4}
 ac_cv_sizeof_long_long=${ac_cv_sizeof_long_long=8}
 ac_cv_sizeof___int64=${ac_cv_sizeof___int64=8}
 ac_cv_sizeof_float=${ac_cv_sizeof_float=4}
@@ -79,6 +97,9 @@ ac_cv_sizeof_uint_least64_t=${ac_cv_sizeof_uint_least64_t=8}
 ac_cv_sizeof_int_fast64_t=${ac_cv_sizeof_int_fast64_t=8}
 ac_cv_sizeof_uint_fast64_t=${ac_cv_sizeof_uint_fast64_t=8}
 
+# Don't cache long since it varies between 32 and 64 bits
+#ac_cv_sizeof_long=${ac_cv_sizeof_long=4}
+
 # Don't cache size_t and off_t because they depend on if -D_LARGE_FILES is used
 #ac_cv_sizeof_size_t=${ac_cv_sizeof_size_t=4}
 #ac_cv_sizeof_off_t=${ac_cv_sizeof_off_t=8}