[llvm-branch-commits] [llvm] [Delinearization] Add validation for large size arrays (PR #169902)

[Michael Kruse via llvm-branch-commits]

================
@@ -755,6 +769,43 @@ bool llvm::validateDelinearizationResult(ScalarEvolution &SE,
     if (!isKnownLessThan(&SE, Subscript, Size))
       return false;
   }
+
+  // The offset computation is as follows:
+  //
+  //   Offset = I_n +
+  //            S_n * I_{n-1} +
+  //            ... +
+  //            (S_2 * ... * S_n) * I_1
+  //
+  // Regarding this as a function from (I_1, I_2, ..., I_n) to integers, it
+  // must be injective. To guarantee it, the above calculation must not
+  // overflow. Since we have already checked that 0 <= I_k < S_k for k = 2..n,
+  // the minimum and maximum values occur in the following cases:
+  //
+  //   Min = [I_1][0]...[0] = S_2 * ... * S_n * I_1
+  //   Max = [I_1][S_2-1]...[S_n-1]
+  //       = (S_2 * ... * S_n) * I_1 +
+  //         (S_2 * ... * S_{n-1}) * (S_2 - 1) +
+  //         ... +
+  //         (S_n - 1)
+  //       = (S_2 * ... * S_n) * I_1 +
----------------
Meinersbur wrote:

```suggestion
  //       = (S_2 * ... * S_n) * I_1 +  /* this is the same as Min expression; if this expression has already been checked for overlflow, need only check the rest of the expression for Max */ 
```
Some cleverness I did not see (`Min == Prod * I_1`). I was expecting someting more symmetric. My suggested comment here is clunky, maby you find something better.

https://github.com/llvm/llvm-project/pull/169902