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

[Teresa Johnson 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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teresajohnson wrote:

I'm not completely sure which map you are referring to, but I started out making a change to add a bool in the ContextNode as to whether its callsite had been assigned to a func clone, but that turns out to be tricky to get right, and this was more targeted and simpler to get right as a fix for the specific problem I was looking at. In particular, we want callsites that we weren't able to update callers for to get assigned to clone 0, which this change ensures. I've added a TODO though to look at adding a mechanism to better identify and distinguish callsite clones that aren't getting assigned to any clone.

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