[Clang] Implement the core language parts of P2786 - Trivial relocation (#127636)

This adds

- The parsing of `trivially_relocatable_if_eligible`,
`replaceable_if_eligible` keywords
- `__builtin_trivially_relocate`, implemented in terms of memmove. In
the future this should
- Add the appropriate start/end lifetime markers that llvm does not have
(`start_lifetime_as`)
     - Add support for ptrauth when that's upstreamed

- the `__builtin_is_cpp_trivially_relocatable` and
`__builtin_is_replaceable` traits


Fixes #127609

Author: cor3ntin

clang/docs/LanguageExtensions.rst

@@ -1680,6 +1680,7 @@ Static assert with user-generated message     __cpp_static_assert >= 202306L   C
 Pack Indexing                                 __cpp_pack_indexing              C++26         C++03
 ``= delete ("should have a reason");``        __cpp_deleted_function           C++26         C++03
 Variadic Friends                              __cpp_variadic_friend            C++26         C++03
+Trivial Relocatability                        __cpp_trivial_relocatability     C++26         C++03
 --------------------------------------------- -------------------------------- ------------- -------------
 Designated initializers (N494)                                                 C99           C89
 Array & element qualification (N2607)                                          C23           C89
@@ -1861,8 +1862,15 @@ The following type trait primitives are supported by Clang. Those traits marked
 * ``__is_trivially_relocatable`` (Clang): Returns true if moving an object
   of the given type, and then destroying the source object, is known to be
   functionally equivalent to copying the underlying bytes and then dropping the
-  source object on the floor. This is true of trivial types and types which
+  source object on the floor. This is true of trivial types,
+  C++26 relocatable types, and types which
   were made trivially relocatable via the ``clang::trivial_abi`` attribute.
+* ``__builtin_is_cpp_trivially_relocatable`` (C++): Returns true if an object
+  is trivially relocatable, as defined by the C++26 standard [meta.unary.prop].
+  Note that when relocating the caller code should ensure that if the object is polymorphic,
+  the dynamic type is of the most derived type. Padding bytes should not be copied.
+* ``__builtin_is_replaceable`` (C++): Returns true if an object
+  is replaceable, as defined by the C++26 standard [meta.unary.prop].
 * ``__is_trivially_equality_comparable`` (Clang): Returns true if comparing two
   objects of the provided type is known to be equivalent to comparing their
   object representations. Note that types containing padding bytes are never
@@ -3722,6 +3730,21 @@ Query for this feature with ``__has_builtin(__builtin_operator_new)`` or
     replaceable global (de)allocation functions, but do support calling at least
     ``::operator new(size_t)`` and ``::operator delete(void*)``.
 
+
+``__builtin_trivially_relocate``
+-----------------------------------
+
+**Syntax**:
+
+.. code-block:: c
+
+  T* __builtin_trivially_relocate(T* dest, T* src, size_t count)
+
+Trivially relocates ``count`` objects of relocatable, complete type ``T``
+from ``src`` to ``dest`` and returns ``dest``.
+This builtin is used to implement ``std::trivially_relocate``.
+
+
 ``__builtin_preserve_access_index``
 -----------------------------------
 

clang/docs/ReleaseNotes.rst

@@ -106,6 +106,8 @@ C++2c Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
 - Implemented `P1061R10 Structured Bindings can introduce a Pack <https://wg21.link/P1061R10>`_.
+- Implemented `P2786R13 Trivial Relocatability <https://wg21.link/P2786R13>`_.
+
 
 - Implemented `P0963R3 Structured binding declaration as a condition <https://wg21.link/P0963R3>`_.
 

clang/include/clang/AST/ASTContext.h

@@ -617,6 +617,20 @@ class ASTContext : public RefCountedBase<ASTContext> {
   using ParameterIndexTable = llvm::DenseMap<const VarDecl *, unsigned>;
   ParameterIndexTable ParamIndices;
 
+public:
+  struct CXXRecordDeclRelocationInfo {
+    unsigned IsRelocatable;
+    unsigned IsReplaceable;
+  };
+  std::optional<CXXRecordDeclRelocationInfo>
+  getRelocationInfoForCXXRecord(const CXXRecordDecl *) const;
+  void setRelocationInfoForCXXRecord(const CXXRecordDecl *,
+                                     CXXRecordDeclRelocationInfo);
+
+private:
+  llvm::DenseMap<const CXXRecordDecl *, CXXRecordDeclRelocationInfo>
+      RelocatableClasses;
+
   ImportDecl *FirstLocalImport = nullptr;
   ImportDecl *LastLocalImport = nullptr;
 

clang/include/clang/AST/DeclCXX.h

@@ -1550,6 +1550,9 @@ class CXXRecordDecl : public RecordDecl {
   /// Returns the destructor decl for this class.
   CXXDestructorDecl *getDestructor() const;
 
