Remove -Wunreachable-code flag

[smontanaro]

BPO	24324
Nosy	@loewis, @smontanaro, @pitrou, @vstinner, @ned-deily, @embray
Files	unreached.diff Python351-no-unreachable-check.diff: Cleaner version of Skip's patch. Just changes configure.ac

^{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 2016-03-08.06:01:59.710>
created_at = <Date 2015-05-29.13:51:17.482>
labels = ['build']
title = 'Remove -Wunreachable-code flag'
updated_at = <Date 2016-03-08.06:01:59.706>
user = 'https://github.com/smontanaro'

bugs.python.org fields:

activity = <Date 2016-03-08.06:01:59.706>
actor = 'ned.deily'
assignee = 'none'
closed = True
closed_date = <Date 2016-03-08.06:01:59.710>
closer = 'ned.deily'
components = ['Build']
creation = <Date 2015-05-29.13:51:17.482>
creator = 'skip.montanaro'
dependencies = []
files = ['40510', '41663']
hgrepos = []
issue_num = 24324
keywords = ['patch']
message_count = 8.0
messages = ['244377', '245035', '251001', '251004', '251017', '261323', '261324', '261325']
nosy_count = 7.0
nosy_names = ['loewis', 'skip.montanaro', 'pitrou', 'vstinner', 'ned.deily', 'python-dev', 'erik.bray']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue24324'
versions = ['Python 3.5', 'Python 3.6']

[smontanaro]

Makefile.pre defines BASECFLAGS to include -Wunreachable-code. When building in the Linux environment available to me (GCC 4.4.6), compilation spews tons of warnings about "warning: will never be executed". According to this StackOverflow thread:

http://stackoverflow.com/questions/14591778/

that flag was always broken in GCC and was eventually removed in 4.5. Maybe we should dispense with it when building Python, at least unless requested by the user.

[smontanaro]

I guess this is an autoconf thing. "@BASECFLAGS@" in Makefile.pre.in seems to expand to "-Wsign-compare -Wunreachable-code" in Makefile.pre.

[embray]

This would definitely be nice to fix. I panicked a bit because of this when I compiled my extension modules against Python 3.5 for the first time.

[vstinner]

Can you please propose a patch?

[smontanaro]

This seems to work for me. Diff against default. I have no idea how old the autoconf setup is here at work. I imagine you'll want to only apply the configure.ac patch and regenerate configure.

[python-dev]

New changeset 61f8f7610a88 by Ned Deily in branch '3.5':
Issue bpo-24324: Do not enable unreachable code warnings when using
https://hg.python.org/cpython/rev/61f8f7610a88

[python-dev]

New changeset 8fecf6e17616 by Ned Deily in branch 'default':
Issue bpo-24324: merge from 3.5
https://hg.python.org/cpython/rev/8fecf6e17616

[ned-deily]

Thanks for the suggested patches. I used a somewhat different approach in an attempt to retain the benefit of the unreachable code warning when using other (non-GCC) compilers, in particular, llvm clang where the warnings do seem to be useful.