[LV] Use Demanded Bits and ValueTracking for reduction type-shrinking

[mcrosier]

Summary

The type-shrinking logic in reduction detection, although narrow in scope, is also rather ad-hoc, which has led to bugs (e.g., PR35734). This patch modifies the approach to rely on the demanded bits and value tracking analyses, if available. We currently perform type-shrinking separately for reductions and other instructions in the loop. Long-term, we should probably think about computing minimal bit widths in a more complete way for the loops we want to vectorize.

Reference: https://bugs.llvm.org/show_bug.cgi?id=35734

[dcaballe]

Hi Matthew,

thanks for taking care of this! I like the general idea of the fix but I have a concern regarding the TODO in line 408:

// TODO: We should not rely on InstCombine to rewrite the reduction in the
//       smaller type. We should just generate a correctly typed expression
//       to begin with.

The cost model is relying on an optimization that will hopefully be applied by InstCombine. I wonder if it would be too complicated to implement the actual type shrinking in LV code gen as part of the fix.
That would better align cost modeling with LV code generation, which is one the major concerns of the current infrastructure.

In the future VPlan infrastructure, we will definitely need to address this optimization as a VPlan-to-VPlan transformation. Thanks for bringing up this issue.
Diego

lib/Transforms/Utils/LoopUtils.cpp

168–169

(I don't know DB/ValueTracking in depth so maybe I'm missing something.)
If I understand correctly, we would try with value tracking in the following scenario:

117    MaxBitWidth = 64
...
127    MaxBitWidth = 33
...
129    MaxBitWidth = 64
...
132    First condition is true
}

But we wouldn't in the following one:

117    MaxBitWidth = 64
...
127    MaxBitWidth = 31
...
129    MaxBitWidth = 32
...
132    First condition is false
}

Is this the expected behavior? In other words, if DB returns a width narrower than the original one and it's later rounded up to the original width (first scenario), could value tracking return an even narrower width?

If so, shouldn't we always try with value tracking, even when MaxBitWidth != DL.getTypeSizeInBits?
Otherwise, shouldn't we skip value tracking for those cases (first scenario)? We could invoke isPowerOf2_64 only once at the end of the function.

386

may been -> may have been?

[mssimpso]

In D42309#984646, @dcaballe wrote:

Hi Matthew,

thanks for taking care of this! I like the general idea of the fix but I have a concern regarding the TODO in line 408:

// TODO: We should not rely on InstCombine to rewrite the reduction in the
//       smaller type. We should just generate a correctly typed expression
//       to begin with.

The cost model is relying on an optimization that will hopefully be applied by InstCombine. I wonder if it would be too complicated to implement the actual type shrinking in LV code gen as part of the fix.
That would better align cost modeling with LV code generation, which is one the major concerns of the current infrastructure.

In the future VPlan infrastructure, we will definitely need to address this optimization as a VPlan-to-VPlan transformation. Thanks for bringing up this issue.
Diego

Yes, my comment was intended to draw attention to something that needs to be addressed in the future VPlan work. I think we will want to generate the actual type-shrunk code instead of inserting trunc/ext pairs, and assuming InstCombine will do what we want it to. We also generate trunc/ext pairs in SLP for type-shrinking, which needs to be addressed as well, I think. As you point out, this will better align the cost model with code generation.

lib/Transforms/Utils/LoopUtils.cpp

168–169

Demanded bits and value tracking will report different bit widths, in general. My thinking was that if demanded bits is able to limit the bit width at all, we use that width. Otherwise, we can try to do something with value tracking, which I think can be a more expensive analysis.

So I think what we should do there is move the isPowerOf2_64 round-ups to the very end. The MaxBitWidth == DL.getTypeSizeInBits(Exit->getType()) condition will then check if demanded bits was able to tell us anything before we do the rounding.

386

Nice catch!

[mcrosier]

ping.

[dcaballe]

Hi Chad,

I was waiting for the comments in lines 132 and 389 to be addressed.
Other than that, LGTM.

Thanks,
Diego

[mcrosier]

Address Diego's comments.

