[llvm] [InstCombine] Fold redundant select guards for ctpop (PR #195443)

[Simon Pilgrim via llvm-commits]

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@@ -4317,6 +4317,16 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   if (Instruction *I = canonicalizeScalarSelectOfVecs(SI, *this))
     return I;
 
+  // Fold: select (icmp ult X, 2), X, ctpop(X)  -->  ctpop(X)
+  // ctpop(0)==0 and ctpop(1)==1, so the guard is always redundant.
+  if (match(FalseVal, m_Intrinsic<Intrinsic::ctpop>(m_Specific(TrueVal))) &&
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RKSimon wrote:

If you update to trunk latest you should be able to use the m_ Ctpop matcher now

https://github.com/llvm/llvm-project/pull/195443