llvm · lukel97 · Sep 15, 2025 · Jul 30, 2025 · Feb 7, 2025 · Jul 30, 2025
diff --git a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
@@ -10,9 +10,19 @@
 // instructions are inserted.
 //
 // The purpose of this optimization is to make the VL argument, for instructions
-// that have a VL argument, as small as possible. This is implemented by
-// visiting each instruction in reverse order and checking that if it has a VL
-// argument, whether the VL can be reduced.
+// that have a VL argument, as small as possible.
+//
+// This is split into a sparse dataflow analysis where we determine what VL is
+// demanded by each instruction first, and then afterwards try to reduce the VL
+// of each instruction if it demands less than its VL operand.
+//
+// The analysis is explained in more detail in the 2025 EuroLLVM Developers'
+// Meeting talk "Accidental Dataflow Analysis: Extending the RISC-V VL
+// Optimizer", which is available on YouTube at
+// https://www.youtube.com/watch?v=Mfb5fRSdJAc
+//
+// The slides for the talk are available at
+// https://llvm.org/devmtg/2025-04/slides/technical_talk/lau_accidental_dataflow.pdf
 //
 //===---------------------------------------------------------------------===//
 
@@ -30,6 +40,27 @@ using namespace llvm;
 
 namespace {
 
+/// Wrapper around MachineOperand that defaults to immediate 0.
+struct DemandedVL {
+  MachineOperand VL;
+  DemandedVL() : VL(MachineOperand::CreateImm(0)) {}
+  DemandedVL(MachineOperand VL) : VL(VL) {}
+  static DemandedVL vlmax() {
+    return DemandedVL(MachineOperand::CreateImm(RISCV::VLMaxSentinel));
+  }
+  bool operator!=(const DemandedVL &Other) const {
+    return !VL.isIdenticalTo(Other.VL);
+  }
+};
+
+static DemandedVL max(const DemandedVL &LHS, const DemandedVL &RHS) {
+  if (RISCV::isVLKnownLE(LHS.VL, RHS.VL))
+    return RHS;
+  if (RISCV::isVLKnownLE(RHS.VL, LHS.VL))
+    return LHS;
+  return DemandedVL::vlmax();
+}
+
 class RISCVVLOptimizer : public MachineFunctionPass {
   const MachineRegisterInfo *MRI;
   const MachineDominatorTree *MDT;
@@ -51,17 +82,17 @@ class RISCVVLOptimizer : public MachineFunctionPass {
   StringRef getPassName() const override { return PASS_NAME; }
 
 private:
-  std::optional<MachineOperand>
-  getMinimumVLForUser(const MachineOperand &UserOp) const;
-  /// Returns the largest common VL MachineOperand that may be used to optimize
-  /// MI. Returns std::nullopt if it failed to find a suitable VL.
-  std::optional<MachineOperand> checkUsers(const MachineInstr &MI) const;
+  DemandedVL getMinimumVLForUser(const MachineOperand &UserOp) const;
+  /// Returns true if the users of \p MI have compatible EEWs and SEWs.
+  bool checkUsers(const MachineInstr &MI) const;
   bool tryReduceVL(MachineInstr &MI) const;
   bool isCandidate(const MachineInstr &MI) const;
+  void transfer(const MachineInstr &MI);
 
   /// For a given instruction, records what elements of it are demanded by
   /// downstream users.
-  DenseMap<const MachineInstr *, std::optional<MachineOperand>> DemandedVLs;
+  DenseMap<const MachineInstr *, DemandedVL> DemandedVLs;
+  SetVector<const MachineInstr *> Worklist;
 };
 
 /// Represents the EMUL and EEW of a MachineOperand.
@@ -813,6 +844,7 @@ static std::optional<OperandInfo> getOperandInfo(const MachineOperand &MO) {
   const MachineInstr &MI = *MO.getParent();
   const RISCVVPseudosTable::PseudoInfo *RVV =
       RISCVVPseudosTable::getPseudoInfo(MI.getOpcode());
+  MI.dump();
   assert(RVV && "Could not find MI in PseudoTable");
 
   std::optional<unsigned> Log2EEW = getOperandLog2EEW(MO);
@@ -847,10 +879,15 @@ static std::optional<OperandInfo> getOperandInfo(const MachineOperand &MO) {
   return OperandInfo(getEMULEqualsEEWDivSEWTimesLMUL(*Log2EEW, MI), *Log2EEW);
 }
 
