Bug 859217 – games-simulation/flightgear-2020.3.13 fails to compile (lto): renderer_legacy.hxx:52:7: error: type struct FGRenderer violates the C++ One Definition Rule [-Werror=odr]

• Status: CONFIRMED
• Product: Gentoo Linux

[Description Agostino Sarubbo 2022-07-19 07:23:31 UTC]

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

Issue: games-simulation/flightgear-2020.3.13 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

[Comment 1 Agostino Sarubbo 2022-07-19 07:23:33 UTC]

Created attachment 792233 [details]
build.log.xz

build log and emerge --info (compressed because it exceeds attachment limit, use 'xzless' to read it)

[Comment 2 Agostino Sarubbo 2022-07-19 07:23:35 UTC]

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


FAILED: src/Main/fgfs 
fatal: not a git repository: '/var/tmp/portage/games-simulation/flightgear-2020.3.13/work/flightgear-2020.3.13/.git'
/var/tmp/portage/games-simulation/flightgear-2020.3.13/work/flightgear-2020.3.13/src/Viewer/renderer_legacy.hxx:52:7: error: type ‘struct FGRenderer’ violates the C++ One Definition Rule [-Werror=odr]

[Comment 3 Agostino Sarubbo 2022-07-26 09:01:56 UTC]

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