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 ?)

I am able to reproduce this issue on my gcc 4.4.7

ef9b356a9deb">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

New changeset 24ba3b0df5e5f2f237d7b23b4017ba12f16320ae by Dong-hee Na in branch 'master':
bpo-41875: Use __builtin_unreachable when possible (GH-22433)
https://github.com/python/cpython/commit/24ba3b0df5e5f2f237d7b23b4017ba12f16320ae

New changeset cca896e13b230a934fdb709b7f10c5451ffc25ba by Miss Islington (bot) in branch '3.9':
bpo-41875: Use __builtin_unreachable when possible (GH-22433)
https://github.com/python/cpython/commit/cca896e13b230a934fdb709b7f10c5451ffc25ba

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

commit eebaa9bfc593d5a46b293c1abd929fbfbfd28199
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>

Lukasz: Would you mind to cherry-pick the commit cca896e13b230a934fdb709b7f10c5451ffc25ba in 3.9.0 final?

Done.

New changeset df71b65a882cc9ddf9cd45ae9f83a04ec441af1e by Łukasz Langa (Miss Islington (bot)) in branch '3.9':
bpo-41875: Use __builtin_unreachable when possible (GH-22433)
https://github.com/python/cpython/commit/df71b65a882cc9ddf9cd45ae9f83a04ec441af1e