[clang][DebugInfo] Revert "emit variable definitions for constant-initialized static data-members"

This commit reverts the changes in llvm#71780
and all of its follow-up patches.

We got reports of the `.debug_names/.debug_gnu_pubnames/gdb_index/etc.`
sections growing by a non-trivial amount for some large projects. While
GCC does index definitions for static data member constants, they
do so *only* for explicitly `constexpr` members. We were indexing
*all* constant-initialized const-static members, which is likely where
the significant size difference comes from. However, only emitting explicitly
`constexpr` variables into the index doesn't seem like a good way
forward, since from clang's perspective `const`-static integrals
are `constexpr` too, and that shouldn't be any different in the debug-info
component. Also, as new code moves to `constexpr` instead of `const` static
for constants, such solution would just delay the growth of the Names index.

To prevent the size regression we revert to not emitting definitions for
static data-members that have no location.

To support access to such constants from LLDB we'll most likely have to
have to make LLDB find the constants by looking at the containing class
first.

Author: Michael137

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -69,29 +69,6 @@ static uint32_t getDeclAlignIfRequired(const Decl *D, const ASTContext &Ctx) {
   return D->hasAttr<AlignedAttr>() ? D->getMaxAlignment() : 0;
 }
 
-/// Given a VarDecl corresponding to either the definition or
-/// declaration of a C++ static data member, if it has a constant
-/// initializer and is evaluatable, return the evaluated value.
-/// Returns std::nullopt otherwise.
-static std::optional<APValue>
-evaluateConstantInitializer(const clang::VarDecl *VD,
-                            const clang::ASTContext &Ctx) {
-  assert(VD != nullptr);
-
-  if (!VD->isStaticDataMember())
-    return std::nullopt;
-
-  if (!VD->isUsableInConstantExpressions(Ctx))
-    return std::nullopt;
-
-  auto const *InitExpr = VD->getAnyInitializer();
-  Expr::EvalResult Result;
-  if (!InitExpr->EvaluateAsConstantExpr(Result, Ctx))
-    return std::nullopt;
-
-  return Result.Val;
-}
-
 CGDebugInfo::CGDebugInfo(CodeGenModule &CGM)
     : CGM(CGM), DebugKind(CGM.getCodeGenOpts().getDebugInfo()),
       DebugTypeExtRefs(CGM.getCodeGenOpts().DebugTypeExtRefs),
@@ -1724,7 +1701,6 @@ CGDebugInfo::CreateRecordStaticField(const VarDecl *Var, llvm::DIType *RecordTy,
   llvm::DIDerivedType *GV = DBuilder.createStaticMemberType(
       RecordTy, VName, VUnit, LineNumber, VTy, Flags, C, Tag, Align);
   StaticDataMemberCache[Var->getCanonicalDecl()].reset(GV);
-  StaticDataMemberDefinitionsToEmit.push_back(Var->getCanonicalDecl());
   return GV;
 }
 
@@ -5628,41 +5604,6 @@ void CGDebugInfo::EmitGlobalVariable(const ValueDecl *VD, const APValue &Init) {
       TemplateParameters, Align));
 }
 
-void CGDebugInfo::EmitGlobalVariable(const VarDecl *VD) {
-  assert(CGM.getCodeGenOpts().hasReducedDebugInfo());
-  if (VD->hasAttr<NoDebugAttr>())
-    return;
-
-  const auto CacheIt = DeclCache.find(VD);
-  if (CacheIt != DeclCache.end())
-    return;
-
-  const auto InitVal = evaluateConstantInitializer(VD, CGM.getContext());
-  if (!InitVal)
-    return;
-
-  llvm::DIFile *Unit = nullptr;
-  llvm::DIScope *DContext = nullptr;
-  unsigned LineNo;
-  StringRef DeclName, LinkageName;
-  QualType T;
-  llvm::MDTuple *TemplateParameters = nullptr;
-  collectVarDeclProps(VD, Unit, LineNo, T, DeclName, LinkageName,
-                      TemplateParameters, DContext);
-
-  auto Align = getDeclAlignIfRequired(VD, CGM.getContext());
-  llvm::DINodeArray Annotations = CollectBTFDeclTagAnnotations(VD);
-  llvm::DIExpression *InitExpr = createConstantValueExpression(VD, *InitVal);
-
-  // Omit linkage name for variable definitions that represent constants.
-  // There hasn't been a need from consumers yet to have it attached.
-  DeclCache[VD].reset(DBuilder.createGlobalVariableExpression(
-      TheCU, DeclName, /* LinkageName */ {}, Unit, LineNo,
-      getOrCreateType(T, Unit), true, true, InitExpr,
-      getOrCreateStaticDataMemberDeclarationOrNull(VD), TemplateParameters,
-      Align, Annotations));
-}
-
 void CGDebugInfo::EmitExternalVariable(llvm::GlobalVariable *Var,
                                        const VarDecl *D) {
   assert(CGM.getCodeGenOpts().hasReducedDebugInfo());
@@ -5867,20 +5808,6 @@ void CGDebugInfo::setDwoId(uint64_t Signature) {
 }
 
 void CGDebugInfo::finalize() {
-  // We can't use a for-each here because `EmitGlobalVariable`
-  // may push new decls into `StaticDataMemberDefinitionsToEmit`,
-  // which would invalidate any iterator.
-  for (size_t i = 0; i < StaticDataMemberDefinitionsToEmit.size(); ++i) {
-    auto const *VD = StaticDataMemberDefinitionsToEmit[i];
-
-    assert(VD && VD->isStaticDataMember());
-
-    if (DeclCache.contains(VD))
-      continue;
-
-    EmitGlobalVariable(VD);
-  }
-
   // Creating types might create further types - invalidating the current
   // element and the size(), so don't cache/reference them.
   for (size_t i = 0; i != ObjCInterfaceCache.size(); ++i) {

clang/lib/CodeGen/CGDebugInfo.h

@@ -161,9 +161,6 @@ class CGDebugInfo {
   llvm::DenseMap<const Decl *, llvm::TypedTrackingMDRef<llvm::DIDerivedType>>
       StaticDataMemberCache;
 
-  /// Keeps track of static data members for which we should emit a definition.
-  std::vector<const VarDecl *> StaticDataMemberDefinitionsToEmit;
-
   using ParamDecl2StmtTy = llvm::DenseMap<const ParmVarDecl *, const Stmt *>;
   using Param2DILocTy =
       llvm::DenseMap<const ParmVarDecl *, llvm::DILocalVariable *>;
@@ -529,9 +526,6 @@ class CGDebugInfo {
   /// Emit a constant global variable's debug info.
   void EmitGlobalVariable(const ValueDecl *VD, const APValue &Init);
 
-  /// Emit debug-info for a variable with a constant initializer.
-  void EmitGlobalVariable(const VarDecl *VD);
-
   /// Emit information about an external variable.
   void EmitExternalVariable(llvm::GlobalVariable *GV, const VarDecl *Decl);
 

clang/test/CodeGenCXX/debug-info-class.cpp

@@ -117,18 +117,11 @@ int main(int argc, char **argv) {
 // CHECK-NOT:                             identifier:
 // CHECK-SAME:                            ){{$}}
 
-// CHECK:      !DIGlobalVariableExpression(var: ![[HDR_VAR:[0-9]+]], expr: !DIExpression(DW_OP_constu, 52, DW_OP_stack_value))
-// CHECK:      ![[HDR_VAR]] = distinct !DIGlobalVariable(name: "HdrSize",
-// CHECK-SAME:                isLocal: true, isDefinition: true, declaration: ![[HDR_VAR_DECL:[0-9]+]])
-// CHECK:      ![[INT:[0-9]+]] = !DIBasicType(name: "int"
-// CHECK:      ![[HDR_VAR_DECL]] = !DIDerivedType(tag: DW_TAG_member, name: "HdrSize"
-
-// CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "A"
-
 // CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "I"
 // CHECK-NOT:              DIFlagFwdDecl
 // CHECK-SAME:             ){{$}}
 
+// CHECK:      ![[INT:[0-9]+]] = !DIBasicType(name: "int"
 // CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "foo"
 // CHECK: !DICompositeType(tag: DW_TAG_class_type, name: "bar"
 // CHECK: !DICompositeType(tag: DW_TAG_union_type, name: "baz"
@@ -194,5 +187,8 @@ int main(int argc, char **argv) {
 // CHECK: [[G_INNER_I]] = !DIDerivedType(tag: DW_TAG_member, name: "j"
 // CHECK-SAME:                           baseType: ![[INT]]
 
+// CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "A"
+// CHECK: !DIDerivedType(tag: DW_TAG_member, name: "HdrSize"
+
 // CHECK: ![[EXCEPTLOC]] = !DILocation(line: 100,
 // CHECK: ![[RETLOC]] = !DILocation(line: 99,

clang/test/CodeGenCXX/debug-info-static-inline-member.cpp

@@ -1,8 +1,8 @@
-// RUN: %clangxx -target x86_64-unknown-unknown -g -gdwarf-4 %s -emit-llvm -S -o - | FileCheck --check-prefixes=CHECK,DWARF4,CPP11,NOT-MS %s
+// RUN: %clangxx -target x86_64-unknown-unknown -g -gdwarf-4 %s -emit-llvm -S -o - | FileCheck --check-prefixes=CHECK,DWARF4,NOT-MS %s
 // RUN: %clangxx -target x86_64-unknown-unknown -g -gdwarf-4 -std=c++98 %s -emit-llvm -S -o - | FileCheck --check-prefixes=CHECK,DWARF4,NOT-MS %s
-// RUN: %clangxx -target x86_64-unknown-unknown -g -gdwarf-4 -std=c++11 %s -emit-llvm -S -o - | FileCheck --check-prefixes=CHECK,DWARF4,CPP11,NOT-MS %s
-// RUN: %clangxx -target x86_64-unknown-unknown -g -gdwarf-5 -std=c++11 %s -emit-llvm -S -o - | FileCheck --check-prefixes=CHECK,DWARF5,CPP11 %s
-// RUN: %clangxx -target x86_64-windows-msvc -g -gdwarf-4 %s -emit-llvm -S -o - | FileCheck --check-prefixes=CHECK,DWARF4,CPP11 %s
+// RUN: %clangxx -target x86_64-unknown-unknown -g -gdwarf-4 -std=c++11 %s -emit-llvm -S -o - | FileCheck --check-prefixes=CHECK,DWARF4,NOT-MS %s
+// RUN: %clangxx -target x86_64-unknown-unknown -g -gdwarf-5 -std=c++11 %s -emit-llvm -S -o - | FileCheck --check-prefixes=CHECK,DWARF5 %s
+// RUN: %clangxx -target x86_64-windows-msvc -g -gdwarf-4 %s -emit-llvm -S -o - | FileCheck --check-prefixes=CHECK,DWARF4 %s
 // PR14471
 
 // CHECK: @{{.*}}a{{.*}} = dso_local global i32 4, align 4, !dbg [[A:![0-9]+]]
@@ -166,15 +166,3 @@ struct y {
 };
 int y::z;
 }
-
-// CHECK:      !DIGlobalVariableExpression(var: ![[CONST_A_VAR:[0-9]+]], expr: !DIExpression(DW_OP_constu, 1, DW_OP_stack_value))
-// CHECK:      ![[CONST_A_VAR]] = distinct !DIGlobalVariable(name: "const_a"
-// CHECK-SAME:                    isLocal: true, isDefinition: true, declaration: ![[CONST_A_DECL]])
-
-// CPP11:      !DIGlobalVariableExpression(var: ![[CONST_B_VAR:[0-9]+]], expr: !DIExpression(DW_OP_constu, {{.*}}, DW_OP_stack_value))
-// CPP11:      ![[CONST_B_VAR]] = distinct !DIGlobalVariable(name: "const_b"
-// CPP11-SAME:                    isLocal: true, isDefinition: true, declaration: ![[CONST_B_DECL]])
-
-// CHECK:      !DIGlobalVariableExpression(var: ![[CONST_C_VAR:[0-9]+]], expr: !DIExpression(DW_OP_constu, 18, DW_OP_stack_value))
-// CHECK:      ![[CONST_C_VAR]] = distinct !DIGlobalVariable(name: "const_c"
-// CHECK-SAME:                    isLocal: true, isDefinition: true, declaration: ![[CONST_C_DECL]])

clang/test/CodeGenCXX/debug-info-static-member.cpp

@@ -142,54 +142,6 @@ static bool ShouldIgnoreArtificialField(llvm::StringRef FieldName) {
          || FieldName.starts_with("_vptr.");
 }
 
