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

[Momchil Velikov via llvm-commits]

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@@ -262,30 +267,124 @@ Cost InstCostVisitor::estimateBranchInst(BranchInst &I) {
   return estimateBasicBlocks(WorkList);
 }
 
+// This function is determining if a PHINode is part of a strongly
+// connected component - that is a chain or graph of PHINodes that all
+// link to each other. That means, if the original input to the chain
+// is a constant, all the other values are also that constant.
+//
+// For example:
+//
+// L1:
+// %a = load %0
+//
+// L4:
+// %c = phi [%a, %L1], [%d, %L2]
+//
+// L2:
+// %d = phi [%e, L3] [%c, L4]
+// L3:
+// %e = phi [%c, L4],[%f, L5]
+// L5:
+// %f = phi [%j, L6], [%h, L7]
+// L6:
+// %j = phi [%h:L7, %j, L6]
+// L7:
+// %h = phi [%g:L6, %c:L4]
+//
+// This is only showing the labels and PHINodes, not the branches that
+// choose the different paths.
+// If some input is not part of the graph, then it's not a strongly
+// connected component.
+//
+// A depth limit is used to avoid extreme recurusion.
+// A max number of incoming phi values ensures that expensive searches
+// are avoided.
+void InstCostVisitor::discoverStronglyConnectedComponent(PHINode *PN,
+                                                         unsigned Depth) {
+  if (Depth > MaxDiscoveryDepth)
+    return;
+
+  if (PN->getNumIncomingValues() > MaxIncomingPhiValues)
+    return;
----------------
momchil-velikov wrote:

Agree. Even if we stop because of one of the above two conditions then the nodes would appear as unknown incoming values and we will fail there (unless we reach these nodes along another path).

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