[PATCH] D98714: [SLP] Add insertelement instructions to vectorizable tree

[Anton Afanasyev via Phabricator via llvm-commits]

anton-afanasyev marked an inline comment as done.
anton-afanasyev added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:4672-4680
+        V = Builder.CreateShuffleVector(V, UndefValue::get(V->getType()), Mask);
+
+        for (unsigned I = 0; I < NumElts; I++)
+          Mask[I] =
+              (I < MinIndex || I >= MaxIndex) ? I : NumElts - MinIndex + I;
+
+        V = Builder.CreateShuffleVector(
----------------
ABataev wrote:
> anton-afanasyev wrote:
> > anton-afanasyev wrote:
> > > ABataev wrote:
> > > > ABataev wrote:
> > > > > Can we use `ShuffleBuilder` here?
> > > > Did you include these costs in the cost model?
> > > They are included in `getScalarizationOverhead()`.
> > Yes, I thought about `ShuffleBuilder`, but here we just need to create two shuffles of special kind for vector resizing. It requires `ShuffleInstructionBuilder` expanding, don't think it's worth it.
> I rather doubt in it. First, you subtract the scalarization overhead cost from the vector cost, but here you need to add the costs of subvector insert and permutation of 2 vectors
>>> Did you include these costs in the cost model?
>> They are included in getScalarizationOverhead().
> I rather doubt in it. First, you subtract the scalarization overhead cost from the vector cost, but here you need to add the costs of subvector insert and permutation of 2 vectors

These subvector inserts are lowered to nop actually, so they cost nothing. We need this code when processing big vector of scalars part-by-part, but every chunk fits the whole vector register (condition `bits >= MinVecRegSize`), so actually there is no inserting. The result consisting of several vector registers is returned then.


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  https://reviews.llvm.org/D98714/new/

https://reviews.llvm.org/D98714