[llvm] ValueTracking: simplify udiv/urem recurrences (PR #108973)

[Nikita Popov via llvm-commits]

================
@@ -1427,16 +1427,23 @@ static void computeKnownBitsFromOperator(const Operator *I,
       // this is sufficient to catch some interesting cases.
       unsigned Opcode = BO->getOpcode();
 
-      // If this is a shift recurrence, we know the bits being shifted in.
-      // We can combine that with information about the start value of the
-      // recurrence to conclude facts about the result.
       switch (Opcode) {
+      // If this is a shift recurrence, we know the bits being shifted in. We
+      // can combine that with information about the start value of the
+      // recurrence to conclude facts about the result. If this is a udiv
+      // recurrence, we know that the result can never exceed either the
+      // numerator or the start value, whichever is greater.
       case Instruction::LShr:
       case Instruction::AShr:
-      case Instruction::Shl: {
+      case Instruction::Shl:
+      case Instruction::UDiv:
         if (BO->getOperand(0) != I)
           break;
+        [[fallthrough]];
 
+      // For a urem recurrence, the result can never exceed the start value. The
+      // start value could either be the numerator or the denominator.
----------------
nikic wrote:

The second sentence should probably refer to the phi / recurrence rather than start value?

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