[llvm-dev] Vectorizing minimum without function attributes

[Nicolau Werneck via llvm-dev]

Greetings,

The LLVM loop vectorizer does a great job handling reductions with the
`min(a, b)` function over an array of integers or floats. This finds the
smallest value of a list exploiting SIMD instructions, and works just as
well as a summation.

Specifically with floats, though, using the `fcmp` instruction, the
vectorization seems to require the function attribute "no-nans-fp-math" to
be set. Just setting instruction flags is not enough. This forces us to
give up on fine-grained control of fast-math in the code in order to
benefit from this vectorization.

How to overcome this? LLVM has intrinsic functions such as `minnum` and
`minimum` (`minnan`) that accurately represent the operation. This could
permit fine-grained control of fast-math flags, although the vectorizer
seems to ignore these intrinsics.

Beyond this specific case, it would be nice to be sure when is it ever
necessary to set these function attributes, e.g.
https://github.com/llvm/llvm-project/blob/8205a814a691bfa62fed911b58b0a306ab5efe31/clang/lib/CodeGen/CGCall.cpp#L1743-L1750

What would be a way to control the vectorization for `min` without having
to rely on that function attribute? And furthermore, could LLVM
optimizations conceivably depend only on instruction flags, and not ever on
function attributes? What would be necessary to achieve this?

Thanks,

-- 
Nicolau Werneck <nwerneck at gmail.com>
http://n <http://nwerneck.sdf.org>ic.hpavc.net
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