[CUDA][HIP] fix virtual dtor host/device attr (llvm#128926)

Currently if CUDA/HIP users use template class with virtual dtor
and std::string data member with C++20 and MSVC. When the template
class is explicitly instantiated, there is error about host
function called by host device function (used to be undefined
symbols in linking stage before member destructors were checked
by deferred diagnostics).

It was caused by clang inferring host/device attributes for
default dtors. Since all dtors of member and parent classes
have implicit host device attrs, clang infers the virtual dtor have
implicit host and device attrs. Since virtual dtor of
explicitly instantiated template class must be emitted,
this causes constexpr dtor of std::string emitted, which
calls a host function which was not emitted on device side.

This is a serious issue since it prevents users from
using std::string with C++20 on Windows.

When inferring host device attr of virtual dtor of explicit
template class instantiation, clang should be conservative
since it is sure to be emitted. Since an implicit host device
function may call a host function, clang cannot assume it is
always available on device. This guarantees dtors that
may call host functions not to have implicit device attr,
therefore will not be emitted on device side.

Fixes: llvm#108548

Fixes: SWDEV-517435

Author: yxsamliu

clang/docs/HIPSupport.rst

@@ -286,6 +286,26 @@ Example Usage
       basePtr->virtualFunction(); // Allowed since obj is constructed in device code
    }
 
+Host and Device Attributes of Default Destructors
+===================================================
+
+If a default destructor does not have explicit host or device attributes,
+clang infers these attributes based on the destructors of its data members
+and base classes. If any conflicts are detected among these destructors,
+clang diagnoses the issue. Otherwise, clang adds an implicit host or device
+attribute according to whether the data members's and base classes's
+destructors can execute on the host or device side.
+
+For explicit template classes with virtual destructors, which must be emitted,
+the inference adopts a conservative approach. In this case, implicit host or
+device attributes from member and base class destructors are ignored. This
+precaution is necessary because, although a constexpr destructor carries
+implicit host or device attributes, a constexpr function may call a
+non-constexpr function, which is by default a host function.
+
+Users can override the inferred host and device attributes of default
+destructors by adding explicit host and device attributes to them.
+
 C++ Standard Parallelism Offload Support: Compiler And Runtime
 ==============================================================
 

clang/lib/Sema/SemaCUDA.cpp

@@ -437,7 +437,9 @@ bool SemaCUDA::inferTargetForImplicitSpecialMember(CXXRecordDecl *ClassDecl,
     if (!SMOR.getMethod())
       continue;
 
-    CUDAFunctionTarget BaseMethodTarget = IdentifyTarget(SMOR.getMethod());
+    CUDAFunctionTarget BaseMethodTarget =
+        IdentifyTarget(SMOR.getMethod(), IsExpVDtor);
+
     if (!InferredTarget) {
       InferredTarget = BaseMethodTarget;
     } else {
@@ -481,7 +483,9 @@ bool SemaCUDA::inferTargetForImplicitSpecialMember(CXXRecordDecl *ClassDecl,
     if (!SMOR.getMethod())
       continue;
 
-    CUDAFunctionTarget FieldMethodTarget = IdentifyTarget(SMOR.getMethod());
+    CUDAFunctionTarget FieldMethodTarget =
+        IdentifyTarget(SMOR.getMethod(), IsExpVDtor);
+
     if (!InferredTarget) {
       InferredTarget = FieldMethodTarget;
     } else {

clang/test/SemaCUDA/dtor.cu

@@ -32,22 +32,24 @@ public:
 template class B<float>;
 }
 
-// The implicit host/device attrs of virtual dtor B<float>::~B() is inferred to
-// have implicit device attr since dtors of its members and parent classes can
-// be executed on device. This causes a diagnostic since B<float>::~B() must
-// be emitted, and it eventually causes host_fun() called on device side.
+// The implicit host/device attrs of virtual dtor ~B() should be
+// conservatively inferred, where constexpr member dtor's should
+// not be considered device since they may call host functions.
+// Therefore B<float>::~B() should not have implicit device attr.
+// However C<float>::~C() should have implicit device attr since
+// it is trivial.
 namespace ExplicitInstantiationDtorNoAttr {
-void host_fun() // dev-note {{'host_fun' declared here}}
+void host_fun()
 {}
 
 template <unsigned>
 constexpr void hd_fun() {
-  host_fun(); // dev-error{{reference to __host__ function 'host_fun' in __host__ __device__ function}}
+  host_fun();
 }
 
 struct A {
-  constexpr ~A() { // dev-note {{called by '~B'}}
-     hd_fun<8>(); // dev-note {{called by '~A'}}
+  constexpr ~A() {
+     hd_fun<8>();
   }
 };