[clang][CodeGen] Fix wrong memcpy size of no_unique_address in FieldMemcpyizer

[zsrkmyn]

Summary

When generating ctor, FieldMemcpyizer wrongly treated zero-sized class members
as what should be copied, and generated wrong memcpy size under some special
circumstances. This patch tries to fix it.

[rjmccall]

LGTM

[MaskRay]

clang/test/CodeGenCXX/no-unique-address-2.cpp

18

{ on the same line of call()

[zsrkmyn]

Thanks for reviewing! Would you mind helping me commit this patch? Thanks :-D

clang/lib/CodeGen/CGClass.cpp

bool isMemcpyableField(FieldDecl *F) const {
         return false;
       Qualifiers Qual = F->getType().getQualifiers();
       if (Qual.hasVolatile() || Qual.hasObjCLifetime())
         return false;
       return true;
     }
 
     void addMemcpyableField(FieldDecl *F) {
+      if (F->isZeroSize(CGF.getContext()))
+        return;
       if (!FirstField)
         addInitialField(F);
       else
         addNextField(F);
     }
 
     CharUnits getMemcpySize(uint64_t FirstByteOffset) const {
       ASTContext &Ctx = CGF.getContext();

clang/test/CodeGenCXX/no-unique-address-2.cpp

+// RUN: %clang_cc1 -std=c++2a %s -emit-llvm -o - -triple x86_64-linux-gnu | FileCheck %s
+
+struct TriviallyCopyable {};
+
+struct NonTriviallyCopyable {
+  NonTriviallyCopyable() = default;
+  NonTriviallyCopyable(const NonTriviallyCopyable&) = default;
+  NonTriviallyCopyable(NonTriviallyCopyable &&) {}
+};
+
+struct Foo {
+  int i;
+  [[no_unique_address]] TriviallyCopyable m;
+  [[no_unique_address]] NonTriviallyCopyable n;
+};
+
+void call() {
+  Foo foo;
+  Foo foo2(static_cast<Foo&&>(foo));
+}
+
+// The memcpy call should copy exact 4 bytes for member 'int i'
+// CHECK: define {{.*}} void @_ZN3FooC2EOS_
+// CHECK:  call void @llvm.memcpy.p0i8.p0i8.i64(i8* {{.+}}, i8* {{.+}}, i64 4, i1 false)
+// CHECK:  call void @_ZN20NonTriviallyCopyableC2EOS_