| //===-- BranchFolding.cpp - Fold machine code branch instructions ---------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file 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 "BranchFolding.h" |
| #include "llvm/Function.h" |
| #include "llvm/CodeGen/Passes.h" |
| #include "llvm/CodeGen/MachineModuleInfo.h" |
| #include "llvm/CodeGen/MachineFunctionPass.h" |
| #include "llvm/CodeGen/MachineJumpTableInfo.h" |
| #include "llvm/CodeGen/RegisterScavenging.h" |
| #include "llvm/Target/TargetInstrInfo.h" |
| #include "llvm/Target/TargetMachine.h" |
| #include "llvm/Target/TargetRegisterInfo.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/ErrorHandling.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include "llvm/ADT/SmallSet.h" |
| #include "llvm/ADT/SetVector.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/ADT/STLExtras.h" |
| #include <algorithm> |
| 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<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", |
| cl::init(cl::BOU_UNSET), cl::Hidden); |
| |
| // Throttle for huge numbers of predecessors (compile speed problems) |
| static cl::opt<unsigned> |
| TailMergeThreshold("tail-merge-threshold", |
| cl::desc("Max number of predecessors to consider tail merging"), |
| cl::init(150), cl::Hidden); |
| |
| // Heuristic for tail merging (and, inversely, tail duplication). |
| // TODO: This should be replaced with a target query. |
| static cl::opt<unsigned> |
| TailMergeSize("tail-merge-size", |
| cl::desc("Min number of instructions to consider tail merging"), |
| cl::init(3), cl::Hidden); |
| |
| namespace { |
| /// BranchFolderPass - Wrap branch folder in a machine function pass. |
| class BranchFolderPass : public MachineFunctionPass, |
| public BranchFolder { |
| public: |
| static char ID; |
| explicit BranchFolderPass(bool defaultEnableTailMerge) |
| : MachineFunctionPass(&ID), BranchFolder(defaultEnableTailMerge) {} |
| |
| virtual bool runOnMachineFunction(MachineFunction &MF); |
| virtual const char *getPassName() const { return "Control Flow Optimizer"; } |
| }; |
| } |
| |
| char BranchFolderPass::ID = 0; |
| |
| FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) { |
| return new BranchFolderPass(DefaultEnableTailMerge); |
| } |
| |
| bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) { |
| return OptimizeFunction(MF, |
| MF.getTarget().getInstrInfo(), |
| MF.getTarget().getRegisterInfo(), |
| getAnalysisIfAvailable<MachineModuleInfo>()); |
| } |
| |
| |
| BranchFolder::BranchFolder(bool defaultEnableTailMerge) { |
| switch (FlagEnableTailMerge) { |
| case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; |
| case cl::BOU_TRUE: EnableTailMerge = true; break; |
| case cl::BOU_FALSE: EnableTailMerge = false; break; |
| } |
| } |
| |
| /// 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!"); |
| DEBUG(dbgs() << "\nRemoving MBB: " << *MBB); |
| |
| MachineFunction *MF = MBB->getParent(); |
| // drop all successors. |
| while (!MBB->succ_empty()) |
| MBB->removeSuccessor(MBB->succ_end()-1); |
| |
| // If there are any labels in the basic block, unregister them from |
| // MachineModuleInfo. |
| if (MMI && !MBB->empty()) { |
| for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); |
| I != E; ++I) { |
| if (I->isLabel()) |
| // The label ID # is always operand #0, an immediate. |
| MMI->InvalidateLabel(I->getOperand(0).getImm()); |
| } |
| } |
| |
| // Remove the block. |
| MF->erase(MBB); |
| } |
| |
| /// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def |
| /// followed by terminators, and if the implicitly defined registers are not |
| /// used by the terminators, remove those implicit_def's. e.g. |
| /// BB1: |
| /// r0 = implicit_def |
| /// r1 = implicit_def |
| /// br |
| /// This block can be optimized away later if the implicit instructions are |
| /// removed. |
| bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) { |
| SmallSet<unsigned, 4> ImpDefRegs; |
| MachineBasicBlock::iterator I = MBB->begin(); |
| while (I != MBB->end()) { |
| if (!I->isImplicitDef()) |
| break; |
| unsigned Reg = I->getOperand(0).getReg(); |
| ImpDefRegs.insert(Reg); |
| for (const unsigned *SubRegs = TRI->getSubRegisters(Reg); |
| unsigned SubReg = *SubRegs; ++SubRegs) |
| ImpDefRegs.insert(SubReg); |
| ++I; |
| } |
| if (ImpDefRegs.empty()) |
| return false; |
| |
| MachineBasicBlock::iterator FirstTerm = I; |
| while (I != MBB->end()) { |
| if (!TII->isUnpredicatedTerminator(I)) |
| return false; |
| // See if it uses any of the implicitly defined registers. |
| for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { |
| MachineOperand &MO = I->getOperand(i); |
| if (!MO.isReg() || !MO.isUse()) |
| continue; |
| unsigned Reg = MO.getReg(); |
| if (ImpDefRegs.count(Reg)) |
| return false; |
| } |
| ++I; |
| } |
| |
| I = MBB->begin(); |
| while (I != FirstTerm) { |
| MachineInstr *ImpDefMI = &*I; |
| ++I; |
| MBB->erase(ImpDefMI); |
| } |
| |
| return true; |
| } |
| |
| /// OptimizeFunction - Perhaps branch folding, tail merging and other |
| /// CFG optimizations on the given function. |
| bool BranchFolder::OptimizeFunction(MachineFunction &MF, |
| const TargetInstrInfo *tii, |
| const TargetRegisterInfo *tri, |
| MachineModuleInfo *mmi) { |
| if (!tii) return false; |
| |
| TII = tii; |
| TRI = tri; |
| MMI = mmi; |
| |
| RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; |
| |
| // Fix CFG. The later algorithms expect it to be right. |
| bool MadeChange = false; |
| for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { |
| MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; |
| SmallVector<MachineOperand, 4> Cond; |
| if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true)) |
| MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); |
| MadeChange |= OptimizeImpDefsBlock(MBB); |
| } |
| |
| bool MadeChangeThisIteration = true; |
| while (MadeChangeThisIteration) { |
| MadeChangeThisIteration = false; |
| MadeChangeThisIteration |= TailMergeBlocks(MF); |
| MadeChangeThisIteration |= OptimizeBranches(MF); |
| MadeChange |= MadeChangeThisIteration; |
| } |
| |
| // See if any jump tables have become mergable or dead as the code generator |
| // did its thing. |
| MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); |
| if (JTI == 0) { |
| delete RS; |
| return MadeChange; |
| } |
| |
| const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); |
| // Figure out how these jump tables should be merged. |
| std::vector<unsigned> 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) { |
| if (JTs[i].MBBs.empty()) |
| JTMapping.push_back(i); |
| else |
| 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. |
| BitVector JTIsLive(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.isJTI()) continue; |
| unsigned NewIdx = JTMapping[Op.getIndex()]; |
| Op.setIndex(NewIdx); |
| |
| // Remember that this JT is live. |
| JTIsLive.set(NewIdx); |
| } |
| } |
| |
| // 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.test(i)) { |
| JTI->RemoveJumpTable(i); |
| MadeChange = true; |
| } |
| |
| delete RS; |
| return MadeChange; |
| } |
| |
| //===----------------------------------------------------------------------===// |
| // 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.getMBB()->getNumber(); |
| break; |
| case MachineOperand::MO_FrameIndex: |
| case MachineOperand::MO_ConstantPoolIndex: |
| case MachineOperand::MO_JumpTableIndex: |
| OperandHash = Op.getIndex(); |
| 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 few instructions in the MBB. For blocks |
| /// with no successors, we hash two instructions, because cross-jumping |
| /// only saves code when at least two instructions are removed (since a |
| /// branch must be inserted). For blocks with a successor, one of the |
| /// two blocks to be tail-merged will end with a branch already, so |
| /// it gains to cross-jump even for one instruction. |
| static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, |
| unsigned minCommonTailLength) { |
| MachineBasicBlock::const_iterator I = MBB->end(); |
| if (I == MBB->begin()) |
| return 0; // Empty MBB. |
| |
| --I; |
| unsigned Hash = HashMachineInstr(I); |
| |
| if (I == MBB->begin() || minCommonTailLength == 1) |
| 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) || |
| // FIXME: This check is dubious. It's used to get around a problem where |
| // people incorrectly expect inline asm directives to remain in the same |
| // relative order. This is untenable because normal compiler |
| // optimizations (like this one) may reorder and/or merge these |
| // directives. |
| I1->isInlineAsm()) { |
| ++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, SmallVector<MachineOperand, 0>()); |
| 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) { |
| MachineFunction &MF = *CurMBB.getParent(); |
| |
| // Create the fall-through block. |
| MachineFunction::iterator MBBI = &CurMBB; |
| MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock()); |
| CurMBB.getParent()->insert(++MBBI, NewMBB); |
| |
| // Move all the successors of this block to the specified block. |
| NewMBB->transferSuccessors(&CurMBB); |
| |
| // 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()); |
| |
| // For targets that use the register scavenger, we must maintain LiveIns. |
| if (RS) { |
| RS->enterBasicBlock(&CurMBB); |
| if (!CurMBB.empty()) |
| RS->forward(prior(CurMBB.end())); |
| BitVector RegsLiveAtExit(TRI->getNumRegs()); |
| RS->getRegsUsed(RegsLiveAtExit, false); |
| for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++) |
| if (RegsLiveAtExit[i]) |
| NewMBB->addLiveIn(i); |
| } |
| |
| 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) { |
| unsigned Time = 0; |
| for (; I != E; ++I) { |
| const TargetInstrDesc &TID = I->getDesc(); |
| if (TID.isCall()) |
| Time += 10; |
| else if (TID.mayLoad() || TID.mayStore()) |
| Time += 2; |
| else |
| ++Time; |
| } |
| return Time; |
| } |
| |
| // CurMBB needs to add an unconditional branch to SuccMBB (we removed these |
| // branches temporarily for tail merging). In the case where CurMBB ends |
| // with a conditional branch to the next block, optimize by reversing the |
| // test and conditionally branching to SuccMBB instead. |
| static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB, |
| const TargetInstrInfo *TII) { |
| MachineFunction *MF = CurMBB->getParent(); |
| MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB)); |
| MachineBasicBlock *TBB = 0, *FBB = 0; |
| SmallVector<MachineOperand, 4> Cond; |
| if (I != MF->end() && |
| !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) { |
| MachineBasicBlock *NextBB = I; |
| if (TBB == NextBB && !Cond.empty() && !FBB) { |
| if (!TII->ReverseBranchCondition(Cond)) { |
| TII->RemoveBranch(*CurMBB); |
| TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); |
| return; |
| } |
| } |
| } |
| TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>()); |
| } |
| |
| bool |
| BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const { |
| if (getHash() < o.getHash()) |
| return true; |
| else if (getHash() > o.getHash()) |
| return false; |
| else if (getBlock()->getNumber() < o.getBlock()->getNumber()) |
| return true; |
| else if (getBlock()->getNumber() > o.getBlock()->getNumber()) |
| return false; |
| else { |
| // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing |
| // an object with itself. |
| #ifndef _GLIBCXX_DEBUG |
| llvm_unreachable("Predecessor appears twice"); |
| #endif |
| return false; |
| } |
| } |
| |
| /// CountTerminators - Count the number of terminators in the given |
| /// block and set I to the position of the first non-terminator, if there |
| /// is one, or MBB->end() otherwise. |
| static unsigned CountTerminators(MachineBasicBlock *MBB, |
| MachineBasicBlock::iterator &I) { |
| I = MBB->end(); |
| unsigned NumTerms = 0; |
| for (;;) { |
| if (I == MBB->begin()) { |
| I = MBB->end(); |
| break; |
| } |
| --I; |
| if (!I->getDesc().isTerminator()) break; |
