[RISCV] Prevent using dummy_reg_pair_with_x0 in more places. (#141273)

Similar to #141261.

These aren't easy to test without write MIR tests in areas we don't
currently have tests. I'm not sure we use X0_Pair anywhere today.

I'm going to try to migrate RISCVMakeCompressible to use copyToReg so we
can share that code instead of basically duplicating it.

I'm still concerned that target independent code may fold an
extract_subreg operation and get an incorrect register if we do start
using X0_Pair. We may have to special case dummy_reg_pair_with_x0 in the
encoder and printer to be safe.

Author: topperc

llvm/lib/Target/RISCV/RISCVExpandPseudoInsts.cpp

@@ -323,6 +323,8 @@ bool RISCVExpandPseudo::expandRV32ZdinxStore(MachineBasicBlock &MBB,
       TRI->getSubReg(MBBI->getOperand(0).getReg(), RISCV::sub_gpr_even);
   Register Hi =
       TRI->getSubReg(MBBI->getOperand(0).getReg(), RISCV::sub_gpr_odd);
+  if (Hi == RISCV::DUMMY_REG_PAIR_WITH_X0)
+    Hi = RISCV::X0;
 
   auto MIBLo = BuildMI(MBB, MBBI, DL, TII->get(RISCV::SW))
                    .addReg(Lo, getKillRegState(MBBI->getOperand(0).isKill()))
@@ -370,6 +372,7 @@ bool RISCVExpandPseudo::expandRV32ZdinxLoad(MachineBasicBlock &MBB,
       TRI->getSubReg(MBBI->getOperand(0).getReg(), RISCV::sub_gpr_even);
   Register Hi =
       TRI->getSubReg(MBBI->getOperand(0).getReg(), RISCV::sub_gpr_odd);
+  assert(Hi != RISCV::DUMMY_REG_PAIR_WITH_X0 && "Cannot write to X0_Pair");
 
   MachineInstrBuilder MIBLo, MIBHi;
 

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

@@ -543,16 +543,21 @@ void RISCVInstrInfo::copyPhysReg(MachineBasicBlock &MBB,
   }
 
   if (RISCV::GPRPairRegClass.contains(DstReg, SrcReg)) {
+    MCRegister EvenReg = TRI->getSubReg(SrcReg, RISCV::sub_gpr_even);
+    MCRegister OddReg = TRI->getSubReg(SrcReg, RISCV::sub_gpr_odd);
+    // We need to correct the odd register of X0_Pair.
+    if (OddReg == RISCV::DUMMY_REG_PAIR_WITH_X0)
+      OddReg == RISCV::X0;
+    assert(DstReg != RISCV::X0_Pair && "Cannot write to X0_Pair");
+
     // Emit an ADDI for both parts of GPRPair.
     BuildMI(MBB, MBBI, DL, get(RISCV::ADDI),
             TRI->getSubReg(DstReg, RISCV::sub_gpr_even))
-        .addReg(TRI->getSubReg(SrcReg, RISCV::sub_gpr_even),
-                getKillRegState(KillSrc))
+        .addReg(EvenReg, getKillRegState(KillSrc))
         .addImm(0);
     BuildMI(MBB, MBBI, DL, get(RISCV::ADDI),
             TRI->getSubReg(DstReg, RISCV::sub_gpr_odd))
-        .addReg(TRI->getSubReg(SrcReg, RISCV::sub_gpr_odd),
-                getKillRegState(KillSrc))
+        .addReg(OddReg, getKillRegState(KillSrc))
         .addImm(0);
     return;
   }