[llvm] [InstCombine] Remove dead phi web (PR #108876)

[Yingwei Zheng via llvm-commits]

================
@@ -976,24 +976,26 @@ Instruction *InstCombinerImpl::foldPHIArgOpIntoPHI(PHINode &PN) {
   return NewCI;
 }
 
-/// Return true if this PHI node is only used by a PHI node cycle that is dead.
-static bool isDeadPHICycle(PHINode *PN,
-                           SmallPtrSetImpl<PHINode *> &PotentiallyDeadPHIs) {
-  if (PN->use_empty()) return true;
-  if (!PN->hasOneUse()) return false;
-
-  // Remember this node, and if we find the cycle, return.
-  if (!PotentiallyDeadPHIs.insert(PN).second)
-    return true;
-
-  // Don't scan crazily complex things.
-  if (PotentiallyDeadPHIs.size() == 16)
-    return false;
-
-  if (PHINode *PU = dyn_cast<PHINode>(PN->user_back()))
-    return isDeadPHICycle(PU, PotentiallyDeadPHIs);
-
-  return false;
+/// Return true if this PHI node is only used by a PHI node web that is dead.
+static bool isDeadPHIWeb(PHINode *PN) {
+  SmallVector<PHINode *, 16> Stack;
+  SmallPtrSet<PHINode *, 16> Visited;
+  Stack.push_back(PN);
+  while (!Stack.empty()) {
+    PHINode *Phi = Stack.pop_back_val();
+    if (!Visited.insert(Phi).second)
+      continue;
+    // Early stop if the set of PHIs is large
+    if (Visited.size() == 16)
+      return false;
+    for (User *Use : Phi->users()) {
+      if (PHINode *PhiUse = dyn_cast<PHINode>(Use))
+        Stack.push_back(PhiUse);
+      else
+        return false;
+    }
+  }
+  return true;
----------------
dtcxzyw wrote:

Why not just delete the whole dead phi web here? Thus we can avoid `O(n^2)` traversals.


https://github.com/llvm/llvm-project/pull/108876