[OpaquePtr] Allow globals with opaque pointer value type

[nikic]

Summary

Do this by making opaque pointers a valid pointer element type, for which we implicitly create an opaque pointer (moving the logic from getPointerTo into PointerType::get).

We'll never create something like a "pointer to opaque pointer", but accept it in the API, because a lot of code reasonably assumes that you can create a pointer to pointer type.

[aeubanks]

lgtm, though invalid-opaque-ptr.ll is failing and should probably be deleted

[dexonsmith]

Do this by making opaque pointers a valid pointer element type, for which we implicitly create an opaque pointer (moving the logic from getPointerTo into PointerType::get).

Is there a way to test this? Maybe with something like this?

@g = global ptr null

In D104902#2840987, @aeubanks wrote:

lgtm, though invalid-opaque-ptr.ll is failing and should probably be deleted

I don't understand why the testcase was deleted. It seems to indicate a hole in the verifier. Isn't this still invalid IR?

define void @f(ptr %a) {
    %b = bitcast ptr %a to ptr*
    ret void
}

[dexonsmith]

In D104902#2841332, @dexonsmith wrote:

Do this by making opaque pointers a valid pointer element type, for which we implicitly create an opaque pointer (moving the logic from getPointerTo into PointerType::get).

Is there a way to test this? Maybe with something like this?

(Ignore that first comment — somehow I missed the chnage to opaque-ptr.ll! Still interested in a verifier change for the invalid IR though...)

[dexonsmith]

In D104902#2841332, @dexonsmith wrote:

In D104902#2840987, @aeubanks wrote:

lgtm, though invalid-opaque-ptr.ll is failing and should probably be deleted

I don't understand why the testcase was deleted. It seems to indicate a hole in the verifier. Isn't this still invalid IR?

define void @f(ptr %a) {
    %b = bitcast ptr %a to ptr*
    ret void
}

Ah, I see -- the parsed IR will be correct do to the change to PointerType::get. So this isn't a verifier bug, it's a parser bug.

Seems like you just need to update:

// Type ::= Type '*'
case lltok::star:
  if (Result->isLabelTy())
    return tokError("basic block pointers are invalid");
  if (Result->isVoidTy())
    return tokError("pointers to void are invalid - use i8* instead");
  if (!PointerType::isValidElementType(Result))
    return tokError("pointer to this type is invalid");

to have:

if (Result->isOpaquePointerTy())
  return tokError("pointer to opaque pointer is invalid");

before the the call to isValidElementType.

You'd need to update the logic in the similar case below for lltok::kw_addrspace as well.

[dexonsmith]

I sent out https://reviews.llvm.org/D104938 to fix this.

llvm/lib/IR/Type.cpp

 //===----------------------------------------------------------------------===//
 
 PointerType *PointerType::get(Type *EltTy, unsigned AddressSpace) {
   assert(EltTy && "Can't get a pointer to <null> type!");
   assert(isValidElementType(EltTy) && "Invalid type for pointer element!");
 
   LLVMContextImpl *CImpl = EltTy->getContext().pImpl;
 
-  if (CImpl->ForceOpaquePointers)
+  // Create opaque pointer for pointer to opaque pointer.
+  if (CImpl->ForceOpaquePointers ||
+      (isa<PointerType>(EltTy) && cast<PointerType>(EltTy)->isOpaque()))
     return get(EltTy->getContext(), AddressSpace);
 
   // Since AddressSpace #0 is the common case, we special case it.
   PointerType *&Entry = AddressSpace == 0 ? CImpl->PointerTypes[EltTy]
      : CImpl->ASPointerTypes[std::make_pair(EltTy, AddressSpace)];
 
   if (!Entry)
     Entry = new (CImpl->Alloc) PointerType(EltTy, AddressSpace);

 }
 
 PointerType::PointerType(LLVMContext &C, unsigned AddrSpace)
     : Type(C, PointerTyID), PointeeTy(nullptr) {
   setSubclassData(AddrSpace);
 }
 
 PointerType *Type::getPointerTo(unsigned AddrSpace) const {
-  // Pointer to opaque pointer is opaque pointer.
-  if (auto *PTy = dyn_cast<PointerType>(this))
-    if (PTy->isOpaque())
-      return PointerType::get(getContext(), AddrSpace);
-
   return PointerType::get(const_cast<Type*>(this), AddrSpace);
 }
 
 bool PointerType::isValidElementType(Type *ElemTy) {
   return !ElemTy->isVoidTy() && !ElemTy->isLabelTy() &&
          !ElemTy->isMetadataTy() && !ElemTy->isTokenTy() &&
-         !ElemTy->isX86_AMXTy() &&
-         !(ElemTy->isPointerTy() && cast<PointerType>(ElemTy)->isOpaque());
+         !ElemTy->isX86_AMXTy();
 }
 
 bool PointerType::isLoadableOrStorableType(Type *ElemTy) {
   return isValidElementType(ElemTy) && !ElemTy->isFunctionTy();
 }

llvm/test/Assembler/invalid-opaque-ptr.ll

-; RUN: not llvm-as < %s -disable-output 2>&1 | FileCheck %s
-
-; CHECK: pointer to this type is invalid
-define void @f(ptr %a) {
-    %b = bitcast ptr %a to ptr*
-    ret void
-}

llvm/test/Assembler/opaque-ptr.ll

 ; RUN: llvm-as < %s | llvm-dis | llvm-as | llvm-dis | FileCheck %s
 ; RUN: verify-uselistorder %s
 
+; CHECK: @global = external global ptr
+@global = external global ptr
+
 ; CHECK: define ptr @f(ptr %a) {
 ; CHECK:     %b = bitcast ptr %a to ptr
 ; CHECK:     ret ptr %b
 define ptr @f(ptr %a) {
     %b = bitcast ptr %a to ptr
     ret ptr %b
 }