[CodeView] Emit the correct TypeIndex for std::nullptr_t

[zturner]

Summary

The TypeIndex used by cl.exe is 0x103, which indicates a SimpleTypeMode of NearPointer (note the absence of the bitness, normally pointers use a mode of SimpleTypeMode::NearPointer32 or SimpleTypeMode::NearPointer64) and a SimpleTypeKind of void. So this is basically a void*, but without a specified size, which makes sense given how std::nullptr_t is defined.

Anyway, we were actually not emitting *anything* for this. When we encountered std::nullptr_t in a DIType, we would actually just emit a TypeIndex of 0, which is obviously wrong.

The check for this whether we're looking at std::nullptr_t looks hacky, but apparently this is the "official" way to do this in DWARF.

I had to udpate some tests in LLDB since they now start working after this change (meaning the old test with broken output would start failing)

[rnk]

Looks good, thanks for the fix.

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

1518

I'd reverse this, so we have a special case for nullptr_t, and then the general case of "unspecified type" gives a type index of zero.

lldb/lit/SymbolFile/NativePDB/function-types-builtins.cpp

 
 // And finally, a function with no arguments.
 auto null = &nullary;
 // CHECK: (void (*)()) null = {{.*}}
 
 // FIXME: These currently don't work because clang-cl emits incorrect debug info
 // for std::nullptr_t.  We should fix these in clang-cl.
 auto rae = &unaryret<decltype(nullptr), 29>;
-// CHECK: rae = <could not resolve type>
+// CHECK: (nullptr_t (*)()) rae = {{.*}}
 auto aae = &unary<decltype(nullptr)>;
-// CHECK: (void (*)(...)) aae = {{.*}}
+// CHECK: (void (*)(nullptr_t)) aae = {{.*}}
 
 int main(int argc, char **argv) {
   return 0;
 }

lldb/source/Plugins/SymbolFile/NativePDB/SymbolFileNativePDB.cpp

if ((pr.getOptions() & PointerOptions::Restrict) != PointerOptions::None)
     pointer_ct = pointer_ct.AddRestrictModifier();
 
   return std::make_shared<Type>(type_uid.toOpaqueId(), m_clang->GetSymbolFile(),
                                 ConstString(), pr.getSize(), nullptr,
                                 LLDB_INVALID_UID, Type::eEncodingIsUID, decl,
                                 pointer_ct, Type::eResolveStateFull);
 }
 
+static bool IsStdNullptrT(TypeIndex ti) {
+  return ti.getSimpleMode() == SimpleTypeMode::NearPointer &&
+         ti.getSimpleKind() == SimpleTypeKind::Void;
+}
+
 lldb::TypeSP SymbolFileNativePDB::CreateSimpleType(TypeIndex ti) {
   if (ti.getSimpleMode() != SimpleTypeMode::Direct) {
+    if (IsStdNullptrT(ti)) {
+      PdbSymUid uid =
+          PdbSymUid::makeTypeSymId(PDB_SymType::BuiltinType, ti, false);
+      CompilerType ct = m_clang->GetBasicType(eBasicTypeNullPtr);
+      Declaration decl;
+      return std::make_shared<Type>(uid.toOpaqueId(), this,
+                                    ConstString("std::nullptr_t"), 0, nullptr,
+                                    LLDB_INVALID_UID, Type::eEncodingIsUID,
+                                    decl, ct, Type::eResolveStateFull);
+    }
+
     PdbSymUid uid =
         PdbSymUid::makeTypeSymId(PDB_SymType::PointerType, ti, false);
     TypeSP direct_sp = GetOrCreateType(ti.makeDirect());
     CompilerType ct = direct_sp->GetFullCompilerType();
     ct = ct.GetPointerType();
     uint32_t pointer_size = 0;
     switch (ti.getSimpleMode()) {
     case SimpleTypeMode::FarPointer32:
     case SimpleTypeMode::NearPointer32:
       pointer_size = 4;
       break;
     case SimpleTypeMode::NearPointer64:
       pointer_size = 8;
       break;
     default:
       // 128-bit and 16-bit pointers unsupported.
       return nullptr;
     }
     Declaration decl;
-    return std::make_shared<Type>(uid.toOpaqueId(), m_clang->GetSymbolFile(),
-                                  ConstString(), pointer_size, nullptr,
-                                  LLDB_INVALID_UID, Type::eEncodingIsUID, decl,
-                                  ct, Type::eResolveStateFull);
+    return std::make_shared<Type>(uid.toOpaqueId(), this, ConstString(),
+                                  pointer_size, nullptr, LLDB_INVALID_UID,
+                                  Type::eEncodingIsUID, decl, ct,
+                                  Type::eResolveStateFull);
   }
 
