[LoopVectorize] Enable vectorisation of early exit loops with live-outs

This work feeds part of PR llvm#88385, and adds support for
vectorising loops with uncountable early exits and outside users of
loop-defined variables.

I've added a new fixupEarlyExitIVUsers to mirror what happens in
fixupIVUsers when patching up outside users of induction variables
in the early exit block. We have to handle these differently for two
reasons:

1. We can't work backwards from the end value in the middle block
because we didn't leave at the last iteration.
2. We need to generate different IR that calculates the vector lane
that triggered the exit, and hence can determine the induction value
at the point we exited.

I've added a new 'null' VPValue as a dummy placeholder to manage
the incoming operands of PHI nodes in the exit block. We can have
situations where one of the incoming values is an induction variable
(or its update) and the other is not. For example, both the latch
and the early exiting block can jump to the same exit block. However,
VPInstruction::generate walks through all predecessors of the PHI
assuming the value is *not* an IV. In order to ensure that we process
the right value for the right incoming block we use this new 'null'
value is a marker to indicate it should be skipped, since it will be
handled separately in fixupIVUsers or fixupEarlyExitIVUsers.

All code for calculating the last value when exiting the loop early
now lives in a new vector.early.exit block, which sits between the
middle.split block and the original exit block. I also had to fix
up the vplan verifier because it assumed that the block containing
a definition always dominated the parent of the user. That's no
longer the case because we can arrive at the exit block via one of
the latch or the early exiting block.

I've added a new ExtractFirstActive VPInstruction that extracts the
first active lane of a vector, i.e. the lane of the vector predicate
that triggered the exit.

Author: david-arm

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -548,6 +548,11 @@ class InnerLoopVectorizer {
                             Value *VectorTripCount, BasicBlock *MiddleBlock,
                             VPTransformState &State);
 
+  void fixupEarlyExitIVUsers(PHINode *OrigPhi, const InductionDescriptor &II,
+                             BasicBlock *VectorEarlyExitBB,
+                             BasicBlock *MiddleBlock, VPlan &Plan,
+                             VPTransformState &State);
+
   /// Iteratively sink the scalarized operands of a predicated instruction into
   /// the block that was created for it.
   void sinkScalarOperands(Instruction *PredInst);
@@ -2775,6 +2780,23 @@ BasicBlock *InnerLoopVectorizer::createVectorizedLoopSkeleton(
   return LoopVectorPreHeader;
 }
 
+static bool isValueIncomingFromBlock(BasicBlock *ExitingBB, Value *V,
+                                     Instruction *UI) {
+  PHINode *PHI = dyn_cast<PHINode>(UI);
+  assert(PHI && "Expected LCSSA form");
+
+  // If this loop has an uncountable early exit then there could be
+  // different users of OrigPhi with either:
+  //   1. Multiple users, because each exiting block (countable or
+  //      uncountable) jumps to the same exit block, or ..
+  //   2. A single user with an incoming value from a countable or
+  //      uncountable exiting block.
+  // In both cases there is no guarantee this came from a countable exiting
+  // block, i.e. the latch.
+  int Index = PHI->getBasicBlockIndex(ExitingBB);
+  return Index != -1 && PHI->getIncomingValue(Index) == V;
+}
+
 // Fix up external users of the induction variable. At this point, we are
 // in LCSSA form, with all external PHIs that use the IV having one input value,
 // coming from the remainder loop. We need those PHIs to also have a correct
@@ -2790,19 +2812,20 @@ void InnerLoopVectorizer::fixupIVUsers(PHINode *OrigPhi,
   // We allow both, but they, obviously, have different values.
 
   DenseMap<Value *, Value *> MissingVals;
+  BasicBlock *OrigLoopLatch = OrigLoop->getLoopLatch();
 
   Value *EndValue = cast<PHINode>(OrigPhi->getIncomingValueForBlock(
                                       OrigLoop->getLoopPreheader()))
                         ->getIncomingValueForBlock(MiddleBlock);
 
