[NFC] OSSACanOwned: Vary rewrite with def kind.

When rewriting uses, determine how that rewriting should proceed based
on the kind of def whose use is being visited.  Direct uses of reborrows
and borrowed-froms never need to be rewritten because every such use is
a use of a guaranteed value and can never require a copy.

Author: nate-chandler

lib/SILOptimizer/Utils/CanonicalizeOSSALifetime.cpp

@@ -1150,7 +1150,6 @@ void CanonicalizeOSSALifetime::rewriteCopies(
   assert(getCurrentDef()->getOwnershipKind() == OwnershipKind::Owned);
 
   InstructionSetVector instsToDelete(getCurrentDef()->getFunction());
-  defUseWorklist.clear();
 
   // Visit each operand in the def-use chain.
   //
@@ -1195,39 +1194,56 @@ void CanonicalizeOSSALifetime::rewriteCopies(
     return true;
   };
 
+  defUseWorklist.initialize(Def::root(getCurrentDef()));
   // Perform a def-use traversal, visiting each use operand.
-  for (auto useIter = getCurrentDef()->use_begin(),
-         endIter = getCurrentDef()->use_end(); useIter != endIter;) {
-    Operand *use = *useIter++;
-    if (!visitUse(use)) {
-      copyLiveUse(use, getCallbacks());
-    }
-  }
   while (auto def = defUseWorklist.pop()) {
-    SILValue value = def->getValue();
-    CopyValueInst *srcCopy = cast<CopyValueInst>(value);
-    // Recurse through copies while replacing their uses.
-    Operand *reusedCopyOp = nullptr;
-    for (auto useIter = srcCopy->use_begin(); useIter != srcCopy->use_end();) {
-      Operand *use = *useIter++;
-      if (!visitUse(use)) {
-        if (!reusedCopyOp && srcCopy->getParent() == use->getParentBlock()) {
-          reusedCopyOp = use;
-        } else {
+    switch (*def) {
+    case Def::Kind::BorrowedFrom:
+    case Def::Kind::Reborrow:
+      // Direct uses of these defs never need to be rewritten.  Being guaranteed
+      // values, none of their direct uses consume an owned value.
+      assert(def.getValue()->getOwnershipKind() == OwnershipKind::Guaranteed);
+      break;
+    case Def::Kind::Root: {
+      SILValue value = def->getValue();
+      for (auto useIter = value->use_begin(), endIter = value->use_end();
+           useIter != endIter;) {
+        Operand *use = *useIter++;
+        if (!visitUse(use)) {
           copyLiveUse(use, getCallbacks());
         }
       }
+      break;
     }
-    if (!(reusedCopyOp && srcCopy->hasOneUse())) {
-      getCallbacks().replaceValueUsesWith(srcCopy, srcCopy->getOperand());
-      if (reusedCopyOp) {
-        reusedCopyOp->set(srcCopy);
-      } else {
-        if (instsToDelete.insert(srcCopy)) {
-          LLVM_DEBUG(llvm::dbgs() << "  Removing " << *srcCopy);
-          ++NumCopiesAndMovesEliminated;
+    case Def::Kind::Copy: {
+      SILValue value = def->getValue();
+      CopyValueInst *srcCopy = cast<CopyValueInst>(value);
+      // Recurse through copies while replacing their uses.
+      Operand *reusedCopyOp = nullptr;
+      for (auto useIter = srcCopy->use_begin();
+           useIter != srcCopy->use_end();) {
+        Operand *use = *useIter++;
+        if (!visitUse(use)) {
+          if (!reusedCopyOp && srcCopy->getParent() == use->getParentBlock()) {
+            reusedCopyOp = use;
+          } else {
+            copyLiveUse(use, getCallbacks());
+          }
         }
       }
+      if (!(reusedCopyOp && srcCopy->hasOneUse())) {
+        getCallbacks().replaceValueUsesWith(srcCopy, srcCopy->getOperand());
+        if (reusedCopyOp) {
+          reusedCopyOp->set(srcCopy);
+        } else {
+          if (instsToDelete.insert(srcCopy)) {
+            LLVM_DEBUG(llvm::dbgs() << "  Removing " << *srcCopy);
+            ++NumCopiesAndMovesEliminated;
+          }
+        }
+      }
+      break;
+    }
     }
   }
   assert(!consumes.hasUnclaimedConsumes());