[clang] [Clang] Warn when `std::atomic_thread_fence` is used with `fsanitize=thread` (PR #166542)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,44 @@
+// No warnings in regular compile
+// RUN: %clang_cc1 -verify=no-tsan %s
+
+// Emits warning with `-fsanitize=thread`
+// RUN: %clang_cc1 -verify=with-tsan -fsanitize=thread %s
+
+// No warnings if `-Wno-tsan` is passed
+// RUN: %clang_cc1 -verify=no-tsan -fsanitize=thread -Wno-tsan %s
+
+// Ignoring function
+// RUN: echo "fun:main" > %t
+// RUN: %clang_cc1 -verify=no-tsan -fsanitize=thread -fsanitize-ignorelist=%t %s
+
+// Ignoring source file
+// RUN: echo "src:%s" > %t
+// RUN: %clang_cc1 -verify=no-tsan -fsanitize=thread -fsanitize-ignorelist=%t %s
+
+// no-tsan-no-diagnostics
+
+namespace std {
+  enum memory_order {
+    memory_order_relaxed,
+    memory_order_consume,
+    memory_order_acquire,
+    memory_order_release,
+    memory_order_acq_rel,
+    memory_order_seq_cst,
+  };
+  void atomic_thread_fence(memory_order) {}
+};
+
+__attribute__((no_sanitize("thread")))
+void ignore_1() {
+  std::atomic_thread_fence(std::memory_order_relaxed);
+}
+
+__attribute__((no_sanitize_thread))
+void ignore_2() {
+  std::atomic_thread_fence(std::memory_order_relaxed);
+}
+
+int main() {
+  std::atomic_thread_fence(std::memory_order_relaxed); // with-tsan-warning {{'std::atomic_thread_fence' is not supported with '-fsanitize=thread'}}
+}
----------------
AaronBallman wrote:

I think other tests that would be interesting to try out would be:
```
__attribute__((no_sanitize("thread")))
void func() {
  auto lam = [](){
    std::atomic_thread_fence(std::memory_order_relaxed); // This should still diagnose, right?
  };
}

void other() {
  auto lam = []() __attribute__((no_sanitize("thread"))) {
    std::atomic_thread_fence(std::memory_order_relaxed); // This should not diagnose, right?
  };
}
```
and we probably should document that there can be false positives, e.g.,
```
inline void inline_func() {
  std::atomic_thread_fence(std::memory_order_relaxed); // Still diagnosed even though it's an inline function
}

__attribute__((no_sanitize("thread"))) void caller() {
  inline_func();
  if (0) {
    std::atomic_thread_fence(std::memory_order_relaxed); // Still diagnosed even though it's unreachable
  }
}
```

https://github.com/llvm/llvm-project/pull/166542