From 4f65ef92c271b6fd65a4a23ea487cc532b0606c4 Mon Sep 17 00:00:00 2001
From: Joe Groff <jgroff@apple.com>
Date: Mon, 22 Aug 2016 13:37:13 -0700
Subject: [PATCH] Runtime: Don't clobber the compiler-emitted layout of empty
 fields.

Otherwise, we try to dirty constant memory for classes emitted as constant-layout by the compiler. Fixes rdar://problem/27951346.
---
 stdlib/public/runtime/Metadata.cpp            |  3 ++
 .../generic_class_empty_field.swift           | 47 +++++++++++++++++++
 2 files changed, 50 insertions(+)
 create mode 100644 test/Interpreter/generic_class_empty_field.swift

diff --git a/stdlib/public/runtime/Metadata.cpp b/stdlib/public/runtime/Metadata.cpp
index 51023676d55d1..6ee8b3413663f 100644
--- a/stdlib/public/runtime/Metadata.cpp
+++ b/stdlib/public/runtime/Metadata.cpp
@@ -1764,6 +1764,9 @@ swift::swift_initClassMetadata_UniversalStrategy(ClassMetadata *self,
 
   // Okay, now do layout.
   for (unsigned i = 0; i != numFields; ++i) {
+    // Skip empty fields.
+    if (fieldOffsets[i] == 0 && fieldLayouts[i].Size == 0)
+      continue;
     auto offset = roundUpToAlignMask(size, fieldLayouts[i].AlignMask);
     fieldOffsets[i] = offset;
     size = offset + fieldLayouts[i].Size;
diff --git a/test/Interpreter/generic_class_empty_field.swift b/test/Interpreter/generic_class_empty_field.swift
new file mode 100644
index 0000000000000..da980ed4459f0
--- /dev/null
+++ b/test/Interpreter/generic_class_empty_field.swift
@@ -0,0 +1,47 @@
+// RUN: rm -rf %t  &&  mkdir %t
+// RUN: %target-build-swift %s -o %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s
+// REQUIRES: executable_test
+
+class Outer {
+  class Foo {
+    var zim = Bar()
+    var bas = Outer()
+  }
+  class Boo {
+    var bas = Outer()
+    var zim = Bar()
+  }
+
+  required init() {}
+}
+
+protocol Initable { init() }
+extension Outer: Initable {}
+
+class GFoo<T: Initable> {
+  var zim = Bar()
+  var bas = T()
+}
+class GBoo<T: Initable> {
+  var bas = T()
+  var zim = Bar()
+}
+class GFos<T: Initable> {
+  var bar = T()
+  var zim = Bar()
+  var bas = T()
+}
+
+struct Bar { }
+
+do {
+  let a = Outer.Foo()
+  let b = Outer.Boo()
+  let c = GFoo<Outer>()
+  let d = GBoo<Outer>()
+  let e = GFos<Outer>()
+}
+
+// CHECK: Job's finished
+print("Job's finished")