__builtin_unreachable error in gcc 4.4.5

[yotamoteuchi]

BPO	41875
Nosy	@vstinner, @ambv, @serhiy-storchaka, @corona10, @miss-islington
PRs	bpo-41875: Use __builtin_unreachable when possible #22433 [3.9] bpo-41875: Use __builtin_unreachable when possible (GH-22433) #22442

^{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 2020-10-04.16:21:07.370>
created_at = <Date 2020-09-28.11:15:51.695>
labels = ['build', '3.9', '3.10', 'release-blocker']
title = '__builtin_unreachable error in gcc 4.4.5'
updated_at = <Date 2020-10-05.16:10:19.666>
user = 'https://bugs.python.org/yotamoteuchi'

bugs.python.org fields:

activity = <Date 2020-10-05.16:10:19.666>
actor = 'lukasz.langa'
assignee = 'none'
closed = True
closed_date = <Date 2020-10-04.16:21:07.370>
closer = 'lukasz.langa'
components = ['Build']
creation = <Date 2020-09-28.11:15:51.695>
creator = 'yota moteuchi'
dependencies = []
files = []
hgrepos = []
issue_num = 41875
keywords = ['patch']
message_count = 8.0
messages = ['377594', '377604', '377626', '377635', '377636', '377637', '377959', '378049']
nosy_count = 6.0
nosy_names = ['vstinner', 'lukasz.langa', 'serhiy.storchaka', 'yota moteuchi', 'corona10', 'miss-islington']
pr_nums = ['22433', '22442']
priority = 'release blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue41875'
versions = ['Python 3.9', 'Python 3.10']

[yotamoteuchi]

While compiling a very recent release of Python (ie. 3.9.0rc2) with a fairly old release of gcc (ie. debian 6.0.6 gcc 4.4.5) I get the following error :

gcc -pthread -c -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -fvisibility=hidden -I./Include/internal -I. -I./Include -DPy_BUILD_CORE -o Parser/pegen/pegen.o Parser/pegen/pegen.c

Parser/pegen/pegen.c: In function ‘_PyPegen_seq_count_dots’:
Parser/pegen/pegen.c:1414: error: implicit declaration of function ‘__builtin_unreachable’

Indeed, this '__builtin_unreachable()' function will exists starting from gcc 4.5.

The configure file could detect this and offer an alternative.

(does a table of the supported gcc release for each python versions exists ?)

[corona10]

I am able to reproduce this issue on my gcc 4.4.7

root@ef9b356a9deb:/home/cpython# gcc --version
gcc (Ubuntu/Linaro 4.4.7-8ubuntu7) 4.4.7
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE

Objects/abstract.c: In function '_PySequence_IterSearch':
Objects/abstract.c:2065: error: implicit declaration of function '__builtin_unreachable'
Makefile:1751: recipe for target 'Objects/abstract.o' failed
make: *** [Objects/abstract.o] Error 1

And with PR 22433, I am success to build and run test.

I 'd like to send backport patch into 3.9 also

[corona10]

New changeset 24ba3b0 by Dong-hee Na in branch 'master':
bpo-41875: Use __builtin_unreachable when possible (GH-22433)
24ba3b0

[miss-islington]

New changeset cca896e by Miss Islington (bot) in branch '3.9':
bpo-41875: Use __builtin_unreachable when possible (GH-22433)
cca896e

[vstinner]

FYI __builtin_unreachable() is used since this change, of bpo-38249:

commit eebaa9b
Author: Serhiy Storchaka <storchaka@gmail.com>
Date: Mon Mar 9 20:49:52 2020 +0200

bpo-38249: Expand Py_UNREACHABLE() to __builtin_unreachable() in the release mode. (GH-16329)

Co-authored-by: Victor Stinner <vstinner@python.org>

[vstinner]

Lukasz: Would you mind to cherry-pick the commit cca896e in 3.9.0 final?