[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, 9 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 5e979c0de97..ef6b84737b0 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -2033,6 +2033,15 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
     return EmitLoadOfLValue(LV, CE->getExprLoc());
   }
 
+  case CK_LValueToRValueBitCast: {
+    LValue SourceLVal = CGF.EmitLValue(E);
+    Address Addr = Builder.CreateElementBitCast(SourceLVal.getAddress(),
+                                                CGF.ConvertTypeForMem(DestTy));
+    LValue DestLV = CGF.MakeAddrLValue(Addr, DestTy);
+    DestLV.setTBAAInfo(TBAAAccessInfo::getMayAliasInfo());
+    return EmitLoadOfLValue(DestLV, CE->getExprLoc());
+  }
+
   case CK_CPointerToObjCPointerCast:
   case CK_BlockPointerToObjCPointerCast:
   case CK_AnyPointerToBlockPointerCast: