[llvm] [FuncSpec] Update function specialization to handle phi-chains (PR #72903)

[Chuanqi Xu via llvm-commits]

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@@ -262,29 +269,113 @@ Cost InstCostVisitor::estimateBranchInst(BranchInst &I) {
   return estimateBasicBlocks(WorkList);
 }
 
+bool InstCostVisitor::discoverTransitivelyIncomingValues(
+    Constant *Const, PHINode *Root, DenseSet<PHINode *> &TransitivePHIs,
+    SmallVectorImpl<PHINode *> &UnknownIncomingValues) {
+
+  SmallVector<PHINode *, 64> WorkList;
+  WorkList.push_back(Root);
+  unsigned Iter = 0;
+
+  while (!WorkList.empty()) {
+    PHINode *PN = WorkList.pop_back_val();
+
+    if (++Iter > MaxDiscoveryIterations ||
+        PN->getNumIncomingValues() > MaxIncomingPhiValues) {
+      // For now just collect the Phi and later we will check whether it is
+      // in the Transitive set.
+      UnknownIncomingValues.push_back(PN);
+      continue;
+      // FIXME: return false here and remove the UnknownIncomingValues entirely.
+    }
+
+    if (!TransitivePHIs.insert(PN).second)
+      continue;
+
+    for (unsigned I = 0, E = PN->getNumIncomingValues(); I != E; ++I) {
+      Value *V = PN->getIncomingValue(I);
+
+      // Disregard self-references and dead incoming values.
+      if (auto *Inst = dyn_cast<Instruction>(V))
+        if (Inst == PN || DeadBlocks.contains(PN->getIncomingBlock(I)))
+          continue;
+
+      if (Constant *C = findConstantFor(V, KnownConstants)) {
+        // Not all incoming values are the same constant. Bail immediately.
+        if (C != Const)
+          return false;
+        continue;
+      }
+
+      if (auto *Phi = dyn_cast<PHINode>(V)) {
+        WorkList.push_back(Phi);
+        continue;
+      }
+
+      // We can't reason about anything else.
+      return false;
+    }
+  }
+  return true;
+}
+
 Constant *InstCostVisitor::visitPHINode(PHINode &I) {
   if (I.getNumIncomingValues() > MaxIncomingPhiValues)
     return nullptr;
 
   bool Inserted = VisitedPHIs.insert(&I).second;
+  SmallVector<PHINode *, 8> UnknownIncomingValues;
+  DenseSet<PHINode *> TransitivePHIs;
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ChuanqiXu9 wrote:

Let's put declaration close to their uses.

https://github.com/llvm/llvm-project/pull/72903