+static bool isTupleInsertInstr(const MachineInstr &MI);
+
 /// Return true if this optimization should consider MI for VL reduction. This
 /// white-list approach simplifies this optimization for instructions that may
 /// have more complex semantics with relation to how it uses VL.
 static bool isSupportedInstr(const MachineInstr &MI) {
+  if (MI.isPHI() || MI.isFullCopy() || isTupleInsertInstr(MI))
+    return true;
+
   const RISCVVPseudosTable::PseudoInfo *RVV =
       RISCVVPseudosTable::getPseudoInfo(MI.getOpcode());
 
@@ -1348,21 +1385,24 @@ bool RISCVVLOptimizer::isCandidate(const MachineInstr &MI) const {
   return true;
 }
 
-std::optional<MachineOperand>
+DemandedVL
 RISCVVLOptimizer::getMinimumVLForUser(const MachineOperand &UserOp) const {
   const MachineInstr &UserMI = *UserOp.getParent();
   const MCInstrDesc &Desc = UserMI.getDesc();
 
+  if (UserMI.isPHI() || UserMI.isFullCopy() || isTupleInsertInstr(UserMI))
+    return DemandedVLs.lookup(&UserMI);
+
   if (!RISCVII::hasVLOp(Desc.TSFlags) || !RISCVII::hasSEWOp(Desc.TSFlags)) {
     LLVM_DEBUG(dbgs() << "  Abort due to lack of VL, assume that"
                          " use VLMAX\n");
-    return std::nullopt;
+    return DemandedVL::vlmax();
   }
 
   if (RISCVII::readsPastVL(
           TII->get(RISCV::getRVVMCOpcode(UserMI.getOpcode())).TSFlags)) {
     LLVM_DEBUG(dbgs() << "  Abort because used by unsafe instruction\n");
-    return std::nullopt;
+    return DemandedVL::vlmax();
   }
 
   unsigned VLOpNum = RISCVII::getVLOpNum(Desc);
@@ -1376,11 +1416,10 @@ RISCVVLOptimizer::getMinimumVLForUser(const MachineOperand &UserOp) const {
   if (UserOp.isTied()) {
     assert(UserOp.getOperandNo() == UserMI.getNumExplicitDefs() &&
            RISCVII::isFirstDefTiedToFirstUse(UserMI.getDesc()));
-    auto DemandedVL = DemandedVLs.lookup(&UserMI);
-    if (!DemandedVL || !RISCV::isVLKnownLE(*DemandedVL, VLOp)) {
+    if (!RISCV::isVLKnownLE(DemandedVLs.lookup(&UserMI).VL, VLOp)) {
       LLVM_DEBUG(dbgs() << "  Abort because user is passthru in "
                            "instruction with demanded tail\n");
-      return std::nullopt;
+      return DemandedVL::vlmax();
     }
   }
 
@@ -1393,11 +1432,8 @@ RISCVVLOptimizer::getMinimumVLForUser(const MachineOperand &UserOp) const {
 
   // If we know the demanded VL of UserMI, then we can reduce the VL it
   // requires.
-  if (auto DemandedVL = DemandedVLs.lookup(&UserMI)) {
-    assert(isCandidate(UserMI));
-    if (RISCV::isVLKnownLE(*DemandedVL, VLOp))
-      return DemandedVL;
-  }
+  if (RISCV::isVLKnownLE(DemandedVLs.lookup(&UserMI).VL, VLOp))
+    return DemandedVLs.lookup(&UserMI);
 
   return VLOp;
 }
@@ -1450,9 +1486,10 @@ static bool isSegmentedStoreInstr(const MachineInstr &MI) {
   }
 }
 
