[llvm] r368565 - [CodeGen] Do the Simple Early Return in block-placement pass to optimize the blocks

[Roman Lebedev via llvm-commits]

This is being recommitted, but that isn't being said in the commit message,
also this is being recommitted but no test is being added?

On Mon, Aug 12, 2019 at 4:14 PM Kang Zhang via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
>
> Author: zhangkang
> Date: Mon Aug 12 06:15:31 2019
> New Revision: 368565
>
> URL: http://llvm.org/viewvc/llvm-project?rev=368565&view=rev
> Log:
> [CodeGen] Do the Simple Early Return in block-placement pass to optimize the blocks
>
> Summary:
>
> In `block-placement` pass, it will create some patterns for unconditional we can do the simple early retrun.
> But the `early-ret` pass is before `block-placement`, we don't want to run it again.
> This patch is to do the simple early return to optimize the blocks at the last of `block-placement`.
>
> Reviewed By: efriedma
>
> Differential Revision: https://reviews.llvm.org/D63972
>
> Modified:
>     llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
>     llvm/trunk/test/CodeGen/PowerPC/block-placement.mir
>
> Modified: llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp?rev=368565&r1=368564&r2=368565&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
> +++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Mon Aug 12 06:15:31 2019
> @@ -2755,6 +2755,7 @@ void MachineBlockPlacement::optimizeBran
>    // cannot because all branches may not be analyzable.
>    // E.g., the target may be able to remove an unconditional branch to
>    // a fallthrough when it occurs after predicated terminators.
> +  SmallVector<MachineBasicBlock*, 4> EmptyBB;
>    for (MachineBasicBlock *ChainBB : FunctionChain) {
>      Cond.clear();
>      MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
> @@ -2774,9 +2775,42 @@ void MachineBlockPlacement::optimizeBran
>          TII->removeBranch(*ChainBB);
>          TII->insertBranch(*ChainBB, FBB, TBB, Cond, dl);
>          ChainBB->updateTerminator();
> +      } else if (Cond.empty() && TBB && ChainBB != TBB && !TBB->empty() &&
> +                 !TBB->canFallThrough()) {
> +        // When ChainBB is unconditional branch to the TBB, and TBB has no
> +        // fallthrough predecessor and fallthrough successor, try to merge
> +        // ChainBB and TBB. This is legal under the one of following conditions:
> +        // 1. ChainBB is empty except for an unconditional branch.
> +        // 2. TBB has only one predecessor.
> +        MachineFunction::iterator I(TBB);
> +        if (((TBB == &*F->begin()) || !std::prev(I)->canFallThrough()) &&
> +             (TailDup.isSimpleBB(ChainBB) || (TBB->pred_size() == 1))) {
> +          TII->removeBranch(*ChainBB);
> +          ChainBB->removeSuccessor(TBB);
> +
> +          // Update the CFG.
> +          while (!TBB->pred_empty()) {
> +            MachineBasicBlock *Pred = *(TBB->pred_end()-1);
> +            Pred->ReplaceUsesOfBlockWith(TBB, ChainBB);
> +          }
> +
> +          for (MachineBasicBlock *Succ : TBB->successors())
> +            ChainBB->addSuccessor(Succ, MBPI->getEdgeProbability(TBB, Succ));
> +
> +          // Move all the instructions of TBB to ChainBB.
> +          ChainBB->splice(ChainBB->end(), TBB, TBB->begin(), TBB->end());
> +          EmptyBB.push_back(TBB);
> +        }
>        }
>      }
>    }
> +
> +  for (auto BB: EmptyBB) {
> +    MLI->removeBlock(BB);
> +    FunctionChain.remove(BB);
> +    BlockToChain.erase(BB);
> +    F->erase(BB);
> +  }
>  }
>
>  void MachineBlockPlacement::alignBlocks() {
> @@ -3096,6 +3130,9 @@ bool MachineBlockPlacement::runOnMachine
>      }
>    }
>
> +  // optimizeBranches() may change the blocks, but we haven't updated the
> +  // post-dominator tree. Because the post-dominator tree won't be used after
> +  // this function and this pass don't preserve the post-dominator tree.
>    optimizeBranches();
>    alignBlocks();
>
>
> Modified: llvm/trunk/test/CodeGen/PowerPC/block-placement.mir
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/PowerPC/block-placement.mir?rev=368565&r1=368564&r2=368565&view=diff
> ==============================================================================
> --- llvm/trunk/test/CodeGen/PowerPC/block-placement.mir (original)
> +++ llvm/trunk/test/CodeGen/PowerPC/block-placement.mir Mon Aug 12 06:15:31 2019
> @@ -209,14 +209,10 @@ body:             |
>      BLR8 implicit $lr8, implicit $rm, implicit killed $x3
>
>    ; CHECK:      bb.5.if.else.i:
> -  ; CHECK:        successors: %bb.11(0x80000000)
> -  ; CHECK:        B %bb.11
> +  ; CHECK-NEXT:   renamable $x3 = LI8 1
> +  ; CHECK-NEXT:   BLR8 implicit $lr8, implicit $rm, implicit killed $x3
>
>    ; CHECK:      bb.8.while.body.i (align 4):
> -  ; CHECK:        successors: %bb.11(0x04000000), %bb.9(0x7c000000)
> -  ; CHECK:        BCC 76, killed renamable $cr0, %bb.11
> -
> -  ; CHECK:      bb.11:
> -  ; CHECK:        renamable $x3 = LI8 1
> -  ; CHECK-NEXT:   BLR8 implicit $lr8, implicit $rm, implicit killed $x3
> +  ; CHECK:        successors: %bb.5(0x04000000), %bb.9(0x7c000000)
> +  ; CHECK:        BCC 76, killed renamable $cr0, %bb.5
>  ...
>
>
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