Warnings when building on AIX

[jelie]

BPO	22463
Nosy	@vadmium

^{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 2016-06-22.03:14:27.381>
created_at = <Date 2014-09-22.17:32:49.452>
labels = ['build']
title = 'Warnings when building on AIX'
updated_at = <Date 2016-06-22.03:14:27.380>
user = 'https://bugs.python.org/jelie'

bugs.python.org fields:

activity = <Date 2016-06-22.03:14:27.380>
actor = 'martin.panter'
assignee = 'none'
closed = True
closed_date = <Date 2016-06-22.03:14:27.381>
closer = 'martin.panter'
components = ['Build']
creation = <Date 2014-09-22.17:32:49.452>
creator = 'jelie'
dependencies = []
files = []
hgrepos = []
issue_num = 22463
keywords = []
message_count = 8.0
messages = ['227283', '268982', '268984', '268996', '269037', '269039', '269040', '269043']
nosy_count = 4.0
nosy_names = ['jelie', 'python-dev', 'martin.panter', 'David.Edelsohn']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue22463'
versions = ['Python 2.7']

[jelie]

Building Python 2.7.8 on AIX 7.1 gives the following warnings:

Parser/pgen.c:282:9: warning: variable 'i' set but not used [-Wunused-but-set-variable]
Include/objimpl.h:164:66: warning: right-hand operand of comma expression has no effect [-Wunused-value]
/home/iulius/autobuild/src/Python-2.7.8/Modules/cPickle.c:4495:8: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
/home/iulius/autobuild/src/Python-2.7.8/Modules/cPickle.c:202:8: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
/home/iulius/autobuild/src/Python-2.7.8/Modules/readline.c:777:1: warning: 'on_completion_display_matches_hook' defined but not used [-Wunused-function]
./pyconfig.h:1182:0: warning: "_POSIX_C_SOURCE" redefined
./pyconfig.h:1204:0: warning: "_XOPEN_SOURCE" redefined
/home/iulius/autobuild/src/Python-2.7.8/Modules/tkappinit.c:29:15: warning: variable 'main_window' set but not used [-Wunused-but-set-variable]
/home/iulius/autobuild/src/Python-2.7.8/Modules/_ctypes/ctypes.h:456:13: warning: 'capsule_destructor_CTYPES_CAPSULE_WCHAR_T' defined but not used [-Wunused-function]
/home/iulius/autobuild/src/Python-2.7.8/Modules/_ctypes/cfield.c:50:29: warning: variable 'length' set but not used [-Wunused-but-set-variable]
/home/iulius/autobuild/src/Python-2.7.8/Modules/_ctypes/ctypes.h:456:13: warning: 'capsule_destructor_CTYPES_CAPSULE_WCHAR_T' defined but not used [-Wunused-function]

[vadmium]

The most serious of these IMO is the signed overflow warnings in Modules/cPickle.c. I get the same warnings if I disable the “gcc -fwrapv” mode:

/media/disk/home/proj/python/cpython/Modules/cPickle.c: In function ‘Unpickler_noload’:
/media/disk/home/proj/python/cpython/Modules/cPickle.c:5462:1: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
Unpickler_noload(Unpicklerobject *self, PyObject *unused)
^
/media/disk/home/proj/python/cpython/Modules/cPickle.c:5462:1: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
/media/disk/home/proj/python/cpython/Modules/cPickle.c:202:8: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
if (clearto >= self->length) return 0;
^
/media/disk/home/proj/python/cpython/Modules/cPickle.c:202:8: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
if (clearto >= self->length) return 0;
^

It is not clear to me where the overflows actually happen. See bpo-1608, bpo-1621, where some of these bugs were fixed, and the “gcc -fwrapv” workaround was enabled. Perhaps your compiler has a similar flag to use as a workaround, though IMO it would be good to fix the code as well.

Most of the other warnings have been fixed or worked around in Python 3. I propose to port the following revisions to Python 2. All but one are still in effect today:

83a55ca935f0: Parser/pgen.c
12e53abec5d0, 0c7fff783b32: Include/objimpl.h
ce9f5ce33ad2: Modules/readline.c
a7a9e0f46b8a: Modules/tkappinit.c (change present in 3.3, 3.4, but eliminated in 3.5)
e2f96bd29252 (partial): Modules/_ctypes/cfield.c
b04557ae099a (partial): Modules/expat/xmlparse.c (assuming this was the cause of the ./pyconfig.h warnings)

That leaves the two warnings about CTYPES_CAPSULE_WCHAR_T. If necessary, they could be worked around by adding an #if condition around the definitions:

#if defined(CTYPES_UNICODE) && !defined(HAVE_USABLE_WCHAR_T)
#    define CTYPES_CAPSULE_WCHAR_T "_ctypes/[. . .].c wchar_t buffer from unicode"
CTYPES_CAPSULE_INSTANTIATE_DESTRUCTOR(CTYPES_CAPSULE_WCHAR_T)
#endif

[vadmium]

Looking closer at cPickle.c (after upgrading my GCC), I think there are two warnings happening, when Pdata_clear() is inlined into each of noload_reduce() and noload_setitem(). I think both cases are false positives; there is no actual signed overflow possible. Ismail Donmez has provided patches that work around this warning in bpo-1621, but IMO, and maybe in the general consensus, we shouldn’t add these workarounds.

[DavidEdelsohn]

These look like false positives or noise to me as well.

[python-dev]

New changeset 0b63465a1796 by Martin Panter in branch '2.7':
Issue bpo-22463: Backport compiler warning fixes and workarounds
https://hg.python.org/cpython/rev/0b63465a1796

[python-dev]

New changeset b1c1c297bead by Martin Panter in branch '2.7':
Issue bpo-22463: Correct #endif placement; patch by Senthil Kumaran
https://hg.python.org/cpython/rev/b1c1c297bead

[python-dev]

New changeset ffe866aa86a8 by Martin Panter in branch '2.7':
Issue bpo-22463: Cure unused function warnings on AIX
https://hg.python.org/cpython/rev/ffe866aa86a8

[vadmium]

FWIW on the current AIX buildbot <http://buildbot.python.org/all/builders/PPC64%20AIX%202.7\> I don’t see any cPickle.c warnings. (Nor on the earliest build available on that buildbot, from Nov 2015.)