Packaging Friendliness: Compiler Flags to Avoid

It is tempting to enable some (generally useful) compiler flags such as -Werror and -march=native in you build by default (such as to discover more problems and generate more efficient code), however in practice these flags may lead to packaging problems instead.


Don’t add -Werror and -march=native to default build flags of your project.


This flag makes compiler reject code with warnings, forcing author to fix all of them and thus write cleaner code, so it does make sense to enable it for local builds and for continuous integration. However, even if a code is warning-free for you right now, you cannot expect it to remain so with future compiler versions or on different compilers/arches/OSes, so with -Werror your code is prone to break in different environments as well as over time, which is best to avoid.

Such fragile code increases burden on maintainers (who are forced to fix warnings in addition to critical problems when doing bulk updates), and leads to unexpected build failures for end users (such as after compiler update). In either case, it will likely be shut up by patching out the -Werror flag instead of fixing the warning, so using -Werror as a way to force consumers to submit warning fixes upstream is not a good idea.

Enabling individual warnings (e.g. -Werror=uninitialized) may be an option, but it requires you to maintain a list of (still incomplete) must-have warnings, keep it up to date and compatible with different compilers.

So instead, consider looking for new warnings in package build logs (available on Repology for some distros, for instance) and try to use as many different environments for you CI as possible.

Example from FreeBSD Ports Collection of patching -Werror out:

--- mpy-cross/Makefile.orig	2021-09-01 14:07:13 UTC
+++ mpy-cross/Makefile
@@ -17,7 +17,7 @@ INC += -I$(BUILD)
 INC += -I$(TOP)
 # compiler settings
-CWARN = -Wall -Werror
+CWARN = -Wall
 CWARN += -Wextra -Wno-unused-parameter -Wpointer-arith
 CFLAGS += -fdata-sections -ffunction-sections -fno-asynchronous-unwind-tables


This flag enables optimizations for the CPU the code is built on. Generally, enabling it allows compiler to generate more effective code, however the problem with packages is that they are built on completely different hardware than they are installed and used on. Not only the code built for different CPU may have performance regressions on users hardware, but it may not even run (usually crashing with SIGILL), because of using unsupported CPU features.

The additional danger of this flag is that it usually comes unnoticed by package maintainers and the problems caused by it are hard to reproduce. The code runs fine on maintainers hardware; it runs fine on package building cluster as well; even packages built on cluster run fine on maintainers hardware. However some user with slightly older hardware will experience (an unobvious to investigate) crash.

Example from FreeBSD Ports Collection of patching -march=native out:

	@${REINPLACE_CMD} -e 's/ -march=native//' \
		${WRKSRC}/cmake/FindAVX.cmake \
		${WRKSRC}/cmake/FindFMA.cmake \