[VPlan] Convert EVL loops to variable-length stepping after dissolution (#147222)

Loop regions require fixed-length steps and rounded-up trip counts, but
after dissolution creates explicit control flow, EVL loops can leverage
variable-length stepping with original trip counts.

This patch adds a post-dissolution transform pass to convert EVL loops
from fixed-length to variable-length stepping .

Author: arcbbb

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7315,6 +7315,8 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
   // Regions are dissolved after optimizing for VF and UF, which completely
   // removes unneeded loop regions first.
   VPlanTransforms::dissolveLoopRegions(BestVPlan);
+  // Canonicalize EVL loops after regions are dissolved.
+  VPlanTransforms::canonicalizeEVLLoops(BestVPlan);
   // Perform the actual loop transformation.
   VPTransformState State(&TTI, BestVF, LI, DT, ILV.AC, ILV.Builder, &BestVPlan,
                          OrigLoop->getParentLoop(),

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -2390,6 +2390,66 @@ bool VPlanTransforms::tryAddExplicitVectorLength(
   return true;
 }
 
+void VPlanTransforms::canonicalizeEVLLoops(VPlan &Plan) {
+  using namespace llvm::VPlanPatternMatch;
+  // Find EVL loop entries by locating VPEVLBasedIVPHIRecipe.
+  // There should be only one EVL PHI in the entire plan.
+  VPEVLBasedIVPHIRecipe *EVLPhi = nullptr;
+
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
+           vp_depth_first_shallow(Plan.getEntry())))
+    for (VPRecipeBase &R : VPBB->phis())
+      if (auto *PhiR = dyn_cast<VPEVLBasedIVPHIRecipe>(&R)) {
+        assert(!EVLPhi && "Found multiple EVL PHIs. Only one expected");
+        EVLPhi = PhiR;
+      }
+
+  // Early return if no EVL PHI is found.
+  if (!EVLPhi)
+    return;
+
+  VPBasicBlock *HeaderVPBB = EVLPhi->getParent();
+  VPValue *EVLIncrement = EVLPhi->getBackedgeValue();
+
+  // Convert EVLPhi to concrete recipe.
+  auto *ScalarR =
+      VPBuilder(EVLPhi).createScalarPhi({EVLPhi->getStartValue(), EVLIncrement},
+                                        EVLPhi->getDebugLoc(), "evl.based.iv");
+  EVLPhi->replaceAllUsesWith(ScalarR);
+  EVLPhi->eraseFromParent();
+
+  // Replace CanonicalIVInc with EVL-PHI increment.
+  auto *CanonicalIV = cast<VPPhi>(&*HeaderVPBB->begin());
+  VPValue *Backedge = CanonicalIV->getIncomingValue(1);
+  assert(match(Backedge,
+               m_c_Binary<Instruction::Add>(m_Specific(CanonicalIV),
+                                            m_Specific(&Plan.getVFxUF()))) &&
+         "Unexpected canonical iv");
+  Backedge->replaceAllUsesWith(EVLIncrement);
+
+  // Remove unused phi and increment.
+  VPRecipeBase *CanonicalIVIncrement = Backedge->getDefiningRecipe();
+  CanonicalIVIncrement->eraseFromParent();
+  CanonicalIV->eraseFromParent();
+
+  // Replace the use of VectorTripCount in the latch-exiting block.
+  // Before: (branch-on-count EVLIVInc, VectorTripCount)
+  // After: (branch-on-count EVLIVInc, TripCount)
+
+  VPBasicBlock *LatchExiting =
+      HeaderVPBB->getPredecessors()[1]->getEntryBasicBlock();
+  auto *LatchExitingBr = cast<VPInstruction>(LatchExiting->getTerminator());
+  // Skip single-iteration loop region
+  if (match(LatchExitingBr, m_BranchOnCond(m_True())))
+    return;
+  assert(LatchExitingBr &&
+         match(LatchExitingBr,
+               m_BranchOnCount(m_VPValue(EVLIncrement),
+                               m_Specific(&Plan.getVectorTripCount()))) &&
+         "Unexpected terminator in EVL loop");
+  LatchExitingBr->setOperand(1, Plan.getTripCount());
+}
+
 void VPlanTransforms::dropPoisonGeneratingRecipes(
     VPlan &Plan,
     const std::function<bool(BasicBlock *)> &BlockNeedsPredication) {
@@ -2721,15 +2781,6 @@ void VPlanTransforms::convertToConcreteRecipes(VPlan &Plan,
   for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
            vp_depth_first_deep(Plan.getEntry()))) {
     for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
-      if (auto *PhiR = dyn_cast<VPEVLBasedIVPHIRecipe>(&R)) {
-        auto *ScalarR = VPBuilder(PhiR).createScalarPhi(
-            {PhiR->getStartValue(), PhiR->getBackedgeValue()},
-            PhiR->getDebugLoc(), "evl.based.iv");
-        PhiR->replaceAllUsesWith(ScalarR);
-        ToRemove.push_back(PhiR);
-        continue;
-      }
-
       if (auto *WidenIVR = dyn_cast<VPWidenIntOrFpInductionRecipe>(&R)) {
         expandVPWidenIntOrFpInduction(WidenIVR, TypeInfo);
         ToRemove.push_back(WidenIVR);

