Hi Yi, > My plan to implement this is by D4521, where GCCBuiltin __builtin_arm_[dmb, dsb, isb] are tested to be correctly selected by Clang when arm64 triple is supplied. OK, I see. There aren't actually any AArch64 builtins yet so it can't be tested. That sounds reasonable. Go for it! Tim. http://reviews.llvm.org/D4516