[clang] [CIR] Implement FlattenCFG for coroutine AwaitOp, CoroBodyOp, and CoReturnOp (PR #203802)

[Erich Keane via cfe-commits]

================
@@ -2110,12 +2118,339 @@ class CIRTryOpFlattening : public mlir::OpRewritePattern<cir::TryOp> {
   }
 };
 
+static void
+collectCoReturnsOps(mlir::Region &region,
+                    llvm::SmallVectorImpl<cir::CoReturnOp> &coReturnOps) {
+  region.walk(
+      [&](cir::CoReturnOp coReturnOp) { coReturnOps.push_back(coReturnOp); });
+}
+
+// Flatten `cir.coroutine`. It runs once every nested op (awaits, scopes,
+// cleanups, ...) has been flattened. The initialSuspend, body and finalSuspend
+// regions become the body of a cleanup scope whose cleanup is the destroy
+// region; the exit region follows the scope:
+//
+//   cir.cleanup.scope {
+//     initialSuspend ---> body ---> finalSuspend
+//                          |             ^
+//                          +-------------+  cir.co_return
+//   } cleanup all {
+//     destroy                               coro.free
+//   }
+//   exit                                    coro.end + cir.return
+//
+// Leaving the scope body always runs the destroy region. In particular, the
+// cir.yield emitted for an await's destroy case leaves through it. A
+// suspended coroutine (cir.coro.exit) must keep its frame, so it branches
+// straight to the exit region.
+class CIRCoroutineFlattening : public mlir::OpRewritePattern<cir::CoroutineOp> {
+  void collectCoroExitsOps(
+      cir::CoroutineOp &coroOp,
+      llvm::SmallVectorImpl<cir::CoroExitOp> &coroExitsOps) const {
+    // cir.coro.exit is produced by flattening a cir.await, which can only
+    // appear in the initialSuspend, body and finalSuspend regions.
+    auto collectCoroExit = [&](cir::CoroExitOp op) {
+      coroExitsOps.push_back(op);
+    };
+    coroOp.getInitialSuspend().walk(collectCoroExit);
+    coroOp.getBody().walk(collectCoroExit);
+    coroOp.getFinalSuspend().walk(collectCoroExit);
+  }
+
+public:
+  using OpRewritePattern<cir::CoroutineOp>::OpRewritePattern;
+
+  mlir::LogicalResult
+  matchAndRewrite(cir::CoroutineOp coroOp,
+                  mlir::PatternRewriter &rewriter) const override {
+
+    assert(!cir::MissingFeatures::coroutineUnwindRegion());
----------------
erichkeane wrote:

This whole function would greatly benefit form 'helpers' to break it up.  See what you can do to make the function call names tell the 'story' of what is going on here.

https://github.com/llvm/llvm-project/pull/203802