[llvm] [memprof] Add CallStackRadixTreeBuilder (PR #93784)

[Teresa Johnson via llvm-commits]

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@@ -905,6 +905,106 @@ struct IndexedMemProfData {
   llvm::MapVector<CallStackId, llvm::SmallVector<FrameId>> CallStackData;
 };
 
+// Construct a radix tree of call stacks.
+//
+// A set of call stacks might look like:
+//
+// CallStackId 1:  f1 -> f2 -> f3
+// CallStackId 2:  f1 -> f2 -> f4 -> f5
+// CallStackId 3:  f1 -> f2 -> f4 -> f6
+// CallStackId 4:  f7 -> f8 -> f9
+//
+// where each fn refers to a stack frame.
+//
+// Since we expect a lot of common prefixes, we can compress the call stacks
+// into a radix tree like:
+//
+// CallStackId 1:  f1 -> f2 -> f3
+//                       |
+// CallStackId 2:        +---> f4 -> f5
+//                             |
+// CallStackId 3:              +---> f6
+//
+// CallStackId 4:  f7 -> f8 -> f9
+//
+// Now, we are interested in retrieving call stacks for a given CallStackId, so
+// we just need a pointer from a given call stack to its parent.  For example,
+// CallStackId 2 would point to CallStackId 1 as a parent.
+//
+// We serialize the radix tree above into a single array along with the length
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teresajohnson wrote:

Note somewhere in here that we typically traverse from leaf to the root of the call stack, which is why they are ordered in that direction.

https://github.com/llvm/llvm-project/pull/93784