[llvm] [PGO] Implement PGO counter promotion for atomic updates (PR #202487)

[Wael Yehia via llvm-commits]

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@@ -567,6 +612,7 @@ class PGOCounterPromoter {
                                         L.getLoopPreheader(), ExitBlocks,
                                         InsertPts, LoopToCandidates, LI);
       Promoter.run(SmallVector<Instruction *, 2>({Cand.first, Cand.second}));
+      Cand = {nullptr, nullptr};
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w2yehia wrote:

so what you're saying is that a new `Loop*` map entry might be added to `LoopToCandidates`, which might rehash, invalidating any references to any `SmallVector<LoadStorePair,8>` object (in particular references to memory within the object, not malloc'ed memory which I presume is "moved").
That makes sense, and that would mean the [existing code](https://github.com/llvm/llvm-project/blob/8433cf6d6ccc49a0fd99ed86c7d94a1fbb2fd1be/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp#L547) is also broken because the iterator over the LoopToCandidates[&L] would get invalidated.
I think today, in the default -fprofile-generate path, every loop gets a counter, so `LoopToCandidates[TargetLoop]` will always find an existing entry in the map (i.e. no rehashing should occur).


https://github.com/llvm/llvm-project/pull/202487