-std::optional<MachineOperand>
-RISCVVLOptimizer::checkUsers(const MachineInstr &MI) const {
-  std::optional<MachineOperand> CommonVL;
+bool RISCVVLOptimizer::checkUsers(const MachineInstr &MI) const {
+  if (MI.isPHI() || MI.isFullCopy() || isTupleInsertInstr(MI))
+    return true;
+
   SmallSetVector<MachineOperand *, 8> Worklist;
   SmallPtrSet<const MachineInstr *, 4> PHISeen;
   for (auto &UserOp : MRI->use_operands(MI.getOperand(0).getReg()))
@@ -1481,7 +1518,7 @@ RISCVVLOptimizer::checkUsers(const MachineInstr &MI) const {
         // whole register group).
         if (!isTupleInsertInstr(CandidateMI) &&
             !isSegmentedStoreInstr(CandidateMI))
-          return std::nullopt;
+          return false;
         Worklist.insert(&UseOp);
       }
       continue;
@@ -1497,23 +1534,9 @@ RISCVVLOptimizer::checkUsers(const MachineInstr &MI) const {
       continue;
     }
 
-    auto VLOp = getMinimumVLForUser(UserOp);
-    if (!VLOp)
-      return std::nullopt;
-
-    // Use the largest VL among all the users. If we cannot determine this
-    // statically, then we cannot optimize the VL.
-    if (!CommonVL || RISCV::isVLKnownLE(*CommonVL, *VLOp)) {
-      CommonVL = *VLOp;
-      LLVM_DEBUG(dbgs() << "    User VL is: " << VLOp << "\n");
-    } else if (!RISCV::isVLKnownLE(*VLOp, *CommonVL)) {
-      LLVM_DEBUG(dbgs() << "    Abort because cannot determine a common VL\n");
-      return std::nullopt;
-    }
-
     if (!RISCVII::hasSEWOp(UserMI.getDesc().TSFlags)) {
       LLVM_DEBUG(dbgs() << "    Abort due to lack of SEW operand\n");
-      return std::nullopt;
+      return false;
     }
 
     std::optional<OperandInfo> ConsumerInfo = getOperandInfo(UserOp);
@@ -1522,7 +1545,7 @@ RISCVVLOptimizer::checkUsers(const MachineInstr &MI) const {
       LLVM_DEBUG(dbgs() << "    Abort due to unknown operand information.\n");
       LLVM_DEBUG(dbgs() << "      ConsumerInfo is: " << ConsumerInfo << "\n");
       LLVM_DEBUG(dbgs() << "      ProducerInfo is: " << ProducerInfo << "\n");
-      return std::nullopt;
+      return false;
     }
 
     if (!OperandInfo::areCompatible(*ProducerInfo, *ConsumerInfo)) {
@@ -1531,11 +1554,11 @@ RISCVVLOptimizer::checkUsers(const MachineInstr &MI) const {
           << "    Abort due to incompatible information for EMUL or EEW.\n");
       LLVM_DEBUG(dbgs() << "      ConsumerInfo is: " << ConsumerInfo << "\n");
       LLVM_DEBUG(dbgs() << "      ProducerInfo is: " << ProducerInfo << "\n");
-      return std::nullopt;
+      return false;
     }
   }
 
-  return CommonVL;
+  return true;
 }
 
 bool RISCVVLOptimizer::tryReduceVL(MachineInstr &MI) const {
@@ -1551,9 +1574,7 @@ bool RISCVVLOptimizer::tryReduceVL(MachineInstr &MI) const {
     return false;
   }
 
-  auto CommonVL = DemandedVLs.lookup(&MI);
-  if (!CommonVL)
-    return false;
+  auto *CommonVL = &DemandedVLs.at(&MI).VL;
 
   assert((CommonVL->isImm() || CommonVL->getReg().isVirtual()) &&
          "Expected VL to be an Imm or virtual Reg");
@@ -1564,7 +1585,7 @@ bool RISCVVLOptimizer::tryReduceVL(MachineInstr &MI) const {
     const MachineInstr *VLMI = MRI->getVRegDef(CommonVL->getReg());
     if (RISCVInstrInfo::isFaultOnlyFirstLoad(*VLMI) &&
         !MDT->dominates(VLMI, &MI))
-      CommonVL = VLMI->getOperand(RISCVII::getVLOpNum(VLMI->getDesc()));
+      CommonVL = &VLMI->getOperand(RISCVII::getVLOpNum(VLMI->getDesc()));
   }
 
   if (!RISCV::isVLKnownLE(*CommonVL, VLOp)) {
@@ -1599,6 +1620,30 @@ bool RISCVVLOptimizer::tryReduceVL(MachineInstr &MI) const {
   return true;
 }
 