llvm/lib/Transforms/Vectorize/VPlanTransforms.h

@@ -209,6 +209,18 @@ struct VPlanTransforms {
   /// Replace loop regions with explicit CFG.
   static void dissolveLoopRegions(VPlan &Plan);
 
+  /// Transform EVL loops to use variable-length stepping after region
+  /// dissolution.
+  ///
+  /// Once loop regions are replaced with explicit CFG, EVL loops can step with
+  /// variable vector lengths instead of fixed lengths. This transformation:
+  ///  * Makes EVL-Phi concrete.
+  //   * Removes CanonicalIV and increment.
+  ///  * Replaces fixed-length stepping (branch-on-cond CanonicalIVInc,
+  ///    VectorTripCount) with variable-length stepping (branch-on-cond
+  ///    EVLIVInc, TripCount).
+  static void canonicalizeEVLLoops(VPlan &Plan);
+
   /// Lower abstract recipes to concrete ones, that can be codegen'd. Use \p
   /// CanonicalIVTy as type for all un-typed live-ins in VPTypeAnalysis.
   static void convertToConcreteRecipes(VPlan &Plan, Type &CanonicalIVTy);

llvm/lib/Transforms/Vectorize/VPlanVerifier.cpp

@@ -17,6 +17,7 @@
 #include "VPlanCFG.h"
 #include "VPlanDominatorTree.h"
 #include "VPlanHelpers.h"
+#include "VPlanPatternMatch.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/TypeSwitch.h"
 
@@ -193,7 +194,13 @@ bool VPlanVerifier::verifyEVLRecipe(const VPInstruction &EVL) const {
             errs() << "EVL used by unexpected VPInstruction\n";
             return false;
           }
-          if (I->getNumUsers() != 1) {
+          // EVLIVIncrement is only used by EVLIV & BranchOnCount.
+          // Having more than two users is unexpected.
+          if ((I->getNumUsers() != 1) &&
+              (I->getNumUsers() != 2 || none_of(I->users(), [&I](VPUser *U) {
+                 using namespace llvm::VPlanPatternMatch;
+                 return match(U, m_BranchOnCount(m_Specific(I), m_VPValue()));
+               }))) {
             errs() << "EVL is used in VPInstruction with multiple users\n";
             return false;
           }

