[PATCH] D110613: [Taildup] Don't tail-duplicate loop header with multiple successors as its latches

[Kai Luo via Phabricator via llvm-commits]

lkail added a comment.

> @lkail @MatzeB any suggestion with the parameter value 128?

I don't think it's a good idea to add such constraint in taildup pass when `-Oz` or `-Os` is not specified. From the perf data pasted by @alexfh in https://reviews.llvm.org/D106056, the compile time regression occurs in `MachineBlockPlacement` when CFG becomes complex. I think we should fix the issue there, i.e., fix the O(n^2) iteration by(There are multiple such usages)

  diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
  index 8a1b4031642d..56c3d3106a19 100644
  --- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
  +++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
  @@ -1905,8 +1905,11 @@ MachineBlockPlacement::TopFallThroughFreq(
         // Check if Top is the best successor of Pred.
         auto TopProb = MBPI->getEdgeProbability(Pred, Top);
         bool TopOK = true;
  -      for (MachineBasicBlock *Succ : Pred->successors()) {
  -        auto SuccProb = MBPI->getEdgeProbability(Pred, Succ);
  +      for (MachineBasicBlock::succ_iterator SI = Pred->succ_begin(),
  +                                          SE = Pred->succ_end();
  +         SI != SE; ++SI) {
  +        MachineBasicBlock *Succ = *SI;
  +        auto SuccProb = MBPI->getEdgeProbability(Pred, SI);
           BlockChain *SuccChain = BlockToChain[Succ];
           // Check if Succ can be placed after Pred.
           // Succ should not be in any chain, or it is the head of some chain.

This might give acceptable compile time.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D110613/new/

https://reviews.llvm.org/D110613