[llvm] [LV] Add support for cmp reductions with decreasing IVs. (PR #140451)

[Florian Hahn via llvm-commits]

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@@ -713,36 +715,66 @@ 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
+
+    // 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 FindLastIV or FindFirstIV");
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fhahn wrote:

Yeah, thought it might be good for extra safety, but dropped it for now, thanks

https://github.com/llvm/llvm-project/pull/140451