[llvm] [SCEV] Fold zext(C+A)<nsw> -> (sext(C) + zext(A))<nsw> if possible. (PR #142599)

[Nikita Popov via llvm-commits]

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@@ -1793,6 +1793,18 @@ const SCEV *ScalarEvolution::getZeroExtendExprImpl(const SCEV *Op, Type *Ty,
       return getAddExpr(Ops, SCEV::FlagNUW, Depth + 1);
     }
 
+    const SCEVConstant *C;
+    const SCEV *A;
+    // zext (C + A)<nsw> -> (sext(C) + zext(A))<nsw> if zext (C + A)<nsw> >=s 0
+    // and A >=s V.
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nikic wrote:

The core issue here seems to be that pushing sext through add nsw is a non-reversible transform. You can convert sext of add nsw to add nsw of sext, but you generally can't go from add nsw of sext to sext of add.

But given that SCEV does that in general, it probably makes sense to still do it here as well.

https://github.com/llvm/llvm-project/pull/142599