[llvm] [RISCV] Print using dummy_reg_pair_with_x0 in more places. (PR #141273)

[Craig Topper via llvm-commits]

https://github.com/topperc created https://github.com/llvm/llvm-project/pull/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 even 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.

>From 7072094c7a8dfd2d9f27f2af56c9032eb68b50eb Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper at sifive.com>
Date: Fri, 23 May 2025 11:18:59 -0700
Subject: [PATCH] [RISCV] Print using dummy_reg_pair_with_x0 in more places.

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 even 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 decompose
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.
---
 llvm/lib/Target/RISCV/RISCVExpandPseudoInsts.cpp |  3 +++
 llvm/lib/Target/RISCV/RISCVInstrInfo.cpp         | 13 +++++++++----
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Target/RISCV/RISCVExpandPseudoInsts.cpp b/llvm/lib/Target/RISCV/RISCVExpandPseudoInsts.cpp
index 4d26f77d4ed2c..d3dce4edb1e75 100644
--- a/llvm/lib/Target/RISCV/RISCVExpandPseudoInsts.cpp
+++ b/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;
 
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
index 590b45172d43c..6367c8c6d04a0 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
+++ b/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;
   }