[RISCV] Implement RISCVInsrInfo::getConstValDefinedInReg

This helper function handles common cases where we can determine a
constant value is being defined in a register. Although it looks like
codegen changes are possible due to this being called in
PeepholeOptimizer, my main motivation is to use this in
describeLoadedValue.

Author: asb

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

@@ -1580,6 +1580,12 @@ RISCVInstrInfo::isCopyInstrImpl(const MachineInstr &MI) const {
   switch (MI.getOpcode()) {
   default:
     break;
+  case RISCV::ADD:
+    if (MI.getOperand(1).isReg() && MI.getOperand(1).getReg() == RISCV::X0)
+      return DestSourcePair{MI.getOperand(0), MI.getOperand(1)};
+    if (MI.getOperand(2).isReg() && MI.getOperand(2).getReg() == RISCV::X0)
+      return DestSourcePair{MI.getOperand(0), MI.getOperand(2)};
+    break;
   case RISCV::ADDI:
     // Operand 1 can be a frameindex but callers expect registers
     if (MI.getOperand(1).isReg() && MI.getOperand(2).isImm() &&
@@ -2555,6 +2561,33 @@ std::optional<RegImmPair> RISCVInstrInfo::isAddImmediate(const MachineInstr &MI,
   return std::nullopt;
 }
 
+bool RISCVInstrInfo::getConstValDefinedInReg(const MachineInstr &MI,
+                                             const Register Reg,
+                                             int64_t &ImmVal) const {
+  // Handle moves of X0.
+  if (auto DestSrc = isCopyInstr(MI)) {
+    if (DestSrc->Source->getReg() != RISCV::X0)
+      return false;
+    const Register DstReg = DestSrc->Destination->getReg();
+    if (DstReg != Reg)
+      return false;
+    ImmVal = 0;
+    return true;
+  }
+
+  if (!(MI.getOpcode() == RISCV::ADDI || MI.getOpcode() == RISCV::ADDIW ||
+        MI.getOpcode() == RISCV::ORI))
+    return false;
+  if (MI.getOperand(0).getReg() != Reg)
+    return false;
+  if (!MI.getOperand(1).isReg() || MI.getOperand(1).getReg() != RISCV::X0)
+    return false;
+  if (!MI.getOperand(2).isImm())
+    return false;
+  ImmVal = MI.getOperand(2).getImm();
+  return true;
+}
+
 // MIR printer helper function to annotate Operands with a comment.
 std::string RISCVInstrInfo::createMIROperandComment(
     const MachineInstr &MI, const MachineOperand &Op, unsigned OpIdx,

llvm/lib/Target/RISCV/RISCVInstrInfo.h

@@ -212,6 +212,9 @@ class RISCVInstrInfo : public RISCVGenInstrInfo {
   std::optional<RegImmPair> isAddImmediate(const MachineInstr &MI,
                                            Register Reg) const override;
 
+  bool getConstValDefinedInReg(const MachineInstr &MI, const Register Reg,
+                               int64_t &ImmVal) const override;
+
   bool findCommutedOpIndices(const MachineInstr &MI, unsigned &SrcOpIdx1,
                              unsigned &SrcOpIdx2) const override;
   MachineInstr *commuteInstructionImpl(MachineInstr &MI, bool NewMI,

llvm/unittests/Target/RISCV/RISCVInstrInfoTest.cpp

@@ -94,6 +94,52 @@ TEST_P(RISCVInstrInfoTest, IsAddImmediate) {
   }
 }
 
+TEST_P(RISCVInstrInfoTest, GetConstValDefinedInReg) {
+  const RISCVInstrInfo *TII = ST->getInstrInfo();
+  DebugLoc DL;
+  int64_t ImmVal;
+
+  auto *MI1 = BuildMI(*MF, DL, TII->get(RISCV::ADD), RISCV::X1)
+                  .addReg(RISCV::X2)
+                  .addReg(RISCV::X3)
+                  .getInstr();
+  EXPECT_FALSE(TII->getConstValDefinedInReg(*MI1, RISCV::X1, ImmVal));
+
+  auto *MI2 = BuildMI(*MF, DL, TII->get(RISCV::ADDI), RISCV::X1)
+                  .addReg(RISCV::X0)
+                  .addImm(-128)
+                  .getInstr();
+  EXPECT_FALSE(TII->getConstValDefinedInReg(*MI2, RISCV::X0, ImmVal));
+  ASSERT_TRUE(TII->getConstValDefinedInReg(*MI2, RISCV::X1, ImmVal));
+  EXPECT_EQ(ImmVal, -128);
+
+  auto *MI3 = BuildMI(*MF, DL, TII->get(RISCV::ORI), RISCV::X2)
+                  .addReg(RISCV::X0)
+                  .addImm(1024)
+                  .getInstr();
+  EXPECT_FALSE(TII->getConstValDefinedInReg(*MI3, RISCV::X0, ImmVal));
+  ASSERT_TRUE(TII->getConstValDefinedInReg(*MI3, RISCV::X2, ImmVal));
+  EXPECT_EQ(ImmVal, 1024);
+
+  if (ST->is64Bit()) {
+    auto *MI4 = BuildMI(*MF, DL, TII->get(RISCV::ADDIW), RISCV::X2)
+                    .addReg(RISCV::X0)
+                    .addImm(512)
+                    .getInstr();
+    EXPECT_FALSE(TII->getConstValDefinedInReg(*MI4, RISCV::X0, ImmVal));
+    ASSERT_TRUE(TII->getConstValDefinedInReg(*MI4, RISCV::X2, ImmVal));
+    EXPECT_EQ(ImmVal, 512);
+  }
+
+  auto *MI5 = BuildMI(*MF, DL, TII->get(RISCV::ADD), RISCV::X1)
+                  .addReg(RISCV::X0)
+                  .addReg(RISCV::X0)
+                  .getInstr();
+  EXPECT_FALSE(TII->getConstValDefinedInReg(*MI5, RISCV::X0, ImmVal));
+  ASSERT_TRUE(TII->getConstValDefinedInReg(*MI5, RISCV::X1, ImmVal));
+  EXPECT_EQ(ImmVal, 0);
+}
+
 TEST_P(RISCVInstrInfoTest, GetMemOperandsWithOffsetWidth) {
   const RISCVInstrInfo *TII = ST->getInstrInfo();
   const TargetRegisterInfo *TRI = ST->getRegisterInfo();