diff options
Diffstat (limited to 'llvm/lib/CodeGen/MachineBlockPlacement.cpp')
| -rw-r--r-- | llvm/lib/CodeGen/MachineBlockPlacement.cpp | 38 |
1 files changed, 38 insertions, 0 deletions
diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp index 797808e9277..a246717cad6 100644 --- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp +++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp @@ -451,6 +451,8 @@ class MachineBlockPlacement : public MachineFunctionPass { void buildChain(const MachineBasicBlock *BB, BlockChain &Chain, BlockFilterSet *BlockFilter = nullptr); + bool canMoveBottomBlockToTop(const MachineBasicBlock *BottomBlock, + const MachineBasicBlock *OldTop); MachineBasicBlock *findBestLoopTop( const MachineLoop &L, const BlockFilterSet &LoopBlockSet); MachineBasicBlock *findBestLoopExit( @@ -1756,6 +1758,39 @@ void MachineBlockPlacement::buildChain( << getBlockName(*Chain.begin()) << "\n"); } +// If bottom of block BB has only one successor OldTop, in most cases it is +// profitable to move it before OldTop, except the following case: +// +// -->OldTop<- +// | . | +// | . | +// | . | +// ---Pred | +// | | +// BB----- +// +// If BB is moved before OldTop, Pred needs a taken branch to BB, and it can't +// layout the other successor below it, so it can't reduce taken branch. +// In this case we keep its original layout. +bool +MachineBlockPlacement::canMoveBottomBlockToTop( + const MachineBasicBlock *BottomBlock, + const MachineBasicBlock *OldTop) { + if (BottomBlock->pred_size() != 1) + return true; + MachineBasicBlock *Pred = *BottomBlock->pred_begin(); + if (Pred->succ_size() != 2) + return true; + + MachineBasicBlock *OtherBB = *Pred->succ_begin(); + if (OtherBB == BottomBlock) + OtherBB = *Pred->succ_rbegin(); + if (OtherBB == OldTop) + return false; + + return true; +} + /// Find the best loop top block for layout. /// /// Look for a block which is strictly better than the loop header for laying @@ -1800,6 +1835,9 @@ MachineBlockPlacement::findBestLoopTop(const MachineLoop &L, if (Pred->succ_size() > 1) continue; + if (!canMoveBottomBlockToTop(Pred, L.getHeader())) + continue; + BlockFrequency PredFreq = MBFI->getBlockFreq(Pred); if (!BestPred || PredFreq > BestPredFreq || (!(PredFreq < BestPredFreq) && |
