[llvm] [SLP] Create groups before sorting Phis (PR #111174)

[Alexey Bataev via llvm-commits]

================
@@ -5448,40 +5448,136 @@ BoUpSLP::getReorderingData(const TreeEntry &TE, bool TopToBottom) {
     if (!TE.ReorderIndices.empty())
       return TE.ReorderIndices;
 
-    auto PHICompare = [&](unsigned I1, unsigned I2) {
-      Value *V1 = TE.Scalars[I1];
-      Value *V2 = TE.Scalars[I2];
-      if (V1 == V2 || (V1->getNumUses() == 0 && V2->getNumUses() == 0))
-        return false;
-      if (V1->getNumUses() < V2->getNumUses())
-        return true;
-      if (V1->getNumUses() > V2->getNumUses())
-        return false;
-      auto *FirstUserOfPhi1 = cast<Instruction>(*V1->user_begin());
-      auto *FirstUserOfPhi2 = cast<Instruction>(*V2->user_begin());
-      if (auto *IE1 = dyn_cast<InsertElementInst>(FirstUserOfPhi1))
-        if (auto *IE2 = dyn_cast<InsertElementInst>(FirstUserOfPhi2)) {
-          if (!areTwoInsertFromSameBuildVector(
-                  IE1, IE2,
-                  [](InsertElementInst *II) { return II->getOperand(0); }))
-            return I1 < I2;
-          return getElementIndex(IE1) < getElementIndex(IE2);
-        }
-      if (auto *EE1 = dyn_cast<ExtractElementInst>(FirstUserOfPhi1))
-        if (auto *EE2 = dyn_cast<ExtractElementInst>(FirstUserOfPhi2)) {
-          if (EE1->getOperand(0) != EE2->getOperand(0))
-            return I1 < I2;
-          return getElementIndex(EE1) < getElementIndex(EE2);
-        }
-      return I1 < I2;
-    };
-    SmallDenseMap<unsigned, unsigned, 16> PhiToId;
+    DenseMap<unsigned, unsigned> PhiToId;
     SmallVector<unsigned> Phis(TE.Scalars.size());
     std::iota(Phis.begin(), Phis.end(), 0);
+
+    BitVector Seen(Phis.size());
+    SmallVector<SmallVector<unsigned>> Groups;
+    Groups.resize(Phis.size(), {});
+
+    for (auto const Phidx : Phis) {
+      // We've already found a group for this Phidx
+      if (Seen.test(Phidx))
+        continue;
----------------
alexey-bataev wrote:

Hmm, all this code is meaningless. Is not it just this:
```
for (unsigned Phidx : seq<unsigned>(TE.Scalars.size())) {
```
?
`Phis` filled using std::iota, so Seen is to needed at all.

https://github.com/llvm/llvm-project/pull/111174