Some improvements to the handling of C11 atomic types:

- Add atomic-to/from-nonatomic cast types
- Emit atomic operations for arithmetic on atomic types
- Emit non-atomic stores for initialisation of atomic types, but atomic stores and loads for every other store / load
- Add a __atomic_init() intrinsic which does a non-atomic store to an _Atomic() type.  This is needed for the corresponding C11 stdatomic.h function.
- Enables the relevant __has_feature() checks.  The feature isn't 100% complete yet, but it's done enough that we want people testing it.

Still to do:

- Make the arithmetic operations on atomic types (e.g. Atomic(int) foo = 1; foo++;) use the correct LLVM intrinsic if one exists, not a loop with a cmpxchg.
- Add a signal fence builtin
- Properly set the fenv state in atomic operations on floating point values
- Correctly handle things like _Atomic(_Complex double) which are too large for an atomic cmpxchg on some platforms (this requires working out what 'correctly' means in this context)
- Fix the many remaining corner cases

llvm-svn: 148242

clang/lib/CodeGen/CGExprComplex.cpp

@@ -358,6 +358,10 @@ ComplexPairTy ComplexExprEmitter::EmitCast(CastExpr::CastKind CK, Expr *Op,
   switch (CK) {
   case CK_Dependent: llvm_unreachable("dependent cast kind in IR gen!");
 
+  // Atomic to non-atomic casts may be more than a no-op for some platforms and
+  // for some types.
+  case CK_AtomicToNonAtomic:
+  case CK_NonAtomicToAtomic:
   case CK_NoOp:
   case CK_LValueToRValue:
   case CK_UserDefinedConversion: