[llvm] ValueTracking: complete matchSimpleRecurrence (PR #108973)

[Yingwei Zheng via llvm-commits]

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@@ -1514,9 +1514,26 @@ static void computeKnownBitsFromOperator(const Operator *I,
                    Known3.isNonNegative())
             Known.makeNonNegative();
         }
-
         break;
       }
+      case Instruction::UDiv:
+      case Instruction::URem:
+        // Result cannot be larger than start value.
+        Known.Zero.setHighBits(Known2.countMinLeadingZeros());
+        break;
+      case Instruction::SDiv:
+      case Instruction::SRem: {
+        // Magnitude of result cannot be larger than that of start value.
+        if (Known2.isNonNegative())
+          Known.Zero.setHighBits(Known2.countMinLeadingZeros());
+        else if (Known2.isNegative())
+          Known.One.setHighBits(Known2.countMinLeadingOnes());
----------------
dtcxzyw wrote:

Is it correct? A counterexample: `for (int i = -9; cond; i /= 7)`.
The phi node should be known to be non-zero. I prefer to drop the `Known2.isNegative()` case.



https://github.com/llvm/llvm-project/pull/108973