[PATCH] D117480: [IR] Extend llvm.vector.reduce.fadd

[Steve Canon via Phabricator via llvm-commits]

scanon added a comment.

In D117480#3248644 <https://reviews.llvm.org/D117480#3248644>, @fhahn wrote:

> I assume ordered here means sequential. The clang builtin specifies a tree-wise reduction order.

It's worth noting that "sequential" is //only// desirable for matching scalar code (which is important, but isn't everything). Explicit vector code essentially always prefers a tree reduction (either even + odd or low + high, depending on arch/uarch, but either is better than sequential). Either normal tree reduction delivers faster results with smaller average errors, and they are still reproducible. Having a defined tree reduction is a great tool for explicit vectorizers.


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