[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@364954 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 97 insertions, 0 deletions

diff --git a/lib/Sema/SemaCast.cpp b/lib/Sema/SemaCast.cpp
index 46d0f57dc7..9ca6e4ab0e 100644
--- a/lib/Sema/SemaCast.cpp
+++ b/lib/Sema/SemaCast.cpp
@@ -87,6 +87,7 @@ namespace {
     void CheckDynamicCast();
     void CheckCXXCStyleCast(bool FunctionalCast, bool ListInitialization);
     void CheckCStyleCast();
+    void CheckBuiltinBitCast();
 
     void updatePartOfExplicitCastFlags(CastExpr *CE) {
       // Walk down from the CE to the OrigSrcExpr, and mark all immediate
@@ -331,6 +332,38 @@ Sema::BuildCXXNamedCast(SourceLocation OpLoc, tok::TokenKind Kind,
   }
 }
 
+ExprResult Sema::ActOnBuiltinBitCastExpr(SourceLocation KWLoc, Declarator &D,
+                                         ExprResult Operand,
+                                         SourceLocation RParenLoc) {
+  assert(!D.isInvalidType());
+
+  TypeSourceInfo *TInfo = GetTypeForDeclaratorCast(D, Operand.get()->getType());
+  if (D.isInvalidType())
+    return ExprError();
+
+  return BuildBuiltinBitCastExpr(KWLoc, TInfo, Operand.get(), RParenLoc);
+}
+
+ExprResult Sema::BuildBuiltinBitCastExpr(SourceLocation KWLoc,
+                                         TypeSourceInfo *TSI, Expr *Operand,
+                                         SourceLocation RParenLoc) {
+  CastOperation Op(*this, TSI->getType(), Operand);
+  Op.OpRange = SourceRange(KWLoc, RParenLoc);
+  TypeLoc TL = TSI->getTypeLoc();
+  Op.DestRange = SourceRange(TL.getBeginLoc(), TL.getEndLoc());
+
+  if (!Operand->isTypeDependent() && !TSI->getType()->isDependentType()) {
+    Op.CheckBuiltinBitCast();
+    if (Op.SrcExpr.isInvalid())
+      return ExprError();
+  }
+
+  BuiltinBitCastExpr *BCE =
+      new (Context) BuiltinBitCastExpr(Op.ResultType, Op.ValueKind, Op.Kind,
+                                       Op.SrcExpr.get(), TSI, KWLoc, RParenLoc);
+  return Op.complete(BCE);
+}
+
 /// Try to diagnose a failed overloaded cast.  Returns true if
 /// diagnostics were emitted.
 static bool tryDiagnoseOverloadedCast(Sema &S, CastType CT,
@@ -2764,6 +2797,43 @@ void CastOperation::CheckCStyleCast() {
     checkCastAlign();
 }
 
+void CastOperation::CheckBuiltinBitCast() {
+  QualType SrcType = SrcExpr.get()->getType();
+  if (SrcExpr.get()->isRValue())
+    SrcExpr = Self.CreateMaterializeTemporaryExpr(SrcType, SrcExpr.get(),
+                                                  /*IsLValueReference=*/false);
+
+  CharUnits DestSize = Self.Context.getTypeSizeInChars(DestType);
+  CharUnits SourceSize = Self.Context.getTypeSizeInChars(SrcType);
+  if (DestSize != SourceSize) {
+    Self.Diag(OpRange.getBegin(), diag::err_bit_cast_type_size_mismatch)
+        << (int)SourceSize.getQuantity() << (int)DestSize.getQuantity();
+    SrcExpr = ExprError();
+    return;
+  }
+
+  if (!DestType.isTriviallyCopyableType(Self.Context)) {
+    Self.Diag(OpRange.getBegin(), diag::err_bit_cast_non_trivially_copyable)
+        << 1;
+    SrcExpr = ExprError();
+    return;
+  }
+
+  if (!SrcType.isTriviallyCopyableType(Self.Context)) {
+    Self.Diag(OpRange.getBegin(), diag::err_bit_cast_non_trivially_copyable)
+        << 0;
+    SrcExpr = ExprError();
+    return;
+  }
+
+  if (Self.Context.hasSameUnqualifiedType(DestType, SrcType)) {
+    Kind = CK_NoOp;
+    return;
+  }
+
+  Kind = CK_LValueToRValueBitCast;
+}
+
 /// DiagnoseCastQual - Warn whenever casts discards a qualifiers, be it either
 /// const, volatile or both.
 static void DiagnoseCastQual(Sema &Self, const ExprResult &SrcExpr,
diff --git a/lib/Sema/SemaExceptionSpec.cpp b/lib/Sema/SemaExceptionSpec.cpp
index 5274532b0f..9fd924a8ca 100644
--- a/lib/Sema/SemaExceptionSpec.cpp
+++ b/lib/Sema/SemaExceptionSpec.cpp
@@ -1201,6 +1201,7 @@ CanThrowResult Sema::canThrow(const Expr *E) {
   case Expr::CoyieldExprClass:
   case Expr::CXXConstCastExprClass:
   case Expr::CXXReinterpretCastExprClass:
+  case Expr::BuiltinBitCastExprClass:
   case Expr::CXXStdInitializerListExprClass:
   case Expr::DesignatedInitExprClass:
   case Expr::DesignatedInitUpdateExprClass:
diff --git a/lib/Sema/TreeTransform.h b/lib/Sema/TreeTransform.h
index 953889c6a0..7edb2a8d80 100644
--- a/lib/Sema/TreeTransform.h
+++ b/lib/Sema/TreeTransform.h
@@ -2652,6 +2652,16 @@ public:
                                                ListInitialization);
   }
 
+  /// Build a new C++ __builtin_bit_cast expression.
+  ///
+  /// By default, performs semantic analysis to build the new expression.
+  /// Subclasses may override this routine to provide different behavior.
+  ExprResult RebuildBuiltinBitCastExpr(SourceLocation KWLoc,
+                                       TypeSourceInfo *TSI, Expr *Sub,
+                                       SourceLocation RParenLoc) {
+    return getSema().BuildBuiltinBitCastExpr(KWLoc, TSI, Sub, RParenLoc);
+  }
+
   /// Build a new C++ typeid(type) expression.
   ///
   /// By default, performs semantic analysis to build the new expression.
@@ -10247,6 +10257,22 @@ TreeTransform<Derived>::TransformCXXNamedCastExpr(CXXNamedCastExpr *E) {
 
 template<typename Derived>
 ExprResult
+TreeTransform<Derived>::TransformBuiltinBitCastExpr(BuiltinBitCastExpr *BCE) {
+  TypeSourceInfo *TSI =
+      getDerived().TransformType(BCE->getTypeInfoAsWritten());
+  if (!TSI)
+    return ExprError();
+
+  ExprResult Sub = getDerived().TransformExpr(BCE->getSubExpr());
+  if (Sub.isInvalid())
+    return ExprError();
+
+  return getDerived().RebuildBuiltinBitCastExpr(BCE->getBeginLoc(), TSI,
+                                                Sub.get(), BCE->getEndLoc());
+}
+
+template<typename Derived>
+ExprResult
 TreeTransform<Derived>::TransformCXXStaticCastExpr(CXXStaticCastExpr *E) {
   return getDerived().TransformCXXNamedCastExpr(E);
 }