[llvm] [ValueTracking] Compute known bits from recursive select/phi (PR #113707)

[via llvm-commits]

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@@ -1566,6 +1566,22 @@ static void computeKnownBitsFromOperator(const Operator *I,
         // Skip direct self references.
         if (IncValue == P) continue;
 
+        // Recurse, but cap the recursion to one level, because we don't
+        // want to waste time spinning around in loops.
+        // TODO: See if we can base recursion limiter on number of incoming phi
+        // edges so we don't overly clamp analysis.
+        unsigned IncDepth = MaxAnalysisRecursionDepth - 1;
+
+        // If the Use is a select of this phi, use the knownbit of the other
+        // operand to break the recursion.
+        if (auto *SI = dyn_cast<SelectInst>(IncValue)) {
+          if (SI->getTrueValue() == P || SI->getFalseValue() == P) {
+            IncValue = SI->getTrueValue() == P ? SI->getFalseValue()
+                                               : SI->getTrueValue();
+            IncDepth = Depth + 1;
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goldsteinn wrote:

Answered in https://github.com/llvm/llvm-project/pull/114689#issuecomment-2453461016, lets keep the conversion in one place.

https://github.com/llvm/llvm-project/pull/113707