-std::optional<DWARFFormValue>
-DWARFASTParserClang::FindConstantOnVariableDefinition(DWARFDIE die) {
-  assert(die.Tag() == DW_TAG_member || die.Tag() == DW_TAG_variable);
-
-  auto *dwarf = die.GetDWARF();
-  if (!dwarf)
-    return {};
-
-  ConstString name{die.GetName()};
-  if (!name)
-    return {};
-
-  auto *CU = die.GetCU();
-  if (!CU)
-    return {};
-
-  DWARFASTParser *dwarf_ast = dwarf->GetDWARFParser(*CU);
-  auto parent_decl_ctx = dwarf_ast->GetDeclContextContainingUIDFromDWARF(die);
-
-  // Make sure we populate the GetDieToVariable cache.
-  VariableList variables;
-  dwarf->FindGlobalVariables(name, parent_decl_ctx, UINT_MAX, variables);
-
-  // The cache contains the variable definition whose DW_AT_specification
-  // points to our declaration DIE. Look up that definition using our
-  // declaration.
-  auto const &die_to_var = dwarf->GetDIEToVariable();
-  auto it = die_to_var.find(die.GetDIE());
-  if (it == die_to_var.end())
-    return {};
-
-  auto var_sp = it->getSecond();
-  assert(var_sp != nullptr);
-
-  if (!var_sp->GetLocationIsConstantValueData())
-    return {};
-
-  auto def = dwarf->GetDIE(var_sp->GetID());
-  auto def_attrs = def.GetAttributes();
-  DWARFFormValue form_value;
-  if (!def_attrs.ExtractFormValueAtIndex(
-          def_attrs.FindAttributeIndex(llvm::dwarf::DW_AT_const_value),
-          form_value))
-    return {};
-
-  return form_value;
-}
-
 TypeSP DWARFASTParserClang::ParseTypeFromClangModule(const SymbolContext &sc,
                                                      const DWARFDIE &die,
                                                      Log *log) {
@@ -2916,23 +2868,11 @@ void DWARFASTParserClang::CreateStaticMemberVariable(
 
   bool unused;
   // TODO: Support float/double static members as well.
-  if (!ct.IsIntegerOrEnumerationType(unused))
+  if (!ct.IsIntegerOrEnumerationType(unused) || !attrs.const_value_form)
     return;
 
-  auto maybe_const_form_value = attrs.const_value_form;
-
-  // Newer versions of Clang don't emit the DW_AT_const_value
-  // on the declaration of an inline static data member. Instead
-  // it's attached to the definition DIE. If that's the case,
-  // try and fetch it.
-  if (!maybe_const_form_value) {
-    maybe_const_form_value = FindConstantOnVariableDefinition(die);
-    if (!maybe_const_form_value)
-      return;
-  }
-
   llvm::Expected<llvm::APInt> const_value_or_err =
-      ExtractIntFromFormValue(ct, *maybe_const_form_value);
+      ExtractIntFromFormValue(ct, *attrs.const_value_form);
   if (!const_value_or_err) {
     LLDB_LOG_ERROR(log, const_value_or_err.takeError(),
                    "Failed to add const value to variable {1}: {0}",

lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp

@@ -413,17 +413,6 @@ class DWARFASTParserClang : public lldb_private::plugin::dwarf::DWARFASTParser {
                        lldb_private::CompilerType &class_clang_type,
                        const lldb::AccessType default_accesibility,
                        lldb_private::ClangASTImporter::LayoutInfo &layout_info);
-
-  /// Tries to find the definition DW_TAG_variable DIE of the the specified
-  /// DW_TAG_member 'die'. If such definition exists, returns the
-  /// DW_AT_const_value of that definition if available. Returns std::nullopt
-  /// otherwise.
-  ///
-  /// In newer versions of clang, DW_AT_const_value attributes are not attached
-  /// to the declaration of a inline static data-member anymore, but rather on
-  /// its definition. This function is used to locate said constant.
-  std::optional<lldb_private::plugin::dwarf::DWARFFormValue>
-  FindConstantOnVariableDefinition(lldb_private::plugin::dwarf::DWARFDIE die);
 };
 
 /// Parsed form of all attributes that are relevant for type reconstruction.