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

[Mel Chen via llvm-commits]

================
@@ -259,17 +267,33 @@ class RecurrenceDescriptor {
     return Kind == RecurKind::FindLastIV;
   }
 
+  /// 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 Kind == RecurKind::FindLastIV ||
+           Kind == RecurKind::FindFirstIVUMin ||
+           Kind == RecurKind::FindFirstIVSMin;
+  }
+
   /// 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();
-    return ConstantInt::get(Ty,
-                            APInt::getSignedMinValue(Ty->getIntegerBitWidth()));
+    if (isFindLastIVRecurrenceKind(Kind)) {
+      return ConstantInt::get(
+          Ty, APInt::getSignedMinValue(Ty->getIntegerBitWidth()));
+    } else if (Kind == RecurKind::FindFirstIVSMin) {
+      return ConstantInt::get(
+          Ty, APInt::getSignedMaxValue(Ty->getIntegerBitWidth()));
+    } else {
+      assert(Kind == RecurKind::FindFirstIVUMin);
+      return ConstantInt::get(Ty, APInt::getMaxValue(Ty->getIntegerBitWidth()));
+    }
   }
----------------
Mel-Chen wrote:

https://github.com/llvm/llvm-project/pull/141788 Try to change the sentinel value to be selected dynamically instead of based on RecurKind statically.
If I'm not mistaken, the values for signed max, unsigned max, signed min, and unsigned min are all distinct.
So making the sentinel value non-static could help with supporting the unsigned version, I think.

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