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

[Alexander Richardson 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);
+}
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arichardson wrote:

As far as I can tell the reason for the AMDGPU code using an addrspacecast is the following comment `// Currently LLVM assumes null pointers always have value 0, which results in incorrectly transformed IR.` so it can't use a `null` literal for all ones.

Looking at the lang-ref I can't actually see anywhere that `ptr addrspace(X) null` is the zero value, so this should probably be clarified in the lagref: https://llvm.org/docs/LangRef.html#t-pointer

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