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Don't use AC_RUN_IFELSE to determine float endian #78766

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rossburton mannequin opened this issue Sep 5, 2018 · 4 comments
Closed

Don't use AC_RUN_IFELSE to determine float endian #78766

rossburton mannequin opened this issue Sep 5, 2018 · 4 comments
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3.8 only security fixes build The build process and cross-build

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@rossburton
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Mannequin

rossburton mannequin commented Sep 5, 2018

BPO 34585
Nosy @tim-one, @mdickinson, @benjaminp, @rossburton, @yan12125
PRs
  • [RFC] bpo-34585: Don't do runtime test to get float byte order #9085
    		•
    Files
  • 0001-Don-t-do-runtime-test-to-get-float-byte-order.patch
    		•

    Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = None
closed_at = <Date 2018-09-19.06:25:56.225>
created_at = <Date 2018-09-05.10:57:22.362>
labels = ['build', '3.8']
title = "Don't use AC_RUN_IFELSE to determine float endian"
updated_at = <Date 2018-09-19.06:49:08.815>
user = 'https://github.com/rossburton'

    bugs.python.org fields:

    activity = <Date 2018-09-19.06:49:08.815>
actor = 'benjamin.peterson'
assignee = 'none'
closed = True
closed_date = <Date 2018-09-19.06:25:56.225>
closer = 'benjamin.peterson'
components = ['Build']
creation = <Date 2018-09-05.10:57:22.362>
creator = 'rossburton'
dependencies = []
files = ['47788']
hgrepos = []
issue_num = 34585
keywords = ['patch']
message_count = 3.0
messages = ['324627', '325705', '325708']
nosy_count = 5.0
nosy_names = ['tim.peters', 'mark.dickinson', 'benjamin.peterson', 'rossburton', 'yan12125']
pr_nums = ['9085']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue34585'
versions = ['Python 3.8']
    @rossburton
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    rossburton mannequin commented Sep 5, 2018

    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.

    @rossburton rossburton mannequin added 3.8 only security fixes build The build process and cross-build labels Sep 5, 2018
    @benjaminp
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    New changeset 2a9c380 by Benjamin Peterson (Ross Burton) in branch 'master':
    closes bpo-34585: Don't do runtime test to get float byte order. (GH-9085)
    2a9c380

    @benjaminp
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    New changeset b3b8cb4 by Benjamin Peterson in branch 'master':
    run autoconf (GH-9411)
    b3b8cb4

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
    @libTorrentUser
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    Unfortunately ax_c_float_words_bigendian.m4 evil magical trickery does not work if certain compiler flags are used. Using gcc and -flto seems to be enough to break it.

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