Try even harder to use the C compiler for assembly

Now as long as you have a C compiler available in the project, it will
be used to compile assembly even if the target contains a C++ compiler
and even if the target contains only assembly and C++ sources.

Earlier, the order in which sources appeared in a target would decide
which compiler would be used.

However, if the project only provides a C++ compiler, that will be
used for compiling assembly sources.

If this breaks your use-case, please tell us.

Includes a test that ensures that all of the above is adhered to.

11 files changed, 126 insertions, 16 deletions

diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index c28a8a4..1f646dc 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -12,15 +12,17 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import copy, os, re
+from collections import OrderedDict
+
 from . import environment
 from . import dependencies
 from . import mlog
-import copy, os, re
 from .mesonlib import File, MesonException
 from .mesonlib import flatten, stringlistify, classify_unity_sources
 from .mesonlib import get_filenames_templates_dict, substitute_values
 from .environment import for_windows, for_darwin
-from .compilers import is_object, clike_langs, lang_suffixes
+from .compilers import is_object, clike_langs, sort_clike, lang_suffixes
 
 known_basic_kwargs = {'install': True,
                       'c_pch': True,
@@ -291,7 +293,7 @@ class BuildTarget(Target):
         self.is_unity = environment.coredata.get_builtin_option('unity')
         self.environment = environment
         self.sources = []
-        self.compilers = {}
+        self.compilers = OrderedDict()
         self.objects = []
         self.external_deps = []
         self.include_dirs = []
@@ -444,6 +446,9 @@ class BuildTarget(Target):
                         if lang not in self.compilers:
                             self.compilers[lang] = compiler
                         break
+            # Re-sort according to clike_langs
+            self.compilers = OrderedDict(sorted(self.compilers.items(),
+                                                key=lambda t: sort_clike(t[0])))
         else:
             # No source files, target consists of only object files of unknown
             # origin. Just add the first clike compiler that we have and hope
diff --git a/mesonbuild/compilers.py b/mesonbuild/compilers.py
index 22fb522..76a9067 100644
--- a/mesonbuild/compilers.py
+++ b/mesonbuild/compilers.py
@@ -57,6 +57,17 @@ clike_suffixes += ('h', 'll', 's')
 
 # All these are only for C-like languages; see `clike_langs` above.
 
+def sort_clike(lang):
+    '''
+    Sorting function to sort the list of languages according to
+    reversed(compilers.clike_langs) and append the unknown langs in the end.
+    The purpose is to prefer C over C++ for files that can be compiled by
+    both such as assembly, C, etc. Also applies to ObjC, ObjC++, etc.
+    '''
+    if lang not in clike_langs:
+        return 1
+    return -clike_langs.index(lang)
+
 def is_header(fname):
     if hasattr(fname, 'fname'):
         fname = fname.fname
diff --git a/mesonbuild/interpreter.py b/mesonbuild/interpreter.py
index 550e2f9..198c758 100644
--- a/mesonbuild/interpreter.py
+++ b/mesonbuild/interpreter.py
@@ -1752,22 +1752,10 @@ class Interpreter(InterpreterBase):
         self.coredata.compiler_options = new_options
         return comp, cross_comp
 
-    @staticmethod
-    def sort_clike(lang):
-        '''
-        Sorting function to sort the list of languages according to
-        reversed(compilers.clike_langs) and append the unknown langs in the end.
-        The purpose is to prefer C over C++ for files that can be compiled by
-        both such as assembly, C, etc. Also applies to ObjC, ObjC++, etc.
-        '''
-        if lang not in compilers.clike_langs:
-            return 1
-        return -compilers.clike_langs.index(lang)
-
     def add_languages(self, args, required):
         success = True
         need_cross_compiler = self.environment.is_cross_build() and self.environment.cross_info.need_cross_compiler()
-        for lang in sorted(args, key=self.sort_clike):
+        for lang in sorted(args, key=compilers.sort_clike):
             lang = lang.lower()
             if lang in self.coredata.compilers:
                 comp = self.coredata.compilers[lang]
diff --git a/run_unittests.py b/run_unittests.py
index a11e3a5..91ccf37 100755
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -878,6 +878,52 @@ class AllPlatformTests(BasePlatformTests):
             self.assertEqual(wcc.get_exelist(), wrappercc)
             self.assertEqual(wlinker.get_exelist(), wrapperlinker)
 
