[coroutines] Add support for llvm.coro.noop intrinsics

Summary:
A recent addition to Coroutines TS (https://wg21.link/p0913) adds a pre-defined coroutine noop_coroutine that does nothing.
To implement this feature, we implemented an llvm.coro.noop intrinsic that returns a coroutine handle to a coroutine that does nothing when resumed or destroyed.

Reviewers: EricWF, modocache, rnk, lewissbaker

Reviewed By: modocache

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45114

llvm-svn: 328986

llvm/docs/Coroutines.rst

@@ -880,6 +880,32 @@ Example:
     %phi = phi i8* [ null, %entry ], [ %alloc, %coro.alloc ]
     %frame = call i8* @llvm.coro.begin(token %id, i8* %phi)
 
+.. _coro.noop:
+
+'llvm.coro.noop' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+::
+
+  declare i8* @llvm.coro.noop()
+
+Overview:
+"""""""""
+
+The '``llvm.coro.noop``' intrinsic returns an address of the coroutine frame of
+a coroutine that does nothing when resumed or destroyed.
+
+Arguments:
+""""""""""
+
+None
+
+Semantics:
+""""""""""
+
+This intrinsic is lowered to refer to a private constant coroutine frame. The
+resume and destroy handlers for this frame are empty functions that do nothing.
+Note that in different translation units llvm.coro.noop may return different pointers.
+
 .. _coro.frame:
 
 'llvm.coro.frame' Intrinsic

llvm/include/llvm/IR/Intrinsics.td

@@ -768,6 +768,7 @@ def int_coro_free : Intrinsic<[llvm_ptr_ty], [llvm_token_ty, llvm_ptr_ty],
 def int_coro_end : Intrinsic<[llvm_i1_ty], [llvm_ptr_ty, llvm_i1_ty], []>;
 
 def int_coro_frame : Intrinsic<[llvm_ptr_ty], [], [IntrNoMem]>;
+def int_coro_noop : Intrinsic<[llvm_ptr_ty], [], [IntrNoMem]>;
 def int_coro_size : Intrinsic<[llvm_anyint_ty], [], [IntrNoMem]>;
 
 def int_coro_save : Intrinsic<[llvm_token_ty], [llvm_ptr_ty], []>;

llvm/lib/Transforms/Coroutines/CoroEarly.cpp

@@ -27,10 +27,12 @@ namespace {
 class Lowerer : public coro::LowererBase {
   IRBuilder<> Builder;
   PointerType *const AnyResumeFnPtrTy;
+  Constant *NoopCoro = nullptr;
 
   void lowerResumeOrDestroy(CallSite CS, CoroSubFnInst::ResumeKind);
   void lowerCoroPromise(CoroPromiseInst *Intrin);
   void lowerCoroDone(IntrinsicInst *II);
+  void lowerCoroNoop(IntrinsicInst *II);
 
 public:
   Lowerer(Module &M)
@@ -103,6 +105,41 @@ void Lowerer::lowerCoroDone(IntrinsicInst *II) {
   II->eraseFromParent();
 }
 
+void Lowerer::lowerCoroNoop(IntrinsicInst *II) {
+  if (!NoopCoro) {
+    LLVMContext &C = Builder.getContext();
+    Module &M = *II->getModule();
+
+    // Create a noop.frame struct type.
+    StructType *FrameTy = StructType::create(C, "NoopCoro.Frame");
+    auto *FramePtrTy = FrameTy->getPointerTo();
+    auto *FnTy = FunctionType::get(Type::getVoidTy(C), FramePtrTy,
+                                   /*IsVarArgs=*/false);
+    auto *FnPtrTy = FnTy->getPointerTo();
+    FrameTy->setBody({FnPtrTy, FnPtrTy});
+
+    // Create a Noop function that does nothing.
+    Function *NoopFn =
+        Function::Create(FnTy, GlobalValue::LinkageTypes::PrivateLinkage,
+                         "NoopCoro.ResumeDestroy", &M);
+    NoopFn->setCallingConv(CallingConv::Fast);
+    auto *Entry = BasicBlock::Create(C, "entry", NoopFn);
+    ReturnInst::Create(C, Entry);
+
+    // Create a constant struct for the frame.
+    Constant* Values[] = {NoopFn, NoopFn};
+    Constant* NoopCoroConst = ConstantStruct::get(FrameTy, Values);
+    NoopCoro = new GlobalVariable(M, NoopCoroConst->getType(), /*isConstant=*/true,
+                                GlobalVariable::PrivateLinkage, NoopCoroConst,
+                                "NoopCoro.Frame.Const");
+  }
+
+  Builder.SetInsertPoint(II);
+  auto *NoopCoroVoidPtr = Builder.CreateBitCast(NoopCoro, Int8Ptr);
+  II->replaceAllUsesWith(NoopCoroVoidPtr);
+  II->eraseFromParent();
+}
+
 // Prior to CoroSplit, calls to coro.begin needs to be marked as NoDuplicate,
 // as CoroSplit assumes there is exactly one coro.begin. After CoroSplit,
 // NoDuplicate attribute will be removed from coro.begin otherwise, it will
@@ -138,6 +175,9 @@ bool Lowerer::lowerEarlyIntrinsics(Function &F) {
         if (cast<CoroEndInst>(&I)->isFallthrough())
           CS.setCannotDuplicate();
         break;
+      case Intrinsic::coro_noop:
+        lowerCoroNoop(cast<IntrinsicInst>(&I));
+        break;
       case Intrinsic::coro_id:
         // Mark a function that comes out of the frontend that has a coro.id
         // with a coroutine attribute.
@@ -192,10 +232,10 @@ struct CoroEarly : public FunctionPass {
   // This pass has work to do only if we find intrinsics we are going to lower
   // in the module.
   bool doInitialization(Module &M) override {
-    if (coro::declaresIntrinsics(M, {"llvm.coro.id", "llvm.coro.destroy",
-                                     "llvm.coro.done", "llvm.coro.end",
-                                     "llvm.coro.free", "llvm.coro.promise",
-                                     "llvm.coro.resume", "llvm.coro.suspend"}))
+    if (coro::declaresIntrinsics(
+            M, {"llvm.coro.id", "llvm.coro.destroy", "llvm.coro.done",
+                "llvm.coro.end", "llvm.coro.noop", "llvm.coro.free",
+                "llvm.coro.promise", "llvm.coro.resume", "llvm.coro.suspend"}))
       L = llvm::make_unique<Lowerer>(M);
     return false;
   }

llvm/lib/Transforms/Coroutines/Coroutines.cpp

@@ -125,9 +125,10 @@ static bool isCoroutineIntrinsicName(StringRef Name) {
   static const char *const CoroIntrinsics[] = {
       "llvm.coro.alloc",   "llvm.coro.begin",   "llvm.coro.destroy",
       "llvm.coro.done",    "llvm.coro.end",     "llvm.coro.frame",
-      "llvm.coro.free",    "llvm.coro.id",      "llvm.coro.param",
-      "llvm.coro.promise", "llvm.coro.resume",  "llvm.coro.save",
-      "llvm.coro.size",    "llvm.coro.subfn.addr", "llvm.coro.suspend",
+      "llvm.coro.free",    "llvm.coro.id",      "llvm.coro.noop",
+      "llvm.coro.param",   "llvm.coro.promise", "llvm.coro.resume",
+      "llvm.coro.save",    "llvm.coro.size",    "llvm.coro.subfn.addr",
+      "llvm.coro.suspend",
   };
   return Intrinsic::lookupLLVMIntrinsicByName(CoroIntrinsics, Name) != -1;
 }

llvm/test/Transforms/Coroutines/coro-early.ll

@@ -1,7 +1,10 @@
-; Tests that CoroEarly pass correctly lowers coro.resume and coro.destroy
-; intrinsics.
+; Tests that CoroEarly pass correctly lowers coro.resume, coro.destroy
+; and other intrinsics managed by this pass.
 ; RUN: opt < %s -S -coro-early | FileCheck %s
 
+; CHECK: %NoopCoro.Frame = type { void (%NoopCoro.Frame*)*, void (%NoopCoro.Frame*)* }
+; CHECK: @NoopCoro.Frame.Const = private constant %NoopCoro.Frame { void (%NoopCoro.Frame*)* @NoopCoro.ResumeDestroy, void (%NoopCoro.Frame*)* @NoopCoro.ResumeDestroy }
+
 ; CHECK-LABEL: @callResume(
 define void @callResume(i8* %hdl) {
 ; CHECK-NEXT: entry
@@ -37,5 +40,21 @@ ehcleanup:
   cleanupret from %0 unwind to caller
 }
 
+
+; CHECK-LABEL: @noop(
+define i8* @noop() {
+; CHECK-NEXT: entry
+entry:
+; CHECK-NEXT: ret i8* bitcast (%NoopCoro.Frame* @NoopCoro.Frame.Const to i8*)
+  %n = call i8* @llvm.coro.noop()
+  ret i8* %n
+}
+
+; CHECK-LABEL: define private fastcc void @NoopCoro.ResumeDestroy(%NoopCoro.Frame*) {
+; CHECK-NEXT: entry
+; CHECK-NEXT:    ret void
+
+
 declare void @llvm.coro.resume(i8*)
 declare void @llvm.coro.destroy(i8*)
+declare i8* @llvm.coro.noop()