Index: cmake/Modules/AddCompilerRT.cmake
===================================================================
--- cmake/Modules/AddCompilerRT.cmake
+++ cmake/Modules/AddCompilerRT.cmake
@@ -342,9 +342,9 @@
   endif()
 endfunction()
 
-# when cross compiling, COMPILER_RT_TEST_COMPILER_CFLAGS help
+# when cross compiling or using a non-default toolchain, COMPILER_RT_TOOLCHAIN_CFLAGS help
 # in compilation and linking of unittests.
-string(REPLACE " " ";" COMPILER_RT_UNITTEST_CFLAGS "${COMPILER_RT_TEST_COMPILER_CFLAGS}")
+string(REPLACE " " ";" COMPILER_RT_UNITTEST_CFLAGS "${COMPILER_RT_TOOLCHAIN_CFLAGS}")
 set(COMPILER_RT_UNITTEST_LINK_FLAGS ${COMPILER_RT_UNITTEST_CFLAGS})
 
 # Unittests support.
@@ -554,6 +554,8 @@
   set(PASSTHROUGH_VARIABLES
     CMAKE_C_COMPILER_TARGET
     CMAKE_CXX_COMPILER_TARGET
+    CMAKE_C_COMPILER_EXTERNAL_TOOLCHAIN
+    CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN
     CMAKE_SHARED_LINKER_FLAGS
     CMAKE_MODULE_LINKER_FLAGS
     CMAKE_EXE_LINKER_FLAGS
Index: cmake/base-config-ix.cmake
===================================================================
--- cmake/base-config-ix.cmake
+++ cmake/base-config-ix.cmake
@@ -219,4 +219,12 @@
     endif()
     set(COMPILER_RT_OS_SUFFIX "")
   endif()
+  # Set up options associated with the toolchain.
+  if(COMPILER_RT_TEST_COMPILER_CFLAGS)
+    # COMPILER_RT_TEST_COMPILER_CFLAGS is used for cross-compiling.
+    set(COMPILER_RT_TOOLCHAIN_CFLAGS ${COMPILER_RT_TEST_COMPILER_CFLAGS})
+  elseif(CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN AND COMPILER_RT_TEST_COMPILER_ID MATCHES "Clang")
+    # Use the non-default toolchain specified for the build when compiling tests.
+    set(COMPILER_RT_TOOLCHAIN_CFLAGS "--gcc-toolchain=${CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN}")
+  endif()
 endmacro()
Index: cmake/config-ix.cmake
===================================================================
--- cmake/config-ix.cmake
+++ cmake/config-ix.cmake
@@ -199,6 +199,7 @@
     if(APPLE)
       list(APPEND ${cflags_out} ${DARWIN_osx_CFLAGS})
     endif()
+    list(APPEND ${cflags_out} ${COMPILER_RT_TOOLCHAIN_CFLAGS})
     string(REPLACE ";" " " ${cflags_out} "${${cflags_out}}")
   endif()
 endmacro()
Index: test/asan/CMakeLists.txt
===================================================================
--- test/asan/CMakeLists.txt
+++ test/asan/CMakeLists.txt
@@ -89,7 +89,7 @@
   foreach(arch ${DARWIN_iossim_ARCHS})
     set(ASAN_TEST_APPLE_PLATFORM "iossim")
     set(ASAN_TEST_TARGET_ARCH ${arch})
-    set(ASAN_TEST_TARGET_CFLAGS "-arch ${arch} -isysroot ${DARWIN_iossim_SYSROOT} ${COMPILER_RT_TEST_COMPILER_CFLAGS}")
+    set(ASAN_TEST_TARGET_CFLAGS "-arch ${arch} -isysroot ${DARWIN_iossim_SYSROOT} ${COMPILER_RT_TOOLCHAIN_CFLAGS}")
     set(ASAN_TEST_CONFIG_SUFFIX "-${arch}-${ASAN_TEST_APPLE_PLATFORM}")
     get_bits_for_arch(${arch} ASAN_TEST_BITS)
     string(TOUPPER ${arch} ARCH_UPPER_CASE)
@@ -106,7 +106,7 @@
   foreach (arch ${DARWIN_ios_ARCHS})
     set(ASAN_TEST_APPLE_PLATFORM "ios")
     set(ASAN_TEST_TARGET_ARCH ${arch})
-    set(ASAN_TEST_TARGET_CFLAGS "-arch ${arch} -isysroot ${DARWIN_ios_SYSROOT} ${COMPILER_RT_TEST_COMPILER_CFLAGS}")
+    set(ASAN_TEST_TARGET_CFLAGS "-arch ${arch} -isysroot ${DARWIN_ios_SYSROOT} ${COMPILER_RT_TOOLCHAIN_CFLAGS}")
     set(ASAN_TEST_CONFIG_SUFFIX "-${arch}-${ASAN_TEST_APPLE_PLATFORM}")
     get_bits_for_arch(${arch} ASAN_TEST_BITS)
     string(TOUPPER ${arch} ARCH_UPPER_CASE)
Index: test/fuzzer/CMakeLists.txt
===================================================================
--- test/fuzzer/CMakeLists.txt
+++ test/fuzzer/CMakeLists.txt
@@ -91,7 +91,7 @@
   foreach(arch ${DARWIN_ios_ARCHS})
     set(LIBFUZZER_TEST_APPLE_PLATFORM "ios")
     set(LIBFUZZER_TEST_TARGET_ARCH ${arch})
-    set(LIBFUZZER_TEST_FLAGS "-arch ${arch} -isysroot ${DARWIN_ios_SYSROOT} ${COMPILER_RT_TEST_COMPILER_CFLAGS}")
+    set(LIBFUZZER_TEST_FLAGS "-arch ${arch} -isysroot ${DARWIN_ios_SYSROOT} ${COMPILER_RT_TOOLCHAIN_CFLAGS}")
     set(LIBFUZZER_TEST_CONFIG_SUFFIX "-${arch}-${LIBFUZZER_TEST_APPLE_PLATFORM}")
     string(TOUPPER ${arch} ARCH_UPPER_CASE)
     set(CONFIG_NAME "IOS${ARCH_UPPER_CASE}Config")
Index: test/lit.common.configured.in
===================================================================
--- test/lit.common.configured.in
+++ test/lit.common.configured.in
@@ -7,7 +7,7 @@
 
