[llvm] [SCEV] Bound the max BE count by the range of End - Start. (PR #222752)

[Nikita Popov via llvm-commits]

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@@ -13378,8 +13364,12 @@ const SCEV *ScalarEvolution::computeMaxBECountForLT(const SCEV *Start,
   MaxEnd = IsSigned ? APIntOps::smax(MaxEnd, MinStart)
                     : APIntOps::umax(MaxEnd, MinStart);
 
-  return getUDivCeilSCEV(getConstant(MaxEnd - MinStart) /* Delta */,
-                         getConstant(StrideForMaxBECount) /* Step */);
+  APInt Delta = MaxEnd - MinStart;
+
+  // Try to refine Delta in case End - Start gives a tighter bound after
+  Delta = APIntOps::umin(Delta, getUnsignedRangeMax(getMinusSCEV(End, Start)));
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nikic wrote:

Do we need both bits of logic (i.e. the getMinusSCEV result is not always more precise)?

https://github.com/llvm/llvm-project/pull/222752