   // An external user of the last iteration's value should see the value that
   // the remainder loop uses to initialize its own IV.
-  Value *PostInc = OrigPhi->getIncomingValueForBlock(OrigLoop->getLoopLatch());
+  Value *PostInc = OrigPhi->getIncomingValueForBlock(OrigLoopLatch);
   for (User *U : PostInc->users()) {
     Instruction *UI = cast<Instruction>(U);
     if (!OrigLoop->contains(UI)) {
-      assert(isa<PHINode>(UI) && "Expected LCSSA form");
-      MissingVals[UI] = EndValue;
+      if (isValueIncomingFromBlock(OrigLoopLatch, PostInc, UI))
+        MissingVals[cast<PHINode>(UI)] = EndValue;
     }
   }
 
@@ -2812,7 +2835,9 @@ void InnerLoopVectorizer::fixupIVUsers(PHINode *OrigPhi,
   for (User *U : OrigPhi->users()) {
     auto *UI = cast<Instruction>(U);
     if (!OrigLoop->contains(UI)) {
-      assert(isa<PHINode>(UI) && "Expected LCSSA form");
+      if (!isValueIncomingFromBlock(OrigLoopLatch, OrigPhi, UI))
+        continue;
+
       IRBuilder<> B(MiddleBlock->getTerminator());
 
       // Fast-math-flags propagate from the original induction instruction.
@@ -2842,18 +2867,6 @@ void InnerLoopVectorizer::fixupIVUsers(PHINode *OrigPhi,
     }
   }
 
-  assert((MissingVals.empty() ||
-          all_of(MissingVals,
-                 [MiddleBlock, this](const std::pair<Value *, Value *> &P) {
-                   return all_of(
-                       predecessors(cast<Instruction>(P.first)->getParent()),
-                       [MiddleBlock, this](BasicBlock *Pred) {
-                         return Pred == MiddleBlock ||
-                                Pred == OrigLoop->getLoopLatch();
-                       });
-                 })) &&
-         "Expected escaping values from latch/middle.block only");
-
   for (auto &I : MissingVals) {
     PHINode *PHI = cast<PHINode>(I.first);
     // One corner case we have to handle is two IVs "chasing" each-other,
@@ -2866,6 +2879,102 @@ void InnerLoopVectorizer::fixupIVUsers(PHINode *OrigPhi,
   }
 }
 
