Bug 864136 – x11-apps/xisxwayland-1 fails to compile (lto): fakexlib.c:56:15: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]

• Product: Gentoo Linux
• Status: RESOLVED FIXED

[Description Agostino Sarubbo 2022-08-07 05:40:03 UTC]

https://blogs.gentoo.org/ago/2020/07/04/gentoo-tinderbox/

Issue: x11-apps/xisxwayland-1 fails to compile (lto).
Discovered on: amd64 (internal ref: lto_tinderbox)

NOTE:
This machine uses lto with CFLAGS=-flto -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing

Here is a bit of explanation:

-Werror=lto-type-mismatch:
User to find possible runtime issues in packages. It likely means the package is unsafe to build & use with LTO.
For projects using the same identifier but with different types across different files, they must be fixed to be consistent across the codebase.

-Werror=odr:
Used to find possible runtime issues in packages. These bugs are a problem anyway but may be even worse when combined with LTO. C++ code must comply with the One Definition Rule (ODR) - see https://en.cppreference.com/w/cpp/language/definition#One_Definition_Rule.

-Werror=strict-aliasing:
Used to find possible runtime issues in packages. These bugs are a problem anyway but may be even worse when combined with LTO.

Workarounds:
- If upstream is friendly and still active, file a bug upstream. For emulators, codecs, games, or multimedia packages, it may be worth just applying a workaround instead, as upstreams sometimes aren't receptive to these bugs (VALID FOR ALL).
- Use the new 'filter-lto' from flag-o-matic.eclass as it's likely to be unsafe with LTO (VALID FOR lto-type-mismatch - odr).
- Fix it yourself if interested, of course (VALID FOR ALL).
- Append-flags -fno-strict-aliasing (VALID FOR strict-aliasing).
- Use memcpy() but a union is sometimes suitable too (VALID FOR strict-aliasing).
- -fstrict-aliasing is implied by -O2, so this must be addressed in some form (VALID FOR strict-aliasing).

See also: https://marc.info/?l=gentoo-dev&m=165639574126280&w=2

[Comment 1 Agostino Sarubbo 2022-08-07 05:40:05 UTC]

Created attachment 798433 [details]
build.log

build log and emerge --info

[Comment 2 Agostino Sarubbo 2022-08-07 05:40:07 UTC]

Error(s) that match a know pattern in addition to what has been reported in the summary:


FAILED: test-xisxwayland.p/test_fakexlib.c.o 
../xisxwayland-1/test/fakexlib.c:56:15: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]

[Comment 3 Matt Turner 2022-08-23 01:53:18 UTC]

The aliasing warning here is a real bug.

[Comment 4 Larry the Git Cow 2022-08-24 14:14:14 UTC]

The bug has been closed via the following commit(s):

https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=8c82714f7c5a4803039aa05717d10143c125aaa5

commit 8c82714f7c5a4803039aa05717d10143c125aaa5
Author:     Matt Turner <[e-mail]>
AuthorDate: 2022-08-24 14:06:47 +0000
Commit:     Matt Turner <[e-mail]>
CommitDate: 2022-08-24 14:08:25 +0000

    x11-apps/xisxwayland: Version bump to 2
    
    Closes: https://bugs.gentoo.org/864136
    Signed-off-by: Matt Turner <[e-mail]>

 x11-apps/xisxwayland/Manifest             |  1 +
 x11-apps/xisxwayland/xisxwayland-2.ebuild | 29 +++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+)