[llvm] [SCEV] BECount to zero if `((-1 + (1 smax %x))<nsw> /u %x)` holds (PR #104580)

[Nikita Popov via llvm-commits]

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@@ -3547,6 +3547,25 @@ const SCEV *ScalarEvolution::getUDivExpr(const SCEV *LHS,
     }
   }
 
+  // ((-1 + (1 smax %x))<nsw> /u %x) evaluates to zero, for any positive %x.
+  if (const auto *AE = dyn_cast<SCEVAddExpr>(LHS);
+      AE && AE->getNumOperands() == 2 && AE->hasNoSignedWrap()) {
+    bool FoundMinusOne = false, FoundOneSMaxRHS = false;
+    for (const SCEV *Op : AE->operands()) {
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nikic wrote:

The constant operand is guaranteed to be first. You don't need to loop or use hasOperand, you know which operand will be which value.

https://github.com/llvm/llvm-project/pull/104580