   PdbSymUid uid = PdbSymUid::makeTypeSymId(PDB_SymType::BuiltinType, ti, false);
   if (ti.getSimpleKind() == SimpleTypeKind::NotTranslated)
     return nullptr;
 
   lldb::BasicType bt = GetCompilerTypeForSimpleKind(ti.getSimpleKind());
   if (bt == lldb::eBasicTypeInvalid)

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

TypeIndex CodeViewDebug::lowerType(const DIType *Ty, const DIType *ClassTy) {
   case dwarf::DW_TAG_enumeration_type:
     return lowerTypeEnum(cast<DICompositeType>(Ty));
   case dwarf::DW_TAG_class_type:
   case dwarf::DW_TAG_structure_type:
     return lowerTypeClass(cast<DICompositeType>(Ty));
   case dwarf::DW_TAG_union_type:
     return lowerTypeUnion(cast<DICompositeType>(Ty));
   case dwarf::DW_TAG_unspecified_type:
+    if (Ty->getName() != "decltype(nullptr)")
       return TypeIndex::None();
+
+    // std::nullptr_t is specifically lowered using the pointer mode that
+    // doesn't indicate bit-width, presumably this is because std::nullptr_t
+    // is supposed to be compatible with any pointer type.
+    return TypeIndex(SimpleTypeKind::Void, SimpleTypeMode::NearPointer);
   default:
     // Use the null type index.
     return TypeIndex();
   }
 }
 
 TypeIndex CodeViewDebug::lowerTypeAlias(const DIDerivedType *Ty) {
   DITypeRef UnderlyingTypeRef = Ty->getBaseType();
static ClassOptions getCommonClassOptions(const DICompositeType *Ty) {
   // Put the Nested flag on a type if it appears immediately inside a tag type.
   // Do not walk the scope chain. Do not attempt to compute ContainsNestedClass
   // here. That flag is only set on definitions, and not forward declarations.
   const DIScope *ImmediateScope = Ty->getScope().resolve();
   if (ImmediateScope && isa<DICompositeType>(ImmediateScope))
     CO |= ClassOptions::Nested;
 
   // Put the Scoped flag on function-local types. MSVC puts this flag for enum
-  // type only when it has an immediate function scope. Clang never puts enums 
-  // inside DILexicalBlock scopes. Enum types, as generated by clang, are 
+  // type only when it has an immediate function scope. Clang never puts enums
+  // inside DILexicalBlock scopes. Enum types, as generated by clang, are
   // always in function, class, or file scopes.
   if (Ty->getTag() == dwarf::DW_TAG_enumeration_type) {
     if (ImmediateScope && isa<DISubprogram>(ImmediateScope))
       CO |= ClassOptions::Scoped;
   } else {
     for (const DIScope *Scope = ImmediateScope; Scope != nullptr;
          Scope = Scope->getScope().resolve()) {
       if (isa<DISubprogram>(Scope)) {

llvm/test/DebugInfo/COFF/types-std-nullptr-t.ll

+; RUN: llc < %s -filetype=obj | llvm-readobj - -codeview | FileCheck %s
+
+; C++ source to regenerate:
+; $ cat foo.cpp
+; decltype(nullptr) NullPtr = nullptr;
+; $ clang hello.cpp -S -emit-llvm -g -gcodeview -o t.ll
+
+; CHECK: CodeViewDebugInfo [
+; CHECK:   Subsection [
+; CHECK:     SubSectionType: Symbols (0xF1)
+; CHECK:     GlobalData {
+; CHECK:       Kind: S_GDATA32 (0x110D)
+; CHECK:       DataOffset: ?NullPtr@@3$$TA+0x0
+; CHECK:       Type: void* (0x103)
+; CHECK:       DisplayName: NullPtr
+; CHECK:       LinkageName: ?NullPtr@@3$$TA
+; CHECK:     }
+
+
+; ModuleID = 'foo.cpp'
+source_filename = "foo.cpp"
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc19.15.26730"
+
+@"?NullPtr@@3$$TA" = dso_local global i8* null, align 8, !dbg !0
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!7, !8, !9, !10}
+!llvm.ident = !{!11}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "NullPtr", linkageName: "?NullPtr@@3$$TA", scope: !2, file: !3, line: 1, type: !6, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !3, producer: "clang version 8.0.0 ", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
+!3 = !DIFile(filename: "foo.cpp", directory: "D:\5Csrc\5Cllvmbuild\5Ccl\5CDebug\5Cx64", checksumkind: CSK_MD5, checksum: "0d5c7c9860a17e584808c03a24a135e6")
+!4 = !{}
+!5 = !{!0}
+!6 = !DIBasicType(tag: DW_TAG_unspecified_type, name: "decltype(nullptr)")
+!7 = !{i32 2, !"CodeView", i32 1}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 2}
+!10 = !{i32 7, !"PIC Level", i32 2}
+!11 = !{!"clang version 8.0.0 "}
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