[Openmp-commits] [clang] [openmp] [Clang][OpenMP] Fix tile/unroll on iterator- and foreach-loops. (PR #91459)

[Michael Kruse via Openmp-commits]

================
@@ -156,9 +156,9 @@ extern "C" void body(...) {}
 // IR-EMPTY:
 // IR-NEXT:  [[FOR_INC]]:
 // IR-NEXT:    %[[TMP34:.+]] = load i32, ptr %[[DOTTILE_0_IV_I]], align 4
-// IR-NEXT:    %[[INC:.+]] = add nsw i32 %[[TMP34]], 1
+// IR-NEXT:    %[[INC:.+]] = add i32 %[[TMP34]], 1
----------------
Meinersbur wrote:

This is due to the change of using the type of `LoopHelper.NumIterations` (an unsigned integer) as the new loop's induction variable (instead of the original loop's iteration variable type). In iterator-based loops that type would be incompatible with `LoopHelper.NumIterations` which remains an integer.

Even if the original loop variable is a signed integer (it could be a (`std::iota_view`)[https://en.cppreference.com/w/cpp/ranges/iota_view] over the same range), using and unsigned type is more correct as long as we start counting at 0. For instance, the loop
```
#pragma omp tile sizes(2)
for (int i = INT_MIN; i < INT_MAX; ++i)
```
would overflow the `nsw` flag.

The "most correct" fix would be to add the `nuw` flag. Unfortunately there is no AST expression we could create that would make CodeGen generate one. Alternatively, I could make generated loops start counting at `INT_MIN`.

https://github.com/llvm/llvm-project/pull/91459