llvm/test/Transforms/LoopVectorize/RISCV/evl-compatible-loops.ll

@@ -23,7 +23,6 @@ define void @test_wide_integer_induction(ptr noalias %a, i64 %N) {
 ; CHECK-NEXT:    [[INDUCTION:%.*]] = add <vscale x 2 x i64> zeroinitializer, [[TMP10]]
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       vector.body:
-; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, [[ENTRY]] ], [ [[IV_NEXT:%.*]], [[FOR_BODY]] ]
 ; CHECK-NEXT:    [[EVL_BASED_IV:%.*]] = phi i64 [ 0, [[ENTRY]] ], [ [[INDEX_EVL_NEXT:%.*]], [[FOR_BODY]] ]
 ; CHECK-NEXT:    [[VEC_IND:%.*]] = phi <vscale x 2 x i64> [ [[INDUCTION]], [[ENTRY]] ], [ [[VEC_IND_NEXT:%.*]], [[FOR_BODY]] ]
 ; CHECK-NEXT:    [[AVL:%.*]] = sub i64 [[N]], [[EVL_BASED_IV]]
@@ -36,9 +35,8 @@ define void @test_wide_integer_induction(ptr noalias %a, i64 %N) {
 ; CHECK-NEXT:    call void @llvm.vp.store.nxv2i64.p0(<vscale x 2 x i64> [[VEC_IND]], ptr align 8 [[TMP14]], <vscale x 2 x i1> splat (i1 true), i32 [[TMP11]])
 ; CHECK-NEXT:    [[TMP16:%.*]] = zext i32 [[TMP11]] to i64
 ; CHECK-NEXT:    [[INDEX_EVL_NEXT]] = add i64 [[TMP16]], [[EVL_BASED_IV]]
-; CHECK-NEXT:    [[IV_NEXT]] = add i64 [[IV]], [[TMP8]]
 ; CHECK-NEXT:    [[VEC_IND_NEXT]] = add <vscale x 2 x i64> [[VEC_IND]], [[BROADCAST_SPLAT]]
-; CHECK-NEXT:    [[TMP17:%.*]] = icmp eq i64 [[IV_NEXT]], [[N_VEC]]
+; CHECK-NEXT:    [[TMP17:%.*]] = icmp eq i64 [[INDEX_EVL_NEXT]], [[N]]
 ; CHECK-NEXT:    br i1 [[TMP17]], label [[MIDDLE_BLOCK:%.*]], label [[FOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
 ; CHECK:       middle.block:
 ; CHECK-NEXT:    br label [[FOR_COND_CLEANUP:%.*]]