+static bool isPhysical(const MachineOperand &MO) {
+  return MO.isReg() && MO.getReg().isPhysical();
+}
+
+static bool isVirtualVec(const MachineOperand &MO) {
+  return MO.isReg() && MO.getReg().isVirtual() &&
+         RISCVRegisterInfo::isRVVRegClass(
+             MO.getParent()->getMF()->getRegInfo().getRegClass(MO.getReg()));
+}
+
+/// Look through \p MI's operands and propagate what it demands to its uses.
+void RISCVVLOptimizer::transfer(const MachineInstr &MI) {
+  if (!isSupportedInstr(MI) || !checkUsers(MI) || any_of(MI.defs(), isPhysical))
+    DemandedVLs[&MI] = DemandedVL::vlmax();
+
+  for (const MachineOperand &MO : make_filter_range(MI.uses(), isVirtualVec)) {
+    const MachineInstr *Def = MRI->getVRegDef(MO.getReg());
+    DemandedVL Prev = DemandedVLs[Def];
+    DemandedVLs[Def] = max(DemandedVLs[Def], getMinimumVLForUser(MO));
+    if (DemandedVLs[Def] != Prev)
+      Worklist.insert(Def);
+  }
+}
+
 bool RISCVVLOptimizer::runOnMachineFunction(MachineFunction &MF) {
   if (skipFunction(MF.getFunction()))
     return false;
@@ -1614,15 +1659,18 @@ bool RISCVVLOptimizer::runOnMachineFunction(MachineFunction &MF) {
 
   assert(DemandedVLs.empty());
 
-  // For each instruction that defines a vector, compute what VL its
-  // downstream users demand.
+  // For each instruction that defines a vector, propagate the VL it
+  // uses to its inputs.
   for (MachineBasicBlock *MBB : post_order(&MF)) {
     assert(MDT->isReachableFromEntry(MBB));
-    for (MachineInstr &MI : reverse(*MBB)) {
-      if (!isCandidate(MI))
-        continue;
-      DemandedVLs.insert({&MI, checkUsers(MI)});
-    }
+    for (MachineInstr &MI : reverse(*MBB))
+      Worklist.insert(&MI);
+  }
+
+  while (!Worklist.empty()) {
+    const MachineInstr *MI = Worklist.front();
+    Worklist.remove(MI);
+    transfer(*MI);
   }
 
   // Then go through and see if we can reduce the VL of any instructions to

diff --git a/llvm/test/CodeGen/RISCV/rvv/reproducer-pr146855.ll b/llvm/test/CodeGen/RISCV/rvv/reproducer-pr146855.ll
@@ -6,15 +6,15 @@ target triple = "riscv64-unknown-linux-gnu"
 define i32 @_ZN4Mesh12rezone_countESt6vectorIiSaIiEERiS3_(<vscale x 4 x i32> %wide.load, <vscale x 4 x i1> %0, <vscale x 4 x i1> %1, <vscale x 4 x i1> %2, <vscale x 4 x i1> %3) #0 {
 ; CHECK-LABEL: _ZN4Mesh12rezone_countESt6vectorIiSaIiEERiS3_:
 ; CHECK:       # %bb.0: # %entry
-; CHECK-NEXT:    vsetvli a0, zero, e32, m2, ta, ma
+; CHECK-NEXT:    vsetivli zero, 0, e32, m2, ta, ma
 ; CHECK-NEXT:    vmv1r.v v8, v0
 ; CHECK-NEXT:    li a0, 0
 ; CHECK-NEXT:    vmv.v.i v10, 0
 ; CHECK-NEXT:    vmv.v.i v12, 0
 ; CHECK-NEXT:    vmv.v.i v14, 0
 ; CHECK-NEXT:  .LBB0_1: # %vector.body
 ; CHECK-NEXT:    # =>This Inner Loop Header: Depth=1
-; CHECK-NEXT:    vsetvli a1, zero, e32, m2, ta, mu
+; CHECK-NEXT:    vsetivli zero, 0, e32, m2, ta, mu
 ; CHECK-NEXT:    vmv1r.v v0, v8
 ; CHECK-NEXT:    slli a0, a0, 2
 ; CHECK-NEXT:    vmv2r.v v16, v10

diff --git a/llvm/test/CodeGen/RISCV/rvv/vl-opt.ll b/llvm/test/CodeGen/RISCV/rvv/vl-opt.ll
@@ -238,3 +238,55 @@ define void @segmented_store_insert_subreg(<vscale x 4 x float> %v0, <vscale x 4
   call void @llvm.riscv.vsseg3(target("riscv.vector.tuple", <vscale x 16 x i8>, 3) %t2, ptr %p, iXLen %vl, iXLen 5)
   ret void
 }
+
+define void @recurrence(<vscale x 4 x i32> %v, ptr %p, iXLen %n, iXLen %vl) {
+; CHECK-LABEL: recurrence:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    vsetvli zero, a2, e32, m2, ta, ma
+; CHECK-NEXT:    vmv.v.i v10, 0
+; CHECK-NEXT:  .LBB16_1: # %loop
+; CHECK-NEXT:    # =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    addi a1, a1, -1
+; CHECK-NEXT:    vadd.vv v10, v10, v8
+; CHECK-NEXT:    bnez a1, .LBB16_1
+; CHECK-NEXT:  # %bb.2: # %exit
+; CHECK-NEXT:    vse32.v v10, (a0)
+; CHECK-NEXT:    ret
+entry:
+  br label %loop
+loop:
+  %iv = phi iXLen [ 0, %entry ], [ %iv.next, %loop ]
+  %phi = phi <vscale x 4 x i32> [ zeroinitializer, %entry ], [ %x, %loop ]
+  %x = add <vscale x 4 x i32> %phi, %v
+  %iv.next = add iXLen %iv, 1
+  %done = icmp eq iXLen %iv.next, %n
+  br i1 %done, label %exit, label %loop
+exit:
+  call void @llvm.riscv.vse(<vscale x 4 x i32> %x, ptr %p, iXLen %vl)
+  ret void
+}
+
+define <vscale x 4 x i32> @join(<vscale x 4 x i32> %v, i1 %cond, iXLen %vl) {
+; CHECK-LABEL: join:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    andi a0, a0, 1
+; CHECK-NEXT:    vsetivli zero, 2, e32, m2, ta, ma
+; CHECK-NEXT:    vadd.vi v8, v8, 1
+; CHECK-NEXT:    beqz a0, .LBB17_2
+; CHECK-NEXT:  # %bb.1: # %foo
+; CHECK-NEXT:    vsetivli zero, 1, e32, m2, ta, ma
+; CHECK-NEXT:    vadd.vi v8, v8, 1
+; CHECK-NEXT:    ret
+; CHECK-NEXT:  .LBB17_2: # %bar
+; CHECK-NEXT:    vadd.vi v8, v8, 2
+; CHECK-NEXT:    ret
+entry:
+  %a = call <vscale x 4 x i32> @llvm.riscv.vadd(<vscale x 4 x i32> poison, <vscale x 4 x i32> %v, iXLen 1, iXLen -1)
+  br i1 %cond, label %foo, label %bar
+foo:
+  %b = call <vscale x 4 x i32> @llvm.riscv.vadd(<vscale x 4 x i32> poison, <vscale x 4 x i32> %a, iXLen 1, iXLen 1)
+  ret <vscale x 4 x i32> %b
+bar:
+  %c = call <vscale x 4 x i32> @llvm.riscv.vadd(<vscale x 4 x i32> poison, <vscale x 4 x i32> %a, iXLen 2, iXLen 2)
+  ret <vscale x 4 x i32> %c
+}