From 5f5692c3ba4bb6ed464803d48905f96e8b7c9478 Mon Sep 17 00:00:00 2001
From: Nate Chandler <nate_chandler@apple.com>
Date: Thu, 6 Apr 2023 13:33:22 -0700
Subject: [PATCH 1/2] [Gardening] Separated debug output lines.

---
 lib/SILOptimizer/Transforms/DeadCodeElimination.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/SILOptimizer/Transforms/DeadCodeElimination.cpp b/lib/SILOptimizer/Transforms/DeadCodeElimination.cpp
index 93c69946382c6..de64b0508c25b 100644
--- a/lib/SILOptimizer/Transforms/DeadCodeElimination.cpp
+++ b/lib/SILOptimizer/Transforms/DeadCodeElimination.cpp
@@ -680,7 +680,8 @@ bool DCE::removeDead() {
         }
         LLVM_DEBUG(llvm::dbgs() << "Replacing branch: ");
         LLVM_DEBUG(Inst->dump());
-        LLVM_DEBUG(llvm::dbgs() << "with jump to: BB" << postDom->getDebugID());
+        LLVM_DEBUG(llvm::dbgs()
+                   << "with jump to: BB" << postDom->getDebugID() << "\n");
 
         replaceBranchWithJump(Inst, postDom);
         Inst->eraseFromParent();

From 38cbc6fde11d4a0f06a13ab08855082c61b22a07 Mon Sep 17 00:00:00 2001
From: Nate Chandler <nate_chandler@apple.com>
Date: Thu, 6 Apr 2023 13:33:50 -0700
Subject: [PATCH 2/2] [DCE] Destroys of live values are live.

As is done for `end_borrow`s, add a dependency from a `destroy_value`s
to its operand so that it is not eliminated if its operand isn't.
---
 .../Transforms/DeadCodeElimination.cpp        |  2 +
 .../dead_code_elimination_ossa.sil            | 45 +++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/lib/SILOptimizer/Transforms/DeadCodeElimination.cpp b/lib/SILOptimizer/Transforms/DeadCodeElimination.cpp
index de64b0508c25b..edd753eb3a694 100644
--- a/lib/SILOptimizer/Transforms/DeadCodeElimination.cpp
+++ b/lib/SILOptimizer/Transforms/DeadCodeElimination.cpp
@@ -280,6 +280,8 @@ void DCE::markLive() {
         if (phi && (phi->isLexical() || hasPointerEscape(phi))) {
           markInstructionLive(&I);
         }
+        // The instruction is live only if it's operand value is also live
+        addReverseDependency(I.getOperand(0), &I);
         break;
       }
       case SILInstructionKind::EndBorrowInst: {
diff --git a/test/SILOptimizer/dead_code_elimination_ossa.sil b/test/SILOptimizer/dead_code_elimination_ossa.sil
index cf773fa01ec2d..92202f804762c 100644
--- a/test/SILOptimizer/dead_code_elimination_ossa.sil
+++ b/test/SILOptimizer/dead_code_elimination_ossa.sil
@@ -8,6 +8,16 @@ import Swift
 public struct S {
   var o: AnyObject
 }
+
+typealias Int1 = Builtin.Int1
+
+class C {}
+
+struct CAndBit {
+    var c: C
+    var bit: Int1
+}
+
 sil @dummy : $@convention(thin) () -> ()
 
 // CHECK-LABEL: sil [ossa] @dead1 :
@@ -362,3 +372,38 @@ bb0(%0 : @guaranteed $S):
   %8 = tuple ()
   return %8 : $()
 }
+
+// CHECK-LABEL: sil [ossa] @dce_destroy_of_live_copy : {{.*}} {
+// CHECK:         [[COPY:%[^,]+]] = copy_value
+// CHECK:         cond_br {{%[^,]+}}, [[LEFT:bb[0-9]+]], [[RIGHT:bb[0-9]+]]
+// CHECK:       [[LEFT]]:
+// CHECK:         destroy_value [[COPY]]
+// CHECK:         br [[EXIT:bb[0-9]+]]
+// CHECK:       [[RIGHT]]:
+// CHECK:         destroy_value [[COPY]]
+// CHECK:         br [[EXIT]]
+// CHECK:       [[EXIT]]:
+// CHECK-LABEL: } // end sil function 'dce_destroy_of_live_copy'
+sil [ossa] @dce_destroy_of_live_copy : $@convention(thin) (Int1) -> () {
+entry(%condition : $Int1):
+  %instance = apply undef() : $@convention(thin) () -> (@owned C)
+  %borrow = begin_borrow %instance : $C
+  %aggregate = struct $CAndBit(%borrow : $C, %condition : $Int1)
+  %copy = copy_value %aggregate : $CAndBit
+  end_borrow %borrow : $C
+  %borrow2 = begin_borrow %copy : $CAndBit
+  %bit = struct_extract %borrow2 : $CAndBit, #CAndBit.bit
+  end_borrow %borrow2 : $CAndBit
+  apply undef(%bit) : $@convention(thin) (Int1) -> ()
+  cond_br %condition, left, right
+left:
+  destroy_value %copy : $CAndBit
+  br exit
+right:
+  destroy_value %copy : $CAndBit
+  br exit
+exit:
+  destroy_value %instance : $C
+  %retval = tuple ()
+  return %retval : $()
+}