[New-bugs-announce] [issue34585] Don't use AC_RUN_IFELSE to determine float endian

[Ross Burton]

New submission from Ross Burton <ross at burtonini.com>:

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.

----------
components: Build
files: 0001-Don-t-do-runtime-test-to-get-float-byte-order.patch
keywords: patch
messages: 324627
nosy: rossburton
priority: normal
severity: normal
status: open
title: Don't use AC_RUN_IFELSE to determine float endian
versions: Python 3.8
Added file: https://bugs.python.org/file47788/0001-Don-t-do-runtime-test-to-get-float-byte-order.patch

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<https://bugs.python.org/issue34585>
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