[llvm] [LV] Use original trip-count as the vector-trip-count if use predicated EVL instructions for tail-folding. (PR #132675)

[Shih-Po Hung via llvm-commits]

arcbbb wrote:

> It is by design. The proposed solution was discussed already long time ago, and it was agreed that the vectorized loop should keep the canonical ("countable") loop form. Our initial implementation was also based on a similar solution, but after the discussion transitioned to the existing approach

That reminds me — fault-only-first loads can process fewer than vl elements, even without raising an exception. In such cases, the assumption that the vector trip count is a multiple of the VF no longer holds.

This suggests that, we might need to remove the use of the vector trip count and canonical iv in the EVL transform, and the loop form will not be canonical.

https://github.com/llvm/llvm-project/pull/132675