[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

1 files changed, 26 insertions, 0 deletions

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 953889c6a02..7edb2a8d809 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/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);
 }