| ++NumTerms; |
| } |
| return NumTerms; |
| } |
| |
| /// ProfitableToMerge - Check if two machine basic blocks have a common tail |
| /// and decide if it would be profitable to merge those tails. Return the |
| /// length of the common tail and iterators to the first common instruction |
| /// in each block. |
| static bool ProfitableToMerge(MachineBasicBlock *MBB1, |
| MachineBasicBlock *MBB2, |
| unsigned minCommonTailLength, |
| unsigned &CommonTailLen, |
| MachineBasicBlock::iterator &I1, |
| MachineBasicBlock::iterator &I2, |
| MachineBasicBlock *SuccBB, |
| MachineBasicBlock *PredBB) { |
| CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2); |
| MachineFunction *MF = MBB1->getParent(); |
| |
| if (CommonTailLen == 0) |
| return false; |
| |
| // It's almost always profitable to merge any number of non-terminator |
| // instructions with the block that falls through into the common successor. |
| if (MBB1 == PredBB || MBB2 == PredBB) { |
| MachineBasicBlock::iterator I; |
| unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I); |
| if (CommonTailLen > NumTerms) |
| return true; |
| } |
| |
| // If one of the blocks can be completely merged and happens to be in |
| // a position where the other could fall through into it, merge any number |
| // of instructions, because it can be done without a branch. |
| // TODO: If the blocks are not adjacent, move one of them so that they are? |
| if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin()) |
| return true; |
| if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin()) |
| return true; |
| |
| // If both blocks have an unconditional branch temporarily stripped out, |
| // count that as an additional common instruction for the following |
| // heuristics. |
| unsigned EffectiveTailLen = CommonTailLen; |
| if (SuccBB && MBB1 != PredBB && MBB2 != PredBB && |
| !MBB1->back().getDesc().isBarrier() && |
| !MBB2->back().getDesc().isBarrier()) |
| ++EffectiveTailLen; |
| |
| // Check if the common tail is long enough to be worthwhile. |
| if (EffectiveTailLen >= minCommonTailLength) |
| return true; |
| |
| // If we are optimizing for code size, 2 instructions in common is enough if |
| // we don't have to split a block. At worst we will be introducing 1 new |
| // branch instruction, which is likely to be smaller than the 2 |
| // instructions that would be deleted in the merge. |
| if (EffectiveTailLen >= 2 && |
| MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) && |
| (I1 == MBB1->begin() || I2 == MBB2->begin())) |
| return true; |
| |
| return false; |
| } |
| |
| /// ComputeSameTails - Look through all the blocks in MergePotentials that have |
| /// hash CurHash (guaranteed to match the last element). Build the vector |
| /// SameTails of all those that have the (same) largest number of instructions |
| /// in common of any pair of these blocks. SameTails entries contain an |
| /// iterator into MergePotentials (from which the MachineBasicBlock can be |
| /// found) and a MachineBasicBlock::iterator into that MBB indicating the |
| /// instruction where the matching code sequence begins. |
| /// Order of elements in SameTails is the reverse of the order in which |
| /// those blocks appear in MergePotentials (where they are not necessarily |
| /// consecutive). |
| unsigned BranchFolder::ComputeSameTails(unsigned CurHash, |
| unsigned minCommonTailLength, |
| MachineBasicBlock *SuccBB, |
| MachineBasicBlock *PredBB) { |
| unsigned maxCommonTailLength = 0U; |
| SameTails.clear(); |
| MachineBasicBlock::iterator TrialBBI1, TrialBBI2; |
| MPIterator HighestMPIter = prior(MergePotentials.end()); |
| for (MPIterator CurMPIter = prior(MergePotentials.end()), |
| B = MergePotentials.begin(); |
| CurMPIter != B && CurMPIter->getHash() == CurHash; |
| --CurMPIter) { |
| for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) { |
| unsigned CommonTailLen; |
| if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(), |
| minCommonTailLength, |
| CommonTailLen, TrialBBI1, TrialBBI2, |
| SuccBB, PredBB)) { |
| if (CommonTailLen > maxCommonTailLength) { |
| SameTails.clear(); |
| maxCommonTailLength = CommonTailLen; |
| HighestMPIter = CurMPIter; |
| SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1)); |
| } |
| if (HighestMPIter == CurMPIter && |
| CommonTailLen == maxCommonTailLength) |
| SameTails.push_back(SameTailElt(I, TrialBBI2)); |
| } |
| if (I == B) |
| break; |
| } |
| } |
| return maxCommonTailLength; |
| } |
| |
| /// RemoveBlocksWithHash - Remove all blocks with hash CurHash from |
| /// MergePotentials, restoring branches at ends of blocks as appropriate. |
| void BranchFolder::RemoveBlocksWithHash(unsigned CurHash, |
| MachineBasicBlock *SuccBB, |
| MachineBasicBlock *PredBB) { |
| MPIterator CurMPIter, B; |
| for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin(); |
| CurMPIter->getHash() == CurHash; |
| --CurMPIter) { |
| // Put the unconditional branch back, if we need one. |
| MachineBasicBlock *CurMBB = CurMPIter->getBlock(); |
| if (SuccBB && CurMBB != PredBB) |
| FixTail(CurMBB, SuccBB, TII); |
| if (CurMPIter == B) |
| break; |
| } |
| if (CurMPIter->getHash() != CurHash) |
| CurMPIter++; |
| MergePotentials.erase(CurMPIter, MergePotentials.end()); |
| } |
| |
| /// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist |
| /// only of the common tail. Create a block that does by splitting one. |
| unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, |
| unsigned maxCommonTailLength) { |
| unsigned commonTailIndex = 0; |
| unsigned TimeEstimate = ~0U; |
| for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { |
| // Use PredBB if possible; that doesn't require a new branch. |
| if (SameTails[i].getBlock() == PredBB) { |
| commonTailIndex = i; |
| break; |
| } |
| // Otherwise, make a (fairly bogus) choice based on estimate of |
| // how long it will take the various blocks to execute. |
| unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(), |
| SameTails[i].getTailStartPos()); |
| if (t <= TimeEstimate) { |
| TimeEstimate = t; |
| commonTailIndex = i; |
| } |
| } |
| |
| MachineBasicBlock::iterator BBI = |
| SameTails[commonTailIndex].getTailStartPos(); |
| MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); |
| |
| DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size " |
| << maxCommonTailLength); |
| |
| MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI); |
| SameTails[commonTailIndex].setBlock(newMBB); |
| SameTails[commonTailIndex].setTailStartPos(newMBB->begin()); |
| |
| // If we split PredBB, newMBB is the new predecessor. |
| if (PredBB == MBB) |
| PredBB = newMBB; |
| |
| return commonTailIndex; |
| } |
| |
| // See if any of the blocks in MergePotentials (which all have a common single |
| // successor, or all have no successor) can be tail-merged. If there is a |
| // successor, any blocks in MergePotentials that are not tail-merged and |
| // are not immediately before Succ must have an unconditional branch to |
| // Succ added (but the predecessor/successor lists need no adjustment). |
| // The lone predecessor of Succ that falls through into Succ, |
| // if any, is given in PredBB. |
| |
| bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB, |
| MachineBasicBlock *PredBB) { |
| bool MadeChange = false; |
| |
| // Except for the special cases below, tail-merge if there are at least |
| // this many instructions in common. |
| unsigned minCommonTailLength = TailMergeSize; |
| |
| DEBUG(dbgs() << "\nTryTailMergeBlocks: "; |
| for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) |
| dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber() |
| << (i == e-1 ? "" : ", "); |
| dbgs() << "\n"; |
| if (SuccBB) { |
| dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n'; |
| if (PredBB) |
| dbgs() << " which has fall-through from BB#" |
| << PredBB->getNumber() << "\n"; |
| } |
| dbgs() << "Looking for common tails of at least " |
| << minCommonTailLength << " instruction" |
| << (minCommonTailLength == 1 ? "" : "s") << '\n'; |
| ); |
| |
| // 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.back().getHash(); |
| |
| // Build SameTails, identifying the set of blocks with this hash code |
| // and with the maximum number of instructions in common. |
| unsigned maxCommonTailLength = ComputeSameTails(CurHash, |
| minCommonTailLength, |
| SuccBB, PredBB); |
| |
| // If we didn't find any pair that has at least minCommonTailLength |
| // instructions in common, remove all blocks with this hash code and retry. |
| if (SameTails.empty()) { |
| RemoveBlocksWithHash(CurHash, SuccBB, PredBB); |
| continue; |
| } |
| |
| // If one of the blocks is the entire common tail (and not the entry |
| // block, which we can't jump to), we can treat all blocks with this same |
| // tail at once. Use PredBB if that is one of the possibilities, as that |
| // will not introduce any extra branches. |
| MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()-> |
| getParent()->begin(); |
| unsigned commonTailIndex = SameTails.size(); |
| // If there are two blocks, check to see if one can be made to fall through |
| // into the other. |
| if (SameTails.size() == 2 && |
| SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) && |
| SameTails[1].tailIsWholeBlock()) |
| commonTailIndex = 1; |
| else if (SameTails.size() == 2 && |
| SameTails[1].getBlock()->isLayoutSuccessor( |
| SameTails[0].getBlock()) && |
| SameTails[0].tailIsWholeBlock()) |
| commonTailIndex = 0; |
| else { |
| // Otherwise just pick one, favoring the fall-through predecessor if |
| // there is one. |
| for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { |
| MachineBasicBlock *MBB = SameTails[i].getBlock(); |
| if (MBB == EntryBB && SameTails[i].tailIsWholeBlock()) |
| continue; |
| if (MBB == PredBB) { |
| commonTailIndex = i; |
| break; |
| } |
| if (SameTails[i].tailIsWholeBlock()) |
| commonTailIndex = i; |
| } |
| } |
| |
| if (commonTailIndex == SameTails.size() || |
| (SameTails[commonTailIndex].getBlock() == PredBB && |
| !SameTails[commonTailIndex].tailIsWholeBlock())) { |
| // None of the blocks consist entirely of the common tail. |
| // Split a block so that one does. |
| commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength); |
| } |
| |
| MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); |
| // MBB is common tail. Adjust all other BB's to jump to this one. |
| // Traversal must be forwards so erases work. |
| DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber() |
| << " for "); |
| for (unsigned int i=0, e = SameTails.size(); i != e; ++i) { |
| if (commonTailIndex == i) |
| continue; |
| DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber() |
| << (i == e-1 ? "" : ", ")); |
| // Hack the end off BB i, making it jump to BB commonTailIndex instead. |
| ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB); |
| // BB i is no longer a predecessor of SuccBB; remove it from the worklist. |
| MergePotentials.erase(SameTails[i].getMPIter()); |
| } |
| DEBUG(dbgs() << "\n"); |
| // We leave commonTailIndex in the worklist in case there are other blocks |
| // that match it with a smaller number of instructions. |
| MadeChange = true; |
| } |
| return MadeChange; |
| } |
| |
| bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { |
| |
| if (!EnableTailMerge) return false; |
| |
| bool MadeChange = false; |
| |
| // First find blocks with no successors. |
| MergePotentials.clear(); |
| for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { |
| if (I->succ_empty()) |
| MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I, 2U), I)); |
| } |
| |
| // See if we can do any tail merging on those. |
| if (MergePotentials.size() < TailMergeThreshold && |
| MergePotentials.size() >= 2) |
| MadeChange |= TryTailMergeBlocks(NULL, NULL); |
| |
| // Look at blocks (IBB) with multiple predecessors (PBB). |
| // We change each predecessor to a canonical form, by |
| // (1) temporarily removing any unconditional branch from the predecessor |
| // to IBB, and |
| // (2) alter conditional branches so they branch to the other block |
| // not IBB; this may require adding back an unconditional branch to IBB |
| // later, where there wasn't one coming in. E.g. |
| // Bcc IBB |
| // fallthrough to QBB |
| // here becomes |
| // Bncc QBB |
| // with a conceptual B to IBB after that, which never actually exists. |
| // With those changes, we see whether the predecessors' tails match, |
| // and merge them if so. We change things out of canonical form and |
| // back to the way they were later in the process. (OptimizeBranches |
| // would undo some of this, but we can't use it, because we'd get into |
| // a compile-time infinite loop repeatedly doing and undoing the same |
| // transformations.) |
| |
| for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end(); |
| I != E; ++I) { |
| if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) { |
| SmallPtrSet<MachineBasicBlock *, 8> UniquePreds; |
| MachineBasicBlock *IBB = I; |
| MachineBasicBlock *PredBB = prior(I); |
| MergePotentials.clear(); |
| for (MachineBasicBlock::pred_iterator P = I->pred_begin(), |
| E2 = I->pred_end(); |
| P != E2; ++P) { |
| MachineBasicBlock *PBB = *P; |
| // Skip blocks that loop to themselves, can't tail merge these. |
| if (PBB == IBB) |
| continue; |
| // Visit each predecessor only once. |
| if (!UniquePreds.insert(PBB)) |
| continue; |
| MachineBasicBlock *TBB = 0, *FBB = 0; |
| SmallVector<MachineOperand, 4> Cond; |
| if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) { |
| // Failing case: IBB is the target of a cbr, and |
| // we cannot reverse the branch. |
| SmallVector<MachineOperand, 4> NewCond(Cond); |
| if (!Cond.empty() && TBB == IBB) { |
| if (TII->ReverseBranchCondition(NewCond)) |
| continue; |
| // This is the QBB case described above |
| if (!FBB) |
| FBB = llvm::next(MachineFunction::iterator(PBB)); |
| } |
| // Failing case: the only way IBB can be reached from PBB is via |
| // exception handling. Happens for landing pads. Would be nice |
| // to have a bit in the edge so we didn't have to do all this. |
| if (IBB->isLandingPad()) { |
| MachineFunction::iterator IP = PBB; IP++; |
| MachineBasicBlock *PredNextBB = NULL; |
| if (IP != MF.end()) |
| PredNextBB = IP; |
| if (TBB == NULL) { |
| if (IBB != PredNextBB) // fallthrough |
| continue; |
| } else if (FBB) { |
| if (TBB != IBB && FBB != IBB) // cbr then ubr |
| continue; |
| } else if (Cond.empty()) { |
| if (TBB != IBB) // ubr |
| continue; |
| } else { |
| if (TBB != IBB && IBB != PredNextBB) // cbr |
| continue; |
| } |
| } |
| // Remove the unconditional branch at the end, if any. |
| if (TBB && (Cond.empty() || FBB)) { |
| TII->RemoveBranch(*PBB); |
| if (!Cond.empty()) |
| // reinsert conditional branch only, for now |
| TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond); |
| } |
| MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB, 1U), |
| *P)); |
| } |
| } |
| if (MergePotentials.size() >= 2) |
| MadeChange |= TryTailMergeBlocks(IBB, PredBB); |
| // Reinsert an unconditional branch if needed. |
| // The 1 below can occur as a result of removing blocks in TryTailMergeBlocks. |
| PredBB = prior(I); // this may have been changed in TryTailMergeBlocks |
| if (MergePotentials.size() == 1 && |
| MergePotentials.begin()->getBlock() != PredBB) |
| FixTail(MergePotentials.begin()->getBlock(), IBB, TII); |
| } |
| } |
| return MadeChange; |
| } |
| |
| //===----------------------------------------------------------------------===// |
| // Branch Optimization |
| //===----------------------------------------------------------------------===// |
| |
| bool BranchFolder::OptimizeBranches(MachineFunction &MF) { |
| bool 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++; |
| MadeChange |= OptimizeBlock(MBB); |
| |
| // If it is dead, remove it. |
| if (MBB->pred_empty()) { |
| RemoveDeadBlock(MBB); |
