Fix DCE of begin_borrow with @guaranteed operand

For the lower most begin_borrows with @guaranteed operands, there
was no @guaranteed phi computation, restructure the code and fix this case.

Author: meg-gupta

lib/SILOptimizer/Transforms/DeadCodeElimination.cpp

@@ -283,37 +283,33 @@ void DCE::markLive() {
         break;
       }
       case SILInstructionKind::BeginBorrowInst: {
-        // Currently we only support borrows of owned values.
-        // Nested borrow handling can be complex in the presence of reborrows.
-        // So it is not handled currently.
         auto *borrowInst = cast<BeginBorrowInst>(&I);
+        // Populate guaranteedPhiDependencies for this borrowInst
+        findGuaranteedPhiDependencies(BorrowedValue(borrowInst));
+        auto disableBorrowDCE = [&](SILValue borrow) {
+          visitTransitiveEndBorrows(borrow, [&](EndBorrowInst *endBorrow) {
+            markInstructionLive(endBorrow);
+          });
+        };
+        // If we have a begin_borrow of a @guaranteed operand, disable DCE'ing
+        // of parent borrow scopes. Dead reborrows needs complex handling, whuch
+        // is why it is disabled for now.
         if (borrowInst->getOperand()->getOwnershipKind() ==
             OwnershipKind::Guaranteed) {
-          // Visit the end_borrows of all the borrow scopes that this
-          // begin_borrow could be borrowing.
           SmallVector<SILValue, 4> roots;
           findGuaranteedReferenceRoots(borrowInst->getOperand(),
                                        /*lookThroughNestedBorrows=*/false,
                                        roots);
+          // Visit the end_borrows of all the borrow scopes that this
+          // begin_borrow could be borrowing, and mark them live.
           for (auto root : roots) {
-            visitTransitiveEndBorrows(root,
-                                      [&](EndBorrowInst *endBorrow) {
-                                        markInstructionLive(endBorrow);
-                                      });
+            disableBorrowDCE(root);
           }
-          continue;
         }
-        // Populate guaranteedPhiDependencies for this borrow
-        findGuaranteedPhiDependencies(BorrowedValue(borrowInst));
-        // Don't optimize a borrow scope if it is lexical or has a pointer
-        // escape.
+        // If we have a lexical borrow scope or a pointer escape, disable DCE.
         if (borrowInst->isLexical() ||
             hasPointerEscape(BorrowedValue(borrowInst))) {
-          // Visit all end_borrows and mark them live
-          visitTransitiveEndBorrows(borrowInst, [&](EndBorrowInst *endBorrow) {
-            markInstructionLive(endBorrow);
-          });
-          continue;
+          disableBorrowDCE(borrowInst);
         }
         break;
       }

test/SILOptimizer/dead_code_elimination_nontrivial_ossa.sil

@@ -529,6 +529,48 @@ bb1(%newborrowi : @guaranteed $Klass, %newborrowo : @guaranteed $Klass):
   return %res : $()
 }
 
+
+sil [ossa] @dce_nestedborrowlifetime4 : $@convention(thin) (@guaranteed Wrapper1, @guaranteed Wrapper1) -> () {
+bb0(%0 : @guaranteed $Wrapper1, %1 : @guaranteed $Wrapper1):
+  cond_br undef, bb1, bb2
+
+bb1:
+  %outer1 = begin_borrow %0 : $Wrapper1
+  %ex1 = struct_extract %outer1 : $Wrapper1, #Wrapper1.val1
+  %ex11 = struct_extract %ex1 : $Wrapper2, #Wrapper2.val2
+  br bb3(%ex11 : $Klass, %outer1 : $Wrapper1)
+
+bb2:
+  %outer2 = begin_borrow %1 : $Wrapper1
+  %ex2 = struct_extract %outer2 : $Wrapper1, #Wrapper1.val1
+  %ex21 = struct_extract %ex2 : $Wrapper2, #Wrapper2.val2
+  br bb3(%ex21 : $Klass, %outer2 : $Wrapper1)
+
+bb3(%phi1 : @guaranteed $Klass, %phi2 : @guaranteed $Wrapper1):
+  %f = function_ref @use_klass : $@convention(thin) (@guaranteed Klass) -> ()
+  apply %f(%phi1) : $@convention(thin) (@guaranteed Klass) -> ()
+  end_borrow %phi2 : $Wrapper1
+  %9999 = tuple()
+  return %9999 : $()
+}
+
+sil [ossa] @dce_nestedborrowlifetime5 : $@convention(thin) (@guaranteed Wrapper1) -> () {
+bb0(%0 : @guaranteed $Wrapper1):
+  %outer1 = begin_borrow %0 : $Wrapper1
+  %inner1 = begin_borrow %outer1 : $Wrapper1
+  %ex1 = struct_extract %inner1 : $Wrapper1, #Wrapper1.val1
+  %ex11 = struct_extract %ex1 : $Wrapper2, #Wrapper2.val2
+  br bb2(%ex11 : $Klass, %inner1 : $Wrapper1)
+
+bb2(%phi1 : @guaranteed $Klass, %phi2 : @guaranteed $Wrapper1):
+  %f = function_ref @use_klass : $@convention(thin) (@guaranteed Klass) -> ()
+  apply %f(%phi1) : $@convention(thin) (@guaranteed Klass) -> ()
+  end_borrow %phi2 : $Wrapper1
+  end_borrow %outer1 : $Wrapper1
+  %9999 = tuple()
+  return %9999 : $()
+}
+
 // CHECK-LABEL: sil [ossa] @infinite_loop :
 // CHECK-NOT: copy_value
 // CHECK-LABEL: } // end sil function 'infinite_loop'