[C++] [Coroutines] Assume the allocation doesn't return nullptr

In case of 'get_return_object_on_allocation_failure' get declared, the
compiler is required to call 'operator new(size_t, nothrow_t)' and the
handle the failure case by calling
'get_return_object_on_allocation_failure()'. But the failure case should
be rare and we can assume the allocation is successful and pass the
information to the optimizer.

clang/lib/CodeGen/CGCoroutine.cpp

@@ -630,6 +630,8 @@ void CodeGenFunction::EmitCoroutineBody(const CoroutineBodyStmt &S) {
     // See if allocation was successful.
     auto *NullPtr = llvm::ConstantPointerNull::get(Int8PtrTy);
     auto *Cond = Builder.CreateICmpNE(AllocateCall, NullPtr);
+    // Expect the allocation to be successful.
+    emitCondLikelihoodViaExpectIntrinsic(Cond, Stmt::LH_Likely);
     Builder.CreateCondBr(Cond, InitBB, RetOnFailureBB);
 
     // If not, return OnAllocFailure object.

clang/test/CodeGenCoroutines/coro-alloc.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -std=c++20 \
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -std=c++20 -O2 \
 // RUN:    -Wno-coroutine-missing-unhandled-exception -emit-llvm %s -o - -disable-llvm-passes \
 // RUN:   | FileCheck %s
 
@@ -228,6 +228,7 @@ extern "C" int f4(promise_on_alloc_failure_tag) {
   // CHECK: %[[SIZE:.+]] = call i64 @llvm.coro.size.i64()
   // CHECK: %[[MEM:.+]] = call noalias noundef ptr @_ZnwmRKSt9nothrow_t(i64 noundef %[[SIZE]], ptr noundef nonnull align 1 dereferenceable(1) @_ZStL7nothrow)
   // CHECK: %[[OK:.+]] = icmp ne ptr %[[MEM]], null
+  // CHECK: call i1 @llvm.expect.i1(i1 %[[OK]], i1 true)
   // CHECK: br i1 %[[OK]], label %[[OKBB:.+]], label %[[ERRBB:.+]]
 
   // CHECK: [[ERRBB]]: