[llvm] [Coroutines] Make CoroElide actually work for (trivial) C++ coroutines (PR #92969)

[Yuxuan Chen via llvm-commits]

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@@ -193,23 +195,51 @@ CoroIdElider::CoroIdElider(CoroIdInst *CoroId, FunctionElideInfo &FEI,
       CoroAllocs.push_back(CA);
   }
 
+  DenseMap<AllocaInst *, CoroBeginInst *> Writes;
+  for (CoroBeginInst *CB : CoroBegins) {
+    for (User *U : CB->users()) {
+      if (auto *Store = dyn_cast<StoreInst>(U)) {
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yuxuanchen1997 wrote:

This alloca is clang generated. (usually just the temporary value of type `Task`.) I am thinking that if we can somehow annotate from the front end which field in a `Task` is a handle, we can potentially avoid having to do a convoluted analysis that's not easy to get right. 

Are you asking whether this `%t = alloca %Task` gets `store`d into another `alloca`ed stack memory? Yes it does. Usually operator co_await does that to create an `Awaiter` object. 

https://github.com/llvm/llvm-project/pull/92969