[clang] [llvm] [coro] Lower `llvm.coro.await.suspend.handle` to resume with tail call (PR #89751)

[Mircea Trofin via llvm-commits]

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@@ -1056,6 +1083,25 @@ void CoroCloner::create() {
   // Set up the new entry block.
   replaceEntryBlock();
 
+  // Turn symmetric transfers into musttail calls.
+  for (CallInst *ResumeCall : Shape.SymmetricTransfers) {
+    ResumeCall = cast<CallInst>(VMap[ResumeCall]);
+    ResumeCall->setCallingConv(NewF->getCallingConv());
+    if (TTI.supportsTailCallFor(ResumeCall)) {
+      // FIXME: Could we support symmetric transfer effectively without
+      // musttail?
+      ResumeCall->setTailCallKind(CallInst::TCK_MustTail);
+    }
+
+    // Put a 'ret void' after the call, and split any remaining instructions to
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mtrofin wrote:

It's not about legality, it's about the fact they wouldn't be executed being a symptom of a bug - e.g. earlier pass (like coverage) believing there is something to cover while there isn't. We could at minimum have some "under build with asserts" behavior that (except for trivial cases like debug/lifetime instructions) says something. 

https://github.com/llvm/llvm-project/pull/89751