[RFC] bpo-34585: Don't do runtime test to get float byte order

Misc/NEWS.d/next/Build/2018-09-18-16-28-31.bpo-34585.CGMu0h.rst

@@ -0,0 +1,3 @@
+Check for floating-point byte order in configure.ac using compilation tests
+instead of executing code, so that these checks work in cross-compiled
+builds.

configure.ac

@@ -4206,74 +4206,24 @@ fi
 # * Check for various properties of floating point *
 # **************************************************
 
-AC_MSG_CHECKING(whether C doubles are little-endian IEEE 754 binary64)
-AC_CACHE_VAL(ac_cv_little_endian_double, [
-AC_RUN_IFELSE([AC_LANG_SOURCE([[
-#include <string.h>
-int main() {
-    double x = 9006104071832581.0;
-    if (memcmp(&x, "\x05\x04\x03\x02\x01\xff\x3f\x43", 8) == 0)
-        return 0;
-    else
-        return 1;
-}
-]])],
-[ac_cv_little_endian_double=yes],
-[ac_cv_little_endian_double=no],
-[ac_cv_little_endian_double=no])])
-AC_MSG_RESULT($ac_cv_little_endian_double)
-if test "$ac_cv_little_endian_double" = yes
-then
-  AC_DEFINE(DOUBLE_IS_LITTLE_ENDIAN_IEEE754, 1,
-  [Define if C doubles are 64-bit IEEE 754 binary format, stored
-   with the least significant byte first])
-fi
-
-AC_MSG_CHECKING(whether C doubles are big-endian IEEE 754 binary64)
-AC_CACHE_VAL(ac_cv_big_endian_double, [
-AC_RUN_IFELSE([AC_LANG_SOURCE([[
-#include <string.h>
-int main() {
-    double x = 9006104071832581.0;
-    if (memcmp(&x, "\x43\x3f\xff\x01\x02\x03\x04\x05", 8) == 0)
-        return 0;
-    else
-        return 1;
-}
-]])],
-[ac_cv_big_endian_double=yes],
-[ac_cv_big_endian_double=no],
-[ac_cv_big_endian_double=no])])
-AC_MSG_RESULT($ac_cv_big_endian_double)
-if test "$ac_cv_big_endian_double" = yes
+AX_C_FLOAT_WORDS_BIGENDIAN
+if test "$ax_cv_c_float_words_bigendian" = "yes"
 then
   AC_DEFINE(DOUBLE_IS_BIG_ENDIAN_IEEE754, 1,
   [Define if C doubles are 64-bit IEEE 754 binary format, stored
    with the most significant byte first])
-fi
-
-# Some ARM platforms use a mixed-endian representation for doubles.
-# While Python doesn't currently have full support for these platforms
-# (see e.g., issue 1762561), we can at least make sure that float <-> string
-# conversions work.
-AC_MSG_CHECKING(whether C doubles are ARM mixed-endian IEEE 754 binary64)
-AC_CACHE_VAL(ac_cv_mixed_endian_double, [
-AC_RUN_IFELSE([AC_LANG_SOURCE([[
-#include <string.h>
-int main() {
-    double x = 9006104071832581.0;
-    if (memcmp(&x, "\x01\xff\x3f\x43\x05\x04\x03\x02", 8) == 0)
-        return 0;
-    else
-        return 1;
-}
-]])],
-[ac_cv_mixed_endian_double=yes],
-[ac_cv_mixed_endian_double=no],
-[ac_cv_mixed_endian_double=no])])
-AC_MSG_RESULT($ac_cv_mixed_endian_double)
-if test "$ac_cv_mixed_endian_double" = yes
+elif test "$ax_cv_c_float_words_bigendian" = "no"
 then
+  AC_DEFINE(DOUBLE_IS_LITTLE_ENDIAN_IEEE754, 1,
+  [Define if C doubles are 64-bit IEEE 754 binary format, stored
+   with the least significant byte first])
+else
+  # Some ARM platforms use a mixed-endian representation for doubles.
+  # While Python doesn't currently have full support for these platforms
+  # (see e.g., issue 1762561), we can at least make sure that float <-> string
+  # conversions work.
+  # FLOAT_WORDS_BIGENDIAN doesnt actually detect this case, but if it's not big
+  # or little, then it must be this?
   AC_DEFINE(DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754, 1,
   [Define if C doubles are 64-bit IEEE 754 binary format, stored
    in ARM mixed-endian order (byte order 45670123)])

m4/ax_c_float_words_bigendian.m4

@@ -0,0 +1,83 @@
+# ===============================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_c_float_words_bigendian.html
+# ===============================================================================
+#
+# SYNOPSIS
+#
+#   AX_C_FLOAT_WORDS_BIGENDIAN([ACTION-IF-TRUE], [ACTION-IF-FALSE], [ACTION-IF-UNKNOWN])
+#
+# DESCRIPTION
+#
+#   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
+#   implementation was inspired by the AC_C_BIGENDIAN macro in autoconf.
+#
+#   The endianness 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.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Daniel Amelang <[email protected]>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 11
+
+AC_DEFUN([AX_C_FLOAT_WORDS_BIGENDIAN],
+  [AC_CACHE_CHECK(whether float word ordering is bigendian,
+                  ax_cv_c_float_words_bigendian, [
+
+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