Reapply "[clang][DebugInfo] Emit DW_AT_object_pointer on function declarations with explicit this" (#123455)

This reverts commit c3a935e3f967f8f22f5db240d145459ee621c1e0.

The only change to the reverted commit is that this also updates
the OCaml bindings according to the C debug-info API changes.

The build failure originally introduced was:
```
FAILED: bindings/ocaml/debuginfo/debuginfo_ocaml.o /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo/debuginfo_ocaml.o
cd /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo && /usr/bin/ocamlfind ocamlc -c /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo/debuginfo_ocaml.c -ccopt "-I/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/bindings/ocaml/debuginfo/../llvm -D_GNU_SOURCE -D_DEBUG -D_GLIBCXX_ASSERTIONS -DEXPENSIVE_CHECKS -D_GLIBCXX_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/b/1/llvm-clang-x86_64-expensive-checks-debian/build/include -I/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/include  -DNDEBUG "
/b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo/debuginfo_ocaml.c: In function ‘llvm_dibuild_create_object_pointer_type’:
/b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo/debuginfo_ocaml.c:620:30: error: too few arguments to function ‘LLVMDIBuilderCreateObjectPointerType’
  620 |   LLVMMetadataRef Metadata = LLVMDIBuilderCreateObjectPointerType(
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo/debuginfo_ocaml.c:23:
/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/include/llvm-c/DebugInfo.h:880:17: note: declared here
  880 | LLVMMetadataRef LLVMDIBuilderCreateObjectPointerType(LLVMDIBuilderRef Builder,
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -2016,13 +2016,15 @@ llvm::DISubroutineType *CGDebugInfo::getOrCreateInstanceMethodType(
   // First element is always return type. For 'void' functions it is NULL.
   Elts.push_back(Args[0]);
 
-  // "this" pointer is always first argument.
+  const bool HasExplicitObjectParameter = ThisPtr.isNull();
-  // ThisPtr may be null if the member function has an explicit 'this'
+
-  // parameter.
+  // "this" pointer is always first argument. For explicit "this"
-  if (!ThisPtr.isNull()) {
+  // parameters, it will already be in Args[1].
+  if (!HasExplicitObjectParameter) {
     llvm::DIType *ThisPtrType = getOrCreateType(ThisPtr, Unit);
     TypeCache[ThisPtr.getAsOpaquePtr()].reset(ThisPtrType);
-    ThisPtrType = DBuilder.createObjectPointerType(ThisPtrType);
+    ThisPtrType =
+        DBuilder.createObjectPointerType(ThisPtrType, /*Implicit=*/true);
     Elts.push_back(ThisPtrType);
   }
 
@@ -2030,6 +2032,13 @@ llvm::DISubroutineType *CGDebugInfo::getOrCreateInstanceMethodType(
   for (unsigned i = 1, e = Args.size(); i != e; ++i)
     Elts.push_back(Args[i]);
 
+  // Attach FlagObjectPointer to the explicit "this" parameter.
+  if (HasExplicitObjectParameter) {
+    assert(Elts.size() >= 2 && Args.size() >= 2 &&
+           "Expected at least return type and object parameter.");
+    Elts[1] = DBuilder.createObjectPointerType(Args[1], /*Implicit=*/false);
+  }
+
   llvm::DITypeRefArray EltTypeArray = DBuilder.getOrCreateTypeArray(Elts);
 
   return DBuilder.createSubroutineType(EltTypeArray, OriginalFunc->getFlags(),
@@ -5118,7 +5127,7 @@ llvm::DIType *CGDebugInfo::CreateSelfType(const QualType &QualTy,
   llvm::DIType *CachedTy = getTypeOrNull(QualTy);
   if (CachedTy)
     Ty = CachedTy;
-  return DBuilder.createObjectPointerType(Ty);
+  return DBuilder.createObjectPointerType(Ty, /*Implicit=*/true);
 }
 
 void CGDebugInfo::EmitDeclareOfBlockDeclRefVariable(

clang/test/CodeGenCXX/debug-info-object-pointer.cpp

@@ -5,12 +5,11 @@
 // CHECK: !DIDerivedType(tag: DW_TAG_pointer_type
 // CHECK-SAME:           flags: DIFlagArtificial | DIFlagObjectPointer
 //
-// // FIXME: DIFlagObjectPointer not attached to the explicit object
-// // argument in the subprogram declaration.
 // CHECK: !DISubprogram(name: "explicit_this",
 //                      flags: DIFlagPrototyped
-// CHECK-NOT: DIFlagObjectPointer
+//
-// CHECK-NOT: DIFlagArtificial
+// CHECK: !DIDerivedType(tag: DW_TAG_rvalue_reference_type
+// CHECK-SAME:           flags: DIFlagObjectPointer)
 //
 // CHECK: !DILocalVariable(name: "this", arg: 1
 // CHECK-SAME:             flags: DIFlagArtificial | DIFlagObjectPointer

llvm/bindings/ocaml/debuginfo/debuginfo_ocaml.c

@@ -616,9 +616,10 @@ value llvm_dibuild_create_member_pointer_type_bytecode(value *argv, int argn) {
   );
 }
 
-value llvm_dibuild_create_object_pointer_type(value Builder, value Type) {
+value llvm_dibuild_create_object_pointer_type(value Builder, value Type,
+                                              value Implicit) {
   LLVMMetadataRef Metadata = LLVMDIBuilderCreateObjectPointerType(
-      DIBuilder_val(Builder), Metadata_val(Type));
+      DIBuilder_val(Builder), Metadata_val(Type), Bool_val(Implicit));
   return to_val(Metadata);
 }
 

llvm/bindings/ocaml/debuginfo/llvm_debuginfo.ml

@@ -398,7 +398,7 @@ external dibuild_create_member_pointer_type :
   = "llvm_dibuild_create_member_pointer_type_bytecode" "llvm_dibuild_create_member_pointer_type_native"
 
 external dibuild_create_object_pointer_type :
-  lldibuilder -> Llvm.llmetadata -> Llvm.llmetadata
+  lldibuilder -> Llvm.llmetadata -> implicit:bool -> Llvm.llmetadata
   = "llvm_dibuild_create_object_pointer_type"
 
 external dibuild_create_qualified_type :

llvm/bindings/ocaml/debuginfo/llvm_debuginfo.mli

@@ -471,10 +471,11 @@ val dibuild_create_member_pointer_type :
     a pointer to member. See LLVMDIBuilderCreateMemberPointerType *)
 
 val dibuild_create_object_pointer_type :
-  lldibuilder -> Llvm.llmetadata -> Llvm.llmetadata
+  lldibuilder -> Llvm.llmetadata -> implicit:bool -> Llvm.llmetadata
 (** [dibuild_create_object_pointer_type dib ty] Create a uniqued DIType* clone
-  with FlagObjectPointer and FlagArtificial set. [dib] is the dibuilder
+  with FlagObjectPointer. [dib] is the dibuilder
-  value and [ty] the underlying type to which this pointer points. *)
+  value and [ty] the underlying type to which this pointer points. If
+  [implicit] is true, also set FlagArtificial. *)
 
 val dibuild_create_qualified_type :
   lldibuilder -> tag:int -> Llvm.llmetadata -> Llvm.llmetadata

llvm/include/llvm-c/DebugInfo.h

@@ -870,13 +870,16 @@ LLVMDIBuilderCreateObjCProperty(LLVMDIBuilderRef Builder,
                                 LLVMMetadataRef Ty);
 
 /**
- * Create a uniqued DIType* clone with FlagObjectPointer and FlagArtificial set.
+ * Create a uniqued DIType* clone with FlagObjectPointer. If \c Implicit
+ * is true, then also set FlagArtificial.
  * \param Builder   The DIBuilder.
  * \param Type      The underlying type to which this pointer points.
+ * \param Implicit  Indicates whether this pointer was implicitly generated
+ *                  (i.e., not spelled out in source).
  */
-LLVMMetadataRef
+LLVMMetadataRef LLVMDIBuilderCreateObjectPointerType(LLVMDIBuilderRef Builder,
-LLVMDIBuilderCreateObjectPointerType(LLVMDIBuilderRef Builder,
+                                                     LLVMMetadataRef Type,
-                                     LLVMMetadataRef Type);
+                                                     LLVMBool Implicit);
 
 /**
  * Create debugging information entry for a qualified

llvm/include/llvm/IR/DIBuilder.h

@@ -662,9 +662,9 @@ namespace llvm {
     /// Create a uniqued clone of \p Ty with FlagArtificial set.
     static DIType *createArtificialType(DIType *Ty);
 
-    /// Create a uniqued clone of \p Ty with FlagObjectPointer and
+    /// Create a uniqued clone of \p Ty with FlagObjectPointer set.
-    /// FlagArtificial set.
+    /// If \p Implicit is true, also set FlagArtificial.
-    static DIType *createObjectPointerType(DIType *Ty);
+    static DIType *createObjectPointerType(DIType *Ty, bool Implicit);
 
     /// Create a permanent forward-declared type.
     DICompositeType *createForwardDecl(unsigned Tag, StringRef Name,

llvm/lib/IR/DIBuilder.cpp

@@ -644,11 +644,15 @@ DIType *DIBuilder::createArtificialType(DIType *Ty) {
   return createTypeWithFlags(Ty, DINode::FlagArtificial);
 }
 
-DIType *DIBuilder::createObjectPointerType(DIType *Ty) {
+DIType *DIBuilder::createObjectPointerType(DIType *Ty, bool Implicit) {
   // FIXME: Restrict this to the nodes where it's valid.
   if (Ty->isObjectPointer())
     return Ty;
-  DINode::DIFlags Flags = DINode::FlagObjectPointer | DINode::FlagArtificial;
+  DINode::DIFlags Flags = DINode::FlagObjectPointer;
+
+  if (Implicit)
+    Flags |= DINode::FlagArtificial;
+
   return createTypeWithFlags(Ty, Flags);
 }
 

llvm/lib/IR/DebugInfo.cpp

@@ -1432,10 +1432,11 @@ LLVMDIBuilderCreateObjCProperty(LLVMDIBuilderRef Builder,
                   PropertyAttributes, unwrapDI<DIType>(Ty)));
 }
 
-LLVMMetadataRef
+LLVMMetadataRef LLVMDIBuilderCreateObjectPointerType(LLVMDIBuilderRef Builder,
-LLVMDIBuilderCreateObjectPointerType(LLVMDIBuilderRef Builder,
+                                                     LLVMMetadataRef Type,
-                                     LLVMMetadataRef Type) {
+                                                     LLVMBool Implicit) {
-  return wrap(unwrap(Builder)->createObjectPointerType(unwrapDI<DIType>(Type)));
+  return wrap(unwrap(Builder)->createObjectPointerType(unwrapDI<DIType>(Type),
+                                                       Implicit));
 }
 
 LLVMMetadataRef