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

[Ramkumar Ramachandra via llvm-commits]

================
@@ -1543,6 +1543,29 @@ static void computeKnownBitsFromOperator(const Operator *I,
         }
         break;
       }
+
+      case Instruction::UDiv: {
+        // For UDiv, the result can never exceed either the numerator, or the
+        // start value, whichever is greater. The case where the PHI is not
+        // the numerator of the UDiv is already handled by other code.
+        if (BO->getOperand(0) != P)
+          break;
+        [[fallthrough]];
+      }
+
+      case Instruction::URem: {
+        // For URem, the result can never exceed the start value.
+        SimplifyQuery RecQ = Q.getWithoutCondContext();
+
+        unsigned OpNum = P->getOperand(0) == R ? 0 : 1;
+        Instruction *RInst = P->getIncomingBlock(OpNum)->getTerminator();
+
+        RecQ.CxtI = RInst;
+        computeKnownBits(R, DemandedElts, Known2, Depth + 1, RecQ);
+        Known.Zero.setHighBits(Known2.countMinLeadingZeros());
----------------
artagnon wrote:

Consider the following loop:

```llvm
loop:
  %iv = phi i64 [9, %entry], [%iv.next, %loop]
  %iv.next = urem i64 3, %iv
  %exitcond = icmp eq %iv.next, 3
  br i1 %exitcond, label %exit, label %loop
```

It has trip-count = 1, backedge-taken-count = 0. It is not UB. Do you agree that the maximum value of %iv is 9, and not 3?

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