[CodeGenObjC] Fix a crash when attempting to copy a zero-sized bit-field in a non-trivial C struct

[erik.pilkington]

Summary

Zero sized bit-fields aren't included in the CGRecordLayout, so we shouldn't be calling EmitLValueForField for them.

rdar://60695105

[ahatanak]

Thanks for fixing this!

clang/lib/CodeGen/CGNonTrivialStruct.cpp

545

Can you add the same check to GenBinaryFuncName::visitVolatileTrivial to avoid encoding the zero-length bit field into the name of the copy constructor/assignment functions? It isn't necessary to generate different functions for ZeroBitfieldin the test case and the following struct:

struct S {
  id strong;
};

clang/test/CodeGenObjC/strong-in-c-struct.m

894

Can you check that the zero-sized field information isn't encoded into the name of the copy assignment function?

[erik.pilkington]

Don't bother including zero length bit-fields in the mangling of the copy/destroy helpers.

[ahatanak]

LGTM

clang/include/clang/AST/NonTrivialTypeVisitor.h

-//===-- NonTrivialTypeVisitor.h - Visitor for non-trivial Types *- C++ --*-===//
+//===-- NonTrivialTypeVisitor.h - Visitor for non-trivial Types -*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file defines the visitor classes that are used to traverse non-trivial

clang/lib/CodeGen/CGNonTrivialStruct.cpp

 
     this->Start = this->End = CharUnits::Zero();
   }
 
   template <class... Ts>
   void visitVolatileTrivial(QualType FT, const FieldDecl *FD, CharUnits Offset,
                             std::array<Address, 2> Addrs) {
     LValue DstLV, SrcLV;
     if (FD) {
+      // No need to copy zero-length bit-fields.
+      if (FD->isZeroLengthBitField(this->CGF->getContext()))
+        return;
+
       QualType RT = QualType(FD->getParent()->getTypeForDecl(), 0);
       llvm::PointerType *PtrTy = this->CGF->ConvertType(RT)->getPointerTo();
       Address DstAddr = this->getAddrWithOffset(Addrs[DstIdx], Offset);
       LValue DstBase = this->CGF->MakeAddrLValue(
           this->CGF->Builder.CreateBitCast(DstAddr, PtrTy), FT);
       DstLV = this->CGF->EmitLValueForField(DstBase, FD);
       Address SrcAddr = this->getAddrWithOffset(Addrs[SrcIdx], Offset);
       LValue SrcBase = this->CGF->MakeAddrLValue(

clang/test/CodeGenObjC/strong-in-c-struct.m

 // CHECK: call void @__destructor_8_s8(i8** %[[V3]])
 // CHECK: call void @func(
 
 void test_volatile_variable_reference(volatile StrongSmall *a) {
   (void)*a;
   func(0);
 }
 
+struct ZeroBitfield {
+  int : 0;
+  id strong;
+};
+
+void test_zero_bitfield() {
+  struct ZeroBitfield volatile a, b;
+  a = b;
+}
+
 #endif /* USESTRUCT */