New compiler function 'symbols_have_underscore_prefix'

Check if the compiler prefixes an underscore to global symbols. This is
useful when linking to compiled assembly code, or other code that does
not have its C symbol mangling handled transparently by the compiler.

C symbol mangling is platform and architecture dependent, and a helper
function is needed to detect it. Eg: Windows 32-bit prefixes underscore,
but 64-bit does not. Linux does not prefix an underscore but OS X does.

mesonbuild/compilers.py

@@ -1058,6 +1058,40 @@ void bar() {
 '''
         return self.compiles(templ % (prefix, typename), env, extra_args, dependencies)
 
+    def symbols_have_underscore_prefix(self, env):
+        '''
+        Check if the compiler prefixes an underscore to global C symbols
+        '''
+        symbol_name = b'meson_uscore_prefix'
+        code = '''#ifdef __cplusplus
+        extern "C" {
+        #endif
+        void ''' + symbol_name.decode() + ''' () {}
+        #ifdef __cplusplus
+        }
+        #endif
+        '''
+        args = self.get_cross_extra_flags(env, compile=True, link=False)
+        args += self.get_compiler_check_args()
+        n = 'symbols_have_underscore_prefix'
+        with self.compile(code, args, compile_only=True) as p:
+            if p.returncode != 0:
+                m = 'BUG: Unable to compile {!r} check: {}'
+                raise RuntimeError(m.format(n, p.stdo))
+            if not os.path.isfile(p.output_name):
+                m = 'BUG: Can\'t find compiled test code for {!r} check'
+                raise RuntimeError(m.format(n))
+            with open(p.output_name, 'rb') as o:
+                for line in o:
+                    # Check if the underscore form of the symbol is somewhere
+                    # in the output file.
+                    if b'_' + symbol_name in line:
+                        return True
+                    # Else, check if the non-underscored form is present
+                    elif symbol_name in line:
+                        return False
+        raise RuntimeError('BUG: {!r} check failed unexpectedly'.format(n))
+
     def find_library(self, libname, env, extra_dirs):
         # First try if we can just add the library as -l.
         code = '''int main(int argc, char **argv) {

mesonbuild/interpreter.py

@@ -623,6 +623,7 @@ class CompilerHolder(InterpreterObject):
                              'has_argument' : self.has_argument_method,
                              'first_supported_argument' : self.first_supported_argument_method,
                              'unittest_args' : self.unittest_args_method,
+                             'symbols_have_underscore_prefix': self.symbols_have_underscore_prefix_method,
                             })
 
     def version_method(self, args, kwargs):
@@ -698,6 +699,13 @@ class CompilerHolder(InterpreterObject):
     def get_id_method(self, args, kwargs):
         return self.compiler.get_id()
 
+    def symbols_have_underscore_prefix_method(self, args, kwargs):
+        '''
+        Check if the compiler prefixes _ (underscore) to global C symbols
+        See: https://en.wikipedia.org/wiki/Name_mangling#C
+        '''
+        return self.compiler.symbols_have_underscore_prefix(self.environment)
+
     def unittest_args_method(self, args, kwargs):
         # At time, only D compilers have this feature.
         if not hasattr(self.compiler, 'get_unittest_args'):