[RISCV] Move vmerge same mask peephole to RISCVVectorPeephole

We currently fold a vmerge.vvm into its true operand if the true operand is a masked pseudo with the same mask.

We can move this over to RISCVVectorPeephole by instead splitting it up into a smaller peephole which converts it to a vmv.v.v first. The existing foldVMV_V_V peephole will then take care of folding it if needed.

This is very similar to the existing all-ones mask peephole and we could potentially do it inside of it. I opted to put it in a separate peephole to make it easier to reason about, given that the duplication is small, but I could be persuaded either way.

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -3833,15 +3833,8 @@ bool RISCVDAGToDAGISel::performCombineVMergeAndVOps(SDNode *N) {
   uint64_t TrueTSFlags = TrueMCID.TSFlags;
   bool HasTiedDest = RISCVII::isFirstDefTiedToFirstUse(TrueMCID);
 
-  bool IsMasked = false;
   const RISCV::RISCVMaskedPseudoInfo *Info =
       RISCV::lookupMaskedIntrinsicByUnmasked(TrueOpc);
-  if (!Info && HasTiedDest) {
-    Info = RISCV::getMaskedPseudoInfo(TrueOpc);
-    IsMasked = true;
-  }
-  assert(!(IsMasked && !HasTiedDest) && "Expected tied dest");
-
   if (!Info)
     return false;
 
@@ -3853,19 +3846,6 @@ bool RISCVDAGToDAGISel::performCombineVMergeAndVOps(SDNode *N) {
       return false;
   }
 
-  // If True is masked then the vmerge must have either the same mask or an all
-  // 1s mask, since we're going to keep the mask from True.
-  if (IsMasked) {
-    // FIXME: Support mask agnostic True instruction which would have an
-    // undef passthru operand.
-    SDValue TrueMask =
-        getMaskSetter(True->getOperand(Info->MaskOpIdx),
-                      True->getOperand(True->getNumOperands() - 1));
-    assert(TrueMask);
-    if (!usesAllOnesMask(Mask, Glue) && getMaskSetter(Mask, Glue) != TrueMask)
-      return false;
-  }
-
   // Skip if True has side effect.
   if (TII->get(TrueOpc).hasUnmodeledSideEffects())
     return false;
@@ -3930,24 +3910,13 @@ bool RISCVDAGToDAGISel::performCombineVMergeAndVOps(SDNode *N) {
       (Mask && !usesAllOnesMask(Mask, Glue)))
     return false;
 
-  // If we end up changing the VL or mask of True, then we need to make sure it
-  // doesn't raise any observable fp exceptions, since changing the active
-  // elements will affect how fflags is set.
-  if (TrueVL != VL || !IsMasked)
-    if (mayRaiseFPException(True.getNode()) &&
-        !True->getFlags().hasNoFPExcept())
-      return false;
+  // Make sure it doesn't raise any observable fp exceptions, since changing the
+  // active elements will affect how fflags is set.
+  if (mayRaiseFPException(True.getNode()) && !True->getFlags().hasNoFPExcept())
+    return false;
 
   SDLoc DL(N);
 
-  // From the preconditions we checked above, we know the mask and thus glue
-  // for the result node will be taken from True.
-  if (IsMasked) {
-    Mask = True->getOperand(Info->MaskOpIdx);
-    Glue = True->getOperand(True->getNumOperands() - 1);
-    assert(Glue.getValueType() == MVT::Glue);
-  }
-
   unsigned MaskedOpc = Info->MaskedPseudo;
 #ifndef NDEBUG
   const MCInstrDesc &MaskedMCID = TII->get(MaskedOpc);
@@ -3977,8 +3946,7 @@ bool RISCVDAGToDAGISel::performCombineVMergeAndVOps(SDNode *N) {
   Ops.push_back(False);
 
   const bool HasRoundingMode = RISCVII::hasRoundModeOp(TrueTSFlags);
-  const unsigned NormalOpsEnd = TrueVLIndex - IsMasked - HasRoundingMode;
-  assert(!IsMasked || NormalOpsEnd == Info->MaskOpIdx);
+  const unsigned NormalOpsEnd = TrueVLIndex - HasRoundingMode;
   Ops.append(True->op_begin() + HasTiedDest, True->op_begin() + NormalOpsEnd);
 
   Ops.push_back(Mask);

llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp

@@ -65,7 +65,8 @@ class RISCVVectorPeephole : public MachineFunctionPass {
   bool convertToVLMAX(MachineInstr &MI) const;
   bool convertToWholeRegister(MachineInstr &MI) const;
   bool convertToUnmasked(MachineInstr &MI) const;
-  bool convertVMergeToVMv(MachineInstr &MI) const;
+  bool convertAllOnesVMergeToVMv(MachineInstr &MI) const;
+  bool convertSameMaskVMergeToVMv(MachineInstr &MI) const;
   bool foldUndefPassthruVMV_V_V(MachineInstr &MI);
   bool foldVMV_V_V(MachineInstr &MI);
 
