[NFC][X86] Sret return register cleanup

[urnathan]

Summary

In working on the in-memory sret tail-calling problem, I ran into this. As I want to adjust this code for sret, I figured it best to get this change in first. It turns out that there are no paths into LowerFormalParms that have already specified the sret register. We always materialize a virtual and then assign it to the physical reg at the point of the return. Thus the 'if (!reg)' body is always reachable. Tested by (a) grepping for setSRetReturnReg calls (there are no others), and (b) building and testing with this assert added.

[rnk]

lgtm

llvm/lib/Target/X86/X86ISelLowering.cpp

for (unsigned I = 0, E = Ins.size(); I != E; ++I) {
     if (CallConv == CallingConv::Swift || CallConv == CallingConv::SwiftTail)
       continue;
 
     // All x86 ABIs require that for returning structs by value we copy the
     // sret argument into %rax/%eax (depending on ABI) for the return. Save
     // the argument into a virtual register so that we can access it from the
     // return points.
     if (Ins[I].Flags.isSRet()) {
-      Register Reg = FuncInfo->getSRetReturnReg();
-      if (!Reg) {
-        MVT PtrTy = getPointerTy(DAG.getDataLayout());
-        Reg = MF.getRegInfo().createVirtualRegister(getRegClassFor(PtrTy));
-        FuncInfo->setSRetReturnReg(Reg);
-      }
+      assert(!FuncInfo->getSRetReturnReg() &&
+             "SRet return has already been set");
+      Register Reg =
+          MF.getRegInfo().createVirtualRegister(getRegClassFor(PtrTy));
       SDValue Copy = DAG.getCopyToReg(DAG.getEntryNode(), dl, Reg, InVals[I]);
       Chain = DAG.getNode(ISD::TokenFactor, dl, MVT::Other, Copy, Chain);
       break;
     }
   }
 
   unsigned StackSize = CCInfo.getNextStackOffset();
   // Align stack specially for tail calls.