[MemAccessUtils] Look thru nested for guar roots.

Previously, findGuaranteedReferenceRoots always stopped searching when
finding a begin_borrow, because it's not an ownership-forwarding
instruction.  Here, it is conditionally allowed to keep search through
the borrowee of that begin_borrow if it itself is guaranteed.

Author: nate-chandler

include/swift/SIL/MemAccessUtils.h

@@ -206,7 +206,7 @@ SILValue findOwnershipReferenceRoot(SILValue ref);
 
 /// Look through all ownership forwarding instructions to find the values which
 /// were originally borrowed.
-void findGuaranteedReferenceRoots(SILValue value,
+void findGuaranteedReferenceRoots(SILValue value, bool lookThroughNestedBorrows,
                                   SmallVectorImpl<SILValue> &roots);
 
 /// Find the aggregate containing the first owned root of the

lib/SIL/Utils/MemAccessUtils.cpp

@@ -861,6 +861,7 @@ SILValue swift::findOwnershipReferenceRoot(SILValue ref) {
 }
 
 void swift::findGuaranteedReferenceRoots(SILValue value,
+                                         bool lookThroughNestedBorrows,
                                          SmallVectorImpl<SILValue> &roots) {
   GraphNodeWorklist<SILValue, 4> worklist;
   auto addAllOperandsToWorklist = [&worklist](SILInstruction *inst) -> bool {
@@ -882,7 +883,16 @@ void swift::findGuaranteedReferenceRoots(SILValue value,
         }
       }
     } else if (auto *inst = value->getDefiningInstruction()) {
-      if (auto *result =
+      if (auto *bbi = dyn_cast<BeginBorrowInst>(inst)) {
+        auto borrowee = bbi->getOperand();
+        if (lookThroughNestedBorrows &&
+            borrowee->getOwnershipKind() == OwnershipKind::Guaranteed) {
+          // A nested borrow, the root guaranteed earlier in the use-def chain.
+          worklist.insert(borrowee);
+        }
+        // The borrowee isn't guaranteed or we aren't looking through nested
+        // borrows.  Fall through to add the begin_borrow to roots.
+      } else if (auto *result =
               dyn_cast<FirstArgOwnershipForwardingSingleValueInst>(inst)) {
         if (result->getNumOperands() > 0) {
           worklist.insert(result->getOperand(0));

lib/SILOptimizer/Transforms/DeadCodeElimination.cpp

@@ -292,7 +292,9 @@ void DCE::markLive() {
           // Visit the end_borrows of all the borrow scopes that this
           // begin_borrow could be borrowing.
           SmallVector<SILValue, 4> roots;
-          findGuaranteedReferenceRoots(borrowInst->getOperand(), roots);
+          findGuaranteedReferenceRoots(borrowInst->getOperand(),
+                                       /*lookThroughNestedBorrows=*/false,
+                                       roots);
           for (auto root : roots) {
             visitTransitiveEndBorrows(root,
                                       [&](EndBorrowInst *endBorrow) {