Optimizer: prevent duplicating blocks with dead-end begin-borrow instructions.

In case the control flow ends in a dead-end block there can be begin-borrow instructions which have no corresponding end-borrow uses.
After duplicating such a block, the re-borrow flags cannot be recomputed correctly for inserted phi arguments.
Therefore just disable duplicating such blocks, e.g. in jump-threading.

Author: eeckstein

include/swift/SIL/OwnershipUtils.h

@@ -618,6 +618,12 @@ struct BorrowedValue {
   /// BorrowScopeIntroducingValue::isLocalScope().
   bool visitLocalScopeEndingUses(function_ref<bool(Operand *)> visitor) const;
 
+  /// Returns false if the value has no scope-ending uses because all control flow
+  /// paths end in dead-end blocks.
+  bool hasLocalScopeEndingUses() const {
+    return !visitLocalScopeEndingUses([](Operand *) { return false; });
+  }
+
   bool isLocalScope() const { return kind.isLocalScope(); }
 
   /// Add this scope's live blocks into the PrunedLiveness result. This

lib/SIL/IR/SILInstruction.cpp

@@ -22,6 +22,7 @@
 #include "swift/SIL/ApplySite.h"
 #include "swift/SIL/DynamicCasts.h"
 #include "swift/SIL/InstructionUtils.h"
+#include "swift/SIL/OwnershipUtils.h"
 #include "swift/SIL/SILBuilder.h"
 #include "swift/SIL/SILCloner.h"
 #include "swift/SIL/SILDebugScope.h"
@@ -1479,6 +1480,17 @@ bool SILInstruction::isTriviallyDuplicatable() const {
       isa<GetAsyncContinuationInst>(this))
     return false;
 
+  // Bail if there are any begin-borrow instructions which have no corresponding
+  // end-borrow uses. This is the case if the control flow ends in a dead-end block.
+  // After duplicating such a block, the re-borrow flags cannot be recomputed
+  // correctly for inserted phi arguments.
+  if (auto *svi  = dyn_cast<SingleValueInstruction>(this)) {
+    if (auto bv = BorrowedValue(lookThroughBorrowedFromDef(svi))) {
+      if (!bv.hasLocalScopeEndingUses())
+        return false;
+    }
+  }
+
   // If you add more cases here, you should also update SILLoop:canDuplicate.
 
   return true;

lib/SILOptimizer/Utils/LoopUtils.cpp

@@ -17,6 +17,7 @@
 #include "swift/SIL/BasicBlockUtils.h"
 #include "swift/SIL/Dominance.h"
 #include "swift/SIL/LoopInfo.h"
+#include "swift/SIL/OwnershipUtils.h"
 #include "swift/SIL/SILArgument.h"
 #include "swift/SIL/SILBasicBlock.h"
 #include "swift/SIL/SILBuilder.h"
@@ -331,6 +332,17 @@ bool swift::canDuplicateLoopInstruction(SILLoop *L, SILInstruction *I) {
       isa<GetAsyncContinuationAddrInst>(I) || isa<GetAsyncContinuationInst>(I))
     return false;
 
+  // Bail if there are any begin-borrow instructions which have no corresponding
+  // end-borrow uses. This is the case if the control flow ends in a dead-end block.
+  // After duplicating such a block, the re-borrow flags cannot be recomputed
+  // correctly for inserted phi arguments.
+  if (auto *svi  = dyn_cast<SingleValueInstruction>(I)) {
+    if (auto bv = BorrowedValue(lookThroughBorrowedFromDef(svi))) {
+      if (!bv.hasLocalScopeEndingUses())
+        return false;
+    }
+  }
+
   // Some special cases above that aren't considered isTriviallyDuplicatable
   // return true early.
   assert(I->isTriviallyDuplicatable() &&

test/SILOptimizer/simplify_cfg_dom_jumpthread.sil

@@ -497,3 +497,64 @@ bb6:
   %r = tuple ()
   return %r : $()
 }
+
+// CHECK-LABEL: sil [ossa] @test_begin_borrow_in_duplicated_block :
+// CHECK:         begin_borrow
+// CHECK:         begin_borrow
+// CHECK:         @reborrow
+// CHECK:       } // end sil function 'test_begin_borrow_in_duplicated_block'
+sil [ossa] @test_begin_borrow_in_duplicated_block : $@convention(thin) (@guaranteed E2, @guaranteed E2) -> @owned E2 {
+bb0(%0 : @guaranteed $E2, %1 : @guaranteed $E2):
+  switch_enum %0 : $E2, case #E2.b!enumelt: bb1, default bb2
+
+bb1(%3 : @guaranteed $C):
+  br bb3
+
+bb2(%4 : @guaranteed $E2):
+  br bb3
+
+bb3:
+  %5 = copy_value %1 : $E2
+  %6 = begin_borrow %5 : $E2
+  switch_enum %0 : $E2, case #E2.c!enumelt: bb4, default bb5
+
+bb4(%8 : @guaranteed $C):
+  br bb6
+
+bb5(%11 : @guaranteed $E2):
+  br bb6
+
+bb6:
+  fix_lifetime %6 : $E2
+  end_borrow %6 : $E2
+  return %5 : $E2
+}
+
+// CHECK-LABEL: sil [ossa] @test_begin_borrow_and_unreachable :
+// CHECK-NOT:     @reborrow
+// CHECK:       } // end sil function 'test_begin_borrow_and_unreachable'
+sil [ossa] @test_begin_borrow_and_unreachable : $@convention(thin) (@guaranteed E2, @guaranteed E2) -> () {
+bb0(%0 : @guaranteed $E2, %1 : @guaranteed $E2):
+  switch_enum %0 : $E2, case #E2.b!enumelt: bb1, default bb2
+
+bb1(%3 : @guaranteed $C):
+  br bb3
+
+bb2(%4 : @guaranteed $E2):
+  br bb3
+
+bb3:
+  %5 = copy_value %1 : $E2
+  %6 = begin_borrow %5 : $E2
+  switch_enum %0 : $E2, case #E2.c!enumelt: bb4, default bb5
+
+bb4(%8 : @guaranteed $C):
+  br bb6
+
+bb5(%11 : @guaranteed $E2):
+  br bb6
+
+bb6:
+  fix_lifetime %6 : $E2
+  unreachable
+}