[3.12] gh-89640: harden float word ordering (#125571 and #126387)

.github/workflows/build.yml

@@ -97,7 +97,7 @@ jobs:
     # reproducible: to get the same tools versions (autoconf, aclocal, ...)
     runs-on: ubuntu-24.04
     container:
-      image: ghcr.io/python/autoconf:2024.10.11.11293396815
+      image: ghcr.io/python/autoconf:2024.10.16.11360930377
     timeout-minutes: 60
     needs: check_source
     if: needs.check_source.outputs.run_tests == 'true'

Misc/NEWS.d/next/Build/2024-10-16-09-37-51.gh-issue-89640.UDsW-j.rst

@@ -0,0 +1,2 @@
+Improve detection of float word ordering on Linux when link-time optimizations
+are enabled.

Misc/NEWS.d/next/Build/2024-11-04-09-42-04.gh-issue-89640.QBv05o.rst

@@ -0,0 +1 @@
+Hard-code float word ordering as little endian on WASM.

Tools/build/regen-configure.sh

@@ -5,7 +5,7 @@ set -e -x
 # The check_generated_files job of .github/workflows/build.yml must kept in
 # sync with this script. Use the same container image than the job so the job
 # doesn't need to run autoreconf in a container.
-IMAGE="ghcr.io/python/autoconf:2024.10.06.11200919239"
+IMAGE="ghcr.io/python/autoconf:2024.10.16.11360930377"
 AUTORECONF="autoreconf -ivf -Werror"
 
 WORK_DIR="/src"

configure.ac

@@ -5679,28 +5679,26 @@ AS_VAR_IF([ac_cv_gcc_asm_for_x64], [yes], [
 # * Check for various properties of floating point *
 # **************************************************
 
-AX_C_FLOAT_WORDS_BIGENDIAN
-if test "$ax_cv_c_float_words_bigendian" = "yes"
-then
-  AC_DEFINE([DOUBLE_IS_BIG_ENDIAN_IEEE754], [1],
-  [Define if C doubles are 64-bit IEEE 754 binary format, stored
-   with the most significant byte first])
-elif test "$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 doesn't 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)])
-fi
+AX_C_FLOAT_WORDS_BIGENDIAN(
+  [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])],
+  [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])],
+  [AS_CASE([$host_cpu],
+           [*arm*], [# 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 doesn't 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)])],
+           [wasm*], [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])])])
 
 # The short float repr introduced in Python 3.1 requires the
 # correctly-rounded string <-> double conversion functions from

pyconfig.h.in

@@ -47,10 +47,6 @@
 /* Define if --enable-ipv6 is specified */
 #undef ENABLE_IPV6
 
-/* Define to 1 if your system stores words within floats with the most
-   significant word first */
-#undef FLOAT_WORDS_BIGENDIAN
-
 /* Define if getpgrp() must be called as getpgrp(0). */
 #undef GETPGRP_HAVE_ARG