[RISCV][RVV] Select unmasked TU RVV pseudos in a DAG post-process

Following D118810 that reduced the size of ISel table,
this patch optimizes allone-masked RVV pseudos with TU policy and
swap them out to their unmasked TU pseudos.

Since the UNDEF merge operand is not preserved, we turn it into TA
pseudo regardless of the policy operand.

Reviewed By: craig.topper, frasercrmck
Differential Revision: https://reviews.llvm.org/D121881

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -2265,33 +2265,52 @@ bool RISCVDAGToDAGISel::doPeepholeMaskedRVV(SDNode *N) {
 
   const MCInstrDesc &MaskedMCID = TII->get(N->getMachineOpcode());
 
+  bool IsTA = true;
   if (RISCVII::hasVecPolicyOp(MaskedMCID.TSFlags)) {
-    // The last operand of the pseudo is the policy op, but we're expecting a
-    // Glue operand last. We may also have a chain.
+    // The last operand of the pseudo is the policy op, but we might have a
+    // Glue operand last. We might also have a chain.
     TailPolicyOpIdx = N->getNumOperands() - 1;
     if (N->getOperand(*TailPolicyOpIdx).getValueType() == MVT::Glue)
       (*TailPolicyOpIdx)--;
     if (N->getOperand(*TailPolicyOpIdx).getValueType() == MVT::Other)
       (*TailPolicyOpIdx)--;
 
-    // If the policy isn't TAIL_AGNOSTIC we can't perform this optimization.
-    if (N->getConstantOperandVal(*TailPolicyOpIdx) != RISCVII::TAIL_AGNOSTIC)
-      return false;
+    if (!(N->getConstantOperandVal(*TailPolicyOpIdx) &
+          RISCVII::TAIL_AGNOSTIC)) {
+      // Keep the true-masked instruction when there is no unmasked TU
+      // instruction
+      if (I->UnmaskedTUPseudo == I->MaskedPseudo && !N->getOperand(0).isUndef())
+        return false;
+      // We can't use TA if the tie-operand is not IMPLICIT_DEF
+      if (!N->getOperand(0).isUndef())
+        IsTA = false;
+    }
   }
 
-  const MCInstrDesc &UnmaskedMCID = TII->get(I->UnmaskedPseudo);
+  if (IsTA) {
+    uint64_t TSFlags = TII->get(I->UnmaskedPseudo).TSFlags;
 
-  // Check that we're dropping the merge operand, the mask operand, and any
-  // policy operand when we transform to this unmasked pseudo.
-  assert(!RISCVII::hasMergeOp(UnmaskedMCID.TSFlags) &&
-         RISCVII::hasDummyMaskOp(UnmaskedMCID.TSFlags) &&
-         !RISCVII::hasVecPolicyOp(UnmaskedMCID.TSFlags) &&
-         "Unexpected pseudo to transform to");
-  (void)UnmaskedMCID;
+    // Check that we're dropping the merge operand, the mask operand, and any
+    // policy operand when we transform to this unmasked pseudo.
+    assert(!RISCVII::hasMergeOp(TSFlags) && RISCVII::hasDummyMaskOp(TSFlags) &&
+           !RISCVII::hasVecPolicyOp(TSFlags) &&
+           "Unexpected pseudo to transform to");
+    (void)TSFlags;
+  } else {
+    uint64_t TSFlags = TII->get(I->UnmaskedTUPseudo).TSFlags;
 
+    // Check that we're dropping the mask operand, and any policy operand
+    // when we transform to this unmasked tu pseudo.
+    assert(RISCVII::hasMergeOp(TSFlags) && RISCVII::hasDummyMaskOp(TSFlags) &&
+           !RISCVII::hasVecPolicyOp(TSFlags) &&
+           "Unexpected pseudo to transform to");
+    (void)TSFlags;
+  }
+
+  unsigned Opc = IsTA ? I->UnmaskedPseudo : I->UnmaskedTUPseudo;
   SmallVector<SDValue, 8> Ops;
-  // Skip the merge operand at index 0.
-  for (unsigned I = 1, E = N->getNumOperands(); I != E; I++) {
+  // Skip the merge operand at index 0 if IsTA
+  for (unsigned I = IsTA, E = N->getNumOperands(); I != E; I++) {
     // Skip the mask, the policy, and the Glue.
     SDValue Op = N->getOperand(I);
     if (I == MaskOpIdx || I == TailPolicyOpIdx ||
@@ -2304,8 +2323,7 @@ bool RISCVDAGToDAGISel::doPeepholeMaskedRVV(SDNode *N) {
   if (auto *TGlued = Glued->getGluedNode())
     Ops.push_back(SDValue(TGlued, TGlued->getNumValues() - 1));
 
