[mips][FastISel] Fix call lowering by bailing out on "fastcc" calls.

[vkalintiris]

Summary

Currently, we support only the MIPS O32 ABI calling convention for call
lowering. With this change we avoid using the O32 calling convetion for
lowering calls marked as using the fast calling convention.

[dsanders]

LGTM with a small test case.

The test case should contain a fastcc call and should just check that it falls back on SelectionDAG.

lib/Target/Mips/MipsFastISel.cpp

 
 bool MipsFastISel::fastLowerCall(CallLoweringInfo &CLI) {
   CallingConv::ID CC = CLI.CallConv;
   bool IsTailCall = CLI.IsTailCall;
   bool IsVarArg = CLI.IsVarArg;
   const Value *Callee = CLI.Callee;
   MCSymbol *Symbol = CLI.Symbol;
 
+  // Do not handle FastCC.
+  if (CC == CallingConv::Fast)
+    return false;
+
   // Allow SelectionDAG isel to handle tail calls.
   if (IsTailCall)
     return false;
 
   // Let SDISel handle vararg functions.
   if (IsVarArg)
     return false;
 
bool MipsFastISel::selectRet(const Instruction *I) {
   if (!FuncInfo.CanLowerReturn)
     return false;
 
   // Build a list of return value registers.
   SmallVector<unsigned, 4> RetRegs;
 
   if (Ret->getNumOperands() > 0) {
     CallingConv::ID CC = F.getCallingConv();
+
+    // Do not handle FastCC.
+    if (CC == CallingConv::Fast)
+      return false;
+
     SmallVector<ISD::OutputArg, 4> Outs;
     GetReturnInfo(F.getReturnType(), F.getAttributes(), Outs, TLI, DL);
 
     // Analyze operands of the call, assigning locations to each operand.
     SmallVector<CCValAssign, 16> ValLocs;
     MipsCCState CCInfo(CC, F.isVarArg(), *FuncInfo.MF, ValLocs,
                        I->getContext());
     CCAssignFn *RetCC = RetCC_Mips;