[llvm] [SLP] Initial vectorization of non-power-of-2 ops. (PR #77790)

[Alexey Bataev via llvm-commits]

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@@ -14798,14 +14843,23 @@ bool SLPVectorizerPass::vectorizeStores(ArrayRef<StoreInst *> Stores,
         continue;
       }
 
+      std::optional<unsigned> NonPowerOf2VF;
+      if (VectorizeNonPowerOf2) {
+        // First try vectorizing with a non-power-of-2 VF. At the moment, only
+        // consider cases where VF + 1 is a power-of-2, i.e. almost all vector
+        // lanes are used.
+        unsigned CandVF = Operands.size();
+        if (isPowerOf2_32(CandVF + 1) && CandVF <= MaxVF) {
+          NonPowerOf2VF = CandVF;
+        }
+      }
+
       unsigned Sz = 1 + Log2_32(MaxVF) - Log2_32(MinVF);
-      SmallVector<unsigned> CandidateVFs(Sz);
-      // FIXME: Is division-by-2 the correct step? Should we assert that the
-      // register size is a power-of-2?
-      unsigned Size = MaxVF;
-      for_each(CandidateVFs, [&](unsigned &VF) {
-        VF = Size;
-        Size /= 2;
+      SmallVector<unsigned> CandidateVFs(Sz + bool(NonPowerOf2VF));
----------------
alexey-bataev wrote:

```suggestion
      SmallVector<unsigned> CandidateVFs(Sz + (NonPowerOf2VF > 0 ? 1 : 0));
```
Better to avoid adding `bool`

https://github.com/llvm/llvm-project/pull/77790