[llvm] [FuncSpec] Update function specialization to handle phi-chains (PR #71442)

[Kiran Chandramohan via llvm-commits]

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@@ -262,30 +267,86 @@ Cost InstCostVisitor::estimateBranchInst(BranchInst &I) {
   return estimateBasicBlocks(WorkList);
 }
 
+void InstCostVisitor::discoverStronglyConnectedComponent(PHINode *PN,
+                                                         unsigned Depth) {
+  if (Depth > MaxDiscoveryDepth)
+    return;
+
+  if (PN->getNumIncomingValues() > MaxIncomingPhiValues)
+    return;
+
+  if (!StronglyConnectedPHIs.insert(PN).second)
+    return;
+
+  for (unsigned I = 0, E = PN->getNumIncomingValues(); I != E; ++I) {
+    Value *V = PN->getIncomingValue(I);
+    if (auto *Phi = dyn_cast<PHINode>(V)) {
+      if (Phi == PN || DeadBlocks.contains(PN->getIncomingBlock(I)))
+        continue;
+      discoverStronglyConnectedComponent(Phi, Depth + 1);
+    }
+  }
+}
+
 Constant *InstCostVisitor::visitPHINode(PHINode &I) {
   if (I.getNumIncomingValues() > MaxIncomingPhiValues)
     return nullptr;
 
   bool Inserted = VisitedPHIs.insert(&I).second;
   Constant *Const = nullptr;
+  SmallVector<PHINode *, 8> UnknownIncomingValues;
 
-  for (unsigned Idx = 0, E = I.getNumIncomingValues(); Idx != E; ++Idx) {
-    Value *V = I.getIncomingValue(Idx);
-    if (auto *Inst = dyn_cast<Instruction>(V))
-      if (Inst == &I || DeadBlocks.contains(I.getIncomingBlock(Idx)))
-        continue;
-    Constant *C = findConstantFor(V, KnownConstants);
-    if (!C) {
-      if (Inserted)
-        PendingPHIs.push_back(&I);
-      return nullptr;
+  auto CanConstantFoldPhi = [&](PHINode *PN) -> bool {
----------------
kiranchandramohan wrote:

Add a comment above detailing when a Phi can be constant folded at a higher level. (It is kind of documented inside the code but having it in one place means you do not have to look further)

https://github.com/llvm/llvm-project/pull/71442