[RISCV] Add TSFlag for reading past VL behaviour. NFCI

llvm/lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h

@@ -139,6 +139,9 @@ enum {
   // 3 -> SEW * 4
   DestEEWShift = ElementsDependOnMaskShift + 1,
   DestEEWMask = 3ULL << DestEEWShift,
+
+  ReadsPastVLShift = DestEEWShift + 2,
+  ReadsPastVLMask = 1ULL << ReadsPastVLShift,
 };
 
 // Helper functions to read TSFlags.
@@ -195,6 +198,12 @@ static inline bool elementsDependOnMask(uint64_t TSFlags) {
   return TSFlags & ElementsDependOnMaskMask;
 }
 
+/// \returns true if the instruction may read elements past VL, e.g.
+/// vslidedown/vrgather
+static inline bool readsPastVL(uint64_t TSFlags) {
+  return TSFlags & ReadsPastVLMask;
+}
+
 static inline unsigned getVLOpNum(const MCInstrDesc &Desc) {
   const uint64_t TSFlags = Desc.TSFlags;
   // This method is only called if we expect to have a VL operand, and all

llvm/lib/Target/RISCV/RISCVInstrFormats.td

@@ -261,6 +261,12 @@ class RVInstCommon<dag outs, dag ins, string opcodestr, string argstr,
   // Indicates the EEW of a vector instruction's destination operand.
   EEW DestEEW = EEWSEWx1;
   let TSFlags{25-24} = DestEEW.Value;
+
+  // Some vector instructions like vslidedown/vrgather will read elements past
+  // VL, and should be marked to make sure RISCVVLOptimizer doesn't reduce its
+  // operands' VLs.
+  bit ReadsPastVL = 0;
+  let TSFlags{26} = ReadsPastVL;
 }
 
 class RVInst<dag outs, dag ins, string opcodestr, string argstr,

llvm/lib/Target/RISCV/RISCVInstrInfoV.td

@@ -1703,28 +1703,29 @@ let Constraints = "@earlyclobber $vd", RVVConstraint = SlideUp in {
 defm VSLIDEUP_V : VSLD_IV_X_I<"vslideup", 0b001110, /*slidesUp=*/true>;
 defm VSLIDE1UP_V : VSLD1_MV_X<"vslide1up", 0b001110>;
 } // Constraints = "@earlyclobber $vd", RVVConstraint = SlideUp
+let ReadsPastVL = 1 in
 defm VSLIDEDOWN_V : VSLD_IV_X_I<"vslidedown", 0b001111, /*slidesUp=*/false>;
-let ElementsDependOn = EltDepsVL in
+let ElementsDependOn = EltDepsVL, ReadsPastVL = 1 in
 defm VSLIDE1DOWN_V : VSLD1_MV_X<"vslide1down", 0b001111>;
 } // Predicates = [HasVInstructions]
 
 let Predicates = [HasVInstructionsAnyF] in {
 let Constraints = "@earlyclobber $vd", RVVConstraint = SlideUp in {
 defm VFSLIDE1UP_V : VSLD1_FV_F<"vfslide1up", 0b001110>;
 } // Constraints = "@earlyclobber $vd", RVVConstraint = SlideUp
-let ElementsDependOn = EltDepsVL in
+let ElementsDependOn = EltDepsVL, ReadsPastVL = 1 in
 defm VFSLIDE1DOWN_V : VSLD1_FV_F<"vfslide1down", 0b001111>;
 } // Predicates = [HasVInstructionsAnyF]
 
 let Predicates = [HasVInstructions] in {
 // Vector Register Gather Instruction
-let Constraints = "@earlyclobber $vd", RVVConstraint = Vrgather in {
+let Constraints = "@earlyclobber $vd", RVVConstraint = Vrgather, ReadsPastVL = 1 in {
 defm VRGATHER_V : VGTR_IV_V_X_I<"vrgather", 0b001100>;
 def VRGATHEREI16_VV : VALUVV<0b001110, OPIVV, "vrgatherei16.vv">,
                       SchedBinaryMC<"WriteVRGatherEI16VV",
                                     "ReadVRGatherEI16VV_data",
                                     "ReadVRGatherEI16VV_index">;
-} // Constraints = "@earlyclobber $vd", RVVConstraint = Vrgather
+} // Constraints = "@earlyclobber $vd", RVVConstraint = Vrgather, ReadsPastVL = 1
 
 // Vector Compress Instruction
 let Constraints = "@earlyclobber $vd", RVVConstraint = Vcompress, ElementsDependOn = EltDepsVLMask in {

llvm/lib/Target/RISCV/RISCVInstrInfoXRivos.td

@@ -58,7 +58,7 @@ class CustomRivosXVI<bits<6> funct6, RISCVVFormat opv, dag outs, dag ins,
 
 let Predicates = [HasVendorXRivosVizip], DecoderNamespace = "XRivos",
   Constraints = "@earlyclobber $vd", RVVConstraint = Vrgather,
-  Inst<6-0> = OPC_CUSTOM_2.Value in  {
+  Inst<6-0> = OPC_CUSTOM_2.Value, ReadsPastVL = 1 in  {
 defm RI_VZIPEVEN_V : VALU_IV_V<"ri.vzipeven", 0b001100>;
 defm RI_VZIPODD_V : VALU_IV_V<"ri.vzipodd", 0b011100>;
 defm RI_VZIP2A_V : VALU_IV_V<"ri.vzip2a", 0b000100>;
@@ -126,6 +126,7 @@ def RI_VINSERT : CustomRivosVXI<0b010000, OPMVX, (outs VR:$vd_wb),
                                 (ins VR:$vd, GPR:$rs1, uimm5:$imm),
                                 "ri.vinsert.v.x", "$vd, $rs1, $imm">;
 
+let ReadsPastVL = 1 in
 def RI_VEXTRACT : CustomRivosXVI<0b010111, OPMVV, (outs GPR:$rd),
                                 (ins VR:$vs2, uimm5:$imm),
                                 "ri.vextract.x.v", "$rd, $vs2, $imm">;

llvm/lib/Target/RISCV/RISCVInstrInfoXSf.td

@@ -74,6 +74,7 @@ class RVInstVCCustom2<bits<4> funct6_hi4, bits<3> funct3, dag outs, dag ins,
   let Uses = [VL, VTYPE];
   let RVVConstraint = NoConstraint;
   let ElementsDependOn = EltDepsVLMask;
+  let ReadsPastVL = 1;
 }
 
 class RVInstVCFCustom2<bits<4> funct6_hi4, bits<3> funct3, dag outs, dag ins,
@@ -98,6 +99,7 @@ class RVInstVCFCustom2<bits<4> funct6_hi4, bits<3> funct3, dag outs, dag ins,
   let Uses = [VL, VTYPE];
   let RVVConstraint = NoConstraint;
   let ElementsDependOn = EltDepsVLMask;
+  let ReadsPastVL = 1;
 }
 
 class VCIXInfo<string suffix, VCIXType type, DAGOperand TyRd,

llvm/lib/Target/RISCV/RISCVInstrInfoXSfmm.td

@@ -65,6 +65,7 @@ class SFInstTileMemOp<dag outs, dag ins, bits<3> nf, RISCVOpcode opcode,
   let Inst{6-0} = opcode.Value;
 
   let Uses = [VTYPE, VL];
+  let ReadsPastVL = 1;
 }
 
 let hasSideEffects = 0, mayLoad = 1, mayStore = 0 in
@@ -94,6 +95,7 @@ class SFInstTileMoveOp<bits<6> funct6, dag outs, dag ins, string opcodestr,
   let Inst{6-0} = OPC_OP_V.Value;
 
   let Uses = [VTYPE, VL];
+  let ReadsPastVL = 1;
 }
 
 let hasSideEffects = 0, mayLoad = 0, mayStore = 0 in
@@ -113,6 +115,7 @@ class SFInstMatmulF<dag outs, dag ins, string opcodestr, string argstr>
   let Inst{6-0} = OPC_OP_VE.Value;
 
   let Uses = [VTYPE, VL];
+  let ReadsPastVL = 1;
 }
 
 let hasSideEffects = 0, mayLoad = 0, mayStore = 0 in
@@ -135,6 +138,7 @@ class SFInstMatmulF8<bit a, bit b, dag outs, dag ins,
   let Inst{6-0} = OPC_OP_VE.Value;
 
   let Uses = [VTYPE, VL];
+  let ReadsPastVL = 1;
 }
 
 
@@ -167,6 +171,7 @@ class SFInstMatmulI8<bit funct6_1, bit a, bit b, dag outs, dag ins,
   let Inst{6-0} = OPC_OP_VE.Value;
 
   let Uses = [VTYPE, VL];
+  let ReadsPastVL = 1;
 }
 
 class I8Encode<bit encoding, string name> {

llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp

@@ -1227,34 +1227,6 @@ static bool isVectorOpUsedAsScalarOp(const MachineOperand &MO) {
   }
 }
 
-/// Return true if MI may read elements past VL.
-static bool mayReadPastVL(const MachineInstr &MI) {
-  const RISCVVPseudosTable::PseudoInfo *RVV =
-      RISCVVPseudosTable::getPseudoInfo(MI.getOpcode());
-  if (!RVV)
-    return true;
-
-  switch (RVV->BaseInstr) {
-  // vslidedown instructions may read elements past VL. They are handled
-  // according to current tail policy.
-  case RISCV::VSLIDEDOWN_VI:
-  case RISCV::VSLIDEDOWN_VX:
-  case RISCV::VSLIDE1DOWN_VX:
-  case RISCV::VFSLIDE1DOWN_VF:
-
-  // vrgather instructions may read the source vector at any index < VLMAX,
-  // regardless of VL.
-  case RISCV::VRGATHER_VI:
-  case RISCV::VRGATHER_VV:
-  case RISCV::VRGATHER_VX:
-  case RISCV::VRGATHEREI16_VV:
-    return true;
-
-  default:
-    return false;
-  }
-}
-
 bool RISCVVLOptimizer::isCandidate(const MachineInstr &MI) const {
   const MCInstrDesc &Desc = MI.getDesc();
   if (!RISCVII::hasVLOp(Desc.TSFlags) || !RISCVII::hasSEWOp(Desc.TSFlags))
@@ -1315,7 +1287,8 @@ RISCVVLOptimizer::getMinimumVLForUser(const MachineOperand &UserOp) const {
     return std::nullopt;
   }
 
-  if (mayReadPastVL(UserMI)) {
+  if (RISCVII::readsPastVL(
+          TII->get(RISCV::getRVVMCOpcode(UserMI.getOpcode())).TSFlags)) {
     LLVM_DEBUG(dbgs() << "    Abort because used by unsafe instruction\n");
     return std::nullopt;
   }