[X86][AArch64][NFC] Simplify querying used argument registers

[void]

Summary

Registers used for arguments are listed as "live-ins" into the starting
basic block. This means we don't have to go through a potentially
expensive search through all possible argument registers when we only
care about used argument registers.

[nickdesaulniers]

llvm/lib/CodeGen/PrologEpilogInserter.cpp

1211

RegisterMaskPair

1230–1234

This could probably be 2 statements rather than 5.

if ((OnlyUsed && !LiveIns[Reg]) || !TRI.isArgumentRegister(MF, Reg))
  continue;

[void]

llvm/lib/CodeGen/PrologEpilogInserter.cpp

1211

What?

1230–1234

That doesn't result in the same logic. If OnlyUsed is true and !LiveIns[Reg] is false, then it will execute the TRI.isArgumentRegister call, which is what I want it to avoid in that situation.

[nickdesaulniers]

llvm/lib/CodeGen/PrologEpilogInserter.cpp

1211

I think the type of LI here is const RegisterMaskPair &.

1230–1234

Ack...oops!

[void]

llvm/lib/CodeGen/PrologEpilogInserter.cpp

1211

Changed to MachineBasicBlock::RegisterMaskPair.

llvm/lib/CodeGen/PrologEpilogInserter.cpp

for (const MachineBasicBlock &MBB : MF)
             continue;
 
           MCRegister Reg = MO.getReg();
           if (AllocatableSet[Reg] && !MO.isImplicit() &&
               (MO.isDef() || MO.isUse()))
             UsedRegs.set(Reg);
         }
 
+  // Get a list of registers that are used.
+  BitVector LiveIns(TRI.getNumRegs());
+  for (const MachineBasicBlock::RegisterMaskPair &LI : MF.front().liveins())
+    LiveIns.set(LI.PhysReg);
+
   BitVector RegsToZero(TRI.getNumRegs());
   for (MCRegister Reg : AllocatableSet.set_bits()) {
     // Skip over fixed registers.
     if (TRI.isFixedRegister(MF, Reg))
       continue;
 
     // Want only general purpose registers.
     if (OnlyGPR && !TRI.isGeneralPurposeRegister(MF, Reg))
       continue;
 
     // Want only used registers.
     if (OnlyUsed && !UsedRegs[Reg])
       continue;
 
     // Want only registers used for arguments.
-    if (OnlyArg && !TRI.isArgumentRegister(MF, Reg))
+    if (OnlyArg) {
+      if (OnlyUsed) {
+        if (!LiveIns[Reg])
+          continue;
+      } else if (!TRI.isArgumentRegister(MF, Reg)) {
         continue;
+      }
+    }
 
     RegsToZero.set(Reg);
   }
 
   // Don't clear registers that are live when leaving the function.
   for (const MachineBasicBlock &MBB : MF)
     for (const MachineInstr &MI : MBB.terminators()) {
       if (!MI.isReturn())