[PATCH] D89821: [SCEV] Match 'zext (trunc A to iB) to iY' as URem.

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

I would expect that you should be able to showcase the effect with `opt -analyze -scalar-evolution`-based test:
https://godbolt.org/z/xddz8d (because there's `zext(A % B) --> zext(A) % zext(B)` `matchURem()`-driven fold)



================
Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:12757
 // 4, A / B becomes X / 8).
 bool ScalarEvolution::matchURem(const SCEV *Expr, const SCEV *&LHS,
                                 const SCEV *&RHS) {
----------------
What if `Expr` isn't an `zext` itself, but it is used by zext (which is the case in the single user of this method)?


================
Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:12760-12761
+  // Try to match 'zext (trunc A to iB) to iY', which is used
+  // for URem with constant power-of-2 second operands. Make sure the size of
+  // the operand A matches the size of the whole expressions.
+  if (const auto *ZExt = dyn_cast<SCEVZeroExtendExpr>(Expr)) {
----------------
What if we had `zext(zext(trunc))`, so we end up with a wider type than A was?



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https://reviews.llvm.org/D89821