Add autoconf macro AX_C_FLOAT_WORDS_BIGENDIAN

The symbol that this macro defines (FLOAT_WORDS_BIGENDIAN) can be used
to make double arithmetic tricks portable.

1 files changed, 65 insertions, 0 deletions

diff --git a/acinclude.m4 b/acinclude.m4
index af73800b..a0eb13ad 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -51,3 +51,68 @@ ifelse([$1],[],,
   AM_CONDITIONAL(ENABLE_GTK_DOC, test x$enable_gtk_doc = xyes)
   AM_CONDITIONAL(GTK_DOC_USE_LIBTOOL, test -n "$LIBTOOL")
 ])
+
+# AX_C_FLOAT_WORDS_BIGENDIAN ([ACTION-IF-TRUE], [ACTION-IF-FALSE],
+#                             [ACTION-IF-UNKNOWN])
+#
+# Checks the ordering of words within a multi-word float. This check
+# is necessary because on some systems (e.g. certain ARM systems), the
+# float word ordering can be different from the byte ordering. In a
+# multi-word float context, "big-endian" implies that the word containing
+# the sign bit is found in the memory location with the lowest address.
+# This implemenation was inspired by the AC_C_BIGENDIAN macro in autoconf.
+# -------------------------------------------------------------------------
+AC_DEFUN([AX_C_FLOAT_WORDS_BIGENDIAN],
+  [AC_CACHE_CHECK(whether float word ordering is bigendian,
+                  ax_cv_c_float_words_bigendian, [
+
+# The endianess is detected by first compiling C code that contains a special
+# double float value, then grepping the resulting object file for certain
+# strings of ascii values. The double is specially crafted to have a
+# binary representation that corresponds with a simple string. In this
+# implementation, the string "noonsees" was selected because the individual
+# word values ("noon" and "sees") are palindromes, thus making this test
+# byte-order agnostic. If grep finds the string "noonsees" in the object
+# file, the target platform stores float words in big-endian order. If grep
+# finds "seesnoon", float words are in little-endian order. If neither value
+# is found, the user is instructed to specify the ordering.
+
+ax_cv_c_float_words_bigendian=unknown
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+
+double d = 90904234967036810337470478905505011476211692735615632014797120844053488865816695273723469097858056257517020191247487429516932130503560650002327564517570778480236724525140520121371739201496540132640109977779420565776568942592.0;
+
+]])], [
+
+if grep noonsees conftest.$ac_objext >/dev/null ; then
+  ax_cv_c_float_words_bigendian=yes
+fi
+if grep seesnoon conftest.$ac_objext >/dev/null ; then
+  if test "$ax_cv_c_float_words_bigendian" = unknown; then
+    ax_cv_c_float_words_bigendian=no
+  else
+    ax_cv_c_float_words_bigendian=unknown
+  fi
+fi
+
+])])
+
+case $ax_cv_c_float_words_bigendian in
+  yes)
+    m4_default([$1],
+      [AC_DEFINE([FLOAT_WORDS_BIGENDIAN], 1,
+                 [Define to 1 if your system stores words within floats
+                  with the most significant word first])]) ;;
+  no)
+    $2 ;;
+  *)
+    m4_default([$3],
+      [AC_MSG_ERROR([
+
+Unknown float word ordering. You need to manually preset
+ax_cv_c_float_words_bigendian=no (or yes) according to your system.
+
+    ])]) ;;
+esac
+
+])# AX_C_FLOAT_WORDS_BIGENDIAN