[RISCV] Add missing emulated TLS support

[brad]

Summary

While getting OpenBSD/riscv64 up and running it was noticed that this bit of code to enable emulated TLS was missing from the RISC-V backend.

[jrtc27]

Why should we bother to support this? Emulated TLS should die, not be supported by new architectures, IMO. We've had properly-defined TLS from the start.

[brad]

OpenBSD requires the emulated TLS support.

[jrtc27]

You should really implement proper TLS. Emulated TLS is a lazy, inefficient hack.

[MaskRay]

+1. Please implement proper ELF TLS and drop reliance on emulated TLS. libclang_rt.builtins-$arch.a has some improper libc calls (e.g. malloc) just because of emutls.

I think we will soon be able to eliminate emutls in the LLVM code base.

[brad]

In D102527#2760631, @jrtc27 wrote:

You should really implement proper TLS. Emulated TLS is a lazy, inefficient hack.

I really wish that were the case. I have been asking the relevant developers for ages.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

                     DAG.getExternalSymbol("__tls_get_addr", Ty),
                     std::move(Args));
 
   return LowerCallTo(CLI).first;
 }
 
 SDValue RISCVTargetLowering::lowerGlobalTLSAddress(SDValue Op,
                                                    SelectionDAG &DAG) const {
+  const GlobalAddressSDNode *GA = cast<GlobalAddressSDNode>(Op);
+  if (DAG.getTarget().useEmulatedTLS())
+    return LowerToTLSEmulatedModel(GA, DAG);
+
   SDLoc DL(Op);
   EVT Ty = Op.getValueType();
   GlobalAddressSDNode *N = cast<GlobalAddressSDNode>(Op);
   int64_t Offset = N->getOffset();
   MVT XLenVT = Subtarget.getXLenVT();
 
   TLSModel::Model Model = getTargetMachine().getTLSModel(N->getGlobal());