+void InnerLoopVectorizer::fixupEarlyExitIVUsers(PHINode *OrigPhi,
+                                                const InductionDescriptor &II,
+                                                BasicBlock *VectorEarlyExitBB,
+                                                BasicBlock *MiddleBlock,
+                                                VPlan &Plan,
+                                                VPTransformState &State) {
+  // There are two kinds of external IV usages - those that use the value
+  // computed in the last iteration (the PHI) and those that use the penultimate
+  // value (the value that feeds into the phi from the loop latch).
+  // We allow both, but they, obviously, have different values.
+  DenseMap<Value *, Value *> MissingVals;
+  BasicBlock *OrigLoopLatch = OrigLoop->getLoopLatch();
+  BasicBlock *EarlyExitingBB = Legal->getUncountableEarlyExitingBlock();
+  Value *PostInc = OrigPhi->getIncomingValueForBlock(OrigLoopLatch);
+
+  // Obtain the canonical IV, since we have to use the most recent value
+  // before exiting the loop early. This is unlike fixupIVUsers, which has
+  // the luxury of using the end value in the middle block.
+  VPBasicBlock *EntryVPBB = Plan.getVectorLoopRegion()->getEntryBasicBlock();
+  // NOTE: We cannot call Plan.getCanonicalIV() here because the original
+  // recipe created whilst building plans is no longer valid.
+  VPHeaderPHIRecipe *CanonicalIVR =
+      cast<VPHeaderPHIRecipe>(&*EntryVPBB->begin());
+  Value *CanonicalIV = State.get(CanonicalIVR->getVPSingleValue(), true);
+
+  // Search for the mask that drove us to exit early.
+  VPBasicBlock *EarlyExitVPBB = Plan.getVectorLoopRegion()->getEarlyExit();
+  VPBasicBlock *MiddleSplitVPBB =
+      cast<VPBasicBlock>(EarlyExitVPBB->getSinglePredecessor());
+  VPInstruction *BranchOnCond =
+      cast<VPInstruction>(MiddleSplitVPBB->getTerminator());
+  assert(BranchOnCond->getOpcode() == VPInstruction::BranchOnCond &&
+         "Expected middle.split block terminator to be a branch-on-cond");
+  VPInstruction *ScalarEarlyExitCond =
+      cast<VPInstruction>(BranchOnCond->getOperand(0));
+  assert(
+      ScalarEarlyExitCond->getOpcode() == VPInstruction::AnyOf &&
+      "Expected middle.split block terminator branch condition to be any-of");
+  VPValue *VectorEarlyExitCond = ScalarEarlyExitCond->getOperand(0);
+  // Finally get the mask that led us into the early exit block.
+  Value *EarlyExitMask = State.get(VectorEarlyExitCond);
+
+  // Calculate the IV step.
+  VPValue *StepVPV = Plan.getSCEVExpansion(II.getStep());
+  assert(StepVPV && "step must have been expanded during VPlan execution");
+  Value *Step = StepVPV->isLiveIn() ? StepVPV->getLiveInIRValue()
+                                    : State.get(StepVPV, VPLane(0));
+
+  auto FixUpPhi = [&](Instruction *UI, bool PostInc) -> Value * {
+    IRBuilder<> B(VectorEarlyExitBB->getTerminator());
+    assert(isa<PHINode>(UI) && "Expected LCSSA form");
+
+    // Fast-math-flags propagate from the original induction instruction.
+    if (isa_and_nonnull<FPMathOperator>(II.getInductionBinOp()))
+      B.setFastMathFlags(II.getInductionBinOp()->getFastMathFlags());
+
+    Type *CtzType = CanonicalIV->getType();
+    Value *Ctz = B.CreateCountTrailingZeroElems(CtzType, EarlyExitMask);
+    Ctz = B.CreateAdd(Ctz, cast<PHINode>(CanonicalIV));
+    if (PostInc)
+      Ctz = B.CreateAdd(Ctz, ConstantInt::get(CtzType, 1));
+
+    Value *Escape = emitTransformedIndex(B, Ctz, II.getStartValue(), Step,
+                                         II.getKind(), II.getInductionBinOp());
+    Escape->setName("ind.early.escape");
+    return Escape;
+  };
+
+  for (User *U : PostInc->users()) {
+    auto *UI = cast<Instruction>(U);
+    if (!OrigLoop->contains(UI)) {
+      if (isValueIncomingFromBlock(EarlyExitingBB, PostInc, UI))
+        MissingVals[UI] = FixUpPhi(UI, true);
+    }
+  }
+
+  for (User *U : OrigPhi->users()) {
+    auto *UI = cast<Instruction>(U);
+    if (!OrigLoop->contains(UI)) {
+      if (isValueIncomingFromBlock(EarlyExitingBB, OrigPhi, UI))
+        MissingVals[UI] = FixUpPhi(UI, false);
+    }
+  }
+
+  for (auto &I : MissingVals) {
+    PHINode *PHI = cast<PHINode>(I.first);
+    // One corner case we have to handle is two IVs "chasing" each-other,
+    // that is %IV2 = phi [...], [ %IV1, %latch ]
+    // In this case, if IV1 has an external use, we need to avoid adding both
+    // "last value of IV1" and "penultimate value of IV2". So, verify that we
+    // don't already have an incoming value for the middle block.
+    if (PHI->getBasicBlockIndex(VectorEarlyExitBB) == -1)
+      PHI->addIncoming(I.second, VectorEarlyExitBB);
+  }
+}
+
 namespace {
 
 struct CSEDenseMapInfo {
@@ -2985,6 +3094,20 @@ void InnerLoopVectorizer::fixVectorizedLoop(VPTransformState &State) {
   PSE.getSE()->forgetLoop(OrigLoop);
   PSE.getSE()->forgetBlockAndLoopDispositions();
 
+  // When dealing with uncountable early exits we create middle.split blocks
+  // between the vector loop region and the exit block. These blocks need
+  // adding to any outer loop.
+  VPRegionBlock *VectorRegion = State.Plan->getVectorLoopRegion();
+  Loop *OuterLoop = OrigLoop->getParentLoop();
+  if (Legal->hasUncountableEarlyExit() && OuterLoop) {
+    BasicBlock *OrigEarlyExitBB = Legal->getUncountableEarlyExitBlock();
+    if (Loop *EEL = LI->getLoopFor(OrigEarlyExitBB)) {
+      BasicBlock *VectorEarlyExitBB =
+          State.CFG.VPBB2IRBB[VectorRegion->getEarlyExit()];
+      EEL->addBasicBlockToLoop(VectorEarlyExitBB, *LI);
+    }
+  }
+
   // After vectorization, the exit blocks of the original loop will have
   // additional predecessors. Invalidate SCEVs for the exit phis in case SE
   // looked through single-entry phis.
@@ -3012,15 +3135,23 @@ void InnerLoopVectorizer::fixVectorizedLoop(VPTransformState &State) {
                    getOrCreateVectorTripCount(nullptr), LoopMiddleBlock, State);
   }
 
+  if (Legal->hasUncountableEarlyExit()) {
+    VPBasicBlock *VectorEarlyExitVPBB =
+        cast<VPBasicBlock>(VectorRegion->getEarlyExit());
+    BasicBlock *VectorEarlyExitBB = State.CFG.VPBB2IRBB[VectorEarlyExitVPBB];
+    for (const auto &Entry : Legal->getInductionVars())
+      fixupEarlyExitIVUsers(Entry.first, Entry.second, VectorEarlyExitBB,
+                            LoopMiddleBlock, Plan, State);
+  }
+
   // Don't apply optimizations below when no vector region remains, as they all
   // require a vector loop at the moment.
-  if (!State.Plan->getVectorLoopRegion())
+  if (!VectorRegion)
     return;
 
   for (Instruction *PI : PredicatedInstructions)
     sinkScalarOperands(&*PI);
 
-  VPRegionBlock *VectorRegion = State.Plan->getVectorLoopRegion();
   VPBasicBlock *HeaderVPBB = VectorRegion->getEntryBasicBlock();
   BasicBlock *HeaderBB = State.CFG.VPBB2IRBB[HeaderVPBB];
 
@@ -8948,6 +9079,10 @@ static void addScalarResumePhis(VPRecipeBuilder &Builder, VPlan &Plan) {
       continue;
     }
 
+    assert(!Plan.getVectorLoopRegion()->getEarlyExit() &&
+           "Cannot handle "
+           "first-order recurrences with uncountable early exits");
+
     // The backedge value provides the value to resume coming out of a loop,
     // which for FORs is a vector whose last element needs to be extracted. The
     // start value provides the value if the loop is bypassed.
@@ -9056,9 +9191,8 @@ collectUsersInExitBlocks(Loop *OrigLoop, VPRecipeBuilder &Builder,
         // Exit values for inductions are computed and updated outside of VPlan
         // and independent of induction recipes.
         // TODO: Compute induction exit values in VPlan.
-        if (isOptimizableIVOrUse(V) &&
-            ExitVPBB->getSinglePredecessor() == MiddleVPBB)
-          continue;
+        if (isOptimizableIVOrUse(V))
+          V = VPValue::getNull();
         ExitUsersToFix.insert(ExitIRI);
         ExitIRI->addOperand(V);
       }
@@ -9085,18 +9219,30 @@ addUsersInExitBlocks(VPlan &Plan,
     for (const auto &[Idx, Op] : enumerate(ExitIRI->operands())) {
       // Pass live-in values used by exit phis directly through to their users
       // in the exit block.
-      if (Op->isLiveIn())
+      if (Op->isLiveIn() || Op->isNull())
         continue;
 
       // Currently only live-ins can be used by exit values from blocks not
       // exiting via the vector latch through to the middle block.
-      if (ExitIRI->getParent()->getSinglePredecessor() != MiddleVPBB)
-        return false;
-
       LLVMContext &Ctx = ExitIRI->getInstruction().getContext();
-      VPValue *Ext = B.createNaryOp(VPInstruction::ExtractFromEnd,
-                                    {Op, Plan.getOrAddLiveIn(ConstantInt::get(
-                                             IntegerType::get(Ctx, 32), 1))});
+      VPValue *Ext;
+      VPBasicBlock *PredVPBB =
+          cast<VPBasicBlock>(ExitIRI->getParent()->getPredecessors()[Idx]);
+      if (PredVPBB != MiddleVPBB) {
+        VPBasicBlock *VectorEarlyExitVPBB =
+            Plan.getVectorLoopRegion()->getEarlyExit();
+        VPBuilder B2(VectorEarlyExitVPBB,
+                     VectorEarlyExitVPBB->getFirstNonPhi());
+        assert(ExitIRI->getParent()->getNumPredecessors() <= 2);
+        VPValue *EarlyExitMask =
+            Plan.getVectorLoopRegion()->getVectorEarlyExitCond();
+        Ext = B2.createNaryOp(VPInstruction::ExtractFirstActive,
+                              {Op, EarlyExitMask});
+      } else {
+        Ext = B.createNaryOp(VPInstruction::ExtractFromEnd,
+                             {Op, Plan.getOrAddLiveIn(ConstantInt::get(
+                                      IntegerType::get(Ctx, 32), 1))});
+      }
       ExitIRI->setOperand(Idx, Ext);
     }
   }

