[llvm] [LoopInterchange] Add metadata to control loop-interchange (PR #127474)

[Ryotaro Kasuga via llvm-commits]

kasuga-fj wrote:

I've been trying to figure out how the follow-up mechanism works, but I'm getting a little confused. It seems to me that `makeFollowupLoopID` doesn't work as expected, e.g., in the following example.

```c
void f(float *a, float x) {
  #pragma clang loop vectorize(enable) unroll_count(8)
  for (int i = 0; i < 1024; i++) {
    a[i] *= x;
  }
}
```

LLVM IR before and after loop-vectorize are in https://godbolt.org/z/bde8va1TG.


For the output IR, the two loops (`vector.body` and `for.body`) have LoopIDs !0 and !5, respectively, these LoopIDs have `!1` in there operand, and it has `!3`, which has the unroll count specified by the pragma. IIUC, the nest of the metadata has unexpectedly become one deeper. In fact, the unroll becomes to be applied by removing `vectorize(enable)`.

https://godbolt.org/z/bzd8d8WxK

So, I think we need to do something like "flatten" MDNodes in `makeFollowupLoopID`. Does such a fix make sense?

https://github.com/llvm/llvm-project/pull/127474