llvm dev - Aug 2020 - [RFC] Replace `unsigned VF` with `ElementCount VF` [NFCI]

Hi,

The feedback I received so far on this patch has been positive. Therefore, I
have removed the `WIP` label and change it into a patch for which I want to do
the formal review process (all comments so far have been addressed).

Kind regards,

Francesco

On 8/11/20, 5:58 PM, "llvm-dev on behalf of Francesco Petrogalli via
llvm-dev" <llvm-dev-bounces at lists.llvm.org on behalf of llvm-dev at
lists.llvm.org> wrote:

    Hi all,

    I am working on replacing `unsigned VF` with `ElementCount VF` in the
vectorizer.

    I have submitted an RFC patch to fabricator just to show how it looks like:
https://reviews.llvm.org/D85794

    Just to be clear. I am not trying to get this patch in as it is, I am just
asking for some feedback on the approach.

    The changes in the vectorizer are numerous, but mostly mechanical.

    I have explained in the description of the patch the changes, which fall in
two categories:

    1. changes outsize the vectorizer, which are needed to be able to query
things like "how many lanes" are in this "vectorization
factor", or to print things to debug streams or remarks.
    2. changes in the vectorizer that are needed to guarantee a _non functional
change_, for example asserting that the ElementCount instance being used to
compute a vector reverse cannot be scalable. The code essentially looks the same
as before this patch, other than for the fact that I have added such assertions.

    I am aware the changes in LoopVectorizer.cpp might be a lot. I am open to
consider different incremental approaches.
    One data point here is that my initial approach was to add the `bool
Scalable` property to the VectorizationFactor class.
    However, using ElementCount instead of the pair {unsigned, bool} for dealing
with the number of lanes inside the VectorizationFactor seemed a natural choice
that was worth investigating.

    Please let me know what you think!

    Kind regards,

    Francesco


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