[llvm] [MemProf] Use radix tree for alloc contexts in bitcode summaries (PR #117066)

[Kazu Hirata via llvm-commits]

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@@ -4195,12 +4198,58 @@ static void writeTypeIdCompatibleVtableSummaryRecord(
   }
 }
 
+// Adds the allocation contexts to the CallStacks map. We simply use the
+// size at the time the context was added as the CallStackId. This works because
+// when we look up the call stacks later on we process the function summaries
+// and their allocation records in the same exact order.
+static void collectMemProfCallStacks(
+    FunctionSummary *FS, std::function<LinearFrameId(unsigned)> GetStackIndex,
+    MapVector<CallStackId, llvm::SmallVector<LinearFrameId>> &CallStacks) {
+  // The interfaces in ProfileData/MemProf.h use a type alias for a stack frame
+  // id offset into the index of the full stack frames. The ModuleSummaryIndex
+  // currently uses unsigned. Make sure these stay in sync.
+  static_assert(std::is_same_v<LinearFrameId, unsigned>);
+  for (auto &AI : FS->allocs()) {
+    for (auto &MIB : AI.MIBs) {
+      SmallVector<unsigned> StackIdIndices;
+      StackIdIndices.reserve(MIB.StackIdIndices.size());
+      for (auto Id : MIB.StackIdIndices)
+        StackIdIndices.push_back(GetStackIndex(Id));
+      // The CallStackId is the size at the time this context was inserted.
+      CallStacks.insert({CallStacks.size(), StackIdIndices});
+    }
+  }
+}
+
+// Build the radix tree from the accumulated CallStacks, write out the resulting
+// linearized radix tree array, and return the map of call stack positions into
+// this array for use when writing the allocation records. The returned map is
+// indexed by a CallStackId which in this case is implicitly determined by the
+// order of function summaries and their allocation infos being written.
+static DenseMap<CallStackId, LinearCallStackId> writeMemoryProfileRadixTree(
+    MapVector<CallStackId, llvm::SmallVector<LinearFrameId>> &CallStacks,
----------------
kazutakahirata wrote:

Did you mean to use an rvalue reference?  I see `std::move` a few lines below.

```suggestion
    MapVector<CallStackId, llvm::SmallVector<LinearFrameId>> &&CallStacks,
```

https://github.com/llvm/llvm-project/pull/117066