[llvm] [BasicAA] Handle wrapping of pointer arithmetic (PR #69116)

[Nikita Popov via llvm-commits]

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@@ -417,7 +417,8 @@ static LinearExpression GetLinearExpression(
                                 Depth + 1, AC, DT);
         E.Offset <<= RHS.getLimitedValue();
         E.Scale <<= RHS.getLimitedValue();
-        E.IsNSW &= NSW;
+        // The nsw flag has different semantics for shift and mul.
+        E.IsNSW = false;
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nikic wrote:

This is conceptually right, but you should keep the nsw flag if you know that either RHS is a constant other than bitwidth-1 or the LHS is a constant other than -1.

Please split off this part of the patch with a minimal test that demonstrates that the current behavior is wrong in this specific edge case.

https://github.com/llvm/llvm-project/pull/69116