[CodeGen] Disable UBSan for coroutine functions

Summary:
As explained in http://lists.llvm.org/pipermail/llvm-dev/2018-March/121924.html,
the LLVM coroutines transforms are not yet able to move the
instructions for UBSan null checking past coroutine suspend boundaries.
For now, disable all UBSan checks when generating code for coroutines
functions.

I also considered an approach where only '-fsanitize=null' would be disabled,
However in practice this led to other LLVM errors when writing object files:
"Cannot represent a difference across sections". For now, disable all
UBSan checks until coroutine transforms are updated to handle them.

Test Plan:
1. check-clang
2. Compile the program in https://gist.github.com/modocache/54a036c3bf9c06882fe85122e105d153
   using the '-fsanitize=null' option and confirm it does not crash
   during LLVM IR generation.

Reviewers: GorNishanov, vsk, eric_niebler, lewissbaker

Reviewed By: vsk

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D44672

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@368675 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CodeGenFunction.cpp

@@ -732,6 +732,15 @@ void CodeGenFunction::StartFunction(GlobalDecl GD,
       SanOpts.Mask &= ~SanitizerKind::CFIUnrelatedCast;
   }
 
+  // Ignore null checks in coroutine functions since the coroutines passes
+  // are not aware of how to move the extra UBSan instructions across the split
+  // coroutine boundaries.
+  if (D && SanOpts.has(SanitizerKind::Null))
+    if (const auto *FD = dyn_cast<FunctionDecl>(D))
+      if (FD->getBody() &&
+          FD->getBody()->getStmtClass() == Stmt::CoroutineBodyStmtClass)
+        SanOpts.Mask &= ~SanitizerKind::Null;
+
   // Apply xray attributes to the function (as a string, for now)
   if (D) {
     if (const auto *XRayAttr = D->getAttr<XRayInstrumentAttr>()) {

test/CodeGenCXX/ubsan-coroutines.cpp

@@ -0,0 +1,49 @@
+// This test merely verifies that emitting the object file does not cause a
+// crash when the LLVM coroutines passes are run.
+// RUN: %clang_cc1 -emit-obj -std=c++2a -fsanitize=null %s -o %t.o
+
+namespace std::experimental {
+template <typename R, typename... T> struct coroutine_traits {
+  using promise_type = typename R::promise_type;
+};
+
+template <class Promise = void> struct coroutine_handle;
+template <> struct coroutine_handle<void> {
+  static coroutine_handle from_address(void *) noexcept;
+  coroutine_handle() = default;
+  template <class PromiseType>
+  coroutine_handle(coroutine_handle<PromiseType>) noexcept;
+};
+template <class Promise> struct coroutine_handle : coroutine_handle<void> {
+  coroutine_handle() = default;
+  static coroutine_handle from_address(void *) noexcept;
+};
+}
+
+struct suspend_always {
+  bool await_ready() noexcept;
+  void await_suspend(std::experimental::coroutine_handle<>) noexcept;
+  void await_resume() noexcept;
+};
+
+struct task {
+  struct promise_type {
+    task get_return_object() { return task(); }
+    suspend_always initial_suspend() { return {}; }
+    suspend_always final_suspend() { return {}; }
+    void return_void() {}
+    void unhandled_exception() {}
+  };
+};
+
+struct awaitable {
+  task await() { (void)co_await *this; }
+  bool await_ready() { return false; }
+  bool await_suspend(std::experimental::coroutine_handle<> awaiter) { return false; }
+  bool await_resume() { return false; }
+};
+
+int main() {
+  awaitable a;
+  a.await();
+}