[gn build] Add support for expensive checks

[MaskRay]

Looks good. Worth waiting for a second opinion

[hans]

Should it also get piped through to here? https://github.com/llvm/llvm-project/blob/31e5f7120bdd2f76337686d9d169b1c00e6ee69c/llvm/utils/gn/secondary/compiler-rt/test/BUILD.gn#L54

[aeubanks]

Pipe through LLVM_ENABLE_EXPENSIVE_CHECKS_PYBOOL as well

[hans]

lgtm

llvm/utils/gn/build/BUILD.gn

 
 config("compiler_defaults") {
   defines = []
 
   if (!llvm_enable_assertions) {
     defines += [ "NDEBUG" ]
   }
 
+  if (llvm_enable_expensive_checks) {
+    defines += [ "EXPENSIVE_CHECKS" ]
+  }
+
   asmflags = target_flags
   cflags = target_flags
   ldflags = target_flags + target_ldflags
 
   if (host_os == "mac" && clang_base_path != "") {
     cflags += [
       "-isysroot",
       mac_sdk_path,

llvm/utils/gn/build/buildflags.gni

 declare_args() {
   # Whether to build with debug information.
   is_debug = false
 }
 
 # args that depend on other args must live in a later declare_args() block.
 declare_args() {
   # Whether to build with optimizations.
   is_optimized = !is_debug
 
   # Whether to enable assertions.
   llvm_enable_assertions = true
+
+  # Whether to enable expensive checks.
+  llvm_enable_expensive_checks = false
 }

llvm/utils/gn/secondary/compiler-rt/test/BUILD.gn

 import("//compiler-rt/target.gni")
 import("//compiler-rt/test/test.gni")
 import("//llvm/triples.gni")
+import("//llvm/utils/gn/build/buildflags.gni")
 import("//llvm/utils/gn/build/libs/zlib/enable.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 import("//llvm/utils/gn/build/write_cmake_config.gni")
 import("//llvm/version.gni")
 
 write_cmake_config("lit_common_configured") {
   input = "lit.common.configured.in"
   output = "$target_gen_dir/lit.common.configured"
values = [
     "COMPILER_RT_INTERCEPT_LIBDISPATCH_PYBOOL=False",
     "COMPILER_RT_RESOLVED_LIBRARY_OUTPUT_DIR=" +
         rebase_path(crt_current_out_dir),
     "COMPILER_RT_EMULATOR=",
     "COMPILER_RT_ASAN_SHADOW_SCALE=",
     "SANITIZER_CAN_USE_CXXABI_PYBOOL=True",
     "COMPILER_RT_HAS_LLD_PYBOOL=True",
     "COMPILER_RT_HAS_GWP_ASAN_PYBOOL=False",
-    "LLVM_ENABLE_EXPENSIVE_CHECKS_PYBOOL=False",
     "HAVE_RPC_XDR_H=0",
     "ANDROID_NDK_VERSION=19",
     "ANDROID_SERIAL_FOR_TESTING=$android_serial_for_testing",
   ]
 
+  if (llvm_enable_expensive_checks) {
+    values += [ "LLVM_ENABLE_EXPENSIVE_CHECKS_PYBOOL=True" ]
+  } else {
+    values += [ "LLVM_ENABLE_EXPENSIVE_CHECKS_PYBOOL=False" ]
+  }
+
   if (host_cpu == "x64") {
     values += [ "HOST_ARCH=x86_64" ]
   } else {
     assert(false, "unimplemented host_cpu " + host_cpu)
   }
 
   if (host_os == "mac") {
     values += [ "HOST_OS=Darwin" ]