-  SDNode *Result =
-      CurDAG->getMachineNode(I->UnmaskedPseudo, SDLoc(N), N->getVTList(), Ops);
+  SDNode *Result = CurDAG->getMachineNode(Opc, SDLoc(N), N->getVTList(), Ops);
   ReplaceUses(N, Result);
 
   return true;

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.h

@@ -191,6 +191,7 @@ struct VLX_VSXPseudo {
 struct RISCVMaskedPseudoInfo {
   uint16_t MaskedPseudo;
   uint16_t UnmaskedPseudo;
+  uint16_t UnmaskedTUPseudo;
   uint8_t MaskOpIdx;
 };
 

llvm/lib/Target/RISCV/RISCVInstrInfoVPseudos.td

@@ -423,16 +423,17 @@ def RISCVVIntrinsicsTable : GenericTable {
   let PrimaryKeyName = "getRISCVVIntrinsicInfo";
 }
 
-class RISCVMaskedPseudo<bits<4> MaskIdx> {
+class RISCVMaskedPseudo<bits<4> MaskIdx, bit HasTU = true> {
   Pseudo MaskedPseudo = !cast<Pseudo>(NAME);
   Pseudo UnmaskedPseudo = !cast<Pseudo>(!subst("_MASK", "", NAME));
+  Pseudo UnmaskedTUPseudo = !if(HasTU, !cast<Pseudo>(!subst("_MASK", "", NAME # "_TU")), MaskedPseudo);
   bits<4> MaskOpIdx = MaskIdx;
 }
 
 def RISCVMaskedPseudosTable : GenericTable {
   let FilterClass = "RISCVMaskedPseudo";
   let CppTypeName = "RISCVMaskedPseudoInfo";
-  let Fields = ["MaskedPseudo", "UnmaskedPseudo", "MaskOpIdx"];
+  let Fields = ["MaskedPseudo", "UnmaskedPseudo", "UnmaskedTUPseudo", "MaskOpIdx"];
   let PrimaryKey = ["MaskedPseudo"];
   let PrimaryKeyName = "getMaskedPseudoInfo";
 }
@@ -1770,7 +1771,7 @@ multiclass VPseudoBinaryM<VReg RetClass,
     let ForceTailAgnostic = true in
     def "_" # MInfo.MX # "_MASK" : VPseudoBinaryMOutMask<RetClass, Op1Class,
                                                          Op2Class, Constraint>,
-                                   RISCVMaskedPseudo</*MaskOpIdx*/ 3>;
+                                   RISCVMaskedPseudo</*MaskOpIdx*/ 3, /*HasTU*/ false>;
   }
 }
 

llvm/test/CodeGen/RISCV/rvv/allone-masked-to-unmasked.ll

@@ -31,14 +31,12 @@ entry:
   ret <vscale x 1 x i8> %a
 }
 
-; FIXME: Use an unmasked TAIL_AGNOSTIC instruction if the tie operand is IMPLICIT_DEF
+; Use an unmasked TAIL_AGNOSTIC instruction if the tie operand is IMPLICIT_DEF
 define <vscale x 1 x i8> @test1(<vscale x 1 x i8> %0, <vscale x 1 x i8> %1, iXLen %2) nounwind {
 ; CHECK-LABEL: test1:
 ; CHECK:       # %bb.0: # %entry
 ; CHECK-NEXT:    vsetvli zero, a0, e8, mf8, ta, mu
-; CHECK-NEXT:    vmset.m v0
-; CHECK-NEXT:    vsetvli zero, zero, e8, mf8, tu, mu
-; CHECK-NEXT:    vadd.vv v8, v8, v9, v0.t
+; CHECK-NEXT:    vadd.vv v8, v8, v9
 ; CHECK-NEXT:    ret
 entry:
   %allone = call <vscale x 1 x i1> @llvm.riscv.vmset.nxv1i1(
@@ -53,14 +51,12 @@ entry:
   ret <vscale x 1 x i8> %a
 }
 
-; FIXME: Use an unmasked TU instruction because of the policy operand
+; Use an unmasked TU instruction because of the policy operand
 define <vscale x 1 x i8> @test2(<vscale x 1 x i8> %0, <vscale x 1 x i8> %1, <vscale x 1 x i8> %2, iXLen %3) nounwind {
 ; CHECK-LABEL: test2:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    vsetvli zero, a0, e8, mf8, ta, mu
-; CHECK-NEXT:    vmset.m v0
-; CHECK-NEXT:    vsetvli zero, zero, e8, mf8, tu, mu
-; CHECK-NEXT:    vadd.vv v8, v9, v10, v0.t
+; CHECK-NEXT:    vsetvli zero, a0, e8, mf8, tu, mu
+; CHECK-NEXT:    vadd.vv v8, v9, v10
 ; CHECK-NEXT:    ret
 entry:
   %allone = call <vscale x 1 x i1> @llvm.riscv.vmset.nxv1i1(