 # Generic config options for all compiler-rt lit tests.
 set_default("target_triple", "@COMPILER_RT_DEFAULT_TARGET_TRIPLE@")
-set_default("target_cflags", "@COMPILER_RT_TEST_COMPILER_CFLAGS@")
+set_default("target_cflags", "@COMPILER_RT_TOOLCHAIN_CFLAGS@")
 set_default("host_arch", "@HOST_ARCH@")
 set_default("target_arch", "@COMPILER_RT_DEFAULT_TARGET_ARCH@")
 set_default("host_os", "@HOST_OS@")
Index: test/tsan/CMakeLists.txt
===================================================================
--- test/tsan/CMakeLists.txt
+++ test/tsan/CMakeLists.txt
@@ -53,7 +53,7 @@
   set(TSAN_TEST_APPLE_PLATFORM "iossim")
   set(arch "x86_64")
   set(TSAN_TEST_TARGET_ARCH ${arch})
-  set(TSAN_TEST_TARGET_CFLAGS "-arch ${arch} -isysroot ${DARWIN_iossim_SYSROOT} ${COMPILER_RT_TEST_COMPILER_CFLAGS}")
+  set(TSAN_TEST_TARGET_CFLAGS "-arch ${arch} -isysroot ${DARWIN_iossim_SYSROOT} ${COMPILER_RT_TOOLCHAIN_CFLAGS}")
   set(TSAN_TEST_CONFIG_SUFFIX "-${arch}-${TSAN_TEST_APPLE_PLATFORM}")
   string(TOUPPER ${arch} ARCH_UPPER_CASE)
   set(CONFIG_NAME "IOSSim${ARCH_UPPER_CASE}Config")
@@ -68,7 +68,7 @@
   set(TSAN_TEST_APPLE_PLATFORM "ios")
   set(arch "arm64")
   set(TSAN_TEST_TARGET_ARCH ${arch})
-  set(TSAN_TEST_TARGET_CFLAGS "-arch ${arch} -isysroot ${DARWIN_ios_SYSROOT} ${COMPILER_RT_TEST_COMPILER_CFLAGS}")
+  set(TSAN_TEST_TARGET_CFLAGS "-arch ${arch} -isysroot ${DARWIN_ios_SYSROOT} ${COMPILER_RT_TOOLCHAIN_CFLAGS}")
   set(TSAN_TEST_CONFIG_SUFFIX "-${arch}-${TSAN_TEST_APPLE_PLATFORM}")
   string(TOUPPER ${arch} ARCH_UPPER_CASE)
   set(CONFIG_NAME "IOS${ARCH_UPPER_CASE}Config")