[mcrosier]

Hi Digeo,
Matt will be out of the office for a few weeks and he asked me to follow up on this patch. Hopefully, the new version addresses all of your concerns.

Regards,
Chad

[dcaballe]

LGTM

include/llvm/Transforms/Utils/LoopUtils.h

 
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/DemandedBits.h"
 #include "llvm/Analysis/EHPersonalities.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Operator.h"
 #include "llvm/IR/ValueHandle.h"
 #include "llvm/Support/Casting.h"
public:
   /// RecurrenceKind.
   static unsigned getRecurrenceBinOp(RecurrenceKind Kind);
 
   /// Returns a Min/Max operation corresponding to MinMaxRecurrenceKind.
   static Value *createMinMaxOp(IRBuilder<> &Builder, MinMaxRecurrenceKind RK,
                                Value *Left, Value *Right);
 
   /// Returns true if Phi is a reduction of type Kind and adds it to the
-  /// RecurrenceDescriptor.
+  /// RecurrenceDescriptor. If either \p DB is non-null or \p AC and \p DT are
+  /// non-null, the minimal bit width needed to compute the reduction will be
+  /// computed.
   static bool AddReductionVar(PHINode *Phi, RecurrenceKind Kind, Loop *TheLoop,
                               bool HasFunNoNaNAttr,
-                              RecurrenceDescriptor &RedDes);
-
-  /// Returns true if Phi is a reduction in TheLoop. The RecurrenceDescriptor is
-  /// returned in RedDes.
+                              RecurrenceDescriptor &RedDes,
+                              DemandedBits *DB = nullptr,
+                              AssumptionCache *AC = nullptr,
+                              DominatorTree *DT = nullptr);
+
+  /// Returns true if Phi is a reduction in TheLoop. The RecurrenceDescriptor
+  /// is returned in RedDes. If either \p DB is non-null or \p AC and \p DT are
+  /// non-null, the minimal bit width needed to compute the reduction will be
+  /// computed.
   static bool isReductionPHI(PHINode *Phi, Loop *TheLoop,
-                             RecurrenceDescriptor &RedDes);
+                             RecurrenceDescriptor &RedDes,
+                             DemandedBits *DB = nullptr,
+                             AssumptionCache *AC = nullptr,
+                             DominatorTree *DT = nullptr);
 
   /// Returns true if Phi is a first-order recurrence. A first-order recurrence
   /// is a non-reduction recurrence relation in which the value of the
   /// recurrence in the current loop iteration equals a value defined in the
   /// previous iteration. \p SinkAfter includes pairs of instructions where the
   /// first will be rescheduled to appear after the second if/when the loop is
   /// vectorized. It may be augmented with additional pairs if needed in order
   /// to handle Phi as a first-order recurrence.
public:
   static bool isIntegerRecurrenceKind(RecurrenceKind Kind);
 
   /// Returns true if the recurrence kind is a floating point kind.
   static bool isFloatingPointRecurrenceKind(RecurrenceKind Kind);
 
   /// Returns true if the recurrence kind is an arithmetic kind.
   static bool isArithmeticRecurrenceKind(RecurrenceKind Kind);
 
-  /// Determines if Phi may have been type-promoted. If Phi has a single user
-  /// that ANDs the Phi with a type mask, return the user. RT is updated to
-  /// account for the narrower bit width represented by the mask, and the AND
-  /// instruction is added to CI.
-  static Instruction *lookThroughAnd(PHINode *Phi, Type *&RT,
-                                     SmallPtrSetImpl<Instruction *> &Visited,
-                                     SmallPtrSetImpl<Instruction *> &CI);
-
-  /// Returns true if all the source operands of a recurrence are either
-  /// SExtInsts or ZExtInsts. This function is intended to be used with
-  /// lookThroughAnd to determine if the recurrence has been type-promoted. The
-  /// source operands are added to CI, and IsSigned is updated to indicate if
-  /// all source operands are SExtInsts.
-  static bool getSourceExtensionKind(Instruction *Start, Instruction *Exit,
-                                     Type *RT, bool &IsSigned,
-                                     SmallPtrSetImpl<Instruction *> &Visited,
-                                     SmallPtrSetImpl<Instruction *> &CI);
-
   /// Returns the type of the recurrence. This type can be narrower than the
   /// actual type of the Phi if the recurrence has been type-promoted.
   Type *getRecurrenceType() { return RecurrenceType; }
 
   /// Returns a reference to the instructions used for type-promoting the
   /// recurrence.
   SmallPtrSet<Instruction *, 8> &getCastInsts() { return CastInsts; }
 

lib/Transforms/Utils/LoopUtils.cpp

 #include "llvm/Analysis/GlobalsModRef.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/LoopPass.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
 #include "llvm/Analysis/ScalarEvolutionExpander.h"
 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/IR/ValueHandle.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/KnownBits.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 
 using namespace llvm;
 using namespace llvm::PatternMatch;
 
 #define DEBUG_TYPE "loop-utils"
 
 bool RecurrenceDescriptor::areAllUsesIn(Instruction *I,
bool RecurrenceDescriptor::isArithmeticRecurrenceKind(RecurrenceKind Kind) {
   case RK_IntegerMult:
   case RK_FloatAdd:
   case RK_FloatMult:
     return true;
   }
   return false;
 }
 
-Instruction *
-RecurrenceDescriptor::lookThroughAnd(PHINode *Phi, Type *&RT,
-                                     SmallPtrSetImpl<Instruction *> &Visited,
-                                     SmallPtrSetImpl<Instruction *> &CI) {
+/// Determines if Phi may have been type-promoted. If Phi has a single user
+/// that ANDs the Phi with a type mask, return the user. RT is updated to
+/// account for the narrower bit width represented by the mask, and the AND
+/// instruction is added to CI.
+static Instruction *lookThroughAnd(PHINode *Phi, Type *&RT,
   if (!Phi->hasOneUse())
     return Phi;
 
   const APInt *M = nullptr;
   Instruction *I, *J = cast<Instruction>(Phi->use_begin()->getUser());
 
   // Matches either I & 2^x-1 or 2^x-1 & I. If we find a match, we update RT
   // with a new integer type of the corresponding bit width.
   if (match(J, m_c_And(m_Instruction(I), m_APInt(M)))) {
     int32_t Bits = (*M + 1).exactLogBase2();
     if (Bits > 0) {
       RT = IntegerType::get(Phi->getContext(), Bits);
       Visited.insert(Phi);
       CI.insert(J);
       return J;
     }
   }
   return Phi;
 }
 
-bool RecurrenceDescriptor::getSourceExtensionKind(
-    Instruction *Start, Instruction *Exit, Type *RT, bool &IsSigned,
-    SmallPtrSetImpl<Instruction *> &Visited,
-    SmallPtrSetImpl<Instruction *> &CI) {
+/// Compute the minimal bit width needed to represent a reduction whose exit
+/// instruction is given by Exit.
+static std::pair<Type *, bool> computeRecurrenceType(Instruction *Exit,
+                                                     DemandedBits *DB,
+                                                     AssumptionCache *AC,
+                                                     DominatorTree *DT) {
+  bool IsSigned = false;
+  const DataLayout &DL = Exit->getModule()->getDataLayout();
+  uint64_t MaxBitWidth = DL.getTypeSizeInBits(Exit->getType());
+
+  if (DB) {
+    // Use the demanded bits analysis to determine the bits that are live out
+    // of the exit instruction, rounding up to the nearest power of two. If the
+    // use of demanded bits results in a smaller bit width, we know the value
+    // must be positive (i.e., IsSigned = false), because if this were not the
+    // case, the sign bit would have been demanded.
+    auto Mask = DB->getDemandedBits(Exit);
+    MaxBitWidth = Mask.getBitWidth() - Mask.countLeadingZeros();
+  }
+
+  if (MaxBitWidth == DL.getTypeSizeInBits(Exit->getType()) && AC && DT) {
+    // If demanded bits wasn't able to limit the bit width, we can try to use
+    // value tracking instead. This can be the case, for example, if the value
+    // may be negative.
+    auto NumSignBits = ComputeNumSignBits(Exit, DL, 0, AC, nullptr, DT);
+    auto NumTypeBits = DL.getTypeSizeInBits(Exit->getType());
+    MaxBitWidth = NumTypeBits - NumSignBits;
+    KnownBits Bits = computeKnownBits(Exit, DL);
+    if (!Bits.isNonNegative()) {
+      // If the value is not known to be non-negative, we set IsSigned to true,
+      // meaning that we will use sext instructions instead of zext
+      // instructions to restore the original type.
+      IsSigned = true;
+      if (!Bits.isNegative())
+        // If the value is not known to be negative, we don't known what the
+        // upper bit is, and therefore, we don't know what kind of extend we
+        // will need. In this case, just increase the bit width by one bit and
+        // use sext.
+        ++MaxBitWidth;
+    }
+  }
+  if (!isPowerOf2_64(MaxBitWidth))
+    MaxBitWidth = NextPowerOf2(MaxBitWidth);
+
+  return std::make_pair(Type::getIntNTy(Exit->getContext(), MaxBitWidth),
+                        IsSigned);
+}
+
+/// Collect cast instructions that can be ignored in the vectorizer's cost
+/// model, given a reduction exit value and the minimal type in which the
+/// reduction can be represented.
+static void collectCastsToIgnore(Loop *TheLoop, Instruction *Exit,
+                                 Type *RecurrenceType,
+                                 SmallPtrSetImpl<Instruction *> &Casts) {
 
   SmallVector<Instruction *, 8> Worklist;
-  bool FoundOneOperand = false;
-  unsigned DstSize = RT->getPrimitiveSizeInBits();
+  SmallPtrSet<Instruction *, 8> Visited;
   Worklist.push_back(Exit);
 
-  // Traverse the instructions in the reduction expression, beginning with the
-  // exit value.
   while (!Worklist.empty()) {
-    Instruction *I = Worklist.pop_back_val();
-    for (Use &U : I->operands()) {
-
-      // Terminate the traversal if the operand is not an instruction, or we
-      // reach the starting value.
-      Instruction *J = dyn_cast<Instruction>(U.get());
-      if (!J || J == Start)
-        continue;
-
-      // Otherwise, investigate the operation if it is also in the expression.
-      if (Visited.count(J)) {
-        Worklist.push_back(J);
+    Instruction *Val = Worklist.pop_back_val();
+    Visited.insert(Val);
+    if (auto *Cast = dyn_cast<CastInst>(Val))
+      if (Cast->getSrcTy() == RecurrenceType) {
+        // If the source type of a cast instruction is equal to the recurrence
+        // type, it will be eliminated, and should be ignored in the vectorizer
+        // cost model.
+        Casts.insert(Cast);
         continue;
       }
 
-      // If the operand is not in Visited, it is not a reduction operation, but
-      // it does feed into one. Make sure it is either a single-use sign- or
-      // zero-extend instruction.
-      CastInst *Cast = dyn_cast<CastInst>(J);
-      bool IsSExtInst = isa<SExtInst>(J);
-      if (!Cast || !Cast->hasOneUse() || !(isa<ZExtInst>(J) || IsSExtInst))
-        return false;
-
-      // Ensure the source type of the extend is no larger than the reduction
-      // type. It is not necessary for the types to be identical.
-      unsigned SrcSize = Cast->getSrcTy()->getPrimitiveSizeInBits();
-      if (SrcSize > DstSize)
-        return false;
-
-      // Furthermore, ensure that all such extends are of the same kind.
-      if (FoundOneOperand) {
-        if (IsSigned != IsSExtInst)
-          return false;
-      } else {
-        FoundOneOperand = true;
-        IsSigned = IsSExtInst;
-      }
-
-      // Lastly, if the source type of the extend matches the reduction type,
-      // add the extend to CI so that we can avoid accounting for it in the
-      // cost model.
-      if (SrcSize == DstSize)
-        CI.insert(Cast);
-    }
-  }
-  return true;
-}
+    // Add all operands to the work list if they are loop-varying values that
+    // we haven't yet visited.
+    for (Value *O : cast<User>(Val)->operands())
+      if (auto *I = dyn_cast<Instruction>(O))
+        if (TheLoop->contains(I) && !Visited.count(I))
+          Worklist.push_back(I);
 
 bool RecurrenceDescriptor::AddReductionVar(PHINode *Phi, RecurrenceKind Kind,
                                            Loop *TheLoop, bool HasFunNoNaNAttr,
-                                           RecurrenceDescriptor &RedDes) {
+                                           RecurrenceDescriptor &RedDes,
+                                           DemandedBits *DB,
+                                           AssumptionCache *AC,
+                                           DominatorTree *DT) {
   if (Phi->getNumIncomingValues() != 2)
     return false;
 
   // Reduction variables are only found in the loop header block.
   if (Phi->getParent() != TheLoop->getHeader())
     return false;
 
   // Obtain the reduction start value from the value that comes from the loop
bool RecurrenceDescriptor::AddReductionVar(PHINode *Phi, RecurrenceKind Kind,
   // pattern or more than just a select and cmp.
   if ((Kind == RK_IntegerMinMax || Kind == RK_FloatMinMax) &&
       NumCmpSelectPatternInst != 2)
     return false;
 
   if (!FoundStartPHI || !FoundReduxOp || !ExitInstruction)
     return false;
 
-  // If we think Phi may have been type-promoted, we also need to ensure that
-  // all source operands of the reduction are either SExtInsts or ZEstInsts. If
-  // so, we will be able to evaluate the reduction in the narrower bit width.
-  if (Start != Phi)
-    if (!getSourceExtensionKind(Start, ExitInstruction, RecurrenceType,
-                                IsSigned, VisitedInsts, CastInsts))
+  if (Start != Phi) {
+    // If the starting value is not the same as the phi node, we speculatively
+    // looked through an 'and' instruction when evaluating a potential
+    // arithmetic reduction to determine if it may have been type-promoted.
+    //
+    // We now compute the minimal bit width that is required to represent the
+    // reduction. If this is the same width that was indicated by the 'and', we
+    // can represent the reduction in the smaller type. The 'and' instruction
+    // will be eliminated since it will essentially be a cast instruction that
+    // can be ignore in the cost model. If we compute a different type than we
+    // did when evaluating the 'and', the 'and' will not be eliminated, and we
+    // will end up with different kinds of operations in the recurrence
+    // expression (e.g., RK_IntegerAND, RK_IntegerADD). We give up if this is
+    // the case.
+    //
+    // The vectorizer relies on InstCombine to perform the actual
+    // type-shrinking. It does this by inserting instructions to truncate the
+    // exit value of the reduction to the width indicated by RecurrenceType and
+    // then extend this value back to the original width. If IsSigned is false,
+    // a 'zext' instruction will be generated; otherwise, a 'sext' will be
+    // used.
+    //
+    // TODO: We should not rely on InstCombine to rewrite the reduction in the
+    //       smaller type. We should just generate a correctly typed expression
+    //       to begin with.
+    Type *ComputedType;
+    std::tie(ComputedType, IsSigned) =
+        computeRecurrenceType(ExitInstruction, DB, AC, DT);
+    if (ComputedType != RecurrenceType)
       return false;
 
+    // The recurrence expression will be represented in a narrower type. If
+    // there are any cast instructions that will be unnecessary, collect them
+    // in CastInsts. Note that the 'and' instruction was already included in
+    // this list.
+    //
+    // TODO: A better way to represent this may be to tag in some way all the
+    //       instructions that are a part of the reduction. The vectorizer cost
+    //       model could then apply the recurrence type to these instructions,
+    //       without needing a white list of instructions to ignore.
+    collectCastsToIgnore(TheLoop, ExitInstruction, RecurrenceType, CastInsts);
+  }
+
   // We found a reduction var if we have reached the original phi node and we
   // only have a single instruction with out-of-loop users.
 
   // The ExitInstruction(Instruction which is allowed to have out-of-loop users)
   // is saved as part of the RecurrenceDescriptor.
 
   // Save the description of this reduction variable.
   RecurrenceDescriptor RD(
if (Insts.count(dyn_cast<Instruction>(*Use)))
       ++NumUses;
     if (NumUses > 1)
       return true;
   }
 
   return false;
 }
 bool RecurrenceDescriptor::isReductionPHI(PHINode *Phi, Loop *TheLoop,
-                                          RecurrenceDescriptor &RedDes) {
+                                          RecurrenceDescriptor &RedDes,
+                                          DemandedBits *DB, AssumptionCache *AC,
+                                          DominatorTree *DT) {
 
   BasicBlock *Header = TheLoop->getHeader();
   Function &F = *Header->getParent();
   bool HasFunNoNaNAttr =
       F.getFnAttribute("no-nans-fp-math").getValueAsString() == "true";
 
-  if (AddReductionVar(Phi, RK_IntegerAdd, TheLoop, HasFunNoNaNAttr, RedDes)) {
+  if (AddReductionVar(Phi, RK_IntegerAdd, TheLoop, HasFunNoNaNAttr, RedDes, DB,
+                      AC, DT)) {
     DEBUG(dbgs() << "Found an ADD reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RK_IntegerMult, TheLoop, HasFunNoNaNAttr, RedDes)) {
+  if (AddReductionVar(Phi, RK_IntegerMult, TheLoop, HasFunNoNaNAttr, RedDes, DB,
+                      AC, DT)) {
     DEBUG(dbgs() << "Found a MUL reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RK_IntegerOr, TheLoop, HasFunNoNaNAttr, RedDes)) {
+  if (AddReductionVar(Phi, RK_IntegerOr, TheLoop, HasFunNoNaNAttr, RedDes, DB,
+                      AC, DT)) {
     DEBUG(dbgs() << "Found an OR reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RK_IntegerAnd, TheLoop, HasFunNoNaNAttr, RedDes)) {
+  if (AddReductionVar(Phi, RK_IntegerAnd, TheLoop, HasFunNoNaNAttr, RedDes, DB,
+                      AC, DT)) {
     DEBUG(dbgs() << "Found an AND reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RK_IntegerXor, TheLoop, HasFunNoNaNAttr, RedDes)) {
+  if (AddReductionVar(Phi, RK_IntegerXor, TheLoop, HasFunNoNaNAttr, RedDes, DB,
+                      AC, DT)) {
     DEBUG(dbgs() << "Found a XOR reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RK_IntegerMinMax, TheLoop, HasFunNoNaNAttr,
-                      RedDes)) {
+  if (AddReductionVar(Phi, RK_IntegerMinMax, TheLoop, HasFunNoNaNAttr, RedDes,
+                      DB, AC, DT)) {
     DEBUG(dbgs() << "Found a MINMAX reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RK_FloatMult, TheLoop, HasFunNoNaNAttr, RedDes)) {
+  if (AddReductionVar(Phi, RK_FloatMult, TheLoop, HasFunNoNaNAttr, RedDes, DB,
+                      AC, DT)) {
     DEBUG(dbgs() << "Found an FMult reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RK_FloatAdd, TheLoop, HasFunNoNaNAttr, RedDes)) {
+  if (AddReductionVar(Phi, RK_FloatAdd, TheLoop, HasFunNoNaNAttr, RedDes, DB,
+                      AC, DT)) {
     DEBUG(dbgs() << "Found an FAdd reduction PHI." << *Phi << "\n");
     return true;
   }
-  if (AddReductionVar(Phi, RK_FloatMinMax, TheLoop, HasFunNoNaNAttr, RedDes)) {
+  if (AddReductionVar(Phi, RK_FloatMinMax, TheLoop, HasFunNoNaNAttr, RedDes, DB,
+                      AC, DT)) {
     DEBUG(dbgs() << "Found an float MINMAX reduction PHI." << *Phi << "\n");
     return true;
   }
   // Not a reduction of known type.
   return false;
 }
 
