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

[Kazu Hirata via llvm-commits]

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@@ -7544,6 +7550,45 @@ void ModuleSummaryIndexBitcodeReader::parseTypeIdCompatibleVtableSummaryRecord(
     parseTypeIdCompatibleVtableInfo(Record, Slot, TypeId);
 }
 
+SmallVector<unsigned> ModuleSummaryIndexBitcodeReader::parseAllocInfoContext(
+    ArrayRef<uint64_t> Record, unsigned &I) {
+  SmallVector<unsigned> StackIdList;
+  // For backwards compatibility with old format before radix tree was
+  // used, simply see if we found a radix tree array record.
+  if (RadixArray.empty()) {
+    unsigned NumStackEntries = Record[I++];
+    assert(Record.size() - I >= NumStackEntries);
+    for (unsigned J = 0; J < NumStackEntries; J++) {
+      assert(Record[I] < StackIds.size());
+      StackIdList.push_back(
+          TheIndex.addOrGetStackIdIndex(StackIds[Record[I++]]));
+    }
+  } else {
+    unsigned RadixIndex = Record[I++];
+    // See the comments above CallStackRadixTreeBuilder in ProfileData/MemProf.h
+    // for a detailed description of the radix tree array format. Briefly, the
+    // first entry will be the number of frames, any negative values are the
+    // negative of the offset of the next frame, and otherwise the frames are in
+    // increasing linear order.
+    assert(RadixIndex < RadixArray.size());
+    unsigned NumStackIds = RadixArray[RadixIndex++];
+    while (NumStackIds--) {
----------------
kazutakahirata wrote:

Likewise, may I suggest `reserve` here?  The main reason for encoding the length in the radix tree array was to enable `reserve`.

```suggestion
    StackIdList.reserve(NumStackIds);
    while (NumStackIds--) {
```

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