compilers: Add support for OpenMP from homebrew with AppleClang

Which requires injecting some extra paths and the `-Xpreprocess` flag,
as well as extra search paths for libomp and the headers.

Fixes: #7435

5 files changed, 62 insertions, 6 deletions

diff --git a/mesonbuild/_typing.py b/mesonbuild/_typing.py
index 05ff2b3..8336c46 100644
--- a/mesonbuild/_typing.py
+++ b/mesonbuild/_typing.py
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: Apache-2.0
 # Copyright 2020 The Meson development team
-# Copyright © 2020-2023 Intel Corporation
+# Copyright © 2020-2024 Intel Corporation
 
 """Meson specific typing helpers.
 
diff --git a/mesonbuild/compilers/c.py b/mesonbuild/compilers/c.py
index 8fda3a5..cbc1bea 100644
--- a/mesonbuild/compilers/c.py
+++ b/mesonbuild/compilers/c.py
@@ -10,6 +10,7 @@ from .. import options
 from .. import mlog
 from ..mesonlib import MesonException, version_compare
 from .c_function_attributes import C_FUNC_ATTRIBUTES
+from .mixins.apple import AppleCompilerMixin
 from .mixins.clike import CLikeCompiler
 from .mixins.ccrx import CcrxCompiler
 from .mixins.xc16 import Xc16Compiler
@@ -187,7 +188,7 @@ class ArmLtdClangCCompiler(ClangCCompiler):
     id = 'armltdclang'
 
 
-class AppleClangCCompiler(ClangCCompiler):
+class AppleClangCCompiler(AppleCompilerMixin, ClangCCompiler):
 
     """Handle the differences between Apple Clang and Vanilla Clang.
 
diff --git a/mesonbuild/compilers/cpp.py b/mesonbuild/compilers/cpp.py
index 9467a35..5e8947b 100644
--- a/mesonbuild/compilers/cpp.py
+++ b/mesonbuild/compilers/cpp.py
@@ -19,6 +19,7 @@ from .compilers import (
     CompileCheckMode,
 )
 from .c_function_attributes import CXX_FUNC_ATTRIBUTES, C_FUNC_ATTRIBUTES
+from .mixins.apple import AppleCompilerMixin
 from .mixins.clike import CLikeCompiler
 from .mixins.ccrx import CcrxCompiler
 from .mixins.ti import TICompiler
@@ -337,7 +338,7 @@ class ArmLtdClangCPPCompiler(ClangCPPCompiler):
     id = 'armltdclang'
 
 
-class AppleClangCPPCompiler(ClangCPPCompiler):
+class AppleClangCPPCompiler(AppleCompilerMixin, ClangCPPCompiler):
 
     _CPP23_VERSION = '>=13.0.0'
     # TODO: We don't know which XCode version will include LLVM 17 yet, so
diff --git a/mesonbuild/compilers/mixins/apple.py b/mesonbuild/compilers/mixins/apple.py
new file mode 100644
index 0000000..98c4bfa
--- /dev/null
+++ b/mesonbuild/compilers/mixins/apple.py
@@ -0,0 +1,57 @@
+# SPDX-License-Identifier: Apache-2.0
+# Copyright © 2024 Intel Corporation
+
+"""Provides mixins for Apple compilers."""
+
+from __future__ import annotations
+import typing as T
+
+from ...mesonlib import MesonException
+
+if T.TYPE_CHECKING:
+    from ..._typing import ImmutableListProtocol
+    from ...environment import Environment
+    from ..compilers import Compiler
+else:
+    # This is a bit clever, for mypy we pretend that these mixins descend from
+    # Compiler, so we get all of the methods and attributes defined for us, but
+    # for runtime we make them descend from object (which all classes normally
+    # do). This gives up DRYer type checking, with no runtime impact
+    Compiler = object
+
+
+class AppleCompilerMixin(Compiler):
+
+    """Handle differences between Vanilla Clang and the Clang shipped with XCode."""
+
+    __BASE_OMP_FLAGS: ImmutableListProtocol[str] = ['-Xpreprocessor', '-fopenmp']
+
+    def openmp_flags(self, env: Environment) -> T.List[str]:
+        """Flags required to compile with OpenMP on Apple.
+
+        The Apple Clang Compiler doesn't have builtin support for OpenMP, it
+        must be provided separately. As such, we need to add the -Xpreprocessor
+        argument so that an external OpenMP can be found.
+
+        :return: A list of arguments
+        """
+        m = env.machines[self.for_machine]
+        assert m is not None, 'for mypy'
+        if m.cpu_family.startswith('x86'):
+            root = '/usr/local'
+        else:
+            root = '/opt/homebrew'
+        return self.__BASE_OMP_FLAGS + [f'-I{root}/opt/libomp/include']
+
+    def openmp_link_flags(self, env: Environment) -> T.List[str]:
+        m = env.machines[self.for_machine]
+        assert m is not None, 'for mypy'
+        if m.cpu_family.startswith('x86'):
+            root = '/usr/local'
+        else:
+            root = '/opt/homebrew'
+
+        link = self.find_library('omp', env, [f'{root}/opt/libomp/lib'])
+        if not link:
+            raise MesonException("Couldn't find libomp")
+        return self.__BASE_OMP_FLAGS + link
diff --git a/test cases/common/184 openmp/meson.build b/test cases/common/184 openmp/meson.build
index 4cbe806..ab09b2c 100644
--- a/test cases/common/184 openmp/meson.build
+++ b/test cases/common/184 openmp/meson.build
@@ -16,9 +16,6 @@ endif
 if cc.get_id() == 'clang' and host_machine.system() == 'windows'
   error('MESON_SKIP_TEST Windows clang does not support OpenMP.')
 endif
-if host_machine.system() == 'darwin'
-  error('MESON_SKIP_TEST macOS does not support OpenMP.')
-endif
 
 openmp = dependency('openmp')
 env = environment()