Add TopDownClosureFunctionOrder based on ClosureScopeAnalysis.

Simple utility for transfersing functions such that parent scopes are always
visited before noescape closures.

Note that recursion is disallowed. Noescape closures are not reentrant.

lib/SILOptimizer/Analysis/ClosureScope.cpp

@@ -14,6 +14,7 @@
 
 #define DEBUG_TYPE "closure-scope"
 
+#include "swift/SIL/SILModule.h"
 #include "swift/SILOptimizer/Analysis/ClosureScope.h"
 
 namespace swift {
@@ -154,4 +155,42 @@ SILAnalysis *createClosureScopeAnalysis(SILModule *M) {
   return new ClosureScopeAnalysis(M);
 }
 
+void TopDownClosureFunctionOrder::visitFunctions(
+    std::function<void(SILFunction *)> visitor) {
+  auto markVisited = [&](SILFunction *F) {
+    bool visitOnce = visited.insert(F).second;
+    assert(visitOnce);
+    (void)visitOnce;
+  };
+  auto allScopesVisited = [&](SILFunction *closureF) {
+    return llvm::all_of(CSA->getClosureScopes(closureF),
+                        [this](SILFunction *F) { return visited.count(F); });
+  };
+  for (auto &F : *CSA->getModule()) {
+    if (!allScopesVisited(&F)) {
+      closureWorklist.insert(&F);
+      continue;
+    }
+    markVisited(&F);
+    visitor(&F);
+  }
+  unsigned numClosures = closureWorklist.size();
+  while (numClosures) {
+    for (auto &closureNode : closureWorklist) {
+      // skip erased closures.
+      if (!closureNode)
+        continue;
+
+      auto closureF = closureNode.getValue();
+      if (!allScopesVisited(closureF))
+        continue;
+
+      markVisited(closureF);
+      visitor(closureF);
+      closureWorklist.erase(closureF);
+      --numClosures;
+    }
+  }
+}
+
 } // namespace swift