[cxx-interop] Zero-initialize C++ structs when calling their default constructors

When Swift imports C structs, it synthesizes an initializer that takes no arguments and zero-initializes the C struct.

When C++ interop is enabled, Clang treats all C structs as if they were C++ structs. This means that some of the C structs will get a default constructor implicitly generated by Clang. This implicit default constructor will not zero-initialize trivial fields of the struct.

This is a common source of confusion and subtle bugs for developers who try to enable C++ interop in existing projects that use C interop and rely on zero-initialization of C structs.

rdar://115909532

Author: egorzhdan

lib/IRGen/GenDecl.cpp

@@ -3476,7 +3476,12 @@ llvm::Constant *swift::irgen::emitCXXConstructorThunkIfNeeded(
   llvm::FunctionType *ctorFnType =
       cast<llvm::FunctionType>(clangFunc->getValueType());
 
-  if (assumedFnType == ctorFnType) {
+  // Only need a thunk if either:
+  // 1. The calling conventions do not match, and we need to pass arguments
+  //    differently.
+  // 2. This is a default constructor, and we need to zero the backing memory of
+  //    the struct.
+  if (assumedFnType == ctorFnType && !ctor->isDefaultConstructor()) {
     return ctorAddress;
   }
 
@@ -3510,14 +3515,34 @@ llvm::Constant *swift::irgen::emitCXXConstructorThunkIfNeeded(
       Args.push_back(i);
   }
 
-  clang::CodeGen::ImplicitCXXConstructorArgs implicitArgs =
-      clang::CodeGen::getImplicitCXXConstructorArgs(IGM.ClangCodeGen->CGM(),
-                                                    ctor);
-  for (size_t i = 0; i < implicitArgs.Prefix.size(); ++i) {
-    Args.insert(Args.begin() + 1 + i, implicitArgs.Prefix[i]);
+  if (assumedFnType != ctorFnType) {
+    clang::CodeGen::ImplicitCXXConstructorArgs implicitArgs =
+        clang::CodeGen::getImplicitCXXConstructorArgs(IGM.ClangCodeGen->CGM(),
+                                                      ctor);
+    for (size_t i = 0; i < implicitArgs.Prefix.size(); ++i) {
+      Args.insert(Args.begin() + 1 + i, implicitArgs.Prefix[i]);
+    }
+    for (const auto &arg : implicitArgs.Suffix) {
+      Args.push_back(arg);
+    }
   }
-  for (const auto &arg : implicitArgs.Suffix) {
-    Args.push_back(arg);
+
+  if (ctor->isDefaultConstructor()) {
+    assert(Args.size() > 0 && "expected at least 1 argument (result address) "
+                              "for default constructor");
+
+    // Zero out the backing memory of the struct.
+    // This makes default initializers for C++ structs behave consistently with
+    // the synthesized empty initializers for C structs. When C++ interop is
+    // enabled in a project, all imported C structs are treated as C++ structs,
+    // which sometimes means that Clang will synthesize a default constructor
+    // for the C++ struct that does not zero out trivial fields of a struct.
+    auto cxxRecord = ctor->getParent();
+    clang::ASTContext &ctx = cxxRecord->getASTContext();
+    auto typeSize = ctx.getTypeSizeInChars(ctx.getRecordType(cxxRecord));
+    subIGF.Builder.CreateMemSet(Args[0],
+                                llvm::ConstantInt::get(subIGF.IGM.Int8Ty, 0),
+                                typeSize.getQuantity(), llvm::MaybeAlign());
   }
 
   auto *call =

test/Interop/Cxx/class/Inputs/constructors.h

@@ -32,6 +32,9 @@ struct ConstructorWithParam {
 struct CopyAndMoveConstructor {
   CopyAndMoveConstructor(const CopyAndMoveConstructor &) = default;
   CopyAndMoveConstructor(CopyAndMoveConstructor &&) = default;
+
+  int value = 123;
+  int *ptr = nullptr;
 };
 
 struct Base {};

test/Interop/Cxx/class/constructors-executable.swift

@@ -44,4 +44,18 @@ CxxConstructorTestSuite.test("TemplatedConstructor") {
   expectEqual(2, instance.value.i)
 }
 
+CxxConstructorTestSuite.test("implicit default ctor") {
+  // Make sure that fields of C++ structs are zeroed out.
+
+  let instance1 = ConstructorWithParam() // calling deprecated implicit ctor
+  expectEqual(0, instance1.x)
+
+  let instance2 = IntWrapper() // calling deprecated implicit ctor
+  expectEqual(0, instance2.x)
+
+  let instance3 = CopyAndMoveConstructor() // calling deprecated implicit ctor
+  expectEqual(0, instance3.value)
+  expectNil(instance3.ptr)
+}
+
 runAllTests()

test/Interop/Cxx/class/constructors-module-interface.swift

@@ -32,6 +32,9 @@
 // CHECK-NEXT: struct CopyAndMoveConstructor {
 // CHECK-NEXT:   @available(*, deprecated, message
 // CHECK-NEXT:   init()
+// CHECK-NEXT:   init(value: Int32, ptr: UnsafeMutablePointer<Int32>!)
+// CHECK-NEXT:   var value: Int32
+// CHECK-NEXT:   var ptr: UnsafeMutablePointer<Int32>!
 // CHECK-NEXT: }
 // CHECK-NEXT: struct Base {
 // CHECK-NEXT:   init()