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

[JunMa via Phabricator via llvm-commits]

junparser added a comment.

In D110613#3054163 <https://reviews.llvm.org/D110613#3054163>, @lkail wrote:

>> @lkail @MatzeB any suggestion with the parameter value 128?
>
> 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, listed one)

It's not only MachineBlockPlacement pass,  we saw Eliminate PHI and Control flow pass also cost much more time.  with the ir file i attached above, it shows

  6.1973 ( 46.3%)   0.0000 (  0.0%)   6.1973 ( 45.8%)   6.2801 ( 45.8%)  Eliminate PHI nodes for register allocation
  2.3774 ( 17.7%)   0.0000 (  0.0%)   2.3774 ( 17.6%)   2.4110 ( 17.6%)  Control Flow Optimizer
  0.9603 (  7.2%)   0.0000 (  0.0%)   0.9603 (  7.1%)   0.9740 (  7.1%)  Branch Probability Basic Block Placement
  0.4236 (  3.2%)   0.0680 ( 50.6%)   0.4916 (  3.6%)   0.4978 (  3.6%)  Early Tail Duplication
  0.4374 (  3.3%)   0.0000 (  0.0%)   0.4374 (  3.2%)   0.4455 (  3.2%)  Machine code sinking



>   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.

Also I have test with this change, It does not work.  The time is not consumed by find in getEdgeProbability


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D110613/new/

https://reviews.llvm.org/D110613