@@ -342,17 +343,14 @@ bool RISCVVectorPeephole::convertToWholeRegister(MachineInstr &MI) const {
   return true;
 }
 
-// Transform (VMERGE_VVM_<LMUL> pt, false, true, allones, vl, sew) to
-// (VMV_V_V_<LMUL> pt, true, vl, sew). It may decrease uses of VMSET.
-bool RISCVVectorPeephole::convertVMergeToVMv(MachineInstr &MI) const {
+static unsigned getVMV_V_VOpcodeForVMERGE_VVM(const MachineInstr &MI) {
 #define CASE_VMERGE_TO_VMV(lmul)                                               \
   case RISCV::PseudoVMERGE_VVM_##lmul:                                         \
-    NewOpc = RISCV::PseudoVMV_V_V_##lmul;                                      \
+    return RISCV::PseudoVMV_V_V_##lmul;                                        \
     break;
-  unsigned NewOpc;
   switch (MI.getOpcode()) {
   default:
-    return false;
+    return 0;
     CASE_VMERGE_TO_VMV(MF8)
     CASE_VMERGE_TO_VMV(MF4)
     CASE_VMERGE_TO_VMV(MF2)
@@ -361,14 +359,68 @@ bool RISCVVectorPeephole::convertVMergeToVMv(MachineInstr &MI) const {
     CASE_VMERGE_TO_VMV(M4)
     CASE_VMERGE_TO_VMV(M8)
   }
+}
 
+/// Convert a PseudoVMERGE_VVM with an all ones mask to a PseudoVMV_V_V.
+///
+/// %x = PseudoVMERGE_VVM %passthru, %false, %true, %allones, sew, vl
+/// ->
+/// %x = PseudoVMV_V_V %passthru, %true, vl, sew, tu_mu
+bool RISCVVectorPeephole::convertAllOnesVMergeToVMv(MachineInstr &MI) const {
+  unsigned NewOpc = getVMV_V_VOpcodeForVMERGE_VVM(MI);
+  if (!NewOpc)
+    return false;
   assert(MI.getOperand(4).isReg() && MI.getOperand(4).getReg() == RISCV::V0);
   if (!isAllOnesMask(V0Defs.lookup(&MI)))
     return false;
 
   MI.setDesc(TII->get(NewOpc));
-  MI.removeOperand(2);  // False operand
-  MI.removeOperand(3);  // Mask operand
+  MI.removeOperand(2); // False operand
+  MI.removeOperand(3); // Mask operand
+  MI.addOperand(
+      MachineOperand::CreateImm(RISCVII::TAIL_UNDISTURBED_MASK_UNDISTURBED));
+
+  // vmv.v.v doesn't have a mask operand, so we may be able to inflate the
+  // register class for the destination and passthru operands e.g. VRNoV0 -> VR
+  MRI->recomputeRegClass(MI.getOperand(0).getReg());
+  if (MI.getOperand(1).getReg() != RISCV::NoRegister)
+    MRI->recomputeRegClass(MI.getOperand(1).getReg());
+  return true;
+}
+
+/// If a PseudoVMERGE_VVM's true operand is a masked pseudo and both have the
+/// same mask, and the masked pseudo's passthru is the same as the false
+/// operand, we can convert the PseudoVMERGE_VVM to a PseudoVMV_V_V.
+///
+/// %true = PseudoVADD_VV_M1_MASK %false, %x, %y, %mask, vl1, sew, policy
+/// %x = PseudoVMERGE_VVM %passthru, %false, %true, %mask, vl2, sew
+/// ->
+/// %true = PseudoVADD_VV_M1_MASK %false, %x, %y, %mask, vl1, sew, policy
+/// %x = PseudoVMV_V_V %passthru, %true, vl2, sew, tu_mu
+bool RISCVVectorPeephole::convertSameMaskVMergeToVMv(MachineInstr &MI) const {
+  unsigned NewOpc = getVMV_V_VOpcodeForVMERGE_VVM(MI);
+  if (!NewOpc)
+    return false;
+  MachineInstr *True = MRI->getVRegDef(MI.getOperand(3).getReg());
+  if (!True || !RISCV::getMaskedPseudoInfo(True->getOpcode()) ||
+      !hasSameEEW(MI, *True))
+    return false;
+
+  // True's passthru needs to be equivalent to False
+  Register TruePassthruReg = True->getOperand(1).getReg();
+  Register FalseReg = MI.getOperand(2).getReg();
+  if (TruePassthruReg != RISCV::NoRegister && TruePassthruReg != FalseReg)
+    return false;
+
+  const MachineInstr *TrueV0Def = V0Defs.lookup(True);
+  const MachineInstr *MIV0Def = V0Defs.lookup(&MI);
+  assert(TrueV0Def->isCopy() && MIV0Def->isCopy());
+  if (TrueV0Def->getOperand(1).getReg() != MIV0Def->getOperand(1).getReg())
+    return false;
+
+  MI.setDesc(TII->get(NewOpc));
+  MI.removeOperand(2); // False operand
+  MI.removeOperand(3); // Mask operand
   MI.addOperand(
       MachineOperand::CreateImm(RISCVII::TAIL_UNDISTURBED_MASK_UNDISTURBED));
 
