EscapeInfo: handle unchecked_addr_cast

It's dangerous to continue walking over an `unchecked_addr_cast` which casts between two different types.
We can only do this if the result is known to be the end of the walk, i.e. the cast result is not used in a relevant way.

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/Utilities/EscapeUtils.swift

@@ -558,6 +558,26 @@ fileprivate struct EscapeWalker<V: EscapeVisitor> : ValueDefUseWalker,
     case is DeallocStackInst, is InjectEnumAddrInst, is FixLifetimeInst, is EndBorrowInst, is EndAccessInst,
          is IsUniqueInst, is DebugValueInst:
       return .continueWalk
+    case let uac as UncheckedAddrCastInst:
+      if uac.type != uac.fromAddress.type {
+        // It's dangerous to continue walking over an `unchecked_addr_cast` which casts between two different types.
+        // We can only do this if the result is known to be the end of the walk, i.e. the cast result is not used
+        // in a relevant way.
+        for uacUse in uac.uses {
+          // Following instructions turned out to appear in code coming from the stdlib.
+          switch uacUse.instruction {
+          case is IsUniqueInst:
+            break
+          case is LoadInst, is LoadBorrowInst:
+            if followLoads(at: path) {
+              return .abortWalk
+            }
+          default:
+            return .abortWalk
+          }
+        }
+      }
+      return walkDownUses(ofAddress: uac, path: path)
     default:
       return isEscaping
     }

test/SILOptimizer/addr_escape_info.sil

@@ -811,3 +811,37 @@ bb0(%0 :  @owned $X):
   return %4 : $()
 }
 
+// CHECK-LABEL: Address escape information for test_unchecked_addr_cast:
+// CHECK:       value:  %1 = alloc_stack $X
+// CHECK-NEXT:  End function test_unchecked_addr_cast
+sil [ossa] @test_unchecked_addr_cast : $@convention(thin) (@owned X) -> @owned XandIntClass {
+bb0(%0 :  @owned $X):
+  %1 = alloc_stack $X
+  store %0 to [init] %1 : $*X
+  fix_lifetime %1 : $*X
+  %4 = unchecked_addr_cast %1 : $*X to $*XandIntClass
+  %5 = is_unique %4 : $*XandIntClass
+  %6 = load_borrow %4 : $*XandIntClass
+  end_borrow %6 : $XandIntClass
+  %8 = load [take] %4 : $*XandIntClass
+  dealloc_stack %1 : $*X
+  return %8 : $XandIntClass
+}
+
+// CHECK-LABEL: Address escape information for test_unchecked_addr_cast_escaping:
+// CHECK:       value:  %1 = alloc_stack $X
+// CHECK-NEXT:  ==>   %6 = apply %5(%4) : $@convention(thin) (@inout X) -> ()
+// CHECK-NEXT:  End function test_unchecked_addr_cast_escaping
+sil [ossa] @test_unchecked_addr_cast_escaping : $@convention(thin) (@owned XandIntClass) -> () {
+bb0(%0 :  @owned $XandIntClass):
+  %1 = alloc_stack $XandIntClass
+  store %0 to [init] %1 : $*XandIntClass
+  fix_lifetime %1 : $*XandIntClass
+  %4 = unchecked_addr_cast %1 : $*XandIntClass to $*X
+  %5 = function_ref @inout_class_argument : $@convention(thin) (@inout X) -> ()
+  %6 = apply %5(%4) : $@convention(thin) (@inout X) -> ()
+  dealloc_stack %1 : $*XandIntClass
+  %8 = tuple ()
+  return %8 : $()
+}
+