[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, 2 insertions, 0 deletions

diff --git a/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp b/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp
index 628d4315bcb..d95fb314f3c 100644
--- a/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp
+++ b/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp
@@ -1687,6 +1687,7 @@ void ExprEngine::Visit(const Stmt *S, ExplodedNode *Pred,
     case Stmt::CXXReinterpretCastExprClass:
     case Stmt::CXXConstCastExprClass:
     case Stmt::CXXFunctionalCastExprClass:
+    case Stmt::BuiltinBitCastExprClass:
     case Stmt::ObjCBridgedCastExprClass: {
       Bldr.takeNodes(Pred);
       const auto *C = cast<CastExpr>(S);
diff --git a/clang/lib/StaticAnalyzer/Core/ExprEngineC.cpp b/clang/lib/StaticAnalyzer/Core/ExprEngineC.cpp
index 3b5a437e3c1..f436650fbdd 100644
--- a/clang/lib/StaticAnalyzer/Core/ExprEngineC.cpp
+++ b/clang/lib/StaticAnalyzer/Core/ExprEngineC.cpp
@@ -380,6 +380,7 @@ void ExprEngine::VisitCast(const CastExpr *CastE, const Expr *Ex,
       case CK_Dependent:
       case CK_ArrayToPointerDecay:
       case CK_BitCast:
+      case CK_LValueToRValueBitCast:
       case CK_AddressSpaceConversion:
       case CK_BooleanToSignedIntegral:
       case CK_IntegralToPointer: