Add computeDominanceFrontier().

A trivial utility that returns the leaf blocks in a dominance subtree.

Author: atrick

include/swift/SIL/Dominance.h

@@ -88,6 +88,19 @@ class DominanceInfo : public DominatorTreeBase {
   }
 };
 
+/// Compute a single block's dominance frontier.
+///
+/// Precondition: no critical edges (OSSA)
+///
+/// Postcondition: each block in \p frontier is dominated by \p root and either
+/// exits the function or has a single successor that is not dominated by \p
+/// root.
+///
+/// With no critical edges, the dominance frontier is identified simply by leaf
+/// blocks in the dominance subtree.
+void computeDominanceFrontier(SILBasicBlock *root, DominanceInfo *domTree,
+                              SmallVectorImpl<SILBasicBlock *> &frontier);
+
 /// Helper class for visiting basic blocks in dominance order, based on a
 /// worklist algorithm. Example usage:
 /// \code

lib/SIL/Utils/Dominance.cpp

@@ -141,3 +141,22 @@ void PostDominanceInfo::verify() const {
     abort();
   }
 }
+
+void
+swift::computeDominanceFrontier(SILBasicBlock *root,
+                                DominanceInfo *domTree,
+                                SmallVectorImpl<SILBasicBlock *> &frontier) {
+  auto *function = root->getParent();
+  assert(function->hasOwnership());
+  assert(frontier.empty());
+
+  DominanceOrder domOrder(root, domTree);
+  while (SILBasicBlock *block = domOrder.getNext()) {
+    DominanceInfoNode *domNode = domTree->getNode(block);
+    if (domNode->isLeaf()) {
+      frontier.push_back(block);
+      continue;
+    }
+    domOrder.pushChildren(block);
+  }
+}