+    def test_always_prefer_c_compiler_for_asm(self):
+        testdir = os.path.join(self.common_test_dir, '141 c cpp and asm')
+        self.init(testdir)
+        commands = {'cpp-asm': {}, 'cpp-c-asm': {}, 'c-cpp-asm': {}}
+        for cmd in self.get_compdb():
+            # Get compiler
+            split = shlex.split(cmd['command'])
+            if split[0] == 'ccache':
+                compiler = split[1]
+            else:
+                compiler = split[0]
+            # Classify commands
+            if 'Icpp-asm' in cmd['command']:
+                if cmd['file'].endswith('.S'):
+                    commands['cpp-asm']['asm'] = compiler
+                elif cmd['file'].endswith('.cpp'):
+                    commands['cpp-asm']['cpp'] = compiler
+                else:
+                    raise AssertionError('{!r} found in cpp-asm?'.format(cmd['command']))
+            elif 'Ic-cpp-asm' in cmd['command']:
+                if cmd['file'].endswith('.S'):
+                    commands['c-cpp-asm']['asm'] = compiler
+                elif cmd['file'].endswith('.c'):
+                    commands['c-cpp-asm']['c'] = compiler
+                elif cmd['file'].endswith('.cpp'):
+                    commands['c-cpp-asm']['cpp'] = compiler
+                else:
+                    raise AssertionError('{!r} found in c-cpp-asm?'.format(cmd['command']))
+            elif 'Icpp-c-asm' in cmd['command']:
+                if cmd['file'].endswith('.S'):
+                    commands['cpp-c-asm']['asm'] = compiler
+                elif cmd['file'].endswith('.c'):
+                    commands['cpp-c-asm']['c'] = compiler
+                elif cmd['file'].endswith('.cpp'):
+                    commands['cpp-c-asm']['cpp'] = compiler
+                else:
+                    raise AssertionError('{!r} found in cpp-c-asm?'.format(cmd['command']))
+            else:
+                raise AssertionError('Unknown command {!r} found'.format(cmd['command']))
+        self.assertEqual(commands['cpp-asm']['asm'], commands['c-cpp-asm']['c'])
+        self.assertEqual(commands['c-cpp-asm']['asm'], commands['c-cpp-asm']['c'])
+        self.assertEqual(commands['cpp-c-asm']['asm'], commands['cpp-c-asm']['c'])
+        self.assertNotEqual(commands['cpp-asm']['asm'], commands['cpp-asm']['cpp'])
+        self.assertNotEqual(commands['c-cpp-asm']['c'], commands['c-cpp-asm']['cpp'])
+        self.assertNotEqual(commands['cpp-c-asm']['c'], commands['cpp-c-asm']['cpp'])
+
 
 class WindowsTests(BasePlatformTests):
     '''
diff --git a/test cases/common/141 c cpp and asm/main.cpp b/test cases/common/141 c cpp and asm/main.cpp
new file mode 100644
index 0000000..c089870
--- /dev/null
+++ b/test cases/common/141 c cpp and asm/main.cpp
@@ -0,0 +1,11 @@
+#include <iostream>
+
+extern "C" {
+  int get_retval(void);
+  int get_cval(void);
+}
+
+int main(int argc, char **argv) {
+  std::cout << "C++ seems to be working." << std::endl;
+  return get_retval();
+}
diff --git a/test cases/common/141 c cpp and asm/meson.build b/test cases/common/141 c cpp and asm/meson.build
new file mode 100644
index 0000000..5004ef8
--- /dev/null
+++ b/test cases/common/141 c cpp and asm/meson.build
@@ -0,0 +1,17 @@
+project('c cpp and asm', 'c', 'cpp')
+
+cpu = host_machine.cpu_family()
+
+supported_cpus = ['arm', 'x86', 'x86_64']
+
+if not supported_cpus.contains(cpu)
+  error('MESON_SKIP_TEST unsupported cpu:' + cpu)
+endif
+
+if meson.get_compiler('c').get_id() == 'msvc'
+  error('MESON_SKIP_TEST MSVC can\'t compile assembly')
+endif
+
+test('test-cpp-asm', executable('cpp-asm', ['main.cpp', 'retval-' + cpu + '.S']))
+test('test-c-cpp-asm', executable('c-cpp-asm', ['somelib.c', 'main.cpp', 'retval-' + cpu + '.S']))
+test('test-cpp-c-asm', executable('cpp-c-asm', ['main.cpp', 'somelib.c', 'retval-' + cpu + '.S']))
diff --git a/test cases/common/141 c cpp and asm/retval-arm.S b/test cases/common/141 c cpp and asm/retval-arm.S
new file mode 100644
index 0000000..8b37197
--- /dev/null
+++ b/test cases/common/141 c cpp and asm/retval-arm.S
@@ -0,0 +1,8 @@
+#include "symbol-underscore.h"
+
+.text
+.globl SYMBOL_NAME(get_retval)
+
+SYMBOL_NAME(get_retval):
+	mov	r0, #0
+	mov	pc, lr
diff --git a/test cases/common/141 c cpp and asm/retval-x86.S b/test cases/common/141 c cpp and asm/retval-x86.S
new file mode 100644
index 0000000..06bd75c
--- /dev/null
+++ b/test cases/common/141 c cpp and asm/retval-x86.S
@@ -0,0 +1,8 @@
+#include "symbol-underscore.h"
+
+.text
+.globl SYMBOL_NAME(get_retval)
+
+SYMBOL_NAME(get_retval):
+	xorl	%eax, %eax
+	retl
diff --git a/test cases/common/141 c cpp and asm/retval-x86_64.S b/test cases/common/141 c cpp and asm/retval-x86_64.S
new file mode 100644
index 0000000..638921e
--- /dev/null
+++ b/test cases/common/141 c cpp and asm/retval-x86_64.S
@@ -0,0 +1,8 @@
+#include "symbol-underscore.h"
+
+.text
+.globl SYMBOL_NAME(get_retval)
+
+SYMBOL_NAME(get_retval):
+	xorl	%eax, %eax
+	retq
diff --git a/test cases/common/141 c cpp and asm/somelib.c b/test cases/common/141 c cpp and asm/somelib.c
new file mode 100644
index 0000000..e585b8e
--- /dev/null
+++ b/test cases/common/141 c cpp and asm/somelib.c
@@ -0,0 +1,3 @@
+int get_cval (void) {
+  return 0;
+}
diff --git a/test cases/common/141 c cpp and asm/symbol-underscore.h b/test cases/common/141 c cpp and asm/symbol-underscore.h
new file mode 100644
index 0000000..508cf50
--- /dev/null
+++ b/test cases/common/141 c cpp and asm/symbol-underscore.h
@@ -0,0 +1,5 @@
+#if defined(__WIN32__) || defined(__APPLE__)
+# define SYMBOL_NAME(name) _##name
+#else
+# define SYMBOL_NAME(name) name
+#endif