[llvm] [SCEV] Distribute zext across (C + Rest) in LoopGuards rewriter (PR #210719)

[Florian Hahn via llvm-commits]

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@@ -16424,6 +16424,30 @@ const SCEV *ScalarEvolution::LoopGuards::rewrite(const SCEV *Expr) const {
         Bitwidth = Bitwidth / 2;
       }
 
+      // Pull a leading constant out of the zext:
+      //   zext(C + X) -> zext(X) - zext(-C), if X >=u -C.
+      const SCEVAddExpr *Add;
+      const APInt *C;
+      if (match(Op, m_scev_Add(Add)) &&
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fhahn wrote:

It should be enough to just run `opt -p loop-unroll` on the IR above, it should unroll on current `main` (https://llvm.godbolt.org/z/sY1ndfKj1) and fail to unroll with the current version of the patch I think

https://github.com/llvm/llvm-project/pull/210719