[OSSACanonicalizeOwned] Only discover defs once.

The utility performs two def-use traversals.  The first determines
liveness from uses.  The second rewrites copies.

Previously, the defs whose uses were analyzed were discovered twice,
once during each traversal.  The non-triviality of the discovery logic
(i.e. the logic determining when to walk into the values produced by the
insructions which were the users of visited uses) opened the possibility
for a divergence between the two discoveries.  This possibility had
indeed been realized--the two discoveries didn't visit exactly the same
uses, and issues ensue.

Here, the defs whose uses are analyzed are discovered only once (and not
discarded as their uses are analyzed) during the first traversal.  The
second traversal reuses the defs discovered in the first traversal,
eliminating the possibility of a def discovery difference.

Furthermore, the defs are traversed in the same order.

Author: nate-chandler

include/swift/SILOptimizer/Utils/CanonicalizeOSSALifetime.h

@@ -345,6 +345,9 @@ class CanonicalizeOSSALifetime final {
   /// The defs derived from currentDef whose uses are added to liveness.
   llvm::SmallSetVector<Def, 8> discoveredDefs;
 
+  /// The indices in discoveredDefs in the order in which they are visited.
+  SmallVector<unsigned, 8> discoveredDefIndices;
+
   /// The blocks that were discovered by PrunedLiveness.
   SmallVector<SILBasicBlock *, 32> discoveredBlocks;
 

lib/SILOptimizer/Utils/CanonicalizeOSSALifetime.cpp

@@ -139,6 +139,7 @@ bool CanonicalizeOSSALifetime::computeCanonicalLiveness() {
     indexWorklist.push_back(discoveredDefs.size() - 1);
   };
   discoveredDefs.clear();
+  discoveredDefIndices.clear();
   addDefToWorklist(Def::root(getCurrentDef()));
   // Only the first level of reborrows need to be consider. All nested inner
   // adjacent reborrows and phis are encapsulated within their lifetimes.
@@ -151,6 +152,7 @@ bool CanonicalizeOSSALifetime::computeCanonicalLiveness() {
   }
   while (!indexWorklist.empty()) {
     auto index = indexWorklist.pop_back_val();
+    discoveredDefIndices.push_back(index);
     auto def = discoveredDefs[index];
     auto value = def.getValue();
     LLVM_DEBUG(llvm::dbgs() << "  Uses of value:\n";
@@ -1159,14 +1161,7 @@ void CanonicalizeOSSALifetime::rewriteCopies(
   assert(getCurrentDef()->getOwnershipKind() == OwnershipKind::Owned);
 
   // Shadow discoveredDefs in order to constrain its uses.
-  auto &discoveredDefs = this->discoveredDefs;
-
-  SmallVector<unsigned, 8> indexWorklist;
-  auto addDefToWorklist = [&](Def def) {
-    if (!discoveredDefs.insert(def))
-      return;
-    indexWorklist.push_back(discoveredDefs.size() - 1);
-  };
+  const auto &discoveredDefs = this->discoveredDefs;
 
   InstructionSetVector instsToDelete(getCurrentDef()->getFunction());
 
@@ -1176,9 +1171,8 @@ void CanonicalizeOSSALifetime::rewriteCopies(
   // it requires a copy.
   auto visitUse = [&](Operand *use) {
     auto *user = use->getUser();
-    // Recurse through copies.
+    // The current def can always be used for a copy_value.
     if (auto *copy = dyn_cast<CopyValueInst>(user)) {
-      addDefToWorklist(Def::copy(copy));
       return true;
     }
     if (destroys.contains(user)) {
@@ -1213,12 +1207,8 @@ void CanonicalizeOSSALifetime::rewriteCopies(
     return true;
   };
 
-  discoveredDefs.clear();
-  addDefToWorklist(Def::root(getCurrentDef()));
   // Perform a def-use traversal, visiting each use operand.
-
-  while (!indexWorklist.empty()) {
-    auto index = indexWorklist.pop_back_val();
+  for (auto index : discoveredDefIndices) {
     auto def = discoveredDefs[index];
     switch (def) {
     case Def::Kind::BorrowedFrom: