[libcxx] Add logic to probe compiler in tests.

[EricWF]

Summary

This patch probes the cxx compiler used during testing by getting it to dump its predefined macros. Based on the value of these macros the compiler name and compiler name + version are added to the available features.

There are three compiler names:

• clang
• apple-clang
• gcc.

The available features added are equivalent to:

• '%s' % compiler_name
• '%s-%s.%s' % compiler_name, major_version, minor_version

This information can be used to XFAIL tests on different compilers / versions.

[EricWF]

Remove brackets to maintain consistent style throughout file.

[jroelofs]

Which tests do you expect to add XFAIL/UNSUPPORTED to based on these new features?

Also... LGTM

[EricWF]

There are some compiler bugs in the handling of _Atomic with trivial types that I want to XFAIL (they were fixed in 3.5 but the most recent apple clang doesn't have the fix).
I also think there are problems with relaxed constexpr in older versions of clang.

Thanks for the review.

test/lit.cfg

 
     def configure(self):
         self.configure_cxx()
+        self.probe_cxx()
         self.configure_triple()
         self.configure_src_root()
         self.configure_obj_root()

             self.lit_config.fatal('must specify user parameter cxx_under_test '
                                   '(e.g., --param=cxx_under_test=clang++)')
 
+    def probe_cxx(self):
+        # Dump all of the predefined macros
+        dump_macro_cmd = [self.cxx, '-dM', '-E', '-x', 'c++', '/dev/null']
+        out, err, rc = lit.util.executeCommand(dump_macro_cmd)
+        if rc != 0:
+            self.lit_config.warning('Failed to dump macros for compiler: %s' %
+                                    self.cxx)
+            return
+        # Create a dict containing all the predefined macros.
+        macros = {}
+        lines = [l.strip() for l in out.split('\n') if l.strip()]
+        for l in lines:
+            assert l.startswith('#define ')
+            l = l[len('#define '):]
+            macro, _, value = l.partition(' ')
+            macros[macro] = value
+        # Add compiler information to available features.
+        compiler_name = None
+        major_ver = minor_ver = None
+        if '__clang__' in macros.keys():
+            compiler_name = 'clang'
+            # Treat apple's llvm fork differently.
+            if '__apple_build_type__' in macros.keys():
+                compiler_name = 'apple-clang'
+            major_ver = macros['__clang_major__']
+            minor_ver = macros['__clang_minor__']
+        elif '__GNUC__' in macros.keys():
+            compiler_name = 'gcc'
+            major_ver = macros['__GNUC__']
+            minor_ver = macros['__GNUC_MINOR__']
+        else:
+            self.lit_config.warning('Failed to detect compiler for cxx: %s' %
+                                    self.cxx)
+        if compiler_name is not None:
+            self.config.available_features.add(compiler_name)
+            self.config.available_features.add('%s-%s.%s' % (
+                compiler_name, major_ver, minor_ver))
+
     def configure_src_root(self):
         self.src_root = self.get_lit_conf(
             'libcxx_src_root', os.path.dirname(self.config.test_source_root))