llvm/test/Transforms/LoopVectorize/RISCV/inloop-reduction.ll

@@ -130,7 +130,6 @@ define i32 @add_i16_i32(ptr nocapture readonly %x, i32 %n) {
 ; IF-EVL-OUTLOOP-NEXT:    [[TMP4:%.*]] = mul nuw i32 [[TMP3]], 4
 ; IF-EVL-OUTLOOP-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; IF-EVL-OUTLOOP:       vector.body:
-; IF-EVL-OUTLOOP-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-OUTLOOP-NEXT:    [[EVL_BASED_IV:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_EVL_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-OUTLOOP-NEXT:    [[VEC_PHI:%.*]] = phi <vscale x 4 x i32> [ zeroinitializer, [[VECTOR_PH]] ], [ [[TMP10:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-OUTLOOP-NEXT:    [[AVL:%.*]] = sub i32 [[N]], [[EVL_BASED_IV]]
@@ -141,8 +140,7 @@ define i32 @add_i16_i32(ptr nocapture readonly %x, i32 %n) {
 ; IF-EVL-OUTLOOP-NEXT:    [[VP_OP:%.*]] = add <vscale x 4 x i32> [[VEC_PHI]], [[TMP9]]
 ; IF-EVL-OUTLOOP-NEXT:    [[TMP10]] = call <vscale x 4 x i32> @llvm.vp.merge.nxv4i32(<vscale x 4 x i1> splat (i1 true), <vscale x 4 x i32> [[VP_OP]], <vscale x 4 x i32> [[VEC_PHI]], i32 [[TMP5]])
 ; IF-EVL-OUTLOOP-NEXT:    [[INDEX_EVL_NEXT]] = add nuw i32 [[TMP5]], [[EVL_BASED_IV]]
-; IF-EVL-OUTLOOP-NEXT:    [[INDEX_NEXT]] = add nuw i32 [[INDEX]], [[TMP4]]
-; IF-EVL-OUTLOOP-NEXT:    [[TMP11:%.*]] = icmp eq i32 [[INDEX_NEXT]], [[N_VEC]]
+; IF-EVL-OUTLOOP-NEXT:    [[TMP11:%.*]] = icmp eq i32 [[INDEX_EVL_NEXT]], [[N]]
 ; IF-EVL-OUTLOOP-NEXT:    br i1 [[TMP11]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
 ; IF-EVL-OUTLOOP:       middle.block:
 ; IF-EVL-OUTLOOP-NEXT:    [[TMP12:%.*]] = call i32 @llvm.vector.reduce.add.nxv4i32(<vscale x 4 x i32> [[TMP10]])
@@ -185,7 +183,6 @@ define i32 @add_i16_i32(ptr nocapture readonly %x, i32 %n) {
 ; IF-EVL-INLOOP-NEXT:    [[TMP4:%.*]] = mul nuw i32 [[TMP3]], 8
 ; IF-EVL-INLOOP-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; IF-EVL-INLOOP:       vector.body:
-; IF-EVL-INLOOP-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-INLOOP-NEXT:    [[EVL_BASED_IV:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_EVL_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-INLOOP-NEXT:    [[VEC_PHI:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[TMP11:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-INLOOP-NEXT:    [[TMP5:%.*]] = sub i32 [[N]], [[EVL_BASED_IV]]
@@ -196,8 +193,7 @@ define i32 @add_i16_i32(ptr nocapture readonly %x, i32 %n) {
 ; IF-EVL-INLOOP-NEXT:    [[TMP10:%.*]] = call i32 @llvm.vp.reduce.add.nxv8i32(i32 0, <vscale x 8 x i32> [[TMP14]], <vscale x 8 x i1> splat (i1 true), i32 [[TMP6]])
 ; IF-EVL-INLOOP-NEXT:    [[TMP11]] = add i32 [[TMP10]], [[VEC_PHI]]
 ; IF-EVL-INLOOP-NEXT:    [[INDEX_EVL_NEXT]] = add nuw i32 [[TMP6]], [[EVL_BASED_IV]]
-; IF-EVL-INLOOP-NEXT:    [[INDEX_NEXT]] = add nuw i32 [[INDEX]], [[TMP4]]
-; IF-EVL-INLOOP-NEXT:    [[TMP12:%.*]] = icmp eq i32 [[INDEX_NEXT]], [[N_VEC]]
+; IF-EVL-INLOOP-NEXT:    [[TMP12:%.*]] = icmp eq i32 [[INDEX_EVL_NEXT]], [[N]]
 ; IF-EVL-INLOOP-NEXT:    br i1 [[TMP12]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
 ; IF-EVL-INLOOP:       middle.block:
 ; IF-EVL-INLOOP-NEXT:    br label [[FOR_COND_CLEANUP_LOOPEXIT:%.*]]
