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

[Alexandros Lamprineas 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,
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labrinea wrote:

We assume they are forming a strongly connected component until we prove otherwise. If all the discovered phis can be constant folded to the same value then they must be strongly connected, in which case visitPHINode succeeds.

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