[clang] [NVPTX][AMDGPU][CodeGen] Fix `local_space nullptr` handling for NVPTX and local/private `nullptr` value for AMDGPU. (PR #78759)

[Matt Arsenault via cfe-commits]

================
@@ -285,6 +289,20 @@ void NVPTXTargetCodeGenInfo::addNVVMMetadata(llvm::GlobalValue *GV,
 bool NVPTXTargetCodeGenInfo::shouldEmitStaticExternCAliases() const {
   return false;
 }
+
+llvm::Constant *
+NVPTXTargetCodeGenInfo::getNullPointer(const CodeGen::CodeGenModule &CGM,
+                                       llvm::PointerType *PT,
+                                       QualType QT) const {
+  auto &Ctx = CGM.getContext();
+  if (PT->getAddressSpace() != Ctx.getTargetAddressSpace(LangAS::opencl_local))
+    return llvm::ConstantPointerNull::get(PT);
+
+  auto NPT = llvm::PointerType::get(
+      PT->getContext(), Ctx.getTargetAddressSpace(LangAS::opencl_generic));
+  return llvm::ConstantExpr::getAddrSpaceCast(
+      llvm::ConstantPointerNull::get(NPT), PT);
+}
----------------
arsenm wrote:

"The semantics of non-zero address spaces are target-specific. Memory access through a non-dereferenceable pointer is undefined behavior in any address space. Pointers with the bit-value 0 are only assumed to be non-dereferenceable in address space 0, unless the function is marked with the null_pointer_is_valid attribute."

https://github.com/llvm/llvm-project/pull/78759