compilers: Try harder to dedup builtin libs

-ldl, -lm, -lpthread should always be de-duplicated, no matter what.

Closes #2150

mesonbuild/compilers/compilers.py

@@ -659,6 +659,9 @@ class CompilerArgs(list):
     # Only UNIX shared libraries require this. Others have a fixed extension.
     dedup1_regex = re.compile(r'([\/\\]|\A)lib.*\.so(\.[0-9]+)?(\.[0-9]+)?(\.[0-9]+)?$')
     dedup1_args = ('-c', '-S', '-E', '-pipe', '-pthread')
+    # In generate_link() we add external libs without de-dup, but we must
+    # *always* de-dup these because they're special arguments to the linker
+    always_dedup_args = ('-ldl', '-lm', '-lpthread')
     compiler = None
 
     def _check_args(self, args):
@@ -793,7 +796,7 @@ def extend_preserving_lflags(self, iterable):
         normal_flags = []
         lflags = []
         for i in iterable:
-            if i.startswith('-l') or i.startswith('-L'):
+            if i not in self.always_dedup_args and (i.startswith('-l') or i.startswith('-L')):
                 lflags.append(i)
             else:
                 normal_flags.append(i)

run_unittests.py

@@ -5039,6 +5039,19 @@ def test_ldflag_dedup(self):
                 max_count = max(max_count, line.count(search_term))
         self.assertEqual(max_count, 1, 'Export dynamic incorrectly deduplicated.')
 
+    def test_compiler_libs_static_dedup(self):
+        testdir = os.path.join(self.unit_test_dir, '55 dedup compiler libs')
+        self.init(testdir)
+        build_ninja = os.path.join(self.builddir, 'build.ninja')
+        with open(build_ninja, 'r', encoding='utf-8') as f:
+            lines = f.readlines()
+        for lib in ('-ldl', '-lm'):
+            for line in lines:
+                if lib not in line:
+                    continue
+                # Assert that
+                self.assertEqual(len(line.split(lib)), 2, msg=(lib, line))
+
 
 def should_run_cross_arm_tests():
     return shutil.which('arm-linux-gnueabihf-gcc') and not platform.machine().lower().startswith('arm')