[llvm] [InstCombine] Fold (X / C) < X and (X >> C) < X into X > 0 (PR #85555)

[Nikita Popov via llvm-commits]

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@@ -7103,6 +7103,39 @@ Instruction *InstCombinerImpl::foldICmpCommutative(ICmpInst::Predicate Pred,
   if (Value *V = foldICmpWithLowBitMaskedVal(Pred, Op0, Op1, Q, *this))
     return replaceInstUsesWith(CxtI, V);
 
+  // Folding (X / Y) pred X => X ~pred 0 for some constant Y other than 0 or 1
+  {
+    const APInt *Divisor;
+    if (match(Op0, m_UDiv(m_Specific(Op1), m_APInt(Divisor))) &&
+        Divisor->ugt(1)) {
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nikic wrote:

Worth noting that this condition could be != 1 rather than > 1, because division by zero is UB anyway. Ultimately makes no difference in terms of behavior though, so your variant is fine as well.

https://github.com/llvm/llvm-project/pull/85555