Currently configure.ac uses AC_RUN_IFELSE to determine the byte order of floats and doubles.  This hurts when cross-compiling because a default is set, resulting in Python silently falling back to sub-optimal codepaths.

A partial improvement would be to not set a fallback, to force the user to set the right byte order explicitly.

However this test can be done without running anything.  autoconf-archive has a macro that uses a carefully constructed double that encodes to an ASCII string in the binary, which is then examined using grep.  Evil genius.

Attached is a POC using this.  Currently the autoconf-archive macro only handles big and little endian not the ARM OABI mixed-endian format, so configure.ac assumes if the byte order is unknown then it's the crazy mixed-endian.

To be honest OABI is so old now, I don't believe anyone actually uses it anymore: everyone still on ARMv4 should have moved to EABI many years ago, and the mixed-endian support could be removed from Py3 in the future.

New changeset 2a9c3805ddedf282881ef7811a561c70b74f80b1 by Benjamin Peterson (Ross Burton) in branch 'master':
closes bpo-34585: Don't do runtime test to get float byte order. (GH-9085)
https://github.com/python/cpython/commit/2a9c3805ddedf282881ef7811a561c70b74f80b1

New changeset b3b8cb419e496629873fa7dda82a01863f58617a by Benjamin Peterson in branch 'master':
run autoconf (GH-9411)
https://github.com/python/cpython/commit/b3b8cb419e496629873fa7dda82a01863f58617a