[IR] Introduce the opaque pointer type

The opaque pointer type is essentially just a normal pointer type with a
null pointee type.

This also adds support for the opaque pointer type to the bitcode
reader/writer, as well as to textual IR.

To avoid confusion with existing pointer types, we disallow creating a
pointer to an opaque pointer.

Opaque pointer types should not be widely used at this point since many
parts of LLVM still do not support them. The next steps are to add some
very simple use cases of opaque pointers to make sure they work, then
start pretending that all pointers are opaque pointers and see what
breaks.

https://lists.llvm.org/pipermail/llvm-dev/2021-May/150359.html

Reviewed By: dblaikie, dexonsmith, pcc

Differential Revision: https://reviews.llvm.org/D101704

1 files changed, 20 insertions, 5 deletions

diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index 7261e30..50fb3d4 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -858,6 +858,12 @@ void ModuleBitcodeWriter::writeTypeTable() {
   Abbv->Add(BitCodeAbbrevOp(0));  // Addrspace = 0
   unsigned PtrAbbrev = Stream.EmitAbbrev(std::move(Abbv));
 
+  // Abbrev for TYPE_CODE_OPAQUE_POINTER.
+  Abbv = std::make_shared<BitCodeAbbrev>();
+  Abbv->Add(BitCodeAbbrevOp(bitc::TYPE_CODE_OPAQUE_POINTER));
+  Abbv->Add(BitCodeAbbrevOp(0)); // Addrspace = 0
+  unsigned OpaquePtrAbbrev = Stream.EmitAbbrev(std::move(Abbv));
+
   // Abbrev for TYPE_CODE_FUNCTION.
   Abbv = std::make_shared<BitCodeAbbrev>();
   Abbv->Add(BitCodeAbbrevOp(bitc::TYPE_CODE_FUNCTION));
@@ -928,12 +934,21 @@ void ModuleBitcodeWriter::writeTypeTable() {
       break;
     case Type::PointerTyID: {
       PointerType *PTy = cast<PointerType>(T);
-      // POINTER: [pointee type, address space]
-      Code = bitc::TYPE_CODE_POINTER;
-      TypeVals.push_back(VE.getTypeID(PTy->getElementType()));
       unsigned AddressSpace = PTy->getAddressSpace();
-      TypeVals.push_back(AddressSpace);
-      if (AddressSpace == 0) AbbrevToUse = PtrAbbrev;
+      if (PTy->isOpaque()) {
+        // OPAQUE_POINTER: [address space]
+        Code = bitc::TYPE_CODE_OPAQUE_POINTER;
+        TypeVals.push_back(AddressSpace);
+        if (AddressSpace == 0)
+          AbbrevToUse = OpaquePtrAbbrev;
+      } else {
+        // POINTER: [pointee type, address space]
+        Code = bitc::TYPE_CODE_POINTER;
+        TypeVals.push_back(VE.getTypeID(PTy->getElementType()));
+        TypeVals.push_back(AddressSpace);
+        if (AddressSpace == 0)
+          AbbrevToUse = PtrAbbrev;
+      }
       break;
     }
     case Type::FunctionTyID: {