[gn build] Allow use_ubsan=true on mac and unbreak use_asan and use_ubsan

[thakis]

Summary

use_ubsan=true seems to Just Work on macOS, so allow it.

https://reviews.llvm.org/D122862 broke use_asan=true and use_ubsan=true builds
on Linux too. This makes this go again by explicitly disabling asan and ubsan
for the baremetal part of the build. See discussion on
https://reviews.llvm.org/D122862 for other possible approaches.

[aeubanks]

I like this solution the best

[pcc]

llvm/utils/gn/build/toolchain/BUILD.gn

256–257

Can we add these lines to the stage2_unix_toolchain template instead, as I mentioned in D122862?

[pcc]

llvm/utils/gn/build/toolchain/BUILD.gn

256–257

On the other review @phosek mentioned that use_asan ought to affect libc++ and so on.

That seems reasonable enough, but we might need to change the build a bit to make that work.

Maybe we can move the sanitizer flags into a default config, and have the sanitizers and builtins libraries subtract that from the config?

[thakis]

llvm/utils/gn/build/toolchain/BUILD.gn

256–257

https://reviews.llvm.org/D122862#3587372 mentions that for libc++, it'd be nice to build with sanitizers enabled.

Which is a good point, but so is yours in https://reviews.llvm.org/D122862#3587325.

I think that means we want to have a stage2_unix_toolchain with sanitizers on and one with sanitizers off, and use the one with sanitizers on for libc++, but the one with sanitizers off for building the sanitizers.

…but that's something I don't want to tackle right now, so I kept this as-is to maintain the status quo from before the baremetal patch for now, and added a FIXME pointing to this comment.

256–257

That's an even better approach. Let's do that! (But again, in another patch :))

[pcc]

LGTM

llvm/utils/gn/build/BUILD.gn

config("compiler_defaults") {
   if (sysroot != "" && current_os != "win" && is_clang) {
     cflags += [ "-Wpoison-system-directories" ]
   }
   assert(
       !use_goma || sysroot != "",
       "goma needs a sysroot: run `llvm/utils/sysroot.py make-fake --out-dir=sysroot` and add `sysroot = \"//sysroot\"` to your args.gn")
 
   if (use_ubsan) {
-    assert(is_clang && current_os == "linux",
-           "ubsan only supported on Linux/Clang")
+    assert(is_clang && (current_os == "ios" || current_os == "linux" ||
+                            current_os == "mac"),
+           "ubsan only supported on iOS/Clang, Linux/Clang or macOS/Clang")
     cflags += [
       "-fsanitize=undefined",
       "-fno-sanitize-recover=all",
     ]
     ldflags += [ "-fsanitize=undefined" ]
   }
 
   if (use_asan) {

llvm/utils/gn/build/toolchain/BUILD.gn

stage2_unix_toolchain("stage2_iossim_x64") {
     }
   }
 }
 
 stage2_unix_toolchain("stage2_baremetal_aarch64") {
   toolchain_args = {
     current_os = "baremetal"
     current_cpu = "arm64"
+    use_asan = false
+    use_ubsan = false
   }
 }
 
 template("win_toolchain") {
   toolchain(target_name) {
     # https://groups.google.com/a/chromium.org/d/msg/gn-dev/F_lv5T-tNDM
     forward_variables_from(invoker.toolchain_args, "*")
     not_needed("*")