[llvm] [SLP][REVEC] Make SLP vectorize shufflevector. (PR #102489)

[Han-Kuan Chen via llvm-commits]

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@@ -268,6 +268,98 @@ static void transformScalarShuffleIndiciesToVector(unsigned VecTyNumElements,
   Mask.swap(NewMask);
 }
 
+/// \returns the number of groups of shufflevector
+/// A group has the following features
+/// 1. All of value in a group are shufflevector.
+/// 2. The mask of all shufflevector is isExtractSubvectorMask.
+/// 3. The mask of all shufflevector uses all of the elements of the source (and
+/// the elements are used in order).
+/// e.g., it is 1 group (%0)
+/// %1 = shufflevector <16 x i8> %0, <16 x i8> poison,
+///    <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7>
+/// %2 = shufflevector <16 x i8> %0, <16 x i8> poison,
+///    <8 x i32> <i32 8, i32 9, i32 10, i32 11, i32 12, i32 13, i32 14, i32 15>
+/// it is 2 groups (%3 and %4)
+/// %5 = shufflevector <8 x i16> %3, <8 x i16> poison,
+///    <4 x i32> <i32 0, i32 1, i32 2, i32 3>
+/// %6 = shufflevector <8 x i16> %3, <8 x i16> poison,
+///    <4 x i32> <i32 4, i32 5, i32 6, i32 7>
+/// %7 = shufflevector <8 x i16> %4, <8 x i16> poison,
+///    <4 x i32> <i32 0, i32 1, i32 2, i32 3>
+/// %8 = shufflevector <8 x i16> %4, <8 x i16> poison,
+///    <4 x i32> <i32 4, i32 5, i32 6, i32 7>
+/// it is 0 group
+/// %12 = shufflevector <8 x i16> %10, <8 x i16> poison,
+///     <4 x i32> <i32 0, i32 1, i32 2, i32 3>
+/// %13 = shufflevector <8 x i16> %11, <8 x i16> poison,
+///     <4 x i32> <i32 0, i32 1, i32 2, i32 3>
+static unsigned getShufflevectorNumGroups(ArrayRef<Value *> VL) {
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HanKuanChen wrote:

It is also used by `getScalarsVectorizationState`.
```
      // REVEC can support non alternate shuffle.
      if (SLPReVec && getShufflevectorNumGroups(VL))
        return TreeEntry::Vectorize;
```
If `getShufflevectorNumGroups` returns non-zero value, it means we can vectorize the shufflevector.

https://github.com/llvm/llvm-project/pull/102489