 bool RecurrenceDescriptor::isFirstOrderRecurrence(

lib/Transforms/Vectorize/LoopVectorize.cpp

 class LoopVectorizationLegality {
 public:
   LoopVectorizationLegality(
       Loop *L, PredicatedScalarEvolution &PSE, DominatorTree *DT,
       TargetLibraryInfo *TLI, AliasAnalysis *AA, Function *F,
       const TargetTransformInfo *TTI,
       std::function<const LoopAccessInfo &(Loop &)> *GetLAA, LoopInfo *LI,
       OptimizationRemarkEmitter *ORE, LoopVectorizationRequirements *R,
-      LoopVectorizeHints *H)
+      LoopVectorizeHints *H, DemandedBits *DB, AssumptionCache *AC)
       : TheLoop(L), PSE(PSE), TLI(TLI), TTI(TTI), DT(DT), GetLAA(GetLAA),
-        ORE(ORE), InterleaveInfo(PSE, L, DT, LI), Requirements(R), Hints(H) {}
+        ORE(ORE), InterleaveInfo(PSE, L, DT, LI), Requirements(R), Hints(H),
+        DB(DB), AC(AC) {}
 
   /// ReductionList contains the reduction descriptors for all
   /// of the reductions that were found in the loop.
   using ReductionList = DenseMap<PHINode *, RecurrenceDescriptor>;
 
   /// InductionList saves induction variables and maps them to the
   /// induction descriptor.
   using InductionList = MapVector<PHINode *, InductionDescriptor>;
private:
   bool HasFunNoNaNAttr = false;
 
   /// Vectorization requirements that will go through late-evaluation.
   LoopVectorizationRequirements *Requirements;
 
   /// Used to emit an analysis of any legality issues.
   LoopVectorizeHints *Hints;
 
+  /// The demanded bits analsyis is used to compute the minimum type size in
+  /// which a reduction can be computed.
+  DemandedBits *DB;
+
+  /// The assumption cache analysis is used to compute the minimum type size in
+  /// which a reduction can be computed.
+  AssumptionCache *AC;
+
   /// While vectorizing these instructions we have to generate a
   /// call to the appropriate masked intrinsic
   SmallPtrSet<const Instruction *, 8> MaskedOp;
 };
 
 /// LoopVectorizationCostModel - estimates the expected speedups due to
 /// vectorization.
 /// In many cases vectorization is not profitable. This can happen because of
for (Instruction &I : *BB) {
         if (Phi->getNumIncomingValues() != 2) {
           ORE->emit(createMissedAnalysis("CFGNotUnderstood", Phi)
                     << "control flow not understood by vectorizer");
           DEBUG(dbgs() << "LV: Found an invalid PHI.\n");
           return false;
         }
 
         RecurrenceDescriptor RedDes;
-        if (RecurrenceDescriptor::isReductionPHI(Phi, TheLoop, RedDes)) {
+        if (RecurrenceDescriptor::isReductionPHI(Phi, TheLoop, RedDes, DB, AC,
+                                                 DT)) {
           if (RedDes.hasUnsafeAlgebra())
             Requirements->addUnsafeAlgebraInst(RedDes.getUnsafeAlgebraInst());
           AllowedExit.insert(RedDes.getLoopExitInstr());
           Reductions[Phi] = RedDes;
           continue;
         }
 
         InductionDescriptor ID;
if (!Hints.allowVectorization(F, L, AlwaysVectorize)) {
     return false;
   }
 
   PredicatedScalarEvolution PSE(*SE, *L);
 
   // Check if it is legal to vectorize the loop.
   LoopVectorizationRequirements Requirements(*ORE);
   LoopVectorizationLegality LVL(L, PSE, DT, TLI, AA, F, TTI, GetLAA, LI, ORE,
-                                &Requirements, &Hints);
+                                &Requirements, &Hints, DB, AC);
   if (!LVL.canVectorize()) {
     DEBUG(dbgs() << "LV: Not vectorizing: Cannot prove legality.\n");
     emitMissedWarning(F, L, Hints, ORE);
     return false;
   }
 
   // Check the function attributes to find out if this function should be
   // optimized for size.

