[llvm] [MemProf] Ensure all callsite clones are assigned a function clone (PR #150735)

[Snehasish Kumar via llvm-commits]

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@@ -4842,6 +4886,27 @@ bool CallsiteContextGraph<DerivedCCG, FuncTy, CallTy>::assignFunctions() {
                                        FuncCloneAssignedToCurCallsiteClone);
           }
         }
+        // If we didn't assign a function clone to this callsite clone yet, e.g.
+        // none of its callers has a non-null call, do the assignment here.
+        // We want to ensure that every callsite clone is assigned to some
+        // function clone, so that the call updates below work as expected.
+        // In particular if this is the original callsite, we want to ensure it
+        // is assigned to the original function, otherwise the original function
+        // will appear available for assignment to other callsite clones,
+        // leading to unintended effects. For one, the unknown and not updated
+        // callers will call into cloned paths leading to the wrong hints,
+        // because they still call the original function (clone 0). Also,
+        // because all callsites start out as being clone 0 by default, we can't
+        // easily distinguish between callsites explicitly assigned to clone 0
+        // vs those never assigned, which can lead to multiple updates of the
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snehasish wrote:

Maybe if we used a struct instead of an integer to indicate the clone number as value in the map, we could hold additional data about whether it was assigned along with the assigned clone number. Would that simplify any of the logic here?

https://github.com/llvm/llvm-project/pull/150735