//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===// // // The LLVM Compiler Infrastructure // // This file was developed by the LLVM research group and is distributed under // the University of Illinois Open Source License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This pass forwards branches to unconditional branches to make them branch // directly to the target block. This pass often results in dead MBB's, which // it then removes. // // Note that this pass must be run after register allocation, it cannot handle // SSA form. // //===----------------------------------------------------------------------===// #define DEBUG_TYPE "branchfolding" #include "llvm/CodeGen/Passes.h" #include "llvm/CodeGen/MachineModuleInfo.h" #include "llvm/CodeGen/MachineFunctionPass.h" #include "llvm/CodeGen/MachineJumpTableInfo.h" #include "llvm/Target/TargetInstrInfo.h" #include "llvm/Target/TargetMachine.h" #include "llvm/Support/CommandLine.h" #include "llvm/Support/Debug.h" #include "llvm/ADT/Statistic.h" #include "llvm/ADT/STLExtras.h" #include using namespace llvm; STATISTIC(NumDeadBlocks, "Number of dead blocks removed"); STATISTIC(NumBranchOpts, "Number of branches optimized"); STATISTIC(NumTailMerge , "Number of block tails merged"); static cl::opt EnableTailMerge("enable-tail-merge", cl::Hidden); namespace { struct BranchFolder : public MachineFunctionPass { virtual bool runOnMachineFunction(MachineFunction &MF); virtual const char *getPassName() const { return "Control Flow Optimizer"; } const TargetInstrInfo *TII; MachineModuleInfo *MMI; bool MadeChange; private: // Tail Merging. bool TailMergeBlocks(MachineFunction &MF); void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, MachineBasicBlock *NewDest); MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB, MachineBasicBlock::iterator BBI1); // Branch optzn. bool OptimizeBranches(MachineFunction &MF); void OptimizeBlock(MachineBasicBlock *MBB); void RemoveDeadBlock(MachineBasicBlock *MBB); bool CanFallThrough(MachineBasicBlock *CurBB); bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable, MachineBasicBlock *TBB, MachineBasicBlock *FBB, const std::vector &Cond); }; } FunctionPass *llvm::createBranchFoldingPass() { return new BranchFolder(); } /// RemoveDeadBlock - Remove the specified dead machine basic block from the /// function, updating the CFG. void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { assert(MBB->pred_empty() && "MBB must be dead!"); MachineFunction *MF = MBB->getParent(); // drop all successors. while (!MBB->succ_empty()) MBB->removeSuccessor(MBB->succ_end()-1); // If there is DWARF info to active, check to see if there are any LABEL // records in the basic block. If so, unregister them from MachineModuleInfo. if (MMI && !MBB->empty()) { for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E; ++I) { if ((unsigned)I->getOpcode() == TargetInstrInfo::LABEL) { // The label ID # is always operand #0, an immediate. MMI->InvalidateLabel(I->getOperand(0).getImm()); } } } // Remove the block. MF->getBasicBlockList().erase(MBB); } bool BranchFolder::runOnMachineFunction(MachineFunction &MF) { TII = MF.getTarget().getInstrInfo(); if (!TII) return false; MMI = getAnalysisToUpdate(); bool EverMadeChange = false; bool MadeChangeThisIteration = true; while (MadeChangeThisIteration) { MadeChangeThisIteration = false; MadeChangeThisIteration |= TailMergeBlocks(MF); MadeChangeThisIteration |= OptimizeBranches(MF); EverMadeChange |= MadeChangeThisIteration; } // See if any jump tables have become mergable or dead as the code generator // did its thing. MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); const std::vector &JTs = JTI->getJumpTables(); if (!JTs.empty()) { // Figure out how these jump tables should be merged. std::vector JTMapping; JTMapping.reserve(JTs.size()); // We always keep the 0th jump table. JTMapping.push_back(0); // Scan the jump tables, seeing if there are any duplicates. Note that this // is N^2, which should be fixed someday. for (unsigned i = 1, e = JTs.size(); i != e; ++i) JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); // If a jump table was merge with another one, walk the function rewriting // references to jump tables to reference the new JT ID's. Keep track of // whether we see a jump table idx, if not, we can delete the JT. std::vector JTIsLive; JTIsLive.resize(JTs.size()); for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); BB != E; ++BB) { for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { MachineOperand &Op = I->getOperand(op); if (!Op.isJumpTableIndex()) continue; unsigned NewIdx = JTMapping[Op.getJumpTableIndex()]; Op.setJumpTableIndex(NewIdx); // Remember that this JT is live. JTIsLive[NewIdx] = true; } } // Finally, remove dead jump tables. This happens either because the // indirect jump was unreachable (and thus deleted) or because the jump // table was merged with some other one. for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) if (!JTIsLive[i]) { JTI->RemoveJumpTable(i); EverMadeChange = true; } } return EverMadeChange; } //===----------------------------------------------------------------------===// // Tail Merging of Blocks //===----------------------------------------------------------------------===// /// HashMachineInstr - Compute a hash value for MI and its operands. static unsigned HashMachineInstr(const MachineInstr *MI) { unsigned Hash = MI->getOpcode(); for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { const MachineOperand &Op = MI->getOperand(i); // Merge in bits from the operand if easy. unsigned OperandHash = 0; switch (Op.getType()) { case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; case MachineOperand::MO_MachineBasicBlock: OperandHash = Op.getMachineBasicBlock()->getNumber(); break; case MachineOperand::MO_FrameIndex: OperandHash = Op.getFrameIndex(); break; case MachineOperand::MO_ConstantPoolIndex: OperandHash = Op.getConstantPoolIndex(); break; case MachineOperand::MO_JumpTableIndex: OperandHash = Op.getJumpTableIndex(); break; case MachineOperand::MO_GlobalAddress: case MachineOperand::MO_ExternalSymbol: // Global address / external symbol are too hard, don't bother, but do // pull in the offset. OperandHash = Op.getOffset(); break; default: break; } Hash += ((OperandHash << 3) | Op.getType()) << (i&31); } return Hash; } /// HashEndOfMBB - Hash the last two instructions in the MBB. We hash two /// instructions, because cross-jumping only saves code when at least two /// instructions are removed (since a branch must be inserted). static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) { MachineBasicBlock::const_iterator I = MBB->end(); if (I == MBB->begin()) return 0; // Empty MBB. --I; unsigned Hash = HashMachineInstr(I); if (I == MBB->begin()) return Hash; // Single instr MBB. --I; // Hash in the second-to-last instruction. Hash ^= HashMachineInstr(I) << 2; return Hash; } /// ComputeCommonTailLength - Given two machine basic blocks, compute the number /// of instructions they actually have in common together at their end. Return /// iterators for the first shared instruction in each block. static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2, MachineBasicBlock::iterator &I1, MachineBasicBlock::iterator &I2) { I1 = MBB1->end(); I2 = MBB2->end(); unsigned TailLen = 0; while (I1 != MBB1->begin() && I2 != MBB2->begin()) { --I1; --I2; if (!I1->isIdenticalTo(I2)) { ++I1; ++I2; break; } ++TailLen; } return TailLen; } /// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything /// after it, replacing it with an unconditional branch to NewDest. This /// returns true if OldInst's block is modified, false if NewDest is modified. void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, MachineBasicBlock *NewDest) { MachineBasicBlock *OldBB = OldInst->getParent(); // Remove all the old successors of OldBB from the CFG. while (!OldBB->succ_empty()) OldBB->removeSuccessor(OldBB->succ_begin()); // Remove all the dead instructions from the end of OldBB. OldBB->erase(OldInst, OldBB->end()); // If OldBB isn't immediately before OldBB, insert a branch to it. if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) TII->InsertBranch(*OldBB, NewDest, 0, std::vector()); OldBB->addSuccessor(NewDest); ++NumTailMerge; } /// SplitMBBAt - Given a machine basic block and an iterator into it, split the /// MBB so that the part before the iterator falls into the part starting at the /// iterator. This returns the new MBB. MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, MachineBasicBlock::iterator BBI1) { // Create the fall-through block. MachineFunction::iterator MBBI = &CurMBB; MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock()); CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB); // Move all the successors of this block to the specified block. while (!CurMBB.succ_empty()) { MachineBasicBlock *S = *(CurMBB.succ_end()-1); NewMBB->addSuccessor(S); CurMBB.removeSuccessor(S); } // Add an edge from CurMBB to NewMBB for the fall-through. CurMBB.addSuccessor(NewMBB); // Splice the code over. NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); return NewMBB; } /// EstimateRuntime - Make a rough estimate for how long it will take to run /// the specified code. static unsigned EstimateRuntime(MachineBasicBlock::iterator I, MachineBasicBlock::iterator E, const TargetInstrInfo *TII) { unsigned Time = 0; for (; I != E; ++I) { const TargetInstrDescriptor &TID = TII->get(I->getOpcode()); if (TID.Flags & M_CALL_FLAG) Time += 10; else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG)) Time += 2; else ++Time; } return Time; } /// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at /// MBB2I and then insert an unconditional branch in the other block. Determine /// which is the best to split static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1, MachineBasicBlock::iterator MBB1I, MachineBasicBlock *MBB2, MachineBasicBlock::iterator MBB2I, const TargetInstrInfo *TII) { // TODO: if we had some notion of which block was hotter, we could split // the hot block, so it is the fall-through. Since we don't have profile info // make a decision based on which will hurt most to split. unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII); unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII); // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the // MBB1 block so it falls through. This will penalize the MBB2 path, but will // have a lower overall impact on the program execution. return MBB1Time < MBB2Time; } bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { MadeChange = false; if (!EnableTailMerge) return false; // Find blocks with no successors. std::vector > MergePotentials; for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { if (I->succ_empty()) MergePotentials.push_back(std::make_pair(HashEndOfMBB(I), I)); } // Sort by hash value so that blocks with identical end sequences sort // together. std::stable_sort(MergePotentials.begin(), MergePotentials.end()); // Walk through equivalence sets looking for actual exact matches. while (MergePotentials.size() > 1) { unsigned CurHash = (MergePotentials.end()-1)->first; unsigned PrevHash = (MergePotentials.end()-2)->first; MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second; // If there is nothing that matches the hash of the current basic block, // give up. if (CurHash != PrevHash) { MergePotentials.pop_back(); continue; } // Determine the actual length of the shared tail between these two basic // blocks. Because the hash can have collisions, it's possible that this is // less than 2. MachineBasicBlock::iterator BBI1, BBI2; unsigned CommonTailLen = ComputeCommonTailLength(CurMBB, (MergePotentials.end()-2)->second, BBI1, BBI2); // If the tails don't have at least two instructions in common, see if there // is anything else in the equivalence class that does match. if (CommonTailLen < 2) { unsigned FoundMatch = ~0U; for (int i = MergePotentials.size()-2; i != -1 && MergePotentials[i].first == CurHash; --i) { CommonTailLen = ComputeCommonTailLength(CurMBB, MergePotentials[i].second, BBI1, BBI2); if (CommonTailLen >= 2) { FoundMatch = i; break; } } // If we didn't find anything that has at least two instructions matching // this one, bail out. if (FoundMatch == ~0U) { MergePotentials.pop_back(); continue; } // Otherwise, move the matching block to the right position. std::swap(MergePotentials[FoundMatch], *(MergePotentials.end()-2)); } MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second; // If neither block is the entire common tail, split the tail of one block // to make it redundant with the other tail. if (CurMBB->begin() != BBI1 && MBB2->begin() != BBI2) { if (0) { // Enable this to disable partial tail merges. MergePotentials.pop_back(); continue; } // Decide whether we want to split CurMBB or MBB2. if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII)) { CurMBB = SplitMBBAt(*CurMBB, BBI1); BBI1 = CurMBB->begin(); MergePotentials.back().second = CurMBB; } else { MBB2 = SplitMBBAt(*MBB2, BBI2); BBI2 = MBB2->begin(); (MergePotentials.end()-2)->second = MBB2; } } if (MBB2->begin() == BBI2) { // Hack the end off CurMBB, making it jump to MBBI@ instead. ReplaceTailWithBranchTo(BBI1, MBB2); // This modifies CurMBB, so remove it from the worklist. MergePotentials.pop_back(); } else { assert(CurMBB->begin() == BBI1 && "Didn't split block correctly?"); // Hack the end off MBB2, making it jump to CurMBB instead. ReplaceTailWithBranchTo(BBI2, CurMBB); // This modifies MBB2, so remove it from the worklist. MergePotentials.erase(MergePotentials.end()-2); } MadeChange = true; } return MadeChange; } //===----------------------------------------------------------------------===// // Branch Optimization //===----------------------------------------------------------------------===// bool BranchFolder::OptimizeBranches(MachineFunction &MF) { MadeChange = false; // Make sure blocks are numbered in order MF.RenumberBlocks(); for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { MachineBasicBlock *MBB = I++; OptimizeBlock(MBB); // If it is dead, remove it. if (MBB->pred_empty()) { RemoveDeadBlock(MBB); MadeChange = true; ++NumDeadBlocks; } } return MadeChange; } /// CorrectExtraCFGEdges - Various pieces of code can cause excess edges in the /// CFG to be inserted. If we have proven that MBB can only branch to DestA and /// DestB, remove any other MBB successors from the CFG. DestA and DestB can /// be null. static bool CorrectExtraCFGEdges(MachineBasicBlock &MBB, MachineBasicBlock *DestA, MachineBasicBlock *DestB, bool isCond, MachineFunction::iterator FallThru) { bool MadeChange = false; bool AddedFallThrough = false; // If this block ends with a conditional branch that falls through to its // successor, set DestB as the successor. if (isCond) { if (DestB == 0 && FallThru != MBB.getParent()->end()) { DestB = FallThru; AddedFallThrough = true; } } else { // If this is an unconditional branch with no explicit dest, it must just be // a fallthrough into DestB. if (DestA == 0 && FallThru != MBB.getParent()->end()) { DestA = FallThru; AddedFallThrough = true; } } MachineBasicBlock::pred_iterator SI = MBB.succ_begin(); while (SI != MBB.succ_end()) { if (*SI == DestA) { DestA = 0; ++SI; } else if (*SI == DestB) { DestB = 0; ++SI; } else { // Otherwise, this is a superfluous edge, remove it. MBB.removeSuccessor(SI); MadeChange = true; } } if (!AddedFallThrough) { assert(DestA == 0 && DestB == 0 && "MachineCFG is missing edges!"); } else if (isCond) { assert(DestA == 0 && "MachineCFG is missing edges!"); } return MadeChange; } /// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to /// 'Old', change the code and CFG so that it branches to 'New' instead. static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB, MachineBasicBlock *Old, MachineBasicBlock *New, const TargetInstrInfo *TII) { assert(Old != New && "Cannot replace self with self!"); MachineBasicBlock::iterator I = BB->end(); while (I != BB->begin()) { --I; if (!TII->isTerminatorInstr(I->getOpcode())) break; // Scan the operands of this machine instruction, replacing any uses of Old // with New. for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) if (I->getOperand(i).isMachineBasicBlock() && I->getOperand(i).getMachineBasicBlock() == Old) I->getOperand(i).setMachineBasicBlock(New); } // Update the successor information. std::vector Succs(BB->succ_begin(), BB->succ_end()); for (int i = Succs.size()-1; i >= 0; --i) if (Succs[i] == Old) { BB->removeSuccessor(Old); BB->addSuccessor(New); } } /// CanFallThrough - Return true if the specified block (with the specified /// branch condition) can implicitly transfer control to the block after it by /// falling off the end of it. This should return false if it can reach the /// block after it, but it uses an explicit branch to do so (e.g. a table jump). /// /// True is a conservative answer. /// bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable, MachineBasicBlock *TBB, MachineBasicBlock *FBB, const std::vector &Cond) { MachineFunction::iterator Fallthrough = CurBB; ++Fallthrough; // If FallthroughBlock is off the end of the function, it can't fall through. if (Fallthrough == CurBB->getParent()->end()) return false; // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. if (!CurBB->isSuccessor(Fallthrough)) return false; // If we couldn't analyze the branch, assume it could fall through. if (BranchUnAnalyzable) return true; // If there is no branch, control always falls through. if (TBB == 0) return true; // If there is some explicit branch to the fallthrough block, it can obviously // reach, even though the branch should get folded to fall through implicitly. if (MachineFunction::iterator(TBB) == Fallthrough || MachineFunction::iterator(FBB) == Fallthrough) return true; // If it's an unconditional branch to some block not the fall through, it // doesn't fall through. if (Cond.empty()) return false; // Otherwise, if it is conditional and has no explicit false block, it falls // through. return FBB == 0; } /// CanFallThrough - Return true if the specified can implicitly transfer /// control to the block after it by falling off the end of it. This should /// return false if it can reach the block after it, but it uses an explicit /// branch to do so (e.g. a table jump). /// /// True is a conservative answer. /// bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { MachineBasicBlock *TBB = 0, *FBB = 0; std::vector Cond; bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond); return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); } /// IsBetterFallthrough - Return true if it would be clearly better to /// fall-through to MBB1 than to fall through into MBB2. This has to return /// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will /// result in infinite loops. static bool IsBetterFallthrough(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2, const TargetInstrInfo &TII) { // Right now, we use a simple heuristic. If MBB2 ends with a call, and // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to // optimize branches that branch to either a return block or an assert block // into a fallthrough to the return. if (MBB1->empty() || MBB2->empty()) return false; MachineInstr *MBB1I = --MBB1->end(); MachineInstr *MBB2I = --MBB2->end(); return TII.isCall(MBB2I->getOpcode()) && !TII.isCall(MBB1I->getOpcode()); } /// OptimizeBlock - Analyze and optimize control flow related to the specified /// block. This is never called on the entry block. void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { MachineFunction::iterator FallThrough = MBB; ++FallThrough; // If this block is empty, make everyone use its fall-through, not the block // explicitly. if (MBB->empty()) { // Dead block? Leave for cleanup later. if (MBB->pred_empty()) return; if (FallThrough == MBB->getParent()->end()) { // TODO: Simplify preds to not branch here if possible! } else { // Rewrite all predecessors of the old block to go to the fallthrough // instead. while (!MBB->pred_empty()) { MachineBasicBlock *Pred = *(MBB->pred_end()-1); ReplaceUsesOfBlockWith(Pred, MBB, FallThrough, TII); } // If MBB was the target of a jump table, update jump tables to go to the // fallthrough instead. MBB->getParent()->getJumpTableInfo()-> ReplaceMBBInJumpTables(MBB, FallThrough); MadeChange = true; } return; } // Check to see if we can simplify the terminator of the block before this // one. MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; std::vector PriorCond; bool PriorUnAnalyzable = TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); if (!PriorUnAnalyzable) { // If the CFG for the prior block has extra edges, remove them. MadeChange |= CorrectExtraCFGEdges(PrevBB, PriorTBB, PriorFBB, !PriorCond.empty(), MBB); // If the previous branch is conditional and both conditions go to the same // destination, remove the branch, replacing it with an unconditional one or // a fall-through. if (PriorTBB && PriorTBB == PriorFBB) { TII->RemoveBranch(PrevBB); PriorCond.clear(); if (PriorTBB != MBB) TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); MadeChange = true; ++NumBranchOpts; return OptimizeBlock(MBB); } // If the previous branch *only* branches to *this* block (conditional or // not) remove the branch. if (PriorTBB == MBB && PriorFBB == 0) { TII->RemoveBranch(PrevBB); MadeChange = true; ++NumBranchOpts; return OptimizeBlock(MBB); } // If the prior block branches somewhere else on the condition and here if // the condition is false, remove the uncond second branch. if (PriorFBB == MBB) { TII->RemoveBranch(PrevBB); TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); MadeChange = true; ++NumBranchOpts; return OptimizeBlock(MBB); } // If the prior block branches here on true and somewhere else on false, and // if the branch condition is reversible, reverse the branch to create a // fall-through. if (PriorTBB == MBB) { std::vector NewPriorCond(PriorCond); if (!TII->ReverseBranchCondition(NewPriorCond)) { TII->RemoveBranch(PrevBB); TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); MadeChange = true; ++NumBranchOpts; return OptimizeBlock(MBB); } } // If this block doesn't fall through (e.g. it ends with an uncond branch or // has no successors) and if the pred falls through into this block, and if // it would otherwise fall through into the block after this, move this // block to the end of the function. // // We consider it more likely that execution will stay in the function (e.g. // due to loops) than it is to exit it. This asserts in loops etc, moving // the assert condition out of the loop body. if (!PriorCond.empty() && PriorFBB == 0 && MachineFunction::iterator(PriorTBB) == FallThrough && !CanFallThrough(MBB)) { bool DoTransform = true; // We have to be careful that the succs of PredBB aren't both no-successor // blocks. If neither have successors and if PredBB is the second from // last block in the function, we'd just keep swapping the two blocks for // last. Only do the swap if one is clearly better to fall through than // the other. if (FallThrough == --MBB->getParent()->end() && !IsBetterFallthrough(PriorTBB, MBB, *TII)) DoTransform = false; // We don't want to do this transformation if we have control flow like: // br cond BB2 // BB1: // .. // jmp BBX // BB2: // .. // ret // // In this case, we could actually be moving the return block *into* a // loop! if (DoTransform && !MBB->succ_empty() && (!CanFallThrough(PriorTBB) || PriorTBB->empty())) DoTransform = false; if (DoTransform) { // Reverse the branch so we will fall through on the previous true cond. std::vector NewPriorCond(PriorCond); if (!TII->ReverseBranchCondition(NewPriorCond)) { DOUT << "\nMoving MBB: " << *MBB; DOUT << "To make fallthrough to: " << *PriorTBB << "\n"; TII->RemoveBranch(PrevBB); TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); // Move this block to the end of the function. MBB->moveAfter(--MBB->getParent()->end()); MadeChange = true; ++NumBranchOpts; return; } } } } // Analyze the branch in the current block. MachineBasicBlock *CurTBB = 0, *CurFBB = 0; std::vector CurCond; bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond); if (!CurUnAnalyzable) { // If the CFG for the prior block has extra edges, remove them. MadeChange |= CorrectExtraCFGEdges(*MBB, CurTBB, CurFBB, !CurCond.empty(), ++MachineFunction::iterator(MBB)); // If this is a two-way branch, and the FBB branches to this block, reverse // the condition so the single-basic-block loop is faster. Instead of: // Loop: xxx; jcc Out; jmp Loop // we want: // Loop: xxx; jncc Loop; jmp Out if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { std::vector NewCond(CurCond); if (!TII->ReverseBranchCondition(NewCond)) { TII->RemoveBranch(*MBB); TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); MadeChange = true; ++NumBranchOpts; return OptimizeBlock(MBB); } } // If this branch is the only thing in its block, see if we can forward // other blocks across it. if (CurTBB && CurCond.empty() && CurFBB == 0 && TII->isBranch(MBB->begin()->getOpcode()) && CurTBB != MBB) { // This block may contain just an unconditional branch. Because there can // be 'non-branch terminators' in the block, try removing the branch and // then seeing if the block is empty. TII->RemoveBranch(*MBB); // If this block is just an unconditional branch to CurTBB, we can // usually completely eliminate the block. The only case we cannot // completely eliminate the block is when the block before this one // falls through into MBB and we can't understand the prior block's branch // condition. if (MBB->empty()) { bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); if (PredHasNoFallThrough || !PriorUnAnalyzable || !PrevBB.isSuccessor(MBB)) { // If the prior block falls through into us, turn it into an // explicit branch to us to make updates simpler. if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && PriorTBB != MBB && PriorFBB != MBB) { if (PriorTBB == 0) { assert(PriorCond.empty() && PriorFBB == 0 && "Bad branch analysis"); PriorTBB = MBB; } else { assert(PriorFBB == 0 && "Machine CFG out of date!"); PriorFBB = MBB; } TII->RemoveBranch(PrevBB); TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); } // Iterate through all the predecessors, revectoring each in-turn. MachineBasicBlock::pred_iterator PI = MBB->pred_begin(); bool DidChange = false; bool HasBranchToSelf = false; while (PI != MBB->pred_end()) { if (*PI == MBB) { // If this block has an uncond branch to itself, leave it. ++PI; HasBranchToSelf = true; } else { DidChange = true; ReplaceUsesOfBlockWith(*PI, MBB, CurTBB, TII); } } // Change any jumptables to go to the new MBB. MBB->getParent()->getJumpTableInfo()-> ReplaceMBBInJumpTables(MBB, CurTBB); if (DidChange) { ++NumBranchOpts; MadeChange = true; if (!HasBranchToSelf) return; } } } // Add the branch back if the block is more than just an uncond branch. TII->InsertBranch(*MBB, CurTBB, 0, CurCond); } } // If the prior block doesn't fall through into this block, and if this // block doesn't fall through into some other block, see if we can find a // place to move this block where a fall-through will happen. if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, PriorTBB, PriorFBB, PriorCond)) { // Now we know that there was no fall-through into this block, check to // see if it has a fall-through into its successor. bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, CurCond); // Check all the predecessors of this block. If one of them has no fall // throughs, move this block right after it. for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), E = MBB->pred_end(); PI != E; ++PI) { // Analyze the branch at the end of the pred. MachineBasicBlock *PredBB = *PI; MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; if (PredBB != MBB && !CanFallThrough(PredBB) && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { // If the current block doesn't fall through, just move it. // If the current block can fall through and does not end with a // conditional branch, we need to append an unconditional jump to // the (current) next block. To avoid a possible compile-time // infinite loop, move blocks only backward in this case. if (CurFallsThru) { MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); CurCond.clear(); TII->InsertBranch(*MBB, NextBB, 0, CurCond); } MBB->moveAfter(PredBB); MadeChange = true; return OptimizeBlock(MBB); } } if (!CurFallsThru) { // Check all successors to see if we can move this block before it. for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), E = MBB->succ_end(); SI != E; ++SI) { // Analyze the branch at the end of the block before the succ. MachineBasicBlock *SuccBB = *SI; MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; std::vector SuccPrevCond; if (SuccBB != MBB && !CanFallThrough(SuccPrev)) { MBB->moveBefore(SuccBB); MadeChange = true; return OptimizeBlock(MBB); } } // Okay, there is no really great place to put this block. If, however, // the block before this one would be a fall-through if this block were // removed, move this block to the end of the function. if (FallThrough != MBB->getParent()->end() && PrevBB.isSuccessor(FallThrough)) { MBB->moveAfter(--MBB->getParent()->end()); MadeChange = true; return; } } } }