[C++2a] Add __builtin_bit_cast, used to implement std::bit_cast

This commit adds a new builtin, __builtin_bit_cast(T, v), which performs a
bit_cast from a value v to a type T. This expression can be evaluated at
compile time under specific circumstances.

The compile time evaluation currently doesn't support bit-fields, but I'm
planning on fixing this in a follow up (some of the logic for figuring this out
is in CodeGen). I'm also planning follow-ups for supporting some more esoteric
types that the constexpr evaluator supports, as well as extending
__builtin_memcpy constexpr evaluation to use the same infrastructure.

rdar://44987528

Differential revision: https://reviews.llvm.org/D62825

llvm-svn: 364954

clang/include/clang-c/Index.h

@@ -2546,7 +2546,11 @@ enum CXCursorKind {
    */
   CXCursor_OMPTargetTeamsDistributeSimdDirective = 279,
 
-  CXCursor_LastStmt = CXCursor_OMPTargetTeamsDistributeSimdDirective,
+  /** C++2a std::bit_cast expression.
+   */
+  CXCursor_BuiltinBitCastExpr = 280,
+
+  CXCursor_LastStmt = CXCursor_BuiltinBitCastExpr,
 
   /**
    * Cursor that represents the translation unit itself.

clang/include/clang/AST/ExprCXX.h

@@ -4716,6 +4716,35 @@ class CoyieldExpr : public CoroutineSuspendExpr {
   }
 };
 
+/// Represents a C++2a __builtin_bit_cast(T, v) expression. Used to implement
+/// std::bit_cast. These can sometimes be evaluated as part of a constant
+/// expression, but otherwise CodeGen to a simple memcpy in general.
+class BuiltinBitCastExpr final
+    : public ExplicitCastExpr,
+      private llvm::TrailingObjects<BuiltinBitCastExpr, CXXBaseSpecifier *> {
+  friend class ASTStmtReader;
+  friend class CastExpr;
+  friend class TrailingObjects;
+
+  SourceLocation KWLoc;
+  SourceLocation RParenLoc;
+
+public:
+  BuiltinBitCastExpr(QualType T, ExprValueKind VK, CastKind CK, Expr *SrcExpr,
+                     TypeSourceInfo *DstType, SourceLocation KWLoc,
+                     SourceLocation RParenLoc)
+      : ExplicitCastExpr(BuiltinBitCastExprClass, T, VK, CK, SrcExpr, 0,
+                         DstType),
+        KWLoc(KWLoc), RParenLoc(RParenLoc) {}
+
+  SourceLocation getBeginLoc() const LLVM_READONLY { return KWLoc; }
+  SourceLocation getEndLoc() const LLVM_READONLY { return RParenLoc; }
+
+  static bool classof(const Stmt *T) {
+    return T->getStmtClass() == BuiltinBitCastExprClass;
+  }
+};
+
 } // namespace clang
 
 #endif // LLVM_CLANG_AST_EXPRCXX_H

clang/include/clang/AST/OperationKinds.def

@@ -66,6 +66,10 @@ CAST_OPERATION(BitCast)
 ///    bool b; reinterpret_cast<char&>(b) = 'a';
 CAST_OPERATION(LValueBitCast)
 
+/// CK_LValueToRValueBitCast - A conversion that causes us to reinterpret an
+/// lvalue as an rvalue of a different type. Created by __builtin_bit_cast.
+CAST_OPERATION(LValueToRValueBitCast)
+
 /// CK_LValueToRValue - A conversion which causes the extraction of
 /// an r-value from the operand gl-value.  The result of an r-value
 /// conversion is always unqualified.

clang/include/clang/AST/RecursiveASTVisitor.h

@@ -2282,6 +2282,10 @@ DEF_TRAVERSE_STMT(CXXStaticCastExpr, {
   TRY_TO(TraverseTypeLoc(S->getTypeInfoAsWritten()->getTypeLoc()));
 })
 
+DEF_TRAVERSE_STMT(BuiltinBitCastExpr, {
+  TRY_TO(TraverseTypeLoc(S->getTypeInfoAsWritten()->getTypeLoc()));
+})
+
 template <typename Derived>
 bool RecursiveASTVisitor<Derived>::TraverseSynOrSemInitListExpr(
     InitListExpr *S, DataRecursionQueue *Queue) {

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -215,6 +215,19 @@ def note_constexpr_memcpy_unsupported : Note<
   "size to copy (%4) is not a multiple of size of element type %3 (%5)|"
   "source is not a contiguous array of at least %4 elements of type %3|"
   "destination is not a contiguous array of at least %4 elements of type %3}2">;
+def note_constexpr_bit_cast_unsupported_type : Note<
+  "constexpr bit_cast involving type %0 is not yet supported">;
+def note_constexpr_bit_cast_unsupported_bitfield : Note<
+  "constexpr bit_cast involving bit-field is not yet supported">;
+def note_constexpr_bit_cast_invalid_type : Note<
+  "bit_cast %select{from|to}0 a %select{|type with a }1"
+  "%select{union|pointer|member pointer|volatile|reference}2 "
+  "%select{type|member}1 is not allowed in a constant expression">;
+def note_constexpr_bit_cast_invalid_subtype : Note<
+  "invalid type %0 is a %select{member|base}1 of %2">;
+def note_constexpr_bit_cast_indet_dest : Note<
+  "indeterminate value can only initialize an object of type 'unsigned char'"
+  "%select{, 'char',|}1 or 'std::byte'; %0 is invalid">;
 
 def warn_integer_constant_overflow : Warning<
   "overflow in expression; result is %0 with type %1">,

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -9734,4 +9734,8 @@ def err_builtin_launder_invalid_arg : Error<
   "%select{non-pointer|function pointer|void pointer}0 argument to "
   "'__builtin_launder' is not allowed">;
 
+def err_bit_cast_non_trivially_copyable : Error<
+  "__builtin_bit_cast %select{source|destination}0 type must be trivially copyable">;
+def err_bit_cast_type_size_mismatch : Error<
+  "__builtin_bit_cast source size does not equal destination size (%0 vs %1)">;
 } // end of sema component.

clang/include/clang/Basic/StmtNodes.td

@@ -192,6 +192,7 @@ def ConvertVectorExpr : DStmt<Expr>;
 def BlockExpr : DStmt<Expr>;
 def OpaqueValueExpr : DStmt<Expr>;
 def TypoExpr : DStmt<Expr>;
+def BuiltinBitCastExpr : DStmt<ExplicitCastExpr>;
 
 // Microsoft Extensions.
 def MSPropertyRefExpr : DStmt<Expr>;

clang/include/clang/Basic/TokenKinds.def

@@ -670,7 +670,7 @@ ALIAS("_pascal"      , __pascal   , KEYBORLAND)
 KEYWORD(__builtin_convertvector   , KEYALL)
 ALIAS("__char16_t"   , char16_t   , KEYCXX)
 ALIAS("__char32_t"   , char32_t   , KEYCXX)
-
+KEYWORD(__builtin_bit_cast        , KEYALL)
 KEYWORD(__builtin_available       , KEYALL)
 
 // Clang-specific keywords enabled only in testing.

clang/include/clang/Parse/Parser.h

@@ -1780,6 +1780,9 @@ class Parser : public CodeCompletionHandler {
   // C++ 5.2p1: C++ Casts
   ExprResult ParseCXXCasts();
 
+  /// Parse a __builtin_bit_cast(T, E), used to implement C++2a std::bit_cast.
+  ExprResult ParseBuiltinBitCast();
+
   //===--------------------------------------------------------------------===//
   // C++ 5.2p1: C++ Type Identification
   ExprResult ParseCXXTypeid();

clang/include/clang/Sema/Sema.h

@@ -5236,6 +5236,13 @@ class Sema {
                                SourceRange AngleBrackets,
                                SourceRange Parens);
 
+  ExprResult ActOnBuiltinBitCastExpr(SourceLocation KWLoc, Declarator &Dcl,
+                                     ExprResult Operand,
+                                     SourceLocation RParenLoc);
+
+  ExprResult BuildBuiltinBitCastExpr(SourceLocation KWLoc, TypeSourceInfo *TSI,
+                                     Expr *Operand, SourceLocation RParenLoc);
+
   ExprResult BuildCXXTypeId(QualType TypeInfoType,
                             SourceLocation TypeidLoc,
                             TypeSourceInfo *Operand,

clang/lib/AST/Expr.cpp

@@ -1862,6 +1862,7 @@ bool CastExpr::CastConsistency() const {
   case CK_FloatingComplexToBoolean:
   case CK_IntegralComplexToBoolean:
   case CK_LValueBitCast:            // -> bool&
+  case CK_LValueToRValueBitCast:
   case CK_UserDefinedConversion:    // operator bool()
   case CK_BuiltinFnToFnPtr:
   case CK_FixedPointToBoolean:
@@ -3506,7 +3507,8 @@ bool Expr::HasSideEffects(const ASTContext &Ctx,
   case CXXStaticCastExprClass:
   case CXXReinterpretCastExprClass:
   case CXXConstCastExprClass:
-  case CXXFunctionalCastExprClass: {
+  case CXXFunctionalCastExprClass:
+  case BuiltinBitCastExprClass: {
     // While volatile reads are side-effecting in both C and C++, we treat them
     // as having possible (not definite) side-effects. This allows idiomatic
     // code to behave without warning, such as sizeof(*v) for a volatile-

clang/lib/AST/ExprClassification.cpp

@@ -343,6 +343,7 @@ static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E) {
   case Expr::CXXReinterpretCastExprClass:
   case Expr::CXXConstCastExprClass:
   case Expr::ObjCBridgedCastExprClass:
+  case Expr::BuiltinBitCastExprClass:
     // Only in C++ can casts be interesting at all.
     if (!Lang.CPlusPlus) return Cl::CL_PRValue;
     return ClassifyUnnamed(Ctx, cast<ExplicitCastExpr>(E)->getTypeAsWritten());