+  /// Returns the destructor decl for this class.
+  bool hasDeletedDestructor() const;
+
   /// Returns true if the class destructor, or any implicitly invoked
   /// destructors are marked noreturn.
   bool isAnyDestructorNoReturn() const { return data().IsAnyDestructorNoReturn; }

clang/include/clang/AST/Type.h

@@ -1133,9 +1133,6 @@ class QualType {
   /// Return true if this is a trivially copyable type
   bool isTriviallyCopyConstructibleType(const ASTContext &Context) const;
 
-  /// Return true if this is a trivially relocatable type.
-  bool isTriviallyRelocatableType(const ASTContext &Context) const;
-
   /// Returns true if it is a class and it might be dynamic.
   bool mayBeDynamicClass() const;
 

clang/include/clang/Basic/Attr.td

@@ -1819,6 +1819,22 @@ def Final : InheritableAttr {
   let Documentation = [InternalOnly];
 }
 
+def TriviallyRelocatable : InheritableAttr {
+  let Spellings = [CustomKeyword<"trivially_relocatable_if_eligible">];
+  let SemaHandler = 0;
+  // Omitted from docs, since this is language syntax, not an attribute, as far
+  // as users are concerned.
+  let Documentation = [InternalOnly];
+}
+
+def Replaceable : InheritableAttr {
+  let Spellings = [CustomKeyword<"replaceable_if_eligible">];
+  let SemaHandler = 0;
+  // Omitted from docs, since this is language syntax, not an attribute, as far
+  // as users are concerned.
+  let Documentation = [InternalOnly];
+}
+
 def MinSize : InheritableAttr {
   let Spellings = [Clang<"minsize">];
   let Subjects = SubjectList<[Function, ObjCMethod], ErrorDiag>;

clang/include/clang/Basic/Builtins.td

@@ -2853,6 +2853,12 @@ def MemMove : LibBuiltin<"string.h"> {
   let AddBuiltinPrefixedAlias = 1;
 }
 
+def BuiltinTriviallyRelocate : Builtin {
+  let Spellings = ["__builtin_trivially_relocate"];
+  let Attributes = [FunctionWithBuiltinPrefix, CustomTypeChecking, NoThrow];
+  let Prototype = "void*(void*, void*, size_t)";
+}
+
 def StrCpy : LibBuiltin<"string.h"> {
   let Spellings = ["strcpy"];
   let Attributes = [NoThrow];

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -1065,12 +1065,24 @@ def ext_ms_abstract_keyword : ExtWarn<
   "'abstract' keyword is a Microsoft extension">,
   InGroup<MicrosoftAbstract>;
 
+def ext_relocatable_keyword : ExtWarn<
+  "'%select{trivially_relocatable_if_eligible|replaceable_if_eligible}0' "
+  "keyword is a C++2c extension">,
+  InGroup<CXX26>;
+def warn_relocatable_keyword  : Warning<
+  "'%select{trivially_relocatable|replaceable}0_if_eligible' "
+  "keyword is incompatible with standards before C++2c">,
+  DefaultIgnore, InGroup<CXXPre26Compat>;
+
 def err_access_specifier_interface : Error<
   "interface types cannot specify '%select{private|protected}0' access">;
 
 def err_duplicate_class_virt_specifier : Error<
   "class already marked '%0'">;
 
+def err_duplicate_class_relocation_specifier : Error<
+  "class already marked '%select{trivially_relocatable_if_eligible|replaceable_if_eligible}0'">;
+
 def err_duplicate_virt_specifier : Error<
   "class member already marked '%0'">;
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -12664,6 +12664,11 @@ def err_builtin_invalid_arg_type: Error<
   "%plural{0:|: }3"
   "%plural{[0,3]:type|:types}1 (was %4)">;
 
+def err_builtin_trivially_relocate_invalid_arg_type: Error <
+  "first%select{||| and second}0 argument%select{|||s}0 to "
+  "'__builtin_trivially_relocate' must be"
+  " %select{a pointer|non-const|relocatable|of the same type}0">;
+
 def err_builtin_matrix_disabled: Error<
   "matrix types extension is disabled. Pass -fenable-matrix to enable it">;
 def err_matrix_index_not_integer: Error<

clang/include/clang/Basic/TokenKinds.def

@@ -557,8 +557,12 @@ TYPE_TRAIT_2(__reference_converts_from_temporary, ReferenceConvertsFromTemporary
 // is not exposed to users.
 TYPE_TRAIT_2(/*EmptySpellingName*/, IsDeducible, KEYCXX)
 TYPE_TRAIT_1(__is_bitwise_cloneable, IsBitwiseCloneable, KEYALL)
+TYPE_TRAIT_1(__builtin_is_cpp_trivially_relocatable, IsCppTriviallyRelocatable, KEYCXX)
+TYPE_TRAIT_1(__builtin_is_replaceable, IsReplaceable, KEYCXX)
 TYPE_TRAIT_1(__builtin_structured_binding_size, StructuredBindingSize, KEYCXX)
 
+
+
 // Embarcadero Expression Traits
 EXPRESSION_TRAIT(__is_lvalue_expr, IsLValueExpr, KEYCXX)
 EXPRESSION_TRAIT(__is_rvalue_expr, IsRValueExpr, KEYCXX)

clang/include/clang/Parse/Parser.h

@@ -264,6 +264,8 @@ class Parser : public CodeCompletionHandler {
   mutable IdentifierInfo *Ident_final;
   mutable IdentifierInfo *Ident_GNU_final;
   mutable IdentifierInfo *Ident_override;
+  mutable IdentifierInfo *Ident_trivially_relocatable_if_eligible;
+  mutable IdentifierInfo *Ident_replaceable_if_eligible;
 
   // C++2a contextual keywords.
   mutable IdentifierInfo *Ident_import;
@@ -3196,6 +3198,16 @@ class Parser : public CodeCompletionHandler {
                                           SourceLocation FriendLoc);
 
   bool isCXX11FinalKeyword() const;
+
+  bool isCXX2CTriviallyRelocatableKeyword(Token Tok) const;
+  bool isCXX2CTriviallyRelocatableKeyword() const;
+  void ParseCXX2CTriviallyRelocatableSpecifier(SourceLocation &TRS);
+
+  bool isCXX2CReplaceableKeyword(Token Tok) const;
+  bool isCXX2CReplaceableKeyword() const;
+  void ParseCXX2CReplaceableSpecifier(SourceLocation &MRS);
+
+  bool isClassCompatibleKeyword(Token Tok) const;
   bool isClassCompatibleKeyword() const;
 
   /// DeclaratorScopeObj - RAII object used in Parser::ParseDirectDeclarator to

clang/include/clang/Sema/Sema.h

@@ -4316,13 +4316,18 @@ class Sema final : public SemaBase {
                                        SourceLocation FinalLoc,
                                        bool IsFinalSpelledSealed,
                                        bool IsAbstract,
+                                       SourceLocation TriviallyRelocatable,
+                                       SourceLocation Replaceable,
                                        SourceLocation LBraceLoc);
 
   /// ActOnTagFinishDefinition - Invoked once we have finished parsing
   /// the definition of a tag (enumeration, class, struct, or union).
   void ActOnTagFinishDefinition(Scope *S, Decl *TagDecl,
                                 SourceRange BraceRange);
 
+  ASTContext::CXXRecordDeclRelocationInfo
+  CheckCXX2CRelocatableAndReplaceable(const clang::CXXRecordDecl *D);
+
   void ActOnTagFinishSkippedDefinition(SkippedDefinitionContext Context);
 
   /// ActOnTagDefinitionError - Invoked when there was an unrecoverable
@@ -8637,6 +8642,18 @@ class Sema final : public SemaBase {
                                                ExprResult &LHS, ExprResult &RHS,
                                                SourceLocation QuestionLoc);
 
+  //// Determines if a type is trivially relocatable
+  /// according to the C++26 rules.
+  // FIXME: This is in Sema because it requires
+  // overload resolution, can we move to ASTContext?
+  bool IsCXXTriviallyRelocatableType(QualType T);
+
+  //// Determines if a type is replaceable
+  /// according to the C++26 rules.
+  // FIXME: This is in Sema because it requires
+  // overload resolution, can we move to ASTContext?
+  bool IsCXXReplaceableType(QualType T);
+
   /// Check the operands of ?: under C++ semantics.
   ///
   /// See C++ [expr.cond]. Note that LHS is never null, even for the GNU x ?: y

clang/lib/AST/ASTContext.cpp

@@ -1688,6 +1688,24 @@ void ASTContext::getOverriddenMethods(
   Overridden.append(OverDecls.begin(), OverDecls.end());
 }
 
+std::optional<ASTContext::CXXRecordDeclRelocationInfo>
+ASTContext::getRelocationInfoForCXXRecord(const CXXRecordDecl *RD) const {
+  assert(RD);
+  CXXRecordDecl *D = RD->getDefinition();
+  auto it = RelocatableClasses.find(D);
+  if (it != RelocatableClasses.end())
+    return it->getSecond();
+  return std::nullopt;
+}
+
+void ASTContext::setRelocationInfoForCXXRecord(
+    const CXXRecordDecl *RD, CXXRecordDeclRelocationInfo Info) {
+  assert(RD);
+  CXXRecordDecl *D = RD->getDefinition();
+  assert(RelocatableClasses.find(D) == RelocatableClasses.end());
+  RelocatableClasses.insert({D, Info});
+}
+
 void ASTContext::addedLocalImportDecl(ImportDecl *Import) {
   assert(!Import->getNextLocalImport() &&
          "Import declaration already in the chain");

clang/lib/AST/Decl.cpp

@@ -4491,6 +4491,7 @@ unsigned FunctionDecl::getMemoryFunctionKind() const {
   case Builtin::BImempcpy:
     return Builtin::BImempcpy;
 
+  case Builtin::BI__builtin_trivially_relocate:
   case Builtin::BI__builtin_memmove:
   case Builtin::BI__builtin___memmove_chk:
   case Builtin::BImemmove:

clang/lib/AST/DeclCXX.cpp

@@ -1556,7 +1556,6 @@ void CXXRecordDecl::addedEligibleSpecialMemberFunction(const CXXMethodDecl *MD,
     if (DD->isNoReturn())
       data().IsAnyDestructorNoReturn = true;
   }
-
   if (!MD->isImplicit() && !MD->isUserProvided()) {
     // This method is user-declared but not user-provided. We can't work
     // out whether it's trivial yet (not until we get to the end of the
@@ -2144,6 +2143,12 @@ CXXDestructorDecl *CXXRecordDecl::getDestructor() const {
   return nullptr;
 }
 
+bool CXXRecordDecl::hasDeletedDestructor() const {
+  if (const CXXDestructorDecl *D = getDestructor())
+    return D->isDeleted();
+  return false;
+}
+
 static bool isDeclContextInNamespace(const DeclContext *DC) {
   while (!DC->isTranslationUnit()) {
     if (DC->isNamespace())

clang/lib/AST/Type.cpp

@@ -2896,29 +2896,6 @@ bool QualType::isTriviallyCopyConstructibleType(
                                      /*IsCopyConstructible=*/true);
 }
 
-bool QualType::isTriviallyRelocatableType(const ASTContext &Context) const {
-  QualType BaseElementType = Context.getBaseElementType(*this);
-
-  if (BaseElementType->isIncompleteType()) {
-    return false;
-  } else if (!BaseElementType->isObjectType()) {
-    return false;
-  } else if (const auto *RD = BaseElementType->getAsRecordDecl()) {
-    return RD->canPassInRegisters();
-  } else if (BaseElementType.isTriviallyCopyableType(Context)) {
-    return true;
-  } else {
-    switch (isNonTrivialToPrimitiveDestructiveMove()) {
-    case PCK_Trivial:
-      return !isDestructedType();
-    case PCK_ARCStrong:
-      return true;
-    default:
-      return false;
-    }
-  }
-}
-
 bool QualType::isNonWeakInMRRWithObjCWeak(const ASTContext &Context) const {
   return !Context.getLangOpts().ObjCAutoRefCount &&
          Context.getLangOpts().ObjCWeak &&

clang/lib/CodeGen/CGBuiltin.cpp

@@ -4210,6 +4210,7 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
     return RValue::get(Dest, *this);
   }
 
+  case Builtin::BI__builtin_trivially_relocate:
   case Builtin::BImemmove:
   case Builtin::BI__builtin_memmove: {
     Address Dest = EmitPointerWithAlignment(E->getArg(0));

clang/lib/Frontend/InitPreprocessor.cpp

@@ -772,6 +772,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   Builder.defineMacro("__cpp_pack_indexing", "202311L");
   Builder.defineMacro("__cpp_deleted_function", "202403L");
   Builder.defineMacro("__cpp_variadic_friend", "202403L");
+  // Builder.defineMacro("__cpp_trivial_relocatability", "202502L");
 
   if (LangOpts.Char8)
     Builder.defineMacro("__cpp_char8_t", "202207L");