[clang] [clang][NFC] Specify Type and ExtQuals as having 16-byte alignment (PR #68377)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll created https://github.com/llvm/llvm-project/pull/68377

While working on LLDB visualizer for `QualType`, I stumbled upon `Type` and `ExtQuals` defined with `alignas(8)`. Such alignment leaves only 3 lower bits available for pointer tagging, whereas `QualType` requires 4 (3 qualifiers + discriminator between `Type *` and `ExtQuals *`). Turns out `Type` and its derived classes are allocated with `TypeAlignment == 16` passed to `Allocate()`. So I'm removing misleading `alignas(8)` and fixing corresponding static asserts. Since they are already allocated with 16-byte alignment, this is a non-functional change.

>From 8503ff3e70e585a4a3597e3ee8111269f48105cc Mon Sep 17 00:00:00 2001
From: Vlad Serebrennikov <serebrennikov.vladislav at gmail.com>
Date: Thu, 5 Oct 2023 13:18:14 +0300
Subject: [PATCH] [clang] Align Type and ExtQuals on 16-byte boundary

---
 clang/include/clang/AST/Type.h | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/clang/include/clang/AST/Type.h b/clang/include/clang/AST/Type.h
index a78d8f60462b231..4e98858f6e9432a 100644
--- a/clang/include/clang/AST/Type.h
+++ b/clang/include/clang/AST/Type.h
@@ -1482,7 +1482,7 @@ class ExtQualsTypeCommonBase {
 /// in three low bits on the QualType pointer; a fourth bit records whether
 /// the pointer is an ExtQuals node. The extended qualifiers (address spaces,
 /// Objective-C GC attributes) are much more rare.
-class ExtQuals : public ExtQualsTypeCommonBase, public llvm::FoldingSetNode {
+class alignas(TypeAlignment) ExtQuals : public ExtQualsTypeCommonBase, public llvm::FoldingSetNode {
   // NOTE: changing the fast qualifiers should be straightforward as
   // long as you don't make 'const' non-fast.
   // 1. Qualifiers:
@@ -1507,6 +1507,9 @@ class ExtQuals : public ExtQualsTypeCommonBase, public llvm::FoldingSetNode {
       : ExtQualsTypeCommonBase(baseType,
                                canon.isNull() ? QualType(this_(), 0) : canon),
         Quals(quals) {
+    static_assert(alignof(decltype(*this)) % TypeAlignment == 0,
+                  "Insufficient alignment!");
+
     assert(Quals.hasNonFastQualifiers()
            && "ExtQuals created with no fast qualifiers");
     assert(!Quals.hasFastQualifiers()
@@ -1594,7 +1597,7 @@ enum class AutoTypeKeyword {
 ///
 /// Types, once created, are immutable.
 ///
-class alignas(8) Type : public ExtQualsTypeCommonBase {
+class alignas(TypeAlignment) Type : public ExtQualsTypeCommonBase {
 public:
   enum TypeClass {
 #define TYPE(Class, Base) Class,
@@ -1982,9 +1985,9 @@ class alignas(8) Type : public ExtQualsTypeCommonBase {
   Type(TypeClass tc, QualType canon, TypeDependence Dependence)
       : ExtQualsTypeCommonBase(this,
                                canon.isNull() ? QualType(this_(), 0) : canon) {
-    static_assert(sizeof(*this) <= 8 + sizeof(ExtQualsTypeCommonBase),
+    static_assert(sizeof(*this) <= 16 + sizeof(ExtQualsTypeCommonBase),
                   "changing bitfields changed sizeof(Type)!");
-    static_assert(alignof(decltype(*this)) % sizeof(void *) == 0,
+    static_assert(alignof(decltype(*this)) % TypeAlignment == 0,
                   "Insufficient alignment!");
     TypeBits.TC = tc;
     TypeBits.Dependence = static_cast<unsigned>(Dependence);
@@ -5348,7 +5351,7 @@ class DeducedType : public Type {
 
 /// Represents a C++11 auto or C++14 decltype(auto) type, possibly constrained
 /// by a type-constraint.
-class alignas(8) AutoType : public DeducedType, public llvm::FoldingSetNode {
+class AutoType : public DeducedType, public llvm::FoldingSetNode {
   friend class ASTContext; // ASTContext creates these
 
   ConceptDecl *TypeConstraintConcept;
@@ -5456,7 +5459,7 @@ class DeducedTemplateSpecializationType : public DeducedType,
 /// TemplateArguments, followed by a QualType representing the
 /// non-canonical aliased type when the template is a type alias
 /// template.
-class alignas(8) TemplateSpecializationType
+class TemplateSpecializationType
     : public Type,
       public llvm::FoldingSetNode {
   friend class ASTContext; // ASTContext creates these
@@ -5872,7 +5875,7 @@ class DependentNameType : public TypeWithKeyword, public llvm::FoldingSetNode {
 /// Represents a template specialization type whose template cannot be
 /// resolved, e.g.
 ///   A<T>::template B<T>
-class alignas(8) DependentTemplateSpecializationType
+class DependentTemplateSpecializationType
     : public TypeWithKeyword,
       public llvm::FoldingSetNode {
   friend class ASTContext; // ASTContext creates these