[RISCV] Use X0 as destination of inserted vsetvli when possible.

[craig.topper]

Summary

We aren't going to connect the result to anything so we might
as well avoid allocating a register.

[HsiangKai]

LGTM.

[HsiangKai]

To separate it from D101938, is it because the modification changes lots of test files?

[craig.topper]

In D102031#2743537, @HsiangKai wrote:

To separate it from D101938, is it because the modification changes lots of test files?

Yes. I was trying to make the test changes from D101938 easy to see.

[frasercrmck]

LGTM too. What a lot of changes.

[craig.topper]

Rebase on new insertion pass

[jrtc27]

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

188

Use the existing AVLReg variable rather than repeating the call?

[craig.topper]

Address review comment.

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

if (VLOp.isImm()) {
     int64_t Imm = VLOp.getImm();
 
     // If we have a previous VSETIVLI, we don't need to insert one. The VTYPE
     // was checked earlier, so just compare the immediate.
     if (PrevVSETVLI && PrevVSETVLI->getOpcode() == RISCV::PseudoVSETIVLI &&
         PrevVSETVLI->getOperand(1).getImm() == Imm)
       return false;
 
-    // TODO: Use X0 as the destination.
-    Register DestReg = MRI->createVirtualRegister(&RISCV::GPRRegClass);
     auto MIB = BuildMI(MBB, MII, DL, TII->get(RISCV::PseudoVSETIVLI))
-                   .addReg(DestReg, RegState::Define | RegState::Dead)
+                   .addReg(RISCV::X0, RegState::Define | RegState::Dead)
                    .addImm(Imm)
                    .addImm(VTypeImm);
     PrevVSETVLI = &*MIB;
 
     return true;
   }
 
   // Otherwise we expect VL to be a GPR.
if (AVLReg.isVirtual() && AVLReg == PrevOutVL)
       return false;
 
     if (PrevVSETVLI->getOpcode() == RISCV::PseudoVSETVLI &&
         AVLReg == PrevVSETVLI->getOperand(1).getReg() &&
         (AVLReg != RISCV::X0 || PrevOutVL != RISCV::X0))
       return false;
 
     return true;
   };
 
   if (needVSETVLI(PrevVSETVLI, AVLReg)) {
-    // TODO: Use X0 as the destination if AVLReg is not X0.
-    Register DestReg = MRI->createVirtualRegister(&RISCV::GPRRegClass);
+    // Use X0 as the DestReg unless AVLReg is X0.
+    Register DestReg = RISCV::X0;
+    if (AVLReg == RISCV::X0)
+      DestReg = MRI->createVirtualRegister(&RISCV::GPRRegClass);
     auto MIB = BuildMI(MBB, MII, DL, TII->get(RISCV::PseudoVSETVLI))
                    .addReg(DestReg, RegState::Define | RegState::Dead)
                    .addReg(AVLReg)
                    .addImm(VTypeImm);
     PrevVSETVLI = &*MIB;
   }
 
   // Clear the GPR, the we don't need it anymore