No deduplication of some linker flags via static_library dependencies

[jrosdahl]

Describe the bug

When a static library has dependencies that add linker flags, those flags (at least in some situations) are not deduplicated. For instance, it happens with static_library(..., dependencies: dependency('openmp') or static_library(..., dependencies: meson.get_compiler('c').find_library('z')).

This of course works correctly, but when you see the command line it looks like something has gone awry , especially in a more complex scenario than the example below (we have around 100 -fopenmp arguments in our build at the moment).

To Reproduce

$ touch empty.c
$ cat meson.build
project('example', 'c')

lib1 = static_library('lib1', 'example.c', dependencies: dependency('openmp'))
lib2 = static_library('lib2', 'example.c', dependencies: dependency('openmp'))
shared_library('example', 'example.c', link_with: [lib1, lib2])
$ meson setup build
[...]
$ grep 'LINK_ARGS.*openmp'  build/build.ninja
LINK_ARGS = -Wl,--as-needed -Wl,--no-undefined -shared -fPIC -Wl,-soname,libexample.so -Wl,--start-group liblib1.a liblib2.a -Wl,--end-group -fopenmp -fopenmp

Note the duplicate -fopenmp arguments.

Expected behavior

-fopenmp arguments are deduplicated.

system parameters

• Rocky Linux 9.7
• Python 3.9.25
• Meson 1.10.1

[eli-schwartz]

We define "safe" dedup'able flags. I would have expected find_library('z') to be deduped, but -fopenmp is not on the list (and can presumably be added... 🙂)

By the way dependency('zlib') knows how to search for paired -lz and zlib.h if there are no pkg-config/CMake finders available for it, so I recommend avoiding find_library for that particular library.

Code for dedup:

meson/mesonbuild/compilers/mixins/clike.py

Lines 50 to 58 in 780bbc0

	class CLikeCompilerArgs(arglist.CompilerArgs):
	prepend_prefixes = ('-I', '-L')
	dedup2_prefixes = ('-I', '-isystem', '-L', '-D', '-U')
	# NOTE: not thorough. A list of potential corner cases can be found in
	# https://github.com/mesonbuild/meson/pull/4593#pullrequestreview-182016038
	dedup1_prefixes = ('-l', '-Wl,-l', '-Wl,-rpath,', '-Wl,-rpath-link,')
	dedup1_suffixes = ('.lib', '.dll', '.so', '.dylib', '.a')
	dedup1_args = ('-c', '-S', '-E', '-pipe', '-pthread', '-Wl,--export-dynamic')

meson/mesonbuild/arglist.py

Lines 22 to 43 in 780bbc0

	class Dedup(enum.Enum):
	"""What kind of deduplication can be done to compiler args.
	OVERRIDDEN - Whether an argument can be 'overridden' by a later argument.
	For example, -DFOO defines FOO and -UFOO undefines FOO. In this case,
	we can safely remove the previous occurrence and add a new one. The
	same is true for include paths and library paths with -I and -L.
	UNIQUE - Arguments that once specified cannot be undone, such as `-c` or
	`-pipe`. New instances of these can be completely skipped.
	NO_DEDUP - When it matters where or how many times on the command-line
	a particular argument is present. This can matter for symbol
	resolution in static or shared libraries, so we cannot de-dup or
	reorder them.
	"""
	NO_DEDUP = 0
	UNIQUE = 1
	OVERRIDDEN = 2
	class CompilerArgs(T.MutableSequence[str]):

[jrosdahl]

By the way dependency('zlib') knows how to search for paired -lz and zlib.h if there are no pkg-config/CMake finders available for it, so I recommend avoiding find_library for that particular library.

Right, it was a bad example since we actually use dependency('zlib'), but we do use .find_library('ws2_32') on Windows, resulting in duplication there as well.