[compiler-rt] [test] Fix using toolchains that rely on Clang default configs (#113491)

The use of CLANG_NO_DEFAULT_CONFIG in the tests was added because some
Linux distributions had a global default config file, that added flags
relating to hardening, which interfere with the sanitizer tests. By
setting CLANG_NO_DEFAULT_CONFIG, the global default config files that
are found are ignored, and the sanitizers get the expected default
compiler behaviour.

(This was #60394, which was
fixed in 8ab7625.)

However, some toolchains may rely on default config files for mandatory
parts required for functioning at all - setting things like sysroots,
-rtlib, -unwindlib, -stdlib, -fuse-ld etc. In such a case we can't
forcibly disable any default config, because it will break the otherwise
working toolchain.

Add a test for whether the compiler works while passing
--no-default-config to it. If the option is accepted and the toolchain
still works while that is set, set CLANG_NO_DEFAULT_CONFIG while running
tests.

(This adds a little bit of inconsistency, as we're testing for the
command line option, while using the environment variable. However doing
compile testing with an environment variable isn't quite as easily
doable, and passing an extra command line flag to all compile commands
while testing, is a bit clumsy - therefore this inconsistency.)

(cherry picked from commit a14a83d)

Author: mstorsjo

compiler-rt/CMakeLists.txt

@@ -39,6 +39,22 @@ include(CompilerRTUtils)
 include(CMakeDependentOption)
 include(GetDarwinLinkerVersion)
 
+include(CheckCXXCompilerFlag)
+
+# Check if we can compile with --no-default-config, or if that omits a config
+# file that is essential for the toolchain to work properly.
+#
+# Using CMAKE_REQUIRED_FLAGS to make sure the flag is used both for compilation
+# and for linking.
+#
+# Doing this test early on, to see if the flag works on the toolchain
+# out of the box. Later on, we end up adding -nostdlib and similar flags
+# to all test compiles, which easily can give false positives on this test.
+set(OLD_CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS}")
+set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --no-default-config")
+check_cxx_compiler_flag("" COMPILER_RT_HAS_NO_DEFAULT_CONFIG_FLAG)
+set(CMAKE_REQUIRED_FLAGS "${OLD_CMAKE_REQUIRED_FLAGS}")
+
 option(COMPILER_RT_BUILD_BUILTINS "Build builtins" ON)
 mark_as_advanced(COMPILER_RT_BUILD_BUILTINS)
 option(COMPILER_RT_DISABLE_AARCH64_FMV "Disable AArch64 Function Multi Versioning support" OFF)

compiler-rt/test/CMakeLists.txt

@@ -12,6 +12,8 @@ pythonize_bool(COMPILER_RT_ENABLE_INTERNAL_SYMBOLIZER)
 
 pythonize_bool(COMPILER_RT_HAS_AARCH64_SME)
 
+pythonize_bool(COMPILER_RT_HAS_NO_DEFAULT_CONFIG_FLAG)
+
 configure_compiler_rt_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.common.configured.in
   ${CMAKE_CURRENT_BINARY_DIR}/lit.common.configured)

compiler-rt/test/lit.common.cfg.py

@@ -980,7 +980,11 @@ def is_windows_lto_supported():
 # default configs for the test runs. In particular, anything hardening
 # related is likely to cause issues with sanitizer tests, because it may
 # preempt something we're looking to trap (e.g. _FORTIFY_SOURCE vs our ASAN).
-config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
+#
+# Only set this if we know we can still build for the target while disabling
+# default configs.
+if config.has_no_default_config_flag:
+    config.environment["CLANG_NO_DEFAULT_CONFIG"] = "1"
 
 if config.has_compiler_rt_libatomic:
   base_lib = os.path.join(config.compiler_rt_libdir, "libclang_rt.atomic%s.so"

compiler-rt/test/lit.common.configured.in

@@ -53,6 +53,7 @@ set_default("test_standalone_build_libs", @COMPILER_RT_TEST_STANDALONE_BUILD_LIB
 set_default("has_compiler_rt_libatomic", @COMPILER_RT_BUILD_STANDALONE_LIBATOMIC_PYBOOL@)
 set_default("aarch64_sme", @COMPILER_RT_HAS_AARCH64_SME_PYBOOL@)
 set_default("darwin_linker_version", "@COMPILER_RT_DARWIN_LINKER_VERSION@")
+set_default("has_no_default_config_flag", @COMPILER_RT_HAS_NO_DEFAULT_CONFIG_FLAG_PYBOOL@)
 # True iff the test suite supports ignoring the test compiler's runtime library path
 # and using `config.compiler_rt_libdir` instead. This only matters when the runtime
 # library paths differ.