Rename cxx-benchmark-unittests target and convert to LIT.

This patch renames the cxx-benchmark-unittests to check-cxx-benchmarks
and converts the target to use LIT in order to make the tests run faster
and provide better output.

In particular this runs each benchmark in a suite one by one, allowing
more parallelism while ensuring output isn't garbage with multiple threads.

Additionally, it adds the CMake flag '-DLIBCXX_BENCHMARK_TEST_ARGS=<list>'
to specify what options are passed when running the benchmarks.

llvm-svn: 346888

1 files changed, 9 insertions, 0 deletions

diff --git a/libcxx/docs/BuildingLibcxx.rst b/libcxx/docs/BuildingLibcxx.rst
index 66cfb7b21ea..12acd0bbd2f 100644
--- a/libcxx/docs/BuildingLibcxx.rst
+++ b/libcxx/docs/BuildingLibcxx.rst
@@ -316,6 +316,15 @@ libc++ Feature Options
   Build the libc++ benchmark tests and the Google Benchmark library needed
   to support them.
 
+.. option:: LIBCXX_BENCHMARK_TEST_ARGS:STRING
+
+  **Default**: ``--benchmark_min_time=0.01``
+
+  A semicolon list of arguments to pass when running the libc++ benchmarks using the
+  ``check-cxx-benchmarks`` rule. By default we run the benchmarks for a very short amount of time,
+  since the primary use of ``check-cxx-benchmarks`` is to get test and sanitizer coverage, not to
+  get accurate measurements.
+
 .. option:: LIBCXX_BENCHMARK_NATIVE_STDLIB:STRING
 
   **Default**:: ``""``