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The SLP vectoriser will fail to vectorise this because it wasn't taught to emit runtime alias analysis checks. This is being addressed by Florian in <a href="https://reviews.llvm.org/D102834" id="LPlnk">https://reviews.llvm.org/D102834</a>. We had many issues
with full unrolling removing opportunities for the loop vectoriser, and the SLP missing some tricks. But <span style="color: rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt;">with
<a href="https://reviews.llvm.org/D102834" style="margin:0px">D102834</a> fixed, I expect this loop to be SLP vectorised and most of this pass ordering problems to disappear.</span></div>
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Philip seems happy with this being part of GVN, and I don't have strong opinions, but GVNHoist seems like the natural place for this. An alternative strategy could thus be to integrate this into GVNHoist, enable it by default but only your simple gvn hoist
algorithm (and disable the rest). That would perhaps then be a first step to address Philip's unhappiness with GVNHoist and restructure and improve it step by step.</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Momchil Velikov <momchil.velikov@gmail.com><br>
<b>Sent:</b> 15 September 2021 12:30<br>
<b>To:</b> Sjoerd Meijer <Sjoerd.Meijer@arm.com><br>
<b>Cc:</b> Momchil Velikov <Momchil.Velikov@arm.com>; llvm-dev@lists.llvm.org <llvm-dev@lists.llvm.org>; Philip Reames <listmail@philipreames.com><br>
<b>Subject:</b> Re: [llvm-dev] [RFC] Simple GVN hoist</font>
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<div class="PlainText">On Wed, Sep 15, 2021 at 12:18 PM Momchil Velikov<br>
<momchil.velikov@gmail.com> wrote:<br>
><br>
> On Tue, Sep 14, 2021 at 7:56 PM Sjoerd Meijer via llvm-dev<br>
> <llvm-dev@lists.llvm.org> wrote:<br>
> > 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.<br>
><br>
> Yes, GVNHoist will do what we need<br>
<br>
[hit send too soon]<br>
<br>
So, GVNHoist will do the hoisting we want.. However, while technically<br>
possible there a few drawbacks in taking the existing GVNHoist<br>
approach:<br>
* GVNHoist is large and algorithmically complex<br>
* Even if enabled, it won't solve this specific issue - after hoisting<br>
of the loads the loop size falls below some unrolling threshold and<br>
the loop is fully unrolled - instead the loop should be<br>
loop-vectorised, IMHO - so we get a pass order issue.<br>
* After unrolling the SLP vectoriser does not vectorise the loop - so<br>
we have to fix the SLP vectoriser too.<br>
<br>
While I think all of these are worthwhile, we have on one hand some<br>
rather simple transformation (the mini-GVNHoist) vs. on the other hand<br>
the actual GVNHoist+pass manager+SLP Vectoriser, which<br>
could easily turn into a time sink with no end in sight.<br>
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