[Coroutines] Less IR for noexcept await_resume

Summary:
In his review of https://reviews.llvm.org/D45860, @GorNishanov suggested
avoiding generating additional exception-handling IR in the case that
the resume function was marked as 'noexcept', and exceptions could not
occur. This implements that suggestion.

Test Plan: `check-clang`

Reviewers: GorNishanov, EricWF

Reviewed By: GorNishanov

Subscribers: cfe-commits, GorNishanov

Differential Revision: https://reviews.llvm.org/D47673

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@335422 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CGCoroutine.cpp

@@ -130,6 +130,16 @@ static SmallString<32> buildSuspendPrefixStr(CGCoroData &Coro, AwaitKind Kind) {
   return Prefix;
 }
 
+static bool memberCallExpressionCanThrow(const Expr *E) {
+  if (const auto *CE = dyn_cast<CXXMemberCallExpr>(E))
+    if (const auto *Proto =
+            CE->getMethodDecl()->getType()->getAs<FunctionProtoType>())
+      if (isNoexceptExceptionSpec(Proto->getExceptionSpecType()) &&
+          Proto->canThrow() == CT_Cannot)
+        return false;
+  return true;
+}
+
 // Emit suspend expression which roughly looks like:
 //
 //   auto && x = CommonExpr();
@@ -217,8 +227,12 @@ static LValueOrRValue emitSuspendExpression(CodeGenFunction &CGF, CGCoroData &Co
 
   // Emit await_resume expression.
   CGF.EmitBlock(ReadyBlock);
+
+  // Exception handling requires additional IR. If the 'await_resume' function
+  // is marked as 'noexcept', we avoid generating this additional IR.
   CXXTryStmt *TryStmt = nullptr;
-  if (Coro.ExceptionHandler && Kind == AwaitKind::Init) {
+  if (Coro.ExceptionHandler && Kind == AwaitKind::Init &&
+      memberCallExpressionCanThrow(S.getResumeExpr())) {
     Coro.ResumeEHVar =
         CGF.CreateTempAlloca(Builder.getInt1Ty(), Prefix + Twine("resume.eh"));
     Builder.CreateFlagStore(true, Coro.ResumeEHVar);
@@ -625,12 +639,20 @@ void CodeGenFunction::EmitCoroutineBody(const CoroutineBodyStmt &S) {
     CurCoro.Data->CurrentAwaitKind = AwaitKind::Normal;
 
     if (CurCoro.Data->ExceptionHandler) {
-      BasicBlock *BodyBB = createBasicBlock("coro.resumed.body");
-      BasicBlock *ContBB = createBasicBlock("coro.resumed.cont");
-      Value *SkipBody =
-          Builder.CreateFlagLoad(CurCoro.Data->ResumeEHVar, "coro.resumed.eh");
-      Builder.CreateCondBr(SkipBody, ContBB, BodyBB);
-      EmitBlock(BodyBB);
+      // If we generated IR to record whether an exception was thrown from
+      // 'await_resume', then use that IR to determine whether the coroutine
+      // body should be skipped.
+      // If we didn't generate the IR (perhaps because 'await_resume' was marked
+      // as 'noexcept'), then we skip this check.
+      BasicBlock *ContBB = nullptr;
+      if (CurCoro.Data->ResumeEHVar) {
+        BasicBlock *BodyBB = createBasicBlock("coro.resumed.body");
+        ContBB = createBasicBlock("coro.resumed.cont");
+        Value *SkipBody = Builder.CreateFlagLoad(CurCoro.Data->ResumeEHVar,
+                                                 "coro.resumed.eh");
+        Builder.CreateCondBr(SkipBody, ContBB, BodyBB);
+        EmitBlock(BodyBB);
+      }
 
       auto Loc = S.getLocStart();
       CXXCatchStmt Catch(Loc, /*exDecl=*/nullptr,
@@ -642,7 +664,8 @@ void CodeGenFunction::EmitCoroutineBody(const CoroutineBodyStmt &S) {
       emitBodyAndFallthrough(*this, S, TryStmt->getTryBlock());
       ExitCXXTryStmt(*TryStmt);
 
-      EmitBlock(ContBB);
+      if (ContBB)
+        EmitBlock(ContBB);
     }
     else {
       emitBodyAndFallthrough(*this, S, S.getBody());

test/CodeGenCoroutines/coro-await-resume-eh.cpp

@@ -18,9 +18,9 @@ struct throwing_awaitable {
   void await_resume() { throw 42; }
 };
 
-struct task {
+struct throwing_task {
   struct promise_type {
-    task get_return_object() { return task{}; }
+    auto get_return_object() { return throwing_task{}; }
     auto initial_suspend() { return throwing_awaitable{}; }
     auto final_suspend() { return coro::suspend_never{}; }
     void return_void() {}
@@ -29,7 +29,7 @@ struct task {
 };
 
 // CHECK-LABEL: define void @_Z1fv()
-task f() {
+throwing_task f() {
   // A variable RESUMETHREW is used to keep track of whether the body
   // of 'await_resume' threw an exception. Exceptions thrown in
   // 'await_resume' are unwound to RESUMELPAD.
@@ -50,7 +50,7 @@ task f() {
   // CHECK: [[RESUMELPAD]]:
   // CHECK: br label %[[RESUMECATCH:.+]]
   // CHECK: [[RESUMECATCH]]:
-  // CHECK: invoke void @_ZN4task12promise_type19unhandled_exceptionEv
+  // CHECK: invoke void @_ZN13throwing_task12promise_type19unhandled_exceptionEv
   // CHECK-NEXT: to label %[[RESUMEENDCATCH:.+]] unwind label
   // CHECK: [[RESUMEENDCATCH]]:
   // CHECK-NEXT: invoke void @__cxa_end_catch()
@@ -67,7 +67,7 @@ task f() {
   // CHECK-NEXT: br i1 %[[RESUMETHREWLOAD]], label %[[RESUMEDCONT:.+]], label %[[RESUMEDBODY:.+]]
 
   // CHECK: [[RESUMEDBODY]]:
-  // CHECK: invoke void @_ZN4task12promise_type11return_voidEv
+  // CHECK: invoke void @_ZN13throwing_task12promise_type11return_voidEv
   // CHECK-NEXT: to label %[[REDUMEDBODYCONT:.+]] unwind label
   // CHECK: [[REDUMEDBODYCONT]]:
   // CHECK-NEXT: br label %[[COROFINAL:.+]]
@@ -76,6 +76,33 @@ task f() {
   // CHECK-NEXT: br label %[[COROFINAL]]
 
   // CHECK: [[COROFINAL]]:
-  // CHECK-NEXT: invoke void @_ZN4task12promise_type13final_suspendEv
+  // CHECK-NEXT: invoke void @_ZN13throwing_task12promise_type13final_suspendEv
+  co_return;
+}
+
+struct noexcept_awaitable {
+  bool await_ready() { return true; }
+  void await_suspend(coro::coroutine_handle<>) {}
+  void await_resume() noexcept {}
+};
+
+struct noexcept_task {
+  struct promise_type {
+    auto get_return_object() { return noexcept_task{}; }
+    auto initial_suspend() { return noexcept_awaitable{}; }
+    auto final_suspend() { return coro::suspend_never{}; }
+    void return_void() {}
+    void unhandled_exception() {}
+  };
+};
+
+// CHECK-LABEL: define void @_Z1gv()
+noexcept_task g() {
+  // If the await_resume function is marked as noexcept, none of the additional
+  // conditions that are present in f() above are added to the IR.
+  // This means that no i1 are stored before or after calling await_resume:
+  // CHECK: init.ready:
+  // CHECK-NEXT: call void @_ZN18noexcept_awaitable12await_resumeEv
+  // CHECK-NOT: store i1 false, i1*
   co_return;
 }