From a2a57dff0c76507c002e79028434bd1ebe43bdb8 Mon Sep 17 00:00:00 2001
From: Erik Eckstein <eeckstein@apple.com>
Date: Mon, 20 Oct 2025 15:56:20 +0200
Subject: [PATCH] SimplifyCFG: insert compensating `end_lifetime` when
 replacing a `switch_enum`

This is a follow-up of https://github.com/swiftlang/swift/pull/84905, which handles non-copyable enums with a deinit correctly.
Also, for copyable enums it's more efficient to use `end_lifetime` than `destroy_value`, because we already know that the enum contains a trivial case.
Therefore no destroy operation is needed.
---
 lib/SILOptimizer/Transforms/SimplifyCFG.cpp |  3 +-
 test/SILOptimizer/simplify_cfg_ossa.sil     | 38 ++++++++++++++++++---
 2 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/lib/SILOptimizer/Transforms/SimplifyCFG.cpp b/lib/SILOptimizer/Transforms/SimplifyCFG.cpp
index a64c91492f722..cf3cc99864b55 100644
--- a/lib/SILOptimizer/Transforms/SimplifyCFG.cpp
+++ b/lib/SILOptimizer/Transforms/SimplifyCFG.cpp
@@ -1795,7 +1795,8 @@ bool SimplifyCFG::simplifySwitchEnumUnreachableBlocks(SwitchEnumInst *SEI) {
   if (Dest->args_empty()) {
     SILBuilderWithScope builder(SEI);
     if (SEI->getOperand()->getOwnershipKind() == OwnershipKind::Owned) {
-      builder.createDestroyValue(SEI->getLoc(), SEI->getOperand());
+      // Note that a `destroy_value` would be wrong for non-copyable enums with deinits.
+      builder.createEndLifetime(SEI->getLoc(), SEI->getOperand());
     }
     builder.createBranch(SEI->getLoc(), Dest);
 
diff --git a/test/SILOptimizer/simplify_cfg_ossa.sil b/test/SILOptimizer/simplify_cfg_ossa.sil
index 917b5c9e0cce3..9bb6480af1d83 100644
--- a/test/SILOptimizer/simplify_cfg_ossa.sil
+++ b/test/SILOptimizer/simplify_cfg_ossa.sil
@@ -1,4 +1,6 @@
-// RUN: %target-sil-opt -sil-print-types -enable-objc-interop -enable-sil-verify-all %s -simplify-cfg | %FileCheck %s
+// RUN: %target-sil-opt -sil-print-types -enable-objc-interop -enable-experimental-feature MoveOnlyEnumDeinits -enable-sil-verify-all %s -simplify-cfg | %FileCheck %s
+
+// REQUIRES: swift_feature_MoveOnlyEnumDeinits
 
 // OSSA form of tests from simplify_cfg.sil and simplify_cfg_simple.sil.
 //
@@ -64,6 +66,12 @@ struct S {
   var b: NonTrivial
 }
 
+enum NCE: ~Copyable {
+  case a
+  case b(AnyObject)
+  deinit
+}
+
 ///////////
 // Tests //
 ///////////
@@ -1944,12 +1952,12 @@ bb3:
   return %t : $()
 }
 
-// CHECK-LABEL: sil [ossa] @insert_compensating_destroy_in_switch_enum_destination_block :
+// CHECK-LABEL: sil [ossa] @insert_compensating_endlifetime_in_switch_enum_destination_block :
 // CHECK:       bb0(%0 : @owned $Optional<AnyObject>):
-// CHECK-NEXT:    destroy_value %0
+// CHECK-NEXT:    end_lifetime %0
 // CHECK-NEXT:    tuple
-// CHECK:       } // end sil function 'insert_compensating_destroy_in_switch_enum_destination_block'
-sil [ossa] @insert_compensating_destroy_in_switch_enum_destination_block : $@convention(thin) (@owned Optional<AnyObject>) -> () {
+// CHECK:       } // end sil function 'insert_compensating_endlifetime_in_switch_enum_destination_block'
+sil [ossa] @insert_compensating_endlifetime_in_switch_enum_destination_block : $@convention(thin) (@owned Optional<AnyObject>) -> () {
 bb0(%0 : @owned $Optional<AnyObject>):
   switch_enum %0, case #Optional.none!enumelt: bb1, case #Optional.some!enumelt: bb2
 
@@ -1962,6 +1970,26 @@ bb2(%4 : @owned $AnyObject):
   unreachable
 }
 
+// CHECK-LABEL: sil [ossa] @insert_compensating_endlifetime_in_switch_enum_destination_block2 :
+// CHECK:       bb0(%0 : @owned $NCE):
+// CHECK-NEXT:    %1 = drop_deinit %0
+// CHECK-NEXT:    end_lifetime %1
+// CHECK-NEXT:    tuple
+// CHECK:       } // end sil function 'insert_compensating_endlifetime_in_switch_enum_destination_block2'
+sil [ossa] @insert_compensating_endlifetime_in_switch_enum_destination_block2 : $@convention(thin) (@owned NCE) -> () {
+bb0(%0 : @owned $NCE):
+  %1 = drop_deinit %0
+  switch_enum %1, case #NCE.a!enumelt: bb1, case #NCE.b!enumelt: bb2
+
+bb1:
+  %15 = tuple ()
+  return %15
+
+bb2(%4 : @owned $AnyObject):
+  destroy_value %4
+  unreachable
+}
+
 // CHECK-LABEL: sil [ossa] @replace_phi_arg_with_borrowed_from_use :
 // CHECK:       bb3([[R:%.*]] : @reborrow $B):
 // CHECK:       bb6([[G:%.*]] : @guaranteed $E):