[LV] Add support for cmp reductions with decreasing IVs using SMin.

Similar to FindLastIV, add FindFirstIV to support select (icmp(), x, y)
reductions where one of x or y is a decreasing induction. This is done
via a new recurrence kind FindFirstIVSMin, which selects the first
 value from the reduction vector using smin instead of the last value
(FindLastIV). It uses signed max as sentinel value. The

Author: fhahn

llvm/include/llvm/Analysis/IVDescriptors.h

@@ -54,6 +54,9 @@ enum class RecurKind {
   FMulAdd,  ///< Sum of float products with llvm.fmuladd(a * b + sum).
   AnyOf,    ///< AnyOf reduction with select(cmp(),x,y) where one of (x,y) is
             ///< loop invariant, and both x and y are integer type.
+  FindFirstIVSMin, /// FindFirst reduction with select(icmp(),x,y) where one of
+                   ///< (x,y) is a decreasing loop induction, and both x and y
+                   ///< are integer type, producing a SMin reduction.
   FindLastIVSMax, ///< FindLast reduction with select(cmp(),x,y) where one of
                   ///< (x,y) is increasing loop induction, and both x and y
                   ///< are integer type, producing a SMax reduction.
@@ -165,13 +168,13 @@ class RecurrenceDescriptor {
   /// Returns a struct describing whether the instruction is either a
   ///   Select(ICmp(A, B), X, Y), or
   ///   Select(FCmp(A, B), X, Y)
-  /// where one of (X, Y) is an increasing loop induction variable, and the
-  /// other is a PHI value.
+  /// where one of (X, Y) is an increasing (FindLast) or decreasing (FindFirst)
+  /// loop induction variable, and the other is a PHI value.
   // TODO: Support non-monotonic variable. FindLast does not need be restricted
   // to increasing loop induction variables.
-  LLVM_ABI static InstDesc isFindLastIVPattern(Loop *TheLoop, PHINode *OrigPhi,
-                                               Instruction *I,
-                                               ScalarEvolution &SE);
+  LLVM_ABI static InstDesc isFindIVPattern(RecurKind Kind, Loop *TheLoop,
+                                           PHINode *OrigPhi, Instruction *I,
+                                           ScalarEvolution &SE);
 
   /// Returns a struct describing if the instruction is a
   /// Select(FCmp(X, Y), (Z = X op PHINode), PHINode) instruction pattern.
@@ -259,6 +262,12 @@ class RecurrenceDescriptor {
     return Kind == RecurKind::AnyOf;
   }
 
+  /// Returns true if the recurrence kind is of the form
+  ///   select(cmp(),x,y) where one of (x,y) is decreasing loop induction.
+  static bool isFindFirstIVRecurrenceKind(RecurKind Kind) {
+    return Kind == RecurKind::FindFirstIVSMin;
+  }
+
   /// Returns true if the recurrence kind is of the form
   ///   select(cmp(),x,y) where one of (x,y) is increasing loop induction.
   static bool isFindLastIVRecurrenceKind(RecurKind Kind) {
@@ -269,22 +278,35 @@ class RecurrenceDescriptor {
   /// Returns true if recurrece kind is a signed redux kind.
   static bool isSignedRecurrenceKind(RecurKind Kind) {
     return Kind == RecurKind::SMax || Kind == RecurKind::SMin ||
+           Kind == RecurKind::FindFirstIVSMin ||
            Kind == RecurKind::FindLastIVSMax;
   }
 
+  /// Returns true if the recurrence kind is of the form
+  ///   select(cmp(),x,y) where one of (x,y) is an increasing or decreasing loop
+  ///   induction.
+  static bool isFindIVRecurrenceKind(RecurKind Kind) {
+    return isFindFirstIVRecurrenceKind(Kind) ||
+           isFindLastIVRecurrenceKind(Kind);
+  }
+
   /// 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() const { return RecurrenceType; }
 
-  /// Returns the sentinel value for FindLastIV recurrences to replace the start
-  /// value.
+  /// Returns the sentinel value for FindFirstIV &FindLastIV recurrences to
+  /// replace the start value.
   Value *getSentinelValue() const {
-    assert(isFindLastIVRecurrenceKind(Kind) && "Unexpected recurrence kind");
     Type *Ty = StartValue->getType();
     unsigned BW = Ty->getIntegerBitWidth();
+    if (isFindLastIVRecurrenceKind(Kind)) {
+      return ConstantInt::get(Ty, isSignedRecurrenceKind(Kind)
+                                      ? APInt::getSignedMinValue(BW)
+                                      : APInt::getMinValue(BW));
+    }
     return ConstantInt::get(Ty, isSignedRecurrenceKind(Kind)
-                                    ? APInt::getSignedMinValue(BW)
-                                    : APInt::getMinValue(BW));
+                                    ? APInt::getSignedMaxValue(BW)
+                                    : APInt::getMaxValue(BW));
   }
 
   /// Returns a reference to the instructions used for type-promoting the

llvm/lib/Analysis/IVDescriptors.cpp

@@ -50,6 +50,7 @@ bool RecurrenceDescriptor::isIntegerRecurrenceKind(RecurKind Kind) {
   case RecurKind::UMax:
   case RecurKind::UMin:
   case RecurKind::AnyOf:
+  case RecurKind::FindFirstIVSMin:
   case RecurKind::FindLastIVSMax:
   case RecurKind::FindLastIVUMax:
     return true;
@@ -684,8 +685,9 @@ RecurrenceDescriptor::isAnyOfPattern(Loop *Loop, PHINode *OrigPhi,
 // value of the data type or a non-constant value by using mask and multiple
 // reduction operations.
 RecurrenceDescriptor::InstDesc
-RecurrenceDescriptor::isFindLastIVPattern(Loop *TheLoop, PHINode *OrigPhi,
-                                          Instruction *I, ScalarEvolution &SE) {
+RecurrenceDescriptor::isFindIVPattern(RecurKind Kind, Loop *TheLoop,
+                                      PHINode *OrigPhi, Instruction *I,
+                                      ScalarEvolution &SE) {
   // TODO: Support the vectorization of FindLastIV when the reduction phi is
   // used by more than one select instruction. This vectorization is only
   // performed when the SCEV of each increasing induction variable used by the
@@ -713,36 +715,68 @@ RecurrenceDescriptor::isFindLastIVPattern(Loop *TheLoop, PHINode *OrigPhi,
       return std::nullopt;
 
     const SCEV *Step = AR->getStepRecurrence(SE);
-    if (!SE.isKnownPositive(Step))
+
+    if (isFindFirstIVRecurrenceKind(Kind)) {
+      if (!SE.isKnownNegative(Step))
+        return std::nullopt;
+    } else if (!SE.isKnownPositive(Step))
       return std::nullopt;
 
     // Keep the minimum value of the recurrence type as the sentinel value.
     // The maximum acceptable range for the increasing induction variable,
     // called the valid range, will be defined as
+
+    const ConstantRange IVRange = SE.getSignedRange(AR);
+    // Keep the minimum (FindLast) or maximum (FindFirst) value of the
+    // recurrence type as the sentinel value. The maximum acceptable range for
+    // the induction variable, called the valid range, will be defined as
     //   [<sentinel value> + 1, <sentinel value>)
-    // where <sentinel value> is [Signed|Unsigned]Min(<recurrence type>)
+    // where <sentinel value> is [Signed|Unsigned]Min(<recurrence type>) for
+    // FindLastIV or [Signed|Unsigned]Max(<recurrence type>) for FindFirstIV.
     // TODO: This range restriction can be lifted by adding an additional
     // virtual OR reduction.
     auto CheckRange = [&](bool IsSigned) {
       const ConstantRange IVRange =
           IsSigned ? SE.getSignedRange(AR) : SE.getUnsignedRange(AR);
       unsigned NumBits = Ty->getIntegerBitWidth();
-      const APInt Sentinel = IsSigned ? APInt::getSignedMinValue(NumBits)
-                                      : APInt::getMinValue(NumBits);
-      const ConstantRange ValidRange =
-          ConstantRange::getNonEmpty(Sentinel + 1, Sentinel);
-      LLVM_DEBUG(dbgs() << "LV: FindLastIV valid range is " << ValidRange
+      ConstantRange ValidRange = ConstantRange::getEmpty(NumBits);
+      if (isFindLastIVRecurrenceKind(Kind)) {
+        APInt Sentinel = IsSigned ? APInt::getSignedMinValue(NumBits)
+                                  : APInt::getMinValue(NumBits);
+        ValidRange = ConstantRange::getNonEmpty(Sentinel + 1, Sentinel);
+      } else {
+        assert(isFindFirstIVRecurrenceKind(Kind) &&
+               "Kind must either be a FindLastIV or FindFirstIV");
+        assert(IsSigned &&
+               "only FindFirstIV with SMax is supported at the moment");
+        ValidRange =
+            ConstantRange::getNonEmpty(APInt::getSignedMinValue(NumBits),
+                                       APInt::getSignedMaxValue(NumBits) - 1);
+      }
+
+      LLVM_DEBUG(dbgs() << "LV: "
+                        << (isFindLastIVRecurrenceKind(Kind) ? "FindLastIV"
+                                                             : "FindFirstIV")
+                        << " valid range is " << ValidRange
                         << ", and the range of " << *AR << " is " << IVRange
                         << "\n");
 
       // Ensure the induction variable does not wrap around by verifying that
       // its range is fully contained within the valid range.
       return ValidRange.contains(IVRange);
     };
+    if (isFindLastIVRecurrenceKind(Kind)) {
+      if (CheckRange(true))
+        return RecurKind::FindLastIVSMax;
+      if (CheckRange(false))
+        return RecurKind::FindLastIVUMax;
+      return std::nullopt;
+    }
+    assert(isFindFirstIVRecurrenceKind(Kind) &&
+           "Kind must either be a FindLastIV or FindFirstIV");
+
     if (CheckRange(true))
-      return RecurKind::FindLastIVSMax;
-    if (CheckRange(false))
-      return RecurKind::FindLastIVUMax;
+      return RecurKind::FindFirstIVSMin;
     return std::nullopt;
   };
 
@@ -888,8 +922,8 @@ RecurrenceDescriptor::InstDesc RecurrenceDescriptor::isRecurrenceInstr(
     if (Kind == RecurKind::FAdd || Kind == RecurKind::FMul ||
         Kind == RecurKind::Add || Kind == RecurKind::Mul)
       return isConditionalRdxPattern(I);
-    if (isFindLastIVRecurrenceKind(Kind) && SE)
-      return isFindLastIVPattern(L, OrigPhi, I, *SE);
+    if (isFindIVRecurrenceKind(Kind) && SE)
+      return isFindIVPattern(Kind, L, OrigPhi, I, *SE);
     [[fallthrough]];
   case Instruction::FCmp:
   case Instruction::ICmp:
@@ -1003,6 +1037,11 @@ bool RecurrenceDescriptor::isReductionPHI(PHINode *Phi, Loop *TheLoop,
     LLVM_DEBUG(dbgs() << "Found a FindLastIV reduction PHI." << *Phi << "\n");
     return true;
   }
+  if (AddReductionVar(Phi, RecurKind::FindFirstIVSMin, TheLoop, FMF, RedDes, DB,
+                      AC, DT, SE)) {
+    LLVM_DEBUG(dbgs() << "Found a FindFirstIV reduction PHI." << *Phi << "\n");
+    return true;
+  }
   if (AddReductionVar(Phi, RecurKind::FMul, TheLoop, FMF, RedDes, DB, AC, DT,
                       SE)) {
     LLVM_DEBUG(dbgs() << "Found an FMult reduction PHI." << *Phi << "\n");
@@ -1150,6 +1189,7 @@ unsigned RecurrenceDescriptor::getOpcode(RecurKind Kind) {
   case RecurKind::Mul:
     return Instruction::Mul;
   case RecurKind::AnyOf:
+  case RecurKind::FindFirstIVSMin:
   case RecurKind::FindLastIVSMax:
   case RecurKind::FindLastIVUMax:
   case RecurKind::Or:

llvm/lib/Transforms/Utils/LoopUtils.cpp

@@ -1227,9 +1227,12 @@ Value *llvm::createFindLastIVReduction(IRBuilderBase &Builder, Value *Src,
                                        RecurKind RdxKind, Value *Start,
                                        Value *Sentinel) {
   bool IsSigned = RecurrenceDescriptor::isSignedRecurrenceKind(RdxKind);
-  Value *MaxRdx = Src->getType()->isVectorTy()
-                      ? Builder.CreateIntMaxReduce(Src, IsSigned)
-                      : Src;
+  Value *MaxRdx =
+      Src->getType()->isVectorTy()
+          ? (RecurrenceDescriptor::isFindLastIVRecurrenceKind(RdxKind)
+                 ? Builder.CreateIntMaxReduce(Src, IsSigned)
+                 : Builder.CreateIntMinReduce(Src, IsSigned))
+          : Src;
   // Correct the final reduction result back to the start value if the maximum
   // reduction is sentinel value.
   Value *Cmp =
@@ -1324,8 +1327,8 @@ Value *llvm::createSimpleReduction(IRBuilderBase &Builder, Value *Src,
 Value *llvm::createSimpleReduction(IRBuilderBase &Builder, Value *Src,
                                    RecurKind Kind, Value *Mask, Value *EVL) {
   assert(!RecurrenceDescriptor::isAnyOfRecurrenceKind(Kind) &&
-         !RecurrenceDescriptor::isFindLastIVRecurrenceKind(Kind) &&
-         "AnyOf or FindLastIV reductions are not supported.");
+         !RecurrenceDescriptor::isFindIVRecurrenceKind(Kind) &&
+         "AnyOf, FindFirstIV and FindLastIV reductions are not supported.");
   Intrinsic::ID Id = getReductionIntrinsicID(Kind);
   auto VPID = VPIntrinsic::getForIntrinsic(Id);
   assert(VPReductionIntrinsic::isVPReduction(VPID) &&