[PATCH] D153697: [LV] Freeze start value for select-reductions.

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added a comment.

> When creating select-reductions, we are introducing multiple new uses of
> the reduction start value. If the start value may be undef or poison,
> then differnet uses (e.g in the final select or vector compares) can
> evaluate to different values, leading to incorrect results.

These `SelectCmp` reductions are essentially boolean `Any` reductions which should use the original reduction start value **only once** - at the reduction epilog, as described in https://reviews.llvm.org/D150851#4440545?
They should also use their alternative value only once, as in `result = (cond(i) for any i) ? startValue : alternativeValue`.
This would also reduce the size of the vectorized/unrolled "accumulator" maintained within the loop, to a minimum.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D153697/new/

https://reviews.llvm.org/D153697