[llvm] [InstCombine] Fold square into comparison with constant (PR #196480)

[via llvm-commits]

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@@ -2195,6 +2195,14 @@ Instruction *InstCombinerImpl::foldICmpMulConstant(ICmpInst &Cmp,
       (Mul->hasNoUnsignedWrap() || Mul->hasNoSignedWrap()))
     return new ICmpInst(Pred, X, ConstantInt::getNullValue(MulTy));
 
+  // If unsigned compare or equality comparison of self multiply and square,
+  // compare square roots
+  if (!Cmp.isSigned() && X == Mul->getOperand(1) && Mul->hasNoUnsignedWrap()) {
+    APInt R = C.sqrt();
+    if (C == R * R)
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eebop wrote:

This can be weakened for `lt`. For instance `x * x < 10` <-> `x < 4`. The exact comparison is only needed in the case of `eq`. (And even then, a further optimization is to note that `x * x == 10` can be reduced to `ret false`)

https://github.com/llvm/llvm-project/pull/196480