SIL: A utility for processing basic blocks in a worklist.

BasicBlockWorklist is basically a combination of a block vector and a block set.
It can be used for typical worklist-processing algorithms.

Author: eeckstein

include/swift/SIL/BasicBlockBits.h

@@ -211,6 +211,63 @@ template <unsigned N> class BasicBlockSetVector {
   }
 };
 
+/// A utility for processing basic blocks in a worklist.
+///
+/// It is basically a combination of a block vector and a block set. It can be
+/// used for typical worklist-processing algorithms.
+template <unsigned N> class BasicBlockWorklist {
+  llvm::SmallVector<SILBasicBlock *, N> worklist;
+  BasicBlockSet visited;
+  
+public:
+  /// Construct an empty worklist.
+  BasicBlockWorklist(SILFunction *function) : visited(function) {}
+
+  /// Initialize the worklist with \p initialBlock.
+  BasicBlockWorklist(SILBasicBlock *initialBlock)
+      : visited(initialBlock->getParent()) {
+    push(initialBlock);
+  }
+
+  /// Pops the last added element from the worklist or returns null, if the
+  /// worklist is empty.
+  SILBasicBlock *pop() {
+    if (worklist.empty())
+      return nullptr;
+    return worklist.pop_back_val();
+  }
+
+  /// Pushes \p block onto the worklist if \p block has never been push before.
+  bool pushIfNotVisited(SILBasicBlock *block) {
+    if (visited.insert(block)) {
+      worklist.push_back(block);
+      return true;
+    }
+    return false;
+  }
+
+  /// Like `pushIfNotVisited`, but requires that \p block has never been on the
+  /// worklist before.
+  void push(SILBasicBlock *block) {
+    assert(!visited.contains(block));
+    visited.insert(block);
+    worklist.push_back(block);
+  }
+
+  /// Like `pop`, but marks the returned block as "unvisited". This means, that
+  /// the block can be pushed onto the worklist again.
+  SILBasicBlock *popAndForget() {
+    if (worklist.empty())
+      return nullptr;
+    SILBasicBlock *block = worklist.pop_back_val();
+    visited.erase(block);
+    return block;
+  }
+
+  /// Returns true if \p block was visited, i.e. has been added to the worklist.
+  bool isVisited(SILBasicBlock *block) const { return visited.contains(block); }
+};
+
 } // namespace swift
 
 #endif