x86: Don't mark live-in callee-saved registers as killed

[Zoxc]

Summary

This makes it possible to use callee-saved registers as arguments to a function. I have a custom calling convention on x64 which does this (it's the System V ABI with RBX added in first place to the allowed registers).

Should such an calling convention be added to LLVM so that we can test for callee-saved register being live-in?

lib/Target/X86/X86FrameLowering.cpp

bool X86FrameLowering::spillCalleeSavedRegisters(
 
   // Push GPRs. It increases frame size.
   unsigned Opc = STI.is64Bit() ? X86::PUSH64r : X86::PUSH32r;
   for (unsigned i = CSI.size(); i != 0; --i) {
     unsigned Reg = CSI[i - 1].getReg();
 
     if (!X86::GR64RegClass.contains(Reg) && !X86::GR32RegClass.contains(Reg))
       continue;
+
+    auto IsLive = MBB.isLiveIn(Reg);
+
+    if (!IsLive) {
       // Add the callee-saved register as live-in. It's killed at the spill.
       MBB.addLiveIn(Reg);
+    }
 
-    BuildMI(MBB, MI, DL, TII.get(Opc)).addReg(Reg, RegState::Kill)
+    // Kill the register if it wasn't live previously.
+    BuildMI(MBB, MI, DL, TII.get(Opc)).addReg(Reg, IsLive ? 0 : RegState::Kill)
       .setMIFlag(MachineInstr::FrameSetup);
   }
 
   // Make XMM regs spilled. X86 does not have ability of push/pop XMM.
   // It can be done by spilling XMMs to stack frame.
   for (unsigned i = CSI.size(); i != 0; --i) {
     unsigned Reg = CSI[i-1].getReg();
     if (X86::GR64RegClass.contains(Reg) || X86::GR32RegClass.contains(Reg))