[PATCH] D103700: [LV] Fix bug when unrolling (only) a loop with non-latch exit

[Gil Rapaport via Phabricator via llvm-commits]

gilr added a comment.

In D103700#2813906 <https://reviews.llvm.org/D103700#2813906>, @reames wrote:

> @Ayal If I'm understanding you even partially correctly, it sounds like you're raising a code quality issue.  That is, we may generate a dead epilogue loop (e.g. with a condition statically known to result in the epilogue being untaken, but emitted as a condition) when we didn't need to.  Is that correct?

No, the emitted code in this case would be explicitly leaving some iterations to the epilogue even if the trip count is a multiple of the unroll factor. So the epilogue is alive and will not be optimized away later.



================
Comment at: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:1567
       return true;
     return InterleaveInfo.requiresScalarEpilogue();
   }
----------------
>To have `Cost->requiresScalarEpilogue()` fully convey this logic, one could directly pass it VF

That seems like the simplest way to both fix the bug and avoid pessimizing the unroll-only case, i.e.:

```
return VF.isVector() && InterleaveInfo.requiresScalarEpilogue();
```


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