[llvm-dev] [RFC] Simple GVN hoist

[Philip Reames via llvm-dev]

On 9/15/21 4:30 AM, Momchil Velikov wrote:
> On Wed, Sep 15, 2021 at 12:18 PM Momchil Velikov
> <momchil.velikov at gmail.com> wrote:
>> On Tue, Sep 14, 2021 at 7:56 PM Sjoerd Meijer via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>>> Fix GVNHoist. I like your patch because it's small and concise, but missing in the RFC is a discussion why we don't try to re-enable GVNHoist. I know it was briefly enabled by default, which was reverted due to correctness (or was it regressions?) problems. But if this belongs in GVNHoist, could this for example be added to GVNHoist, and only this part enabled? Not sure if that's possible as I haven't looked at GVNHoist.
>> Yes, GVNHoist will do what we need
> [hit send too soon]
>
> So, GVNHoist will do the hoisting we want.. However, while technically
> possible there a few drawbacks in taking the existing GVNHoist
> approach:
> * GVNHoist is large and algorithmically complex
> * Even if enabled, it won't solve this specific issue - after hoisting
> of the loads the loop size falls below some unrolling threshold and
> the loop is fully unrolled - instead the loop should be
> loop-vectorised, IMHO - so we get a pass order issue.
> * After unrolling the SLP vectoriser does not vectorise the loop - so
> we have to fix the SLP vectoriser too.
>
> While I think all of these are worthwhile, we have on one hand some
> rather simple transformation (the mini-GVNHoist) vs. on the other hand
> the actual  GVNHoist+pass manager+SLP Vectoriser, which
> could easily turn into a time sink with no end in sight.

Your argument here actually makes me less convinced.  :)

In general, we should not be duplicating transform functionality just 
because of pass ordering issues, or deficiencies in other passes.  We 
should be fixing those issues, since they are likely common to other 
workloads as well.

If my opinion of GVNHoist was anything thing other than terrible, this 
would probably have convinced my that we should pursue the GVNHoist option.

Philip