diff --git a/compiler-rt/test/CMakeLists.txt b/compiler-rt/test/CMakeLists.txt
--- a/compiler-rt/test/CMakeLists.txt
+++ b/compiler-rt/test/CMakeLists.txt
@@ -1,6 +1,13 @@
 # Needed for lit support in standalone builds.
 include(AddLLVM)
 
+option(COMPILER_RT_TEST_BUILT_LIBS
+  "When testing use the just built runtime libraries instead of the compiler's. \
+  This option has no effect when the compiler-rt build library directory is  \
+  the same as the compiler's resource library directory."
+  ON)
+pythonize_bool(COMPILER_RT_TEST_BUILT_LIBS)
+
 pythonize_bool(LLVM_ENABLE_EXPENSIVE_CHECKS)
 configure_compiler_rt_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.common.configured.in
diff --git a/compiler-rt/test/builtins/Unit/lit.site.cfg.py.in b/compiler-rt/test/builtins/Unit/lit.site.cfg.py.in
--- a/compiler-rt/test/builtins/Unit/lit.site.cfg.py.in
+++ b/compiler-rt/test/builtins/Unit/lit.site.cfg.py.in
@@ -7,6 +7,7 @@
 config.is_msvc = @MSVC_PYBOOL@
 config.builtins_is_msvc = @BUILTINS_IS_MSVC_PYBOOL@
 config.builtins_lit_source_features = "@BUILTINS_LIT_SOURCE_FEATURES@"
+config.test_suite_supports_testing_built_libs = True
 
 # Load common config for all compiler-rt lit tests.
 lit_config.load_config(config, "@COMPILER_RT_BINARY_DIR@/test/lit.common.configured")
diff --git a/compiler-rt/test/lit.common.cfg.py b/compiler-rt/test/lit.common.cfg.py
--- a/compiler-rt/test/lit.common.cfg.py
+++ b/compiler-rt/test/lit.common.cfg.py
@@ -21,6 +21,41 @@
   import pipes
   sh_quote = pipes.quote
 
+def find_compiler_libdir():
+  """
+    Returns the path to library resource directory used
+    by the compiler.
+  """
+  if config.compiler_id != 'Clang':
+    # TODO: Support other compilers.
+    return None
+  clang_cmd = [
+    config.clang.strip(),
+    '-print-file-name=lib',
+  ]
+  try:
+    result = subprocess.run(
+      clang_cmd,
+      stdout=subprocess.PIPE,
+      stderr=subprocess.PIPE,
+      check=True
+    )
+  except subprocess.CalledProcessError as e:
+    msg = f'Failed to run {clang_cmd}\nrc:{e.returncode}\nstdout:{e.stdout}\ne.stderr{e.stderr}'
+    lit_config.fatal(msg)
+  path = result.stdout.decode().strip()
+  if not os.path.exists(path):
+    lit_config.fatal(f'Path reported by clang does not exist: {path}')
+  path = os.path.realpath(path)
+
+  # TODO: Support other platforms
+  if config.host_os == 'Darwin':
+    path = os.path.join(path, 'darwin')
+  else:
+    return None
+  return os.path.realpath(path)
+
+
 # Choose between lit's internal shell pipeline runner and a real shell.  If
 # LIT_USE_INTERNAL_SHELL is in the environment, we use that as an override.
 use_lit_shell = os.environ.get("LIT_USE_INTERNAL_SHELL")
@@ -63,6 +98,35 @@
 # Add compiler ID to the list of available features.
 config.available_features.add(compiler_id)
 
+# Ask the compiler for the path to libraries it is going to use. If this
+# doesn't match config.compiler_rt_libdir then it means we will be testing the
+# compiler's own runtime libraries rather than the ones we built.
+# Warn about about this and handle appropriately.
+compiler_libdir = find_compiler_libdir()
+if compiler_libdir:
+  compiler_rt_libdir_real = os.path.realpath(config.compiler_rt_libdir)
+  if compiler_libdir != compiler_rt_libdir_real:
+    lit_config.warning(
+      'Compiler lib dir != compiler-rt lib dir\n'
+      f'Compiler libdir: {compiler_libdir}\n'
+      f'compiler-rt lib dir: {compiler_rt_libdir_real}')
+    if config.test_built_libs:
+      # Use just built runtime libraries
+      if not config.test_suite_supports_testing_built_libs:
+        # Test suite doesn't support this configuration.
+        lit_config.fatal(
+            'The compiler being used for testing is not using runtime '
+            'libraries created by this build but testing is configured to test '
+            'the built libraries (COMPILER_RT_TEST_BUILT_LIBS is ON). '
+            'Testing cannot proceed. To fix this modify this test suite to support this testing configuration.'
+            'Alternatively set COMPILER_RT_TEST_BUILT_LIBS to OFF to test the '
+            'runtime libraries of the compiler instead.'
+        )
+    else:
+      # Use Compiler's resource library directory instead.
+      config.compiler_rt_libdir = compiler_libdir
+    lit_config.warning(f'Testing using libraries in {config.compiler_rt_libdir}')
+
 # If needed, add cflag for shadow scale.
 if config.asan_shadow_scale != '':
   config.target_cflags += " -mllvm -asan-mapping-scale=" + config.asan_shadow_scale
diff --git a/compiler-rt/test/lit.common.configured.in b/compiler-rt/test/lit.common.configured.in
--- a/compiler-rt/test/lit.common.configured.in
+++ b/compiler-rt/test/lit.common.configured.in
@@ -46,6 +46,10 @@
 set_default("have_rpc_xdr_h", @HAVE_RPC_XDR_H@)
 set_default("gwp_asan", @COMPILER_RT_HAS_GWP_ASAN_PYBOOL@)
 set_default("expensive_checks", @LLVM_ENABLE_EXPENSIVE_CHECKS_PYBOOL@)
+set_default("test_built_libs", @COMPILER_RT_TEST_BUILT_LIBS_PYBOOL@)
+# True iff the test suite will use just built libs even if the compiler
+# defaults to using its own different libs.
+set_default("test_suite_supports_testing_built_libs", False)
 config.available_features.add('target-is-%s' % config.target_arch)
 
 if config.enable_per_target_runtime_dir: