[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:4670-4671
+        // Create shuffle to resize vector
+        for (unsigned I = 0; I < NumElts; I++)
+          Mask[I] = (I < NumScalars) ? I : UndefMaskElem;
+        V = Builder.CreateShuffleVector(V, UndefValue::get(V->getType()), Mask);
----------------
ABataev wrote:
> anton-afanasyev wrote:
> > ABataev wrote:
> > > ```
> > > SmallVector<int, 16> Mask(NumElts, UndefMaskElem);
> > > std::iota(Mask.begin(), std::next(Mask.begin(), NumScalars), 0); 
> > > ```
> > > 
> > > Also, I think just `std::iota(Mask.begin(), Mask.end(), 0); ` shall work too
> > Thanks, changed to `std::iota(Mask.begin(), std::next(Mask.begin(), NumScalars), 0);`
> > 
> > Yes, `std::iota(Mask.begin(), Mask.end(), 0);` gives the same result, but wouldn't it lead to redundant code lowered?
> `std::iota(Mask.begin(), Mask.end(), 0);` will produce the same code as `std::iota(Mask.begin(), std::next(Mask.begin(), NumScalars), 0);` but only if `NumScalars <= Mask.size() * 2`. Otherwise the compiler may crash in some cases. So, better to keep `std::iota(Mask.begin(), std::next(Mask.begin(), NumScalars), 0);`
`Mask.size()` is just `NumElts` and `NumScalars <= NumElts` (is it worth to add assert for this?), so `NumScalars <= Mask.size() * 2` for all cases.

What I did mean by "the same result" is that shuffle
```
%a = shufflevector <2 x float> %b, <2 x float> undef, <4 x i32> <i32 0, i32 1, i32 undef, i32 undef>
```
is equivalent to
```
%a = shufflevector <2 x float> %b, <2 x float> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
```
(here `NumScalars == 2` and `NumElts == 4`).


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

https://reviews.llvm.org/D98714