Revert "[DenseMap] Do not align pointer sentinel values (NFC) (#146595)"

This reverts commit 7a6435b.

This causes ubsan failures when the sentinel pointers are upcast
using static_cast<>, which checks alignment.

Author: nikic

clang/lib/AST/APValue.cpp

@@ -187,14 +187,14 @@ APValue::LValueBase::operator bool () const {
 clang::APValue::LValueBase
 llvm::DenseMapInfo<clang::APValue::LValueBase>::getEmptyKey() {
   clang::APValue::LValueBase B;
-  B.Ptr = DenseMapInfo<clang::APValue::LValueBase::PtrTy>::getEmptyKey();
+  B.Ptr = DenseMapInfo<const ValueDecl*>::getEmptyKey();
   return B;
 }
 
 clang::APValue::LValueBase
 llvm::DenseMapInfo<clang::APValue::LValueBase>::getTombstoneKey() {
   clang::APValue::LValueBase B;
-  B.Ptr = DenseMapInfo<clang::APValue::LValueBase::PtrTy>::getTombstoneKey();
+  B.Ptr = DenseMapInfo<const ValueDecl*>::getTombstoneKey();
   return B;
 }
 

llvm/include/llvm/ADT/DenseMapInfo.h

@@ -56,13 +56,30 @@ struct DenseMapInfo {
   //static bool isEqual(const T &LHS, const T &RHS);
 };
 
-template <typename T> struct DenseMapInfo<T *> {
+// Provide DenseMapInfo for all pointers. Come up with sentinel pointer values
+// that are aligned to alignof(T) bytes, but try to avoid requiring T to be
+// complete. This allows clients to instantiate DenseMap<T*, ...> with forward
+// declared key types. Assume that no pointer key type requires more than 4096
+// bytes of alignment.
+template<typename T>
+struct DenseMapInfo<T*> {
+  // The following should hold, but it would require T to be complete:
+  // static_assert(alignof(T) <= (1 << Log2MaxAlign),
+  //               "DenseMap does not support pointer keys requiring more than "
+  //               "Log2MaxAlign bits of alignment");
+  static constexpr uintptr_t Log2MaxAlign = 12;
+
   static inline T* getEmptyKey() {
-    // We assume that raw pointers do not carry alignment requirements.
-    return reinterpret_cast<T *>(-1);
+    uintptr_t Val = static_cast<uintptr_t>(-1);
+    Val <<= Log2MaxAlign;
+    return reinterpret_cast<T*>(Val);
   }
 
-  static inline T *getTombstoneKey() { return reinterpret_cast<T *>(-2); }
+  static inline T* getTombstoneKey() {
+    uintptr_t Val = static_cast<uintptr_t>(-2);
+    Val <<= Log2MaxAlign;
+    return reinterpret_cast<T*>(Val);
+  }
 
   static unsigned getHashValue(const T *PtrVal) {
     return (unsigned((uintptr_t)PtrVal) >> 4) ^

llvm/include/llvm/ADT/PointerUnion.h

@@ -286,19 +286,13 @@ struct PointerLikeTypeTraits<PointerUnion<PTs...>> {
 // Teach DenseMap how to use PointerUnions as keys.
 template <typename ...PTs> struct DenseMapInfo<PointerUnion<PTs...>> {
   using Union = PointerUnion<PTs...>;
-  using FirstTypeTraits = PointerLikeTypeTraits<
-      typename pointer_union_detail::GetFirstType<PTs...>::type>;
+  using FirstInfo =
+      DenseMapInfo<typename pointer_union_detail::GetFirstType<PTs...>::type>;
 
-  static inline Union getEmptyKey() {
-    uintptr_t Val = static_cast<uintptr_t>(-1);
-    Val <<= FirstTypeTraits::NumLowBitsAvailable;
-    return FirstTypeTraits::getFromVoidPointer(reinterpret_cast<void *>(Val));
-  }
+  static inline Union getEmptyKey() { return Union(FirstInfo::getEmptyKey()); }
 
   static inline Union getTombstoneKey() {
-    uintptr_t Val = static_cast<uintptr_t>(-2);
-    Val <<= FirstTypeTraits::NumLowBitsAvailable;
-    return FirstTypeTraits::getFromVoidPointer(reinterpret_cast<void *>(Val));
+    return Union(FirstInfo::getTombstoneKey());
   }
 
   static unsigned getHashValue(const Union &UnionVal) {

mlir/include/mlir/Support/TypeID.h

@@ -395,12 +395,11 @@ namespace llvm {
 template <>
 struct DenseMapInfo<mlir::TypeID> {
   static inline mlir::TypeID getEmptyKey() {
-    // Shift by 3 to satisfy the TypeID alignment requirement.
-    void *pointer = reinterpret_cast<void *>(uintptr_t(-1) << 3);
+    void *pointer = llvm::DenseMapInfo<void *>::getEmptyKey();
     return mlir::TypeID::getFromOpaquePointer(pointer);
   }
   static inline mlir::TypeID getTombstoneKey() {
-    void *pointer = reinterpret_cast<void *>(uintptr_t(-2) << 3);
+    void *pointer = llvm::DenseMapInfo<void *>::getTombstoneKey();
     return mlir::TypeID::getFromOpaquePointer(pointer);
   }
   static unsigned getHashValue(mlir::TypeID val) {

mlir/lib/Bindings/Python/NanobindUtils.h

@@ -408,12 +408,11 @@ namespace llvm {
 template <>
 struct DenseMapInfo<MlirTypeID> {
   static inline MlirTypeID getEmptyKey() {
-    // Shift by 3 to satisfy the TypeID alignment requirement.
-    void *pointer = reinterpret_cast<void *>(uintptr_t(-1) << 3);
+    auto *pointer = llvm::DenseMapInfo<void *>::getEmptyKey();
     return mlirTypeIDCreate(pointer);
   }
   static inline MlirTypeID getTombstoneKey() {
-    void *pointer = reinterpret_cast<void *>(uintptr_t(-2) << 3);
+    auto *pointer = llvm::DenseMapInfo<void *>::getTombstoneKey();
     return mlirTypeIDCreate(pointer);
   }
   static inline unsigned getHashValue(const MlirTypeID &val) {