test/Transforms/LoopVectorize/reduction-small-size.ll

 ; RUN: opt < %s -force-vector-width=4 -force-vector-interleave=1 -loop-vectorize -S | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 
 ; CHECK-LABEL: @PR34687(
 ; CHECK:       vector.body:
 ; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, %vector.ph ], [ [[INDEX_NEXT:%.*]], %[[LATCH:.*]] ]
 ; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <4 x i32> [ zeroinitializer, %vector.ph ], [ [[TMP17:%.*]], %[[LATCH]] ]
 ; CHECK:       [[LATCH]]:
 ; CHECK:         [[TMP13:%.*]] = and <4 x i32> [[VEC_PHI]], <i32 255, i32 255, i32 255, i32 255>
 ; CHECK-NEXT:    [[TMP14:%.*]] = add nuw nsw <4 x i32> [[TMP13]], {{.*}}
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add i32 [[INDEX]], 4
 ; CHECK:         [[TMP16:%.*]] = trunc <4 x i32> [[TMP14]] to <4 x i8>
 ; CHECK-NEXT:    [[TMP17]] = zext <4 x i8> [[TMP16]] to <4 x i32>
 ; CHECK-NEXT:    br i1 {{.*}}, label %middle.block, label %vector.body
 ;
-define void @PR34687(i1 %c, i32 %x, i32 %n) {
+define i8 @PR34687(i1 %c, i32 %x, i32 %n) {
 entry:
   br label %for.body
 
 for.body:
   %i = phi i32 [ 0, %entry ], [ %i.next, %if.end ]
   %r = phi i32 [ 0, %entry ], [ %r.next, %if.end ]
   br i1 %c, label %if.then, label %if.end
 
 if.then:
   %tmp0 = sdiv i32 undef, undef
   br label %if.end
 
 if.end:
   %tmp1 = and i32 %r, 255
   %i.next = add nsw i32 %i, 1
   %r.next = add nuw nsw i32 %tmp1, %x
   %cond = icmp eq i32 %i.next, %n
   br i1 %cond, label %for.end, label %for.body
 
 for.end:
   %tmp2 = phi i32 [ %r.next, %if.end ]
-  ret void
+  %tmp3 = trunc i32 %tmp2 to i8
+  ret i8 %tmp3
+}
+
+; CHECK-LABEL: @PR35734(
+; CHECK:       vector.ph:
+; CHECK:         [[TMP3:%.*]] = insertelement <4 x i32> zeroinitializer, i32 %y, i32 0
+; CHECK-NEXT:    br label %vector.body
+; CHECK:       vector.body:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, %vector.ph ], [ [[INDEX_NEXT:%.*]], %vector.body ]
+; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <4 x i32> [ [[TMP3]], %vector.ph ], [ [[TMP9:%.*]], %vector.body ]
+; CHECK:         [[TMP5:%.*]] = and <4 x i32> [[VEC_PHI]], <i32 1, i32 1, i32 1, i32 1>
+; CHECK-NEXT:    [[TMP6:%.*]] = add <4 x i32> [[TMP5]], <i32 -1, i32 -1, i32 -1, i32 -1>
+; CHECK-NEXT:    [[INDEX_NEXT]] = add i32 [[INDEX]], 4
+; CHECK:         [[TMP8:%.*]] = trunc <4 x i32> [[TMP6]] to <4 x i1>
+; CHECK-NEXT:    [[TMP9]] = sext <4 x i1> [[TMP8]] to <4 x i32>
+; CHECK-NEXT:    br i1 {{.*}}, label %middle.block, label %vector.body
+;
+define i32 @PR35734(i32 %x, i32 %y) {
+entry:
+  br label %for.body
+
+for.body:
+  %i = phi i32 [ %x, %entry ], [ %i.next, %for.body ]
+  %r = phi i32 [ %y, %entry ], [ %r.next, %for.body ]
+  %tmp0 = and i32 %r, 1
+  %r.next = add i32 %tmp0, -1
+  %i.next = add nsw i32 %i, 1
+  %cond = icmp sgt i32 %i, 77
+  br i1 %cond, label %for.end, label %for.body
+
+for.end:
+  %tmp1 = phi i32 [ %r.next, %for.body ]
+  ret i32 %tmp1
 }