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -83,6 +83,9 @@ Value *VPLane::getAsRuntimeExpr(IRBuilderBase &Builder,
   llvm_unreachable("Unknown lane kind");
 }
 
+static VPValue NullValue;
+VPValue *VPValue::Null = &NullValue;
+
 VPValue::VPValue(const unsigned char SC, Value *UV, VPDef *Def)
     : SubclassID(SC), UnderlyingVal(UV), Def(Def) {
   if (Def)

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -1225,6 +1225,9 @@ class VPInstruction : public VPRecipeWithIRFlags,
     // Returns a scalar boolean value, which is true if any lane of its single
     // operand is true.
     AnyOf,
+    // Extracts the first active lane of a vector, where the first operand is
+    // the predicate, and the second operand is the vector to extract.
+    ExtractFirstActive,
   };
 
 private:
@@ -3651,6 +3654,13 @@ class VPRegionBlock : public VPBlockBase {
   /// VPRegionBlock.
   VPBlockBase *Exiting;
 
+  /// Hold the Early Exit block of the SEME region, if one exists.
+  VPBasicBlock *EarlyExit;
+
+  /// If one exists, this keeps track of the vector early mask that triggered
+  /// the early exit.
+  VPValue *VectorEarlyExitCond;
+
   /// An indicator whether this region is to generate multiple replicated
   /// instances of output IR corresponding to its VPBlockBases.
   bool IsReplicator;
@@ -3659,6 +3669,7 @@ class VPRegionBlock : public VPBlockBase {
   VPRegionBlock(VPBlockBase *Entry, VPBlockBase *Exiting,
                 const std::string &Name = "", bool IsReplicator = false)
       : VPBlockBase(VPRegionBlockSC, Name), Entry(Entry), Exiting(Exiting),
+        EarlyExit(nullptr), VectorEarlyExitCond(nullptr),
         IsReplicator(IsReplicator) {
     assert(Entry->getPredecessors().empty() && "Entry block has predecessors.");
     assert(Exiting->getSuccessors().empty() && "Exit block has successors.");
@@ -3667,6 +3678,7 @@ class VPRegionBlock : public VPBlockBase {
   }
   VPRegionBlock(const std::string &Name = "", bool IsReplicator = false)
       : VPBlockBase(VPRegionBlockSC, Name), Entry(nullptr), Exiting(nullptr),
+        EarlyExit(nullptr), VectorEarlyExitCond(nullptr),
         IsReplicator(IsReplicator) {}
 
   ~VPRegionBlock() override {}
@@ -3700,6 +3712,22 @@ class VPRegionBlock : public VPBlockBase {
     ExitingBlock->setParent(this);
   }
 
+  /// Sets the early exit vector mask.
+  void setVectorEarlyExitCond(VPValue *V) {
+    assert(!VectorEarlyExitCond);
+    VectorEarlyExitCond = V;
+  }
+
+  /// Gets the early exit vector mask
+  VPValue *getVectorEarlyExitCond() const { return VectorEarlyExitCond; }
+
+  /// Set the vector early exit block
+  void setEarlyExit(VPBasicBlock *ExitBlock) { EarlyExit = ExitBlock; }
+
+  /// Get the vector early exit block
+  const VPBasicBlock *getEarlyExit() const { return EarlyExit; }
+  VPBasicBlock *getEarlyExit() { return EarlyExit; }
+
   /// Returns the pre-header VPBasicBlock of the loop region.
   VPBasicBlock *getPreheaderVPBB() {
     assert(!isReplicator() && "should only get pre-header of loop regions");