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Bug 860462 - [guru] sci-libs/mfem-4.4 fails to compile (lto): ncmesh.cpp:2677:32: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
Summary: [guru] sci-libs/mfem-4.4 fails to compile (lto): ncmesh.cpp:2677:32: error: d...
Status: RESOLVED FIXED
Alias: None
Product: GURU
Classification: Unclassified
Component: Package issues (show other bugs)
Hardware: All Linux
: Normal normal
Assignee: Alessandro Barbieri
URL:
Whiteboard:
Keywords:
Depends on:
Blocks:
 
Reported: 2022-07-24 07:59 UTC by Agostino Sarubbo
Modified: 2024-03-25 20:54 UTC (History)
1 user (show)

See Also:
Package list:
Runtime testing required: ---

Attachments
build.log (build.log,76.77 KB, text/plain)
2022-07-24 07:59 UTC, Agostino Sarubbo 		Details
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Description Agostino Sarubbo gentoo-dev 2022-07-24 07:59:48 UTC 
https://blogs.gentoo.org/ago/2020/07/04/gentoo-tinderbox/

Issue: sci-libs/mfem-4.4 fails to compile (lto).
Discovered on: amd64 (internal ref: guru_tinderbox)

NOTE:
This machine uses lto with CFLAGS=-flto -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing
Comment 1 Agostino Sarubbo gentoo-dev 2022-07-24 07:59:50 UTC 
Created attachment 793817 [details]
build.log

build log and emerge --info
Comment 2 Agostino Sarubbo gentoo-dev 2022-07-24 07:59:51 UTC 
Error(s) that match a know pattern in addition to what has been reported in the summary:


FAILED: CMakeFiles/mfem.dir/mesh/ncmesh.cpp.o 
/var/tmp/portage/sci-libs/mfem-4.4/work/mfem-4.4/mesh/ncmesh.cpp:2677:32: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
Comment 3 Agostino Sarubbo gentoo-dev 2022-07-26 09:09:00 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