@@ -352,7 +348,6 @@ define i32 @smin(ptr %a, i64 %n, i32 %start) {
 ; IF-EVL-OUTLOOP-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <vscale x 4 x i32> [[BROADCAST_SPLATINSERT]], <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
 ; IF-EVL-OUTLOOP-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; IF-EVL-OUTLOOP:       vector.body:
-; IF-EVL-OUTLOOP-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-OUTLOOP-NEXT:    [[EVL_BASED_IV:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_EVL_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-OUTLOOP-NEXT:    [[VEC_PHI:%.*]] = phi <vscale x 4 x i32> [ [[BROADCAST_SPLAT]], [[VECTOR_PH]] ], [ [[TMP15:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-OUTLOOP-NEXT:    [[AVL:%.*]] = sub i64 [[N]], [[EVL_BASED_IV]]
@@ -364,8 +359,7 @@ define i32 @smin(ptr %a, i64 %n, i32 %start) {
 ; IF-EVL-OUTLOOP-NEXT:    [[TMP15]] = call <vscale x 4 x i32> @llvm.vp.merge.nxv4i32(<vscale x 4 x i1> splat (i1 true), <vscale x 4 x i32> [[TMP14]], <vscale x 4 x i32> [[VEC_PHI]], i32 [[TMP9]])
 ; IF-EVL-OUTLOOP-NEXT:    [[TMP16:%.*]] = zext i32 [[TMP9]] to i64
 ; IF-EVL-OUTLOOP-NEXT:    [[INDEX_EVL_NEXT]] = add i64 [[TMP16]], [[EVL_BASED_IV]]
-; IF-EVL-OUTLOOP-NEXT:    [[INDEX_NEXT]] = add i64 [[INDEX]], [[TMP8]]
-; IF-EVL-OUTLOOP-NEXT:    [[TMP17:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
+; IF-EVL-OUTLOOP-NEXT:    [[TMP17:%.*]] = icmp eq i64 [[INDEX_EVL_NEXT]], [[N]]
 ; IF-EVL-OUTLOOP-NEXT:    br i1 [[TMP17]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP5:![0-9]+]]
 ; IF-EVL-OUTLOOP:       middle.block:
 ; IF-EVL-OUTLOOP-NEXT:    [[TMP18:%.*]] = call i32 @llvm.vector.reduce.smin.nxv4i32(<vscale x 4 x i32> [[TMP15]])
@@ -402,7 +396,6 @@ define i32 @smin(ptr %a, i64 %n, i32 %start) {
 ; IF-EVL-INLOOP-NEXT:    [[TMP8:%.*]] = mul nuw i64 [[TMP7]], 4
 ; IF-EVL-INLOOP-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; IF-EVL-INLOOP:       vector.body:
-; IF-EVL-INLOOP-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-INLOOP-NEXT:    [[EVL_BASED_IV:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_EVL_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-INLOOP-NEXT:    [[VEC_PHI:%.*]] = phi i32 [ [[START:%.*]], [[VECTOR_PH]] ], [ [[RDX_MINMAX:%.*]], [[VECTOR_BODY]] ]
 ; IF-EVL-INLOOP-NEXT:    [[AVL:%.*]] = sub i64 [[N]], [[EVL_BASED_IV]]
@@ -413,9 +406,8 @@ define i32 @smin(ptr %a, i64 %n, i32 %start) {
 ; IF-EVL-INLOOP-NEXT:    [[RDX_MINMAX]] = call i32 @llvm.smin.i32(i32 [[TMP13]], i32 [[VEC_PHI]])
 ; IF-EVL-INLOOP-NEXT:    [[TMP14:%.*]] = zext i32 [[TMP9]] to i64
 ; IF-EVL-INLOOP-NEXT:    [[INDEX_EVL_NEXT]] = add i64 [[TMP14]], [[EVL_BASED_IV]]
-; IF-EVL-INLOOP-NEXT:    [[INDEX_NEXT]] = add i64 [[INDEX]], [[TMP8]]
-; IF-EVL-INLOOP-NEXT:    [[TMP15:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
-; IF-EVL-INLOOP-NEXT:    br i1 [[TMP15]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP5:![0-9]+]]
+; IF-EVL-INLOOP-NEXT:    [[TMP10:%.*]] = icmp eq i64 [[INDEX_EVL_NEXT]], [[N]]
+; IF-EVL-INLOOP-NEXT:    br i1 [[TMP10]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP5:![0-9]+]]
 ; IF-EVL-INLOOP:       middle.block:
 ; IF-EVL-INLOOP-NEXT:    br label [[FOR_END:%.*]]
 ; IF-EVL-INLOOP:       scalar.ph: