[llvm] [MemProf] Support cloning through recursive cycles (PR #127429)

[Teresa Johnson via llvm-commits]

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@@ -3546,13 +3625,99 @@ void CallsiteContextGraph<DerivedCCG, FuncTy, CallTy>::identifyClones(
     //
     // Then check if by cloning node at least one of the callee edges will be
     // disambiguated by splitting out different context ids.
+    //
+    // However, always do the cloning if this is a backedge, in which case we
+    // have not yet cloned along this caller edge.
     assert(CallerEdge->AllocTypes != (uint8_t)AllocationType::None);
     assert(Node->AllocTypes != (uint8_t)AllocationType::None);
-    if (allocTypeToUse(CallerAllocTypeForAlloc) ==
+    if (!CallerEdge->IsBackedge &&
+        allocTypeToUse(CallerAllocTypeForAlloc) ==
             allocTypeToUse(Node->AllocTypes) &&
         allocTypesMatch<DerivedCCG, FuncTy, CallTy>(
-            CalleeEdgeAllocTypesForCallerEdge, Node->CalleeEdges))
+            CalleeEdgeAllocTypesForCallerEdge, Node->CalleeEdges)) {
       continue;
+    }
+
+    if (CallerEdge->IsBackedge) {
+      // We should only mark these if cloning recursive contexts, where we
+      // need to do this deferral.
+      assert(CloneRecursiveContexts);
+      DeferredBackedges++;
+    }
+
+    // If this is a backedge, we now do recursive cloning starting from its
+    // caller since we may have moved unambiguous caller contexts to a clone
+    // of this Node in a previous iteration of the current loop, giving more
+    // opportunity for cloning through the backedge. Because we sorted the
+    // caller edges earlier so that cold caller edges are first, we would have
+    // visited and cloned this node for any unamibiguously cold non-recursive
+    // callers before any ambiguous backedge callers. Note that we don't do this
+    // if the caller is already cloned or visited during cloning (e.g. via a
+    // different context path from the allocation).
+    // TODO: Can we do better in the case where the caller was already visited?
+    if (CallerEdge->IsBackedge && !CallerEdge->Caller->CloneOf &&
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teresajohnson wrote:

Sounds good, I will look into that as a follow up

https://github.com/llvm/llvm-project/pull/127429