[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

2 files changed, 4 insertions, 0 deletions

diff --git a/clang/tools/libclang/CIndex.cpp b/clang/tools/libclang/CIndex.cpp
index c9d4c6f0a48..b3d2974b43b 100644
--- a/clang/tools/libclang/CIndex.cpp
+++ b/clang/tools/libclang/CIndex.cpp
@@ -5194,6 +5194,8 @@ CXString clang_getCursorKindSpelling(enum CXCursorKind Kind) {
       return cxstring::createRef("CallExpr");
   case CXCursor_ObjCMessageExpr:
       return cxstring::createRef("ObjCMessageExpr");
+  case CXCursor_BuiltinBitCastExpr:
+    return cxstring::createRef("BuiltinBitCastExpr");
   case CXCursor_UnexposedStmt:
       return cxstring::createRef("UnexposedStmt");
   case CXCursor_DeclStmt:
diff --git a/clang/tools/libclang/CXCursor.cpp b/clang/tools/libclang/CXCursor.cpp
index 0d80cdb9af1..422c2dacf75 100644
--- a/clang/tools/libclang/CXCursor.cpp
+++ b/clang/tools/libclang/CXCursor.cpp
@@ -716,6 +716,8 @@ CXCursor cxcursor::MakeCXCursor(const Stmt *S, const Decl *Parent,
   case Stmt::OMPTargetTeamsDistributeSimdDirectiveClass:
     K = CXCursor_OMPTargetTeamsDistributeSimdDirective;
     break;
+  case Stmt::BuiltinBitCastExprClass:
+    K = CXCursor_BuiltinBitCastExpr;
   }
 
   CXCursor C = { K, 0, { Parent, S, TU } };