Add ValueLifetimeBoundary::visitInsertionPoints

Given a computed ValueLifetimeBoundary, visit all the points at which
the lifetime needs to be terminated, e.g. via and end_borrow or
destroy_value.

Especially useful for creating a borrow scope over guaranteed uses.

This completely decouples the DeadBlocks analysis from the liveness
analysis.

It will allow phasing out the complex and bug-prone
ValueLifetimeAnalysis::Frontier API.

Author: atrick

include/swift/SILOptimizer/Utils/ValueLifetime.h

@@ -48,6 +48,14 @@ namespace swift {
 struct ValueLifetimeBoundary {
   SmallVector<SILInstruction *, 8> lastUsers;
   SmallVector<SILBasicBlock *, 8> boundaryEdges;
+
+  /// Visit the point at which a lifetime-ending instruction must be inserted,
+  /// exclusing dead-end blocks. This is only useful when it is known that none
+  /// of the lastUsers ends the lifetime, for example when creating a new borrow
+  /// scope to enclose all uses.
+  void visitInsertionPoints(
+      llvm::function_ref<void(SILBasicBlock::iterator insertPt)> visitor,
+      DeadEndBlocks *deBlocks = nullptr);
 };
 
 /// Computes the lifetime frontier for a given value with respect to a

lib/SILOptimizer/Utils/ValueLifetime.cpp

@@ -18,6 +18,31 @@
 
 using namespace swift;
 
+void ValueLifetimeBoundary::visitInsertionPoints(
+    llvm::function_ref<void(SILBasicBlock::iterator insertPt)> visitor,
+    DeadEndBlocks *deBlocks) {
+  for (SILInstruction *user : lastUsers) {
+    if (!isa<TermInst>(user)) {
+      visitor(std::next(user->getIterator()));
+      continue;
+    }
+    auto *predBB = user->getParent();
+    for (SILBasicBlock *succ : predBB->getSuccessors()) {
+      if (deBlocks && deBlocks->isDeadEnd(succ))
+        continue;
+
+      assert(succ->getSinglePredecessorBlock() == predBB);
+      visitor(succ->begin());
+    }
+  }
+  for (SILBasicBlock *edge : boundaryEdges) {
+    if (deBlocks && deBlocks->isDeadEnd(edge))
+      continue;
+
+    visitor(edge->begin());
+  }
+}
+
 void ValueLifetimeAnalysis::propagateLiveness() {
   bool defIsInstruction = defValue.is<SILInstruction *>();
   assert(liveBlocks.empty() && "frontier computed twice");