[6.0] IRGen: don't stack promote classes for which the layout has not a fixed size

lib/IRGen/GenClass.cpp

@@ -780,6 +780,8 @@ static llvm::Value *stackPromote(IRGenFunction &IGF,
     return nullptr;
   if (!FieldLayout.isFixedLayout())
     return nullptr;
+  if (!FieldLayout.isFixedSize())
+    return nullptr;
 
   // Calculate the total size needed.
   // The first part is the size of the class itself.

test/SILOptimizer/stack-promotion-crash.swift

@@ -0,0 +1,53 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-build-swift -O -wmo -parse-as-library -emit-module -emit-module-path=%t/XMod.swiftmodule -module-name=XMod %t/xmod.swift -I%t -c -o %t/xmod.o
+// RUN: %target-build-swift -O -wmo -module-name=Main -I%t %t/main.swift -c -o %t/main.o
+// RUN: %target-swiftc_driver %t/main.o %t/xmod.o -o %t/a.out
+// RUN: %target-codesign %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s
+
+//--- module.modulemap
+
+module CModule {
+  header "c-header.h"
+  export *
+}
+
+
+//--- c-header.h
+
+struct CS {
+  int x;
+};
+
+//--- xmod.swift
+
+@_implementationOnly import CModule
+
+// The layout of this class does not include the C-imported CS field.
+// Therefore it must not be stack-promoted in `testit`. Otherwise the reserved stack space
+// would be wrong and the executable would crash.
+final public class X {
+  public var i: Int
+  var cs: CS? = nil
+
+  public init(_ i: Int) { self.i = i }
+}
+
+//--- main.swift
+
+import XMod
+
+@inline(never)
+func getit(_ x: X, _ y: X) -> Int {
+  return x.i + y.i
+}
+
+@inline(never)
+public func testit() -> Int {
+  return getit(X(27), X(11))
+}
+
+// CHECK: 38
+print(testit())
+