[RFC][libc++][benchmark] Enable benchmark optimizations.

libcxx/utils/libcxx/test/dsl.py

@@ -511,6 +511,28 @@ def pretty(self, config, litParams):
         return "add {} to %{{compile_flags}}".format(self._getFlag(config))
 
 
+class AddBenchmarkCompileFlag(ConfigAction):
+    """
+    This action adds the given flag to the %{benchmark_flags} substitution.
+
+    The flag can be a string or a callable, in which case it is called with the
+    configuration to produce the actual flag (as a string).
+    """
+
+    def __init__(self, flag):
+        self._getFlag = lambda config: flag(config) if callable(flag) else flag
+
+    def applyTo(self, config):
+        flag = self._getFlag(config)
+        _ensureFlagIsSupported(config, flag)
+        config.substitutions = _appendToSubstitution(
+            config.substitutions, "%{benchmark_flags}", flag
+        )
+
+    def pretty(self, config, litParams):
+        return "add {} to %{{benchmark_flags}}".format(self._getFlag(config))
+
+
 class AddLinkFlag(ConfigAction):
     """
     This action appends the given flag to the %{link_flags} substitution.

libcxx/utils/libcxx/test/params.py

@@ -195,11 +195,12 @@ def getSuitableClangTidy(cfg):
     ),
     Parameter(
         name="optimization",
-        choices=["none", "speed", "size"],
+        choices=["default", "none", "speed", "size"],
         type=str,
         help="The optimization level to use when compiling the test suite.",
-        default="none",
+        default="default", # defaults, test -> none, benchmarks -> speed
         actions=lambda opt: filter(None, [
+            AddBenchmarkCompileFlag(lambda cfg: getSpeedOptimizationFlag(cfg)) if opt == "default" else None,
             AddCompileFlag(lambda cfg: getSpeedOptimizationFlag(cfg)) if opt == "speed" else None,
             AddCompileFlag(lambda cfg: getSizeOptimizationFlag(cfg)) if opt == "size" else None,
             AddFeature(f'optimization={opt}'),