[clang][NFC] Refactor clang::Linkage (#71049)

This patch introduces a new enumerator `Invalid = 0`, shifting other enumerators by +1. Contrary to how it might sound, this actually affirms status quo of how this enum is stored in `clang::Decl`:
```
  /// If 0, we have not computed the linkage of this declaration.
  /// Otherwise, it is the linkage + 1.
  mutable unsigned CacheValidAndLinkage : 3;
```
This patch makes debuggers to not be mistaken about enumerator stored in this bit-field. It also converts `clang::Linkage` to a scoped enum.

clang-tools-extra/clang-doc/Serialize.cpp

@@ -257,8 +257,8 @@ static bool isPublic(const clang::AccessSpecifier AS,
                      const clang::Linkage Link) {
   if (AS == clang::AccessSpecifier::AS_private)
     return false;
-  else if ((Link == clang::Linkage::ModuleLinkage) ||
-           (Link == clang::Linkage::ExternalLinkage))
+  else if ((Link == clang::Linkage::Module) ||
+           (Link == clang::Linkage::External))
     return true;
   return false; // otherwise, linkage is some form of internal linkage
 }

clang-tools-extra/clangd/Quality.cpp

@@ -274,7 +274,8 @@ computeScope(const NamedDecl *D) {
     return SymbolRelevanceSignals::ClassScope;
   // ExternalLinkage threshold could be tweaked, e.g. module-visible as global.
   // Avoid caching linkage if it may change after enclosing code completion.
-  if (hasUnstableLinkage(D) || D->getLinkageInternal() < ExternalLinkage)
+  if (hasUnstableLinkage(D) || llvm::to_underlying(D->getLinkageInternal()) <
+                                   llvm::to_underlying(Linkage::External))
     return SymbolRelevanceSignals::FileScope;
   return SymbolRelevanceSignals::GlobalScope;
 }

clang-tools-extra/clangd/SemanticHighlighting.cpp

@@ -617,7 +617,8 @@ std::optional<HighlightingModifier> scopeModifier(const NamedDecl *D) {
   if (DC->isTranslationUnit() && D->isTemplateParameter())
     return std::nullopt;
   // ExternalLinkage threshold could be tweaked, e.g. module-visible as global.
-  if (D->getLinkageInternal() < ExternalLinkage)
+  if (llvm::to_underlying(D->getLinkageInternal()) <
+      llvm::to_underlying(Linkage::External))
     return HighlightingModifier::FileScope;
   return HighlightingModifier::GlobalScope;
 }

clang/include/clang/AST/DeclBase.h

@@ -49,7 +49,7 @@ class ExternalSourceSymbolAttr;
 class FunctionDecl;
 class FunctionType;
 class IdentifierInfo;
-enum Linkage : unsigned char;
+enum class Linkage : unsigned char;
 class LinkageSpecDecl;
 class Module;
 class NamedDecl;
@@ -335,7 +335,6 @@ class alignas(8) Decl {
   unsigned IdentifierNamespace : 14;
 
   /// If 0, we have not computed the linkage of this declaration.
-  /// Otherwise, it is the linkage + 1.
   mutable unsigned CacheValidAndLinkage : 3;
 
   /// Allocate memory for a deserialized declaration.
@@ -386,7 +385,7 @@ class alignas(8) Decl {
         Implicit(false), Used(false), Referenced(false),
         TopLevelDeclInObjCContainer(false), Access(AS_none), FromASTFile(0),
         IdentifierNamespace(getIdentifierNamespaceForKind(DK)),
-        CacheValidAndLinkage(0) {
+        CacheValidAndLinkage(llvm::to_underlying(Linkage::Invalid)) {
     if (StatisticsEnabled) add(DK);
   }
 
@@ -395,7 +394,7 @@ class alignas(8) Decl {
         Used(false), Referenced(false), TopLevelDeclInObjCContainer(false),
         Access(AS_none), FromASTFile(0),
         IdentifierNamespace(getIdentifierNamespaceForKind(DK)),
-        CacheValidAndLinkage(0) {
+        CacheValidAndLinkage(llvm::to_underlying(Linkage::Invalid)) {
     if (StatisticsEnabled) add(DK);
   }
 
@@ -405,11 +404,11 @@ class alignas(8) Decl {
   void updateOutOfDate(IdentifierInfo &II) const;
 
   Linkage getCachedLinkage() const {
-    return Linkage(CacheValidAndLinkage - 1);
+    return static_cast<Linkage>(CacheValidAndLinkage);
   }
 
   void setCachedLinkage(Linkage L) const {
-    CacheValidAndLinkage = L + 1;
+    CacheValidAndLinkage = llvm::to_underlying(L);
   }
 
   bool hasCachedLinkage() const {

clang/include/clang/AST/Type.h

@@ -2043,7 +2043,7 @@ class alignas(TypeAlignment) Type : public ExtQualsTypeCommonBase {
     TypeBits.Dependence = static_cast<unsigned>(Dependence);
     TypeBits.CacheValid = false;
     TypeBits.CachedLocalOrUnnamed = false;
-    TypeBits.CachedLinkage = NoLinkage;
+    TypeBits.CachedLinkage = llvm::to_underlying(Linkage::Invalid);
     TypeBits.FromAST = false;
   }
 

clang/include/clang/Basic/Linkage.h

@@ -14,21 +14,25 @@
 #ifndef LLVM_CLANG_BASIC_LINKAGE_H
 #define LLVM_CLANG_BASIC_LINKAGE_H
 
+#include "llvm/Support/ErrorHandling.h"
 #include <utility>
 
 namespace clang {
 
 /// Describes the different kinds of linkage
 /// (C++ [basic.link], C99 6.2.2) that an entity may have.
-enum Linkage : unsigned char {
+enum class Linkage : unsigned char {
+  // Linkage hasn't been computed.
+  Invalid = 0,
+
   /// No linkage, which means that the entity is unique and
   /// can only be referred to from within its scope.
-  NoLinkage = 0,
+  None,
 
   /// Internal linkage, which indicates that the entity can
   /// be referred to from within the translation unit (but not other
   /// translation units).
-  InternalLinkage,
+  Internal,
 
   /// External linkage within a unique namespace.
   ///
@@ -37,21 +41,21 @@ enum Linkage : unsigned char {
   /// their names are unique to this translation unit, which is
   /// equivalent to having internal linkage from the code-generation
   /// point of view.
-  UniqueExternalLinkage,
+  UniqueExternal,
 
   /// No linkage according to the standard, but is visible from other
   /// translation units because of types defined in a inline function.
-  VisibleNoLinkage,
+  VisibleNone,
 
   /// Module linkage, which indicates that the entity can be referred
   /// to from other translation units within the same module, and indirectly
   /// from arbitrary other translation units through inline functions and
   /// templates in the module interface.
-  ModuleLinkage,
+  Module,
 
   /// External linkage, which indicates that the entity can
   /// be referred to from other translation units.
-  ExternalLinkage
+  External
 };
 
 /// Describes the different kinds of language linkage
@@ -84,22 +88,33 @@ inline bool isUniqueGVALinkage(GVALinkage L) {
 }
 
 inline bool isExternallyVisible(Linkage L) {
-  return L >= VisibleNoLinkage;
+  switch (L) {
+  case Linkage::Invalid:
+    llvm_unreachable("Linkage hasn't been computed!");
+  case Linkage::None:
+  case Linkage::Internal:
+  case Linkage::UniqueExternal:
+    return false;
+  case Linkage::VisibleNone:
+  case Linkage::Module:
+  case Linkage::External:
+    return true;
+  }
 }
 
 inline Linkage getFormalLinkage(Linkage L) {
   switch (L) {
-  case UniqueExternalLinkage:
-    return ExternalLinkage;
-  case VisibleNoLinkage:
-    return NoLinkage;
+  case Linkage::UniqueExternal:
+    return Linkage::External;
+  case Linkage::VisibleNone:
+    return Linkage::None;
   default:
     return L;
   }
 }
 
 inline bool isExternalFormalLinkage(Linkage L) {
-  return getFormalLinkage(L) == ExternalLinkage;
+  return getFormalLinkage(L) == Linkage::External;
 }
 
 /// Compute the minimum linkage given two linkages.
@@ -111,13 +126,13 @@ inline bool isExternalFormalLinkage(Linkage L) {
 /// special cases for when VisibleNoLinkage would lose the visible bit and
 /// become NoLinkage.
 inline Linkage minLinkage(Linkage L1, Linkage L2) {
-  if (L2 == VisibleNoLinkage)
+  if (L2 == Linkage::VisibleNone)
     std::swap(L1, L2);
-  if (L1 == VisibleNoLinkage) {
-    if (L2 == InternalLinkage)
-      return NoLinkage;
-    if (L2 == UniqueExternalLinkage)
-      return NoLinkage;
+  if (L1 == Linkage::VisibleNone) {
+    if (L2 == Linkage::Internal)
+      return Linkage::None;
+    if (L2 == Linkage::UniqueExternal)
+      return Linkage::None;
   }
   return L1 < L2 ? L1 : L2;
 }

clang/include/clang/Basic/Visibility.h

@@ -15,6 +15,7 @@
 #define LLVM_CLANG_BASIC_VISIBILITY_H
 
 #include "clang/Basic/Linkage.h"
+#include "llvm/ADT/STLForwardCompat.h"
 #include <cassert>
 #include <cstdint>
 
@@ -56,10 +57,11 @@ class LinkageInfo {
 
   void setVisibility(Visibility V, bool E) { visibility_ = V; explicit_ = E; }
 public:
-  LinkageInfo() : linkage_(ExternalLinkage), visibility_(DefaultVisibility),
-                  explicit_(false) {}
+  LinkageInfo()
+      : linkage_(llvm::to_underlying(Linkage::External)),
+        visibility_(DefaultVisibility), explicit_(false) {}
   LinkageInfo(Linkage L, Visibility V, bool E)
-    : linkage_(L), visibility_(V), explicit_(E) {
+      : linkage_(llvm::to_underlying(L)), visibility_(V), explicit_(E) {
     assert(getLinkage() == L && getVisibility() == V &&
            isVisibilityExplicit() == E && "Enum truncated!");
   }
@@ -68,23 +70,23 @@ class LinkageInfo {
     return LinkageInfo();
   }
   static LinkageInfo internal() {
-    return LinkageInfo(InternalLinkage, DefaultVisibility, false);
+    return LinkageInfo(Linkage::Internal, DefaultVisibility, false);
   }
   static LinkageInfo uniqueExternal() {
-    return LinkageInfo(UniqueExternalLinkage, DefaultVisibility, false);
+    return LinkageInfo(Linkage::UniqueExternal, DefaultVisibility, false);
   }
   static LinkageInfo none() {
-    return LinkageInfo(NoLinkage, DefaultVisibility, false);
+    return LinkageInfo(Linkage::None, DefaultVisibility, false);
   }
   static LinkageInfo visible_none() {
-    return LinkageInfo(VisibleNoLinkage, DefaultVisibility, false);
+    return LinkageInfo(Linkage::VisibleNone, DefaultVisibility, false);
   }
 
-  Linkage getLinkage() const { return (Linkage)linkage_; }
+  Linkage getLinkage() const { return static_cast<Linkage>(linkage_); }
   Visibility getVisibility() const { return (Visibility)visibility_; }
   bool isVisibilityExplicit() const { return explicit_; }
 
-  void setLinkage(Linkage L) { linkage_ = L; }
+  void setLinkage(Linkage L) { linkage_ = llvm::to_underlying(L); }
 
   void mergeLinkage(Linkage L) {
     setLinkage(minLinkage(getLinkage(), L));
@@ -96,10 +98,10 @@ class LinkageInfo {
   void mergeExternalVisibility(Linkage L) {
     Linkage ThisL = getLinkage();
     if (!isExternallyVisible(L)) {
-      if (ThisL == VisibleNoLinkage)
-        ThisL = NoLinkage;
-      else if (ThisL == ExternalLinkage)
-        ThisL = UniqueExternalLinkage;
+      if (ThisL == Linkage::VisibleNone)
+        ThisL = Linkage::None;
+      else if (ThisL == Linkage::External)
+        ThisL = Linkage::UniqueExternal;
     }
     setLinkage(ThisL);
   }

clang/lib/AST/APValue.cpp

@@ -1115,7 +1115,7 @@ LinkageInfo LinkageComputer::getLVForValue(const APValue &V,
 
   auto MergeLV = [&](LinkageInfo MergeLV) {
     LV.merge(MergeLV);
-    return LV.getLinkage() == InternalLinkage;
+    return LV.getLinkage() == Linkage::Internal;
   };
   auto Merge = [&](const APValue &V) {
     return MergeLV(getLVForValue(V, computation));

clang/lib/AST/Decl.cpp

@@ -1204,12 +1204,11 @@ Linkage NamedDecl::getFormalLinkage() const {
   // [basic.namespace.general]/p2
   //   A namespace is never attached to a named module and never has a name with
   //   module linkage.
-  if (isInModulePurview(this) &&
-      InternalLinkage == ExternalLinkage &&
+  if (isInModulePurview(this) && InternalLinkage == Linkage::External &&
       !isExportedFromModuleInterfaceUnit(
           cast<NamedDecl>(this->getCanonicalDecl())) &&
       !isa<NamespaceDecl>(this))
-    InternalLinkage = ModuleLinkage;
+    InternalLinkage = Linkage::Module;
 
   return clang::getFormalLinkage(InternalLinkage);
 }
@@ -1337,7 +1336,7 @@ LinkageInfo LinkageComputer::getLVForClosure(const DeclContext *DC,
   // visible, then the lambda is too. We apply the same rules to blocks.
   if (!isExternallyVisible(OwnerLV.getLinkage()))
     return LinkageInfo::none();
-  return LinkageInfo(VisibleNoLinkage, OwnerLV.getVisibility(),
+  return LinkageInfo(Linkage::VisibleNone, OwnerLV.getVisibility(),
                      OwnerLV.isVisibilityExplicit());
 }
 
@@ -1382,7 +1381,7 @@ LinkageInfo LinkageComputer::getLVForLocalDecl(const NamedDecl *D,
 
       if (const VarDecl *Prev = Var->getPreviousDecl()) {
         LinkageInfo PrevLV = getLVForDecl(Prev, computation);
-        if (PrevLV.getLinkage())
+        if (PrevLV.getLinkage() != Linkage::Invalid)
           LV.setLinkage(PrevLV.getLinkage());
         LV.mergeVisibility(PrevLV);
       }
@@ -1433,14 +1432,14 @@ LinkageInfo LinkageComputer::getLVForLocalDecl(const NamedDecl *D,
             computation.isValueVisibility()
                 ? Context.getLangOpts().getValueVisibilityMode()
                 : Context.getLangOpts().getTypeVisibilityMode();
-        return LinkageInfo(VisibleNoLinkage, globalVisibility,
+        return LinkageInfo(Linkage::VisibleNone, globalVisibility,
                            /*visibilityExplicit=*/false);
       }
     }
   }
   if (!isExternallyVisible(LV.getLinkage()))
     return LinkageInfo::none();
-  return LinkageInfo(VisibleNoLinkage, LV.getVisibility(),
+  return LinkageInfo(Linkage::VisibleNone, LV.getVisibility(),
                      LV.isVisibilityExplicit());
 }
 
@@ -1921,7 +1920,20 @@ bool NamedDecl::declarationReplaces(NamedDecl *OldD, bool IsKnownNewer) const {
 }
 
 bool NamedDecl::hasLinkage() const {
-  return getFormalLinkage() != NoLinkage;
+  switch (getFormalLinkage()) {
+  case Linkage::Invalid:
+    llvm_unreachable("Linkage hasn't been computed!");
+  case Linkage::None:
+    return false;
+  case Linkage::Internal:
+    return true;
+  case Linkage::UniqueExternal:
+  case Linkage::VisibleNone:
+    llvm_unreachable("Non-formal linkage is not allowed here!");
+  case Linkage::Module:
+  case Linkage::External:
+    return true;
+  }
 }
 
 NamedDecl *NamedDecl::getUnderlyingDeclImpl() {

clang/lib/AST/ItaniumMangle.cpp

@@ -708,7 +708,7 @@ ItaniumMangleContextImpl::getEffectiveDeclContext(const Decl *D) {
 }
 
 bool ItaniumMangleContextImpl::isInternalLinkageDecl(const NamedDecl *ND) {
-  if (ND && ND->getFormalLinkage() == InternalLinkage &&
+  if (ND && ND->getFormalLinkage() == Linkage::Internal &&
       !ND->isExternallyVisible() &&
       getEffectiveDeclContext(ND)->isFileContext() &&
       !ND->isInAnonymousNamespace())
@@ -790,7 +790,7 @@ bool ItaniumMangleContextImpl::shouldMangleCXXName(const NamedDecl *D) {
     if (DC->isFunctionOrMethod() && D->hasLinkage())
       while (!DC->isFileContext())
         DC = getEffectiveParentContext(DC);
-    if (DC->isTranslationUnit() && D->getFormalLinkage() != InternalLinkage &&
+    if (DC->isTranslationUnit() && D->getFormalLinkage() != Linkage::Internal &&
         !CXXNameMangler::shouldHaveAbiTags(*this, VD) &&
         !isa<VarTemplateSpecializationDecl>(VD) &&
         !VD->getOwningModuleForLinkage())

clang/lib/AST/MicrosoftMangle.cpp

@@ -539,9 +539,8 @@ bool MicrosoftMangleContextImpl::shouldMangleCXXName(const NamedDecl *D) {
       while (!DC->isNamespace() && !DC->isTranslationUnit())
         DC = getEffectiveParentContext(DC);
 
-    if (DC->isTranslationUnit() && D->getFormalLinkage() == InternalLinkage &&
-        !isa<VarTemplateSpecializationDecl>(D) &&
-        D->getIdentifier() != nullptr)
+    if (DC->isTranslationUnit() && D->getFormalLinkage() == Linkage::Internal &&
+        !isa<VarTemplateSpecializationDecl>(D) && D->getIdentifier() != nullptr)
       return false;
   }