[llvm] [memprof] Simplify readMemprof (NFC) (PR #119930)

[Snehasish Kumar via llvm-commits]

================
@@ -969,10 +968,20 @@ readMemprof(Module &M, Function &F, IndexedInstrProfReader *MemProfReader,
   // Build maps of the location hash to all profile data with that leaf location
   // (allocation info and the callsites).
   std::map<uint64_t, std::set<const AllocationInfo *>> LocHashToAllocInfo;
-  // For the callsites we need to record the index of the associated frame in
-  // the frame array (see comments below where the map entries are added).
-  std::map<uint64_t, std::set<std::pair<const std::vector<Frame> *, unsigned>>>
-      LocHashToCallSites;
+  // A thin wrapper around ArrayRef<Frame> to facilitate std::set<CallStackRef>.
+  struct CallStackRef : public ArrayRef<Frame> {
+    CallStackRef(ArrayRef<Frame> CS, unsigned Pos)
+        : ArrayRef<Frame>(CS.drop_front(Pos)) {}
+    // std::set requires std::less.
----------------
snehasish wrote:

std::set indexing with pointers is inherently unstable. Can we use an unordered container here and simplify this?

https://github.com/llvm/llvm-project/pull/119930