Index: include/llvm/IR/Intrinsics.td
===================================================================
--- include/llvm/IR/Intrinsics.td
+++ include/llvm/IR/Intrinsics.td
@@ -840,6 +840,7 @@
 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_size_chk : Intrinsic<[], [llvm_anyint_ty], []>;
 
 def int_coro_save : Intrinsic<[llvm_token_ty], [llvm_ptr_ty], []>;
 def int_coro_suspend : Intrinsic<[llvm_i8_ty], [llvm_token_ty, llvm_i1_ty], []>;
Index: lib/Transforms/Coroutines/CoroCleanup.cpp
===================================================================
--- lib/Transforms/Coroutines/CoroCleanup.cpp
+++ lib/Transforms/Coroutines/CoroCleanup.cpp
@@ -79,6 +79,9 @@
       case Intrinsic::coro_subfn_addr:
         lowerSubFn(Builder, cast<CoroSubFnInst>(II));
         break;
+      case Intrinsic::coro_size_chk:
+        // Just remove it.
+        break;
       }
       II->eraseFromParent();
       Changed = true;
Index: lib/Transforms/Coroutines/CoroElide.cpp
===================================================================
--- lib/Transforms/Coroutines/CoroElide.cpp
+++ lib/Transforms/Coroutines/CoroElide.cpp
@@ -196,6 +196,19 @@
   ResumeAddr.clear();
   DestroyAddr.clear();
 
+  bool HasSizeChecksIntrinsics = false;
+
+  // TODO: For now, just the mere presense of the coro_size_chk disables
+  // the Halo optimization. We need to investigate if less blunt approach
+  // is possible.
+  for (Instruction &I : instructions(CoroId->getFunction()))
+    if (isa<CoroSizeChkInst>(&I)) {
+      LLVM_DEBUG(dbgs() << "disabling Halo optimization "
+                           " due to presence of coro_size_chk\n");
+      HasSizeChecksIntrinsics = false;
+      break;
+    }
+
   // Collect all coro.begin and coro.allocs associated with this coro.id.
   for (User *U : CoroId->users()) {
     if (auto *CB = dyn_cast<CoroBeginInst>(U))
@@ -243,7 +256,7 @@
 
   replaceWithConstant(DestroyAddrConstant, DestroyAddr);
 
-  if (ShouldElide) {
+  if (ShouldElide && !HasSizeChecksIntrinsics) {
     auto *FrameTy = getFrameType(cast<Function>(ResumeAddrConstant));
     elideHeapAllocations(CoroId->getFunction(), FrameTy, AA);
     coro::replaceCoroFree(CoroId, /*Elide=*/true);
Index: lib/Transforms/Coroutines/CoroInstr.h
===================================================================
--- lib/Transforms/Coroutines/CoroInstr.h
+++ lib/Transforms/Coroutines/CoroInstr.h
@@ -299,6 +299,32 @@
   }
 };
 
+
+/// This represents the llvm.coro.size instruction.
+class LLVM_LIBRARY_VISIBILITY CoroSizeChkInst : public IntrinsicInst {
+  enum { SizeEst };
+
+  ConstantInt *getRawIndex() const {
+    return cast<ConstantInt>(getArgOperand(SizeEst));
+  }
+public:
+  bool isConstant() const {
+    return isa<ConstantInt>(getArgOperand(SizeEst));
+  }
+
+  int64_t getSizeEstimate() const {
+    return getRawIndex()->getValue().getSExtValue();
+  }
+
+  // Methods to support type inquiry through isa, cast, and dyn_cast:
+  static bool classof(const IntrinsicInst *I) {
+    return I->getIntrinsicID() == Intrinsic::coro_size_chk;
+  }
+  static bool classof(const Value *V) {
+    return isa<IntrinsicInst>(V) && classof(cast<IntrinsicInst>(V));
+  }
+};
+
 /// This represents the llvm.coro.end instruction.
 class LLVM_LIBRARY_VISIBILITY CoroEndInst : public IntrinsicInst {
   enum { FrameArg, UnwindArg };
Index: lib/Transforms/Coroutines/CoroInternal.h
===================================================================
--- lib/Transforms/Coroutines/CoroInternal.h
+++ lib/Transforms/Coroutines/CoroInternal.h
@@ -68,6 +68,7 @@
   CoroBeginInst *CoroBegin;
   SmallVector<CoroEndInst *, 4> CoroEnds;
   SmallVector<CoroSizeInst *, 2> CoroSizes;
+  SmallVector<CoroSizeChkInst *, 1> CoroSizeChks;
   SmallVector<CoroSuspendInst *, 4> CoroSuspends;
 
   // Field Indexes for known coroutine frame fields.
Index: lib/Transforms/Coroutines/CoroSplit.cpp
===================================================================
--- lib/Transforms/Coroutines/CoroSplit.cpp
+++ lib/Transforms/Coroutines/CoroSplit.cpp
@@ -364,6 +364,20 @@
     CS->replaceAllUsesWith(SizeConstant);
     CS->eraseFromParent();
   }
+
+  for (CoroSizeChkInst *CS : Shape.CoroSizeChks) {
+    if (!CS->isConstant())
+      report_fatal_error("In function '" + CS->getFunction()->getName() +
+        "' with coroutine frame size " + Twine(Size) +
+        " front end size estimate is not a constant.");
+    else if ((int64_t)Size > CS->getSizeEstimate())
+      report_fatal_error("In function '" + CS->getFunction()->getName() +
+        "' coroutine frame size " + Twine(Size) + " exceeds the frontend estimate of "
+        + Twine(CS->getSizeEstimate()) + ".");
+
+    // Note that we do not remove the check intrinsic, so that it can be
+    // used to inhibit Halo optimization.
+  }
 }
 
 // Create a global constant array containing pointers to functions provided and
Index: lib/Transforms/Coroutines/Coroutines.cpp
===================================================================
--- lib/Transforms/Coroutines/Coroutines.cpp
+++ lib/Transforms/Coroutines/Coroutines.cpp
@@ -249,6 +249,9 @@
       case Intrinsic::coro_size:
         CoroSizes.push_back(cast<CoroSizeInst>(II));
         break;
+      case Intrinsic::coro_size_chk:
+        CoroSizeChks.push_back(cast<CoroSizeChkInst>(II));
+        break;
       case Intrinsic::coro_frame:
         CoroFrames.push_back(cast<CoroFrameInst>(II));
         break;