Fix some liveout handling related to tail calls, see comments.

I don't think this ever resulted in problems on x86, but it
would on ARM.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@105509 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/LiveVariables.cpp

@@ -609,7 +609,12 @@ bool LiveVariables::runOnMachineFunction(MachineFunction &mf) {
 
     // Finally, if the last instruction in the block is a return, make sure to
     // mark it as using all of the live-out values in the function.
-    if (!MBB->empty() && MBB->back().getDesc().isReturn()) {
+    // Things marked both call and return are tail calls; do not do this for
+    // them.  The tail callee need not take the same registers as input
+    // that it produces as output, and there are dependencies for its input
+    // registers elsewhere.
+    if (!MBB->empty() && MBB->back().getDesc().isReturn()
+        && !MBB->back().getDesc().isCall()) {
       MachineInstr *Ret = &MBB->back();
 
       for (MachineRegisterInfo::liveout_iterator

lib/Target/X86/X86ISelLowering.cpp

@@ -2150,17 +2150,12 @@ X86TargetLowering::LowerCall(SDValue Chain, SDValue Callee,
     Ops.push_back(InFlag);
 
   if (isTailCall) {
-    // If this is the first return lowered for this function, add the regs
-    // to the liveout set for the function.
-    if (MF.getRegInfo().liveout_empty()) {
-      SmallVector<CCValAssign, 16> RVLocs;
-      CCState CCInfo(CallConv, isVarArg, getTargetMachine(), RVLocs,
-                     *DAG.getContext());
-      CCInfo.AnalyzeCallResult(Ins, RetCC_X86);
-      for (unsigned i = 0; i != RVLocs.size(); ++i)
-        if (RVLocs[i].isRegLoc())
-          MF.getRegInfo().addLiveOut(RVLocs[i].getLocReg());
-    }
+    // We used to do:
+    //// If this is the first return lowered for this function, add the regs
+    //// to the liveout set for the function.
+    // This isn't right, although it's probably harmless on x86; liveouts
+    // should be computed from returns not tail calls.  Consider a void
+    // function making a tail call to a function returning int.
     return DAG.getNode(X86ISD::TC_RETURN, dl,
                        NodeTys, &Ops[0], Ops.size());
   }