[llvm] VectorCombine: refactor foldShuffleToIdentity (NFC) (PR #92766)

[David Green via llvm-commits]

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@@ -1668,6 +1668,86 @@ bool VectorCombine::foldShuffleOfShuffles(Instruction &I) {
   return true;
 }
 
+using InstLane = std::pair<Value *, int>;
+
+static InstLane lookThroughShuffles(Value *V, int Lane) {
+  while (auto *SV = dyn_cast<ShuffleVectorInst>(V)) {
+    unsigned NumElts =
+        cast<FixedVectorType>(SV->getOperand(0)->getType())->getNumElements();
+    int M = SV->getMaskValue(Lane);
+    if (M < 0)
+      return {nullptr, PoisonMaskElem};
+    if (static_cast<unsigned>(M) < NumElts) {
+      V = SV->getOperand(0);
+      Lane = M;
+    } else {
+      V = SV->getOperand(1);
+      Lane = M - NumElts;
+    }
+  }
+  return InstLane{V, Lane};
+}
+
+static SmallVector<InstLane>
+generateInstLaneVectorFromOperand(ArrayRef<InstLane> Item, int Op) {
+  SmallVector<InstLane> NItem;
+  for (InstLane IL : Item) {
+    auto [V, Lane] = IL;
+    InstLane OpLane =
+        V ? lookThroughShuffles(cast<Instruction>(V)->getOperand(Op), Lane)
+          : InstLane{nullptr, PoisonMaskElem};
+    NItem.emplace_back(OpLane);
+  }
+  return NItem;
+}
+
+static Value *generateNewInstTree(ArrayRef<InstLane> Item, FixedVectorType *Ty,
+                                  const SmallPtrSet<Value *, 4> &IdentityLeafs,
+                                  const SmallPtrSet<Value *, 4> &SplatLeafs,
+                                  IRBuilder<> &Builder) {
+  auto [FrontV, FrontLane] = Item.front();
+
+  if (IdentityLeafs.contains(FrontV) &&
+      all_of(drop_begin(enumerate(Item)), [Item](const auto &E) {
+        Value *FrontV = Item.front().first;
----------------
davemgreen wrote:

What do you mean by 'destructed'? Is it not a pointer?

https://github.com/llvm/llvm-project/pull/92766