[AllocBoxToStack] Restore isDeadEnd check.

The rewrite was missing the intentional omission of `dealloc_stack`s
corresponding to `[dead_end]` `dealloc_box`es.  Add the necessary
bridging to get to parity with the original.

Without this check, `dealloc_box [dead_end]`s are promoted to
`dealloc_stack`s but the memory projected out of such `alloc_box`s need
not be valid.

rdar://159271158

Author: nate-chandler

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/AllocBoxToStack.swift

@@ -365,6 +365,10 @@ private func createAllocStack(for allocBox: AllocBoxInst, flags: Flags, _ contex
       if  !unboxedType.isTrivial(in: allocBox.parentFunction), !(destroy is DeallocBoxInst) {
         builder.createDestroyAddr(address: stackLocation)
       }
+      if let dbi = destroy as? DeallocBoxInst, dbi.isDeadEnd {
+        // Don't bother to create dealloc_stack instructions in dead-ends.
+        return
+      }
       builder.createDeallocStack(asi)
     }
   }

SwiftCompilerSources/Sources/SIL/Instruction.swift

@@ -699,7 +699,9 @@ final public class DeallocRefInst : Instruction, UnaryInstruction, Deallocation
 
 final public class DeallocPartialRefInst : Instruction, Deallocation {}
 
-final public class DeallocBoxInst : Instruction, UnaryInstruction, Deallocation {}
+final public class DeallocBoxInst : Instruction, UnaryInstruction, Deallocation {
+  public var isDeadEnd: Bool { bridged.DeallocBoxInst_isDeadEnd() }
+}
 
 final public class DeallocExistentialBoxInst : Instruction, UnaryInstruction, Deallocation {}
 

include/swift/SIL/SILBridging.h

@@ -827,6 +827,7 @@ struct BridgedInstruction {
   BRIDGED_INLINE bool CopyAddrInst_isInitializationOfDest() const;
   BRIDGED_INLINE void CopyAddrInst_setIsTakeOfSrc(bool isTakeOfSrc) const;
   BRIDGED_INLINE void CopyAddrInst_setIsInitializationOfDest(bool isInitializationOfDest) const;
+  BRIDGED_INLINE bool DeallocBoxInst_isDeadEnd() const;
   BRIDGED_INLINE bool ExplicitCopyAddrInst_isTakeOfSrc() const;
   BRIDGED_INLINE bool ExplicitCopyAddrInst_isInitializationOfDest() const;
   BRIDGED_INLINE SwiftInt MarkUninitializedInst_getKind() const;

include/swift/SIL/SILBridgingImpl.h

@@ -1508,6 +1508,10 @@ void BridgedInstruction::CopyAddrInst_setIsInitializationOfDest(bool isInitializ
       isInitializationOfDest ? swift::IsInitialization : swift::IsNotInitialization);
 }
 
+bool BridgedInstruction::DeallocBoxInst_isDeadEnd() const {
+  return getAs<swift::DeallocBoxInst>()->isDeadEnd();
+}
+
 bool BridgedInstruction::ExplicitCopyAddrInst_isTakeOfSrc() const {
   return getAs<swift::ExplicitCopyAddrInst>()->isTakeOfSrc();
 }

test/SILOptimizer/allocbox_to_stack_lifetime.sil

@@ -54,7 +54,6 @@ bb0(%0 : $Int):
 // CHECK:         [[STACK:%[^,]+]] = alloc_stack
 // CHECK:         cond_br undef, [[DIE:bb[0-9]+]]
 // CHECK:       [[DIE]]:
-// CHECK-NEXT:    dealloc_stack [[STACK]]
 // CHECK-NEXT:    unreachable
 // CHECK-LABEL: } // end sil function 'keep_dead_end'
 sil [ossa] @keep_dead_end : $@convention(thin) () -> () {