[ShrinkBorrowScope] Avoid spurious mutation.

If an end_borrow cannot be hoisted, do not rewrite it.

Author: nate-chandler

lib/SILOptimizer/Utils/ShrinkBorrowScope.cpp

@@ -76,20 +76,34 @@ class ShrinkBorrowScope {
   // The instructions from which the shrinking starts, the scope ending
   // instructions, keyed off the block in which they appear.
   llvm::SmallDenseMap<SILBasicBlock *, SILInstruction *> startingInstructions;
+  // The end _borrow instructions for this borrow scope that existed before
+  // ShrinkBorrowScope ran and which were not modified.
+  llvm::SmallPtrSet<SILInstruction *, 8> reusedEndBorrowInsts;
+
+  // Whether ShrinkBorrowScope made any changes to the function.
+  //
+  // It could have made one of the following sorts of changes:
+  // - deleted an end_borrow
+  // - created an end_borrow
+  // - rewrote the operand of an instruction
+  //   - ApplySite
+  //   - begin_borrow
+  //   - copy_value
+  bool madeChange;
 
 public:
   ShrinkBorrowScope(BeginBorrowInst *bbi, InstructionDeleter &deleter,
                     SmallVectorImpl<CopyValueInst *> &modifiedCopyValueInsts)
       : introducer(bbi), deleter(deleter),
-        modifiedCopyValueInsts(modifiedCopyValueInsts) {}
+        modifiedCopyValueInsts(modifiedCopyValueInsts), madeChange(false) {}
 
   bool run();
 
   bool populateUsers();
   bool initializeWorklist();
   void findBarriers();
-  void rewrite();
-  void createEndBorrow(SILInstruction *insertionPoint);
+  bool rewrite();
+  bool createEndBorrow(SILInstruction *insertionPoint);
 
   bool isBarrierApply(SILInstruction *instruction) {
     // For now, treat every apply (that doesn't use the borrowed value) as a
@@ -165,8 +179,10 @@ class ShrinkBorrowScope {
           if (auto *cvi = dyn_cast<CopyValueInst>(argument)) {
             cvi->setOperand(borrowee);
             modifiedCopyValueInsts.push_back(cvi);
+            madeChange = true;
           } else {
             apply.setArgument(index, borrowee);
+            madeChange = true;
           }
         }
         return true;
@@ -175,12 +191,14 @@ class ShrinkBorrowScope {
             canReplaceValueWithBorrowedValue(bbi->getOperand())) {
           auto borrowee = introducer->getOperand();
           bbi->setOperand(borrowee);
+          madeChange = true;
           return true;
         }
       } else if (auto *cvi = dyn_cast<CopyValueInst>(instruction)) {
         if (canReplaceValueWithBorrowedValue(cvi->getOperand())) {
           auto borrowee = introducer->getOperand();
           cvi->setOperand(borrowee);
+          madeChange = true;
           modifiedCopyValueInsts.push_back(cvi);
           return true;
         }
@@ -205,9 +223,9 @@ bool ShrinkBorrowScope::run() {
 
   findBarriers();
 
-  rewrite();
+  madeChange |= rewrite();
 
-  return true;
+  return madeChange;
 }
 
 bool ShrinkBorrowScope::populateUsers() {
@@ -311,31 +329,50 @@ void ShrinkBorrowScope::findBarriers() {
   }
 }
 
-void ShrinkBorrowScope::rewrite() {
-  // Remove all the original end_borrow instructions.
-  for (auto pair : startingInstructions) {
-    deleter.forceDelete(pair.getSecond());
-  }
+bool ShrinkBorrowScope::rewrite() {
+  bool createdBorrow = false;
 
   // Insert the new end_borrow instructions that occur after deinit barriers.
   for (auto pair : barrierInstructions) {
     auto *insertionPoint = getNextInstruction(pair.second);
-    createEndBorrow(insertionPoint);
+    createdBorrow |= createEndBorrow(insertionPoint);
   }
 
   // Insert the new end_borrow instructions that occur at the beginning of
   // blocks which we couldn't hoist out of.
   for (auto *block : barredBlocks) {
     auto *insertionPoint = &*block->begin();
-    createEndBorrow(insertionPoint);
+    createdBorrow |= createEndBorrow(insertionPoint);
+  }
+
+  if (createdBorrow) {
+    // Remove all the original end_borrow instructions.
+    for (auto pair : startingInstructions) {
+      if (reusedEndBorrowInsts.contains(pair.second)) {
+        continue;
+      }
+      deleter.forceDelete(pair.getSecond());
+    }
   }
+
+  return createdBorrow;
 }
 
-void ShrinkBorrowScope::createEndBorrow(SILInstruction *insertionPoint) {
+bool ShrinkBorrowScope::createEndBorrow(SILInstruction *insertionPoint) {
+  if (auto *ebi = dyn_cast<EndBorrowInst>(insertionPoint)) {
+    llvm::SmallDenseMap<SILBasicBlock *, SILInstruction *>::iterator location;
+    if ((location = llvm::find_if(startingInstructions, [&](auto pair) -> bool {
+           return pair.second == insertionPoint;
+         })) != startingInstructions.end()) {
+      reusedEndBorrowInsts.insert(location->second);
+      return false;
+    }
+  }
   auto builder = SILBuilderWithScope(insertionPoint);
   builder.createEndBorrow(
       RegularLocation::getAutoGeneratedLocation(insertionPoint->getLoc()),
       introducer);
+  return true;
 }
 
 bool swift::shrinkBorrowScope(