[libcxx] Add special warning flag detection logic to compiler.py

[EricWF]

Summary

Detecting -Wno-<warning> flags can be tricky with GCC (See https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html). This patch adds a special addWarningFlagIfSupported(<flag>) method to the test compiler object that can be used to add warning flags. The goal of this patch is to help get the test suite running with more warnings.

[jroelofs]

the addWarningFlagIfSupported part of this patch LGTM.

test/libcxx/compiler.py

88

this looks unused to me.

105

this looks unused to me.

[EricWF]

Those methods are currently unused but are you opposed to me adding them in this patch? They will be useful for compiling short tests to check for compiler features.

[jroelofs]

I'd say put them in when you have actual uses for them....

test/libcxx/compiler.py

195

... you could at least use compileString here.

[EricWF]

Remove unused "CompileString" functions. The reason they can't be used to implement addWarningFlagIfSupported is because we have to fuss with the flags before invoking the compiler .

test/libcxx/compiler.py

             return True
         else:
             return False
+
+    def addWarningFlagIfSupported(self, flag):
+        """
+        addWarningFlagIfSupported - Add a warning flag if the compiler
+        supports it. Unlike addCompileFlagIfSupported, this function detects
+        when "-Wno-<warning>" flags are unsupported. If flag is a
+        "-Wno-<warning>" GCC will not emit an unknown option diagnostic unless
+        another error is triggered during compilation.
+        """
+        assert isinstance(flag, str)
+        if not flag.startswith('-Wno-'):
+            return self.addCompileFlagIfSupported(flag)
+        flags = ['-Werror', flag]
+        cmd = self.compileCmd('-', os.devnull, flags)
+        # Remove '-v' because it will cause the command line invocation
+        # to be printed as part of the error output.
+        # TODO(EricWF): Are there other flags we need to worry about?
+        if '-v' in cmd:
+            cmd.remove('-v')
+        out, err, rc = lit.util.executeCommand(cmd, input='#error\n')
+        assert rc != 0
+        if flag in err:
+            return False
+        self.compile_flags += [flag]
+        return True

test/libcxx/test/config.py

                 '-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER',
                 '-Wall', '-Werror'
             ]
-            self.cxx.addCompileFlagIfSupported('-Wno-attributes')
-            if self.cxx.type == 'clang' or self.cxx.type == 'apple-clang':
-                self.cxx.addCompileFlagIfSupported('-Wno-pessimizing-move')
-                self.cxx.addCompileFlagIfSupported('-Wno-c++11-extensions')
-                self.cxx.addCompileFlagIfSupported('-Wno-user-defined-literals')
+            self.cxx.addWarningFlagIfSupported('-Wno-attributes')
+            self.cxx.addWarningFlagIfSupported('-Wno-pessimizing-move')
+            self.cxx.addWarningFlagIfSupported('-Wno-c++11-extensions')
+            self.cxx.addWarningFlagIfSupported('-Wno-user-defined-literals')
             std = self.get_lit_conf('std', None)
             if std in ['c++98', 'c++03']:
                 # The '#define static_assert' provided by libc++ in C++03 mode
                 # causes an unused local typedef whenever it is used.
-                self.cxx.addCompileFlagIfSupported('-Wno-unused-local-typedef')
+                self.cxx.addWarningFlagIfSupported('-Wno-unused-local-typedef')
 
     def configure_sanitizer(self):
         san = self.get_lit_conf('use_sanitizer', '').strip()