@@ -622,7 +674,8 @@ bool RISCVVectorPeephole::runOnMachineFunction(MachineFunction &MF) {
       Changed |= tryToReduceVL(MI);
       Changed |= convertToUnmasked(MI);
       Changed |= convertToWholeRegister(MI);
-      Changed |= convertVMergeToVMv(MI);
+      Changed |= convertAllOnesVMergeToVMv(MI);
+      Changed |= convertSameMaskVMergeToVMv(MI);
       if (foldUndefPassthruVMV_V_V(MI)) {
         Changed |= true;
         continue; // MI is erased

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-strided-load-store-asm.ll

@@ -62,12 +62,11 @@ define void @gather_masked(ptr noalias nocapture %A, ptr noalias nocapture reado
 ; CHECK-NEXT:    li a4, 5
 ; CHECK-NEXT:  .LBB1_1: # %vector.body
 ; CHECK-NEXT:    # =>This Inner Loop Header: Depth=1
-; CHECK-NEXT:    vmv1r.v v9, v8
-; CHECK-NEXT:    vsetvli zero, a3, e8, m1, ta, mu
-; CHECK-NEXT:    vlse8.v v9, (a1), a4, v0.t
-; CHECK-NEXT:    vle8.v v10, (a0)
-; CHECK-NEXT:    vadd.vv v9, v10, v9
-; CHECK-NEXT:    vse8.v v9, (a0)
+; CHECK-NEXT:    vsetvli zero, a3, e8, m1, ta, ma
+; CHECK-NEXT:    vlse8.v v8, (a1), a4, v0.t
+; CHECK-NEXT:    vle8.v v9, (a0)
+; CHECK-NEXT:    vadd.vv v8, v9, v8
+; CHECK-NEXT:    vse8.v v8, (a0)
 ; CHECK-NEXT:    addi a0, a0, 32
 ; CHECK-NEXT:    addi a1, a1, 160
 ; CHECK-NEXT:    bne a0, a2, .LBB1_1
@@ -344,12 +343,11 @@ define void @scatter_masked(ptr noalias nocapture %A, ptr noalias nocapture read
 ; CHECK-NEXT:    li a4, 5
 ; CHECK-NEXT:  .LBB7_1: # %vector.body
 ; CHECK-NEXT:    # =>This Inner Loop Header: Depth=1
-; CHECK-NEXT:    vsetvli zero, a3, e8, m1, ta, mu
-; CHECK-NEXT:    vle8.v v9, (a1)
-; CHECK-NEXT:    vmv1r.v v10, v8
-; CHECK-NEXT:    vlse8.v v10, (a0), a4, v0.t
-; CHECK-NEXT:    vadd.vv v9, v10, v9
-; CHECK-NEXT:    vsse8.v v9, (a0), a4, v0.t
+; CHECK-NEXT:    vsetvli zero, a3, e8, m1, ta, ma
+; CHECK-NEXT:    vle8.v v8, (a1)
+; CHECK-NEXT:    vlse8.v v9, (a0), a4, v0.t
+; CHECK-NEXT:    vadd.vv v8, v9, v8
+; CHECK-NEXT:    vsse8.v v8, (a0), a4, v0.t
 ; CHECK-NEXT:    addi a1, a1, 32
 ; CHECK-NEXT:    addi a0, a0, 160
 ; CHECK-NEXT:    bne a1, a2, .LBB7_1

llvm/test/CodeGen/RISCV/rvv/rvv-peephole-vmerge-to-vmv.mir

@@ -76,13 +76,13 @@ body: |
     ; CHECK-LABEL: name: same_mask
     ; CHECK: liveins: $v8, $v9, $v0
     ; CHECK-NEXT: {{  $}}
-    ; CHECK-NEXT: %pt:vrnov0 = COPY $v8
+    ; CHECK-NEXT: %pt:vr = COPY $v8
     ; CHECK-NEXT: %false:vrnov0 = COPY $v9
     ; CHECK-NEXT: %mask:vr = COPY $v0
     ; CHECK-NEXT: $v0 = COPY %mask
     ; CHECK-NEXT: %true:vrnov0 = PseudoVADD_VV_M1_MASK %false, $noreg, $noreg, $v0, 4, 5 /* e32 */, 0 /* tu, mu */
     ; CHECK-NEXT: $v0 = COPY %mask
-    ; CHECK-NEXT: %x:vrnov0 = PseudoVMERGE_VVM_M1 %pt, %false, %true, $v0, 8, 5 /* e32 */
+    ; CHECK-NEXT: %x:vr = PseudoVMV_V_V_M1 %pt, %true, 8, 5 /* e32 */, 0 /* tu, mu */
     %pt:vrnov0 = COPY $v8
     %false:vrnov0 = COPY $v9
     %mask:vr = COPY $v0