[CMake] Support compiler-rt builtins library in tests

[phosek]

Summary

We're building tests with -nostdlib which means that we need to
explicitly include the builtins library. When using libgcc (default)
we can simply include -lgcc_s on the link line, but when using
compiler-rt builtins we need a complete path to the builtins library.

This path is already available in CMake as <PROJECT>_BUILTINS_LIBRARY,
so we just need to pass that path to lit and if config.compiler_rt is
true, link it to the test.

Prior to this patch, running tests when compiler-rt is being used as
the builtins library was broken as all tests would fail to link, but
with this change running tests when compiler-rt bultins library is
being used should be supported.

[phosek]

Ping?

[ldionne]

Can you document how that LIT setting works and what is its intended use case?

[phosek]

In D56701#1383157, @ldionne wrote:

Can you document how that LIT setting works and what is its intended use case?

Done

[ldionne]

In D56701#1383291, @phosek wrote:

In D56701#1383157, @ldionne wrote:

Can you document how that LIT setting works and what is its intended use case?

Done

I meant document it in libcxx/docs/TestingLibcxx.rst.

[phosek]

In D56701#1383293, @ldionne wrote:

In D56701#1383291, @phosek wrote:

In D56701#1383157, @ldionne wrote:

Can you document how that LIT setting works and what is its intended use case?

Done

I meant document it in libcxx/docs/TestingLibcxx.rst.

Sorry about that, I wasn't aware of that file. I've also added documentation for llvm_unwinder and compiler_rt which were missing.

[ldionne]

libcxx/docs/TestingLibcxx.rst

190 ↗

(On Diff #185111)

In light of this.. would it make sense to remove compiler_rt in favour of just builtins_library? Basically, compiler_rt == True if and only if Bool(builtins_library).

[ldionne]

Thanks for the cleanup!

docs/TestingLibcxx.rst

.. option:: use_sanitizer=<sanitizer name>
   building libc++ then that sanitizer will be used by default.
 
 .. option:: color_diagnostics
 
   Enable the use of colorized compile diagnostics. If the color_diagnostics
   option is specified or the environment variable LIBCXX_COLOR_DIAGNOSTICS is
   present then color diagnostics will be enabled.
 
+.. option:: llvm_unwinder
+
+  Enable the use of LLVM unwinder instead of libgcc.
+
+.. option:: builtins_library
+
+  Path to the builtins library to use instead of libgcc.
+
 
 Environment Variables
 ---------------------
 
 .. envvar:: LIBCXX_SITE_CONFIG=<path/to/lit.site.cfg>
 
   Specify the site configuration to use when running the tests.
   Also see `libcxx_site_config`.

test/lit.site.cfg.in

 config.gcc_toolchain            = "@LIBCXX_GCC_TOOLCHAIN@"
 config.generate_coverage        = @LIBCXX_GENERATE_COVERAGE@
 config.target_info              = "@LIBCXX_TARGET_INFO@"
 config.test_linker_flags        = "@LIBCXX_TEST_LINKER_FLAGS@"
 config.test_compiler_flags      = "@LIBCXX_TEST_COMPILER_FLAGS@"
 
 config.executor                 = "@LIBCXX_EXECUTOR@"
 config.llvm_unwinder            = @LIBCXXABI_USE_LLVM_UNWINDER@
-config.compiler_rt              = @LIBCXX_USE_COMPILER_RT@
+config.builtins_library         = "@LIBCXX_BUILTINS_LIBRARY@"
 config.has_libatomic            = @LIBCXX_HAS_ATOMIC_LIB@
 config.use_libatomic            = @LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB@
 config.debug_build              = @LIBCXX_DEBUG_BUILD@
 config.libcxxabi_shared         = @LIBCXXABI_ENABLE_SHARED@
 config.cxx_ext_threads          = @LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY@
 
 # Let the main config do the real work.
 config.loaded_site_config = True
 lit_config.load_config(config, "@LIBCXX_SOURCE_DIR@/test/lit.cfg")

utils/libcxx/test/target_info.py

def add_cxx_link_flags(self, flags):
             flags += ['-lpthread']
             if not shared_libcxx or enable_fs:
               flags += ['-lrt']
         flags += ['-lc']
         if llvm_unwinder:
             flags += ['-lunwind', '-ldl']
         else:
             flags += ['-lgcc_s']
-        compiler_rt = self.full_config.get_lit_bool('compiler_rt', False)
-        if not compiler_rt:
+        builtins_lib = self.full_config.get_lit_conf('builtins_library')
+        if builtins_lib:
+            flags += [builtins_lib]
+        else:
             flags += ['-lgcc']
         use_libatomic = self.full_config.get_lit_bool('use_libatomic', False)
         if use_libatomic:
             flags += ['-latomic']
         san = self.full_config.get_lit_conf('use_sanitizer', '').strip()
         if san:
             # The libraries and their order are taken from the
             # linkSanitizerRuntimeDeps function in