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Bug 854360

Summary: app-editors/emacs-18.59-r14 fails to compile (lto): alloc.c:1085:20: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
Product: Gentoo Linux Reporter: Agostino Sarubbo <ago>
Component: Current packagesAssignee: GNU Emacs project <gnu-emacs>
Status: IN_PROGRESS ---    
Severity: normal Keywords: NeedPatch
Priority: Normal    
Version: unspecified   
Hardware: All   
OS: Linux   
See Also: https://bugs.gentoo.org/show_bug.cgi?id=839405
Whiteboard:
Package list:
Runtime testing required: ---
Bug Depends on:    
Bug Blocks: 618550    
Attachments: build.log

Description Agostino Sarubbo gentoo-dev 2022-06-26 07:03:23 UTC
https://blogs.gentoo.org/ago/2020/07/04/gentoo-tinderbox/

Issue: app-editors/emacs-18.59-r14 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 gentoo-dev 2022-06-26 07:03:25 UTC
Created attachment 787682 [details]
build.log

build log and emerge --info
Comment 2 Sam James archtester Gentoo Infrastructure gentoo-dev Security 2022-06-26 07:04:30 UTC
For older Emacs, let's just use -fno-strict-aliasing.
Comment 3 Ulrich Müller gentoo-dev 2022-06-26 10:22:38 UTC
I must admit that I'm getting tired of bugs of this type. It's a strange idea to compile emacs:18 with that combination of CFLAGS. Also this isn't a real issue, otherwise users would have reported it earlier.

Maybe time to retire slot 18 (as well as 23 to 26), or move it to the emacs overlay?
Comment 4 Larry the Git Cow gentoo-dev 2022-06-26 10:34:21 UTC
The bug has been referenced in the following commit(s):

https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=f32b8cd1cdedb9e74ecbd8a07244eaf5250a9e04

commit f32b8cd1cdedb9e74ecbd8a07244eaf5250a9e04
Author:     Ulrich Müller <ulm@gentoo.org>
AuthorDate: 2022-06-26 10:33:19 +0000
Commit:     Ulrich Müller <ulm@gentoo.org>
CommitDate: 2022-06-26 10:34:15 +0000

    app-editors/emacs: Compile Emacs 18 with -fno-strict-aliasing
    
    Bug: https://bugs.gentoo.org/854360
    Suggested-by: Sam James <sam@gentoo.org>
    Signed-off-by: Ulrich Müller <ulm@gentoo.org>

 app-editors/emacs/emacs-18.59-r14.ebuild | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
Comment 5 Agostino Sarubbo gentoo-dev 2022-07-26 09:00:14 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