CFLAGS/LDFLAGS containing `"-Xpreprocessor <foobar>" from dependencies are breaking ARGS and LINK_ARGS in build.ninja

[dimpase]

Describe the bug
dependencies might have flag combos such as -Xpreprocessor -fopenmp in pkg-config files (one example would be OpenBLAS on
macOS in Homebrew, see https://formulae.brew.sh/formula/openblas). With certain extra dependencies we see -Xpreprocessor getting
separated from -fopenmp in build.ninja ARGS and/or LINK_ARGS.

A short reproducer showing incorrect LINK_ARGS generation attached. There one gets -Xpreprocessor -fopenmp swapped around.

build tp: c_LINKER tp.p/tp.c.o | /usr/lib64/libopenblas.so
 LINK_ARGS = -Wl,--as-needed -Wl,--no-undefined -fopenmp -Xpreprocessor /usr/lib64/libopenblas.so -framework Foobar

In some other cases on sagemath/sage#41626 we see -Xpreprocessor getting to the end of ARGS or LINK_FLAGS,
or right in front of -framework.

To Reproduce
On a x86_64 Linux (you might need to modify openblas.cp on different platforms)
create a project by putting the following meson.build, tp.c and (modified) openblas.pc (these files are also
attached at the end as a tar archive)

project(
  'tp',
  'c',
  meson_version : '>= 1.3.0',
  version : '0.1',
  default_options : ['warning_level=3'],
)

openmp = dependency('openmp', required: false, disabler: true)
blas = dependency('openblas')
dependencies = [
openmp, blas
]

sources = [
  'tp.c',
]

exe = executable(
  'tp',
  sources,
  dependencies : dependencies,
  install : true,
)

test('basic', exe)

tp.c:

#include <omp.h>
#include <openblas/cblas.h>

#define PROJECT_NAME "tp"

int main(int argc, char **argv) {
    return 0;
}

openblas.pc

libdir=/usr/lib64
libprefix=
libnamesuffix=
libsuffix=
includedir=/usr/include/openblas
omp_opt=-Xpreprocessor -fopenmp
openblas_config= USE_64BITINT= DYNAMIC_ARCH= DYNAMIC_OLDER= NO_CBLAS= NO_LAPACK= NO_LAPACKE= NO_AFFINITY=1 USE_OPENMP=1 HASWELL MAX_THREADS=64
version=0.3.33
extralib=-lm -lpthread -lgfortran -lm -lpthread -lgfortran -lgomp
Name: openblas
Description: OpenBLAS is an optimized BLAS library based on GotoBLAS2 1.13 BSD version
Version: ${version}
URL: https://github.com/xianyi/OpenBLAS
Libs: ${omp_opt} -L${libdir} -l${libprefix}openblas${libsuffix}${libnamesuffix} -framework Foobar
Libs.private: ${extralib}
Cflags: ${omp_opt} -I${includedir}  -framework Foobar

and run

export PKG_CONFIG_PATH=.
meson setup build

Then inspect the generated build/build.ninja.

Expected behavior
Preserved token -Xpreprocessor -fopenmp in LINK_ARGS

system parameters

• Is this a cross build or just a plain native build (for the same computer)? - plain.
• what operating system (e.g. MacOS Catalina, Windows 10, CentOS 8.0, Ubuntu 18.04, etc.): macOS 26, Linux x86_64 (Gentoo)
• what Python version are you using e.g. 3.8.0 - 3.13.13.
• what meson --version: 1.11.1
• what ninja --version if it's a Ninja build: n/a (no need to run ninja to see the error)

bugpro.tgz

[dimpase]

@tobiasdiez

[bonzini]

This is not trivial to fix... the root cause is that CompilerArgs does not know about -Xpreprocessor, but knows about -fopenmp. The place where to teach Meson about it (and about -Xlinker perhaps) would be CompilerArgs.__iadd__. Another possibility is to early-rewrite -Xpreprocessor -fopenmp into -Xpreprocessor=-fopenmp or -Wp,-fopenmp.

Ultimately, this would be fixed via something like #14748, which however is much more ambitious.

[dimpase]

@bonzini don't you have a similar problem with -framework Foobar macOS options? How do you achieve that such tokens don't get separated?

[bonzini]

The problem is that -fopenmp is treated specially.

[eli-schwartz]

Specifically,

meson/mesonbuild/compilers/mixins/clike.py

Lines 50 to 68 in ea995d7

	class CLikeCompilerArgs(arglist.CompilerArgs):
	# Note: ``-isystem`` is deliberately absent from prepend_prefixes.
	# Because ``-isystem`` is appended by __iadd__ rather than
	# prepended, the reversed iteration in the ninja backend causes
	# directories listed *last* in include_directories() to appear
	# *first* on the command line (and thus be searched first). This
	# is the opposite convention from ``-I``, where first-listed means
	# first-searched. Existing projects (e.g. systemd) rely on this
	# ``-isystem`` ordering, so adding ``-isystem`` to prepend_prefixes
	# would silently break them by reversing their include priority.
	# See NinjaBackend._generate_single_compile_target_args().
	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', '-fopenmp', '-qopenmp')

[bonzini]

... and more precisely, #15503 is why it was done (I replied quickly and couldn't look it up before). However, it may be better to revert it because this is a regression.