| MadeChange = true; |
| ++NumDeadBlocks; |
| } |
| } |
| return MadeChange; |
| } |
| |
| |
| /// 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) { |
| // 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; |
| |
| // If there is a clear successor ordering we make sure that one block |
| // will fall through to the next |
| if (MBB1->isSuccessor(MBB2)) return true; |
| if (MBB2->isSuccessor(MBB1)) return false; |
| |
| MachineInstr *MBB1I = --MBB1->end(); |
| MachineInstr *MBB2I = --MBB2->end(); |
| return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall(); |
| } |
| |
| /// OptimizeBlock - Analyze and optimize control flow related to the specified |
| /// block. This is never called on the entry block. |
| bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { |
| bool MadeChange = false; |
| MachineFunction &MF = *MBB->getParent(); |
| ReoptimizeBlock: |
| |
| MachineFunction::iterator FallThrough = MBB; |
| ++FallThrough; |
| |
| // If this block is empty, make everyone use its fall-through, not the block |
| // explicitly. Landing pads should not do this since the landing-pad table |
| // points to this block. Blocks with their addresses taken shouldn't be |
| // optimized away. |
| if (MBB->empty() && !MBB->isLandingPad() && !MBB->hasAddressTaken()) { |
| // Dead block? Leave for cleanup later. |
| if (MBB->pred_empty()) return MadeChange; |
| |
| if (FallThrough == MF.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); |
| Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); |
| } |
| // If MBB was the target of a jump table, update jump tables to go to the |
| // fallthrough instead. |
| if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) |
| MJTI->ReplaceMBBInJumpTables(MBB, FallThrough); |
| MadeChange = true; |
| } |
| return MadeChange; |
| } |
| |
| // 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; |
| SmallVector<MachineOperand, 4> PriorCond; |
| bool PriorUnAnalyzable = |
| TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); |
| if (!PriorUnAnalyzable) { |
| // If the CFG for the prior block has extra edges, remove them. |
| MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, |
| !PriorCond.empty()); |
| |
| // 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; |
| goto ReoptimizeBlock; |
| } |
| |
| // If the previous block unconditionally falls through to this block and |
| // this block has no other predecessors, move the contents of this block |
| // into the prior block. This doesn't usually happen when SimplifyCFG |
| // has been used, but it can happen if tail merging splits a fall-through |
| // predecessor of a block. |
| // This has to check PrevBB->succ_size() because EH edges are ignored by |
| // AnalyzeBranch. |
| if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 && |
| PrevBB.succ_size() == 1 && |
| !MBB->hasAddressTaken()) { |
| DEBUG(dbgs() << "\nMerging into block: " << PrevBB |
| << "From MBB: " << *MBB); |
| PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end()); |
| PrevBB.removeSuccessor(PrevBB.succ_begin());; |
| assert(PrevBB.succ_empty()); |
| PrevBB.transferSuccessors(MBB); |
| MadeChange = true; |
| return MadeChange; |
| } |
| |
| // 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; |
| goto ReoptimizeBlock; |
| } |
| |
| // 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; |
| goto ReoptimizeBlock; |
| } |
| |
| // 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) { |
| SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); |
| if (!TII->ReverseBranchCondition(NewPriorCond)) { |
| TII->RemoveBranch(PrevBB); |
| TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); |
| MadeChange = true; |
| ++NumBranchOpts; |
| goto ReoptimizeBlock; |
| } |
| } |
| |
| // If this block has no successors (e.g. it is a return block or ends with |
| // a call to a no-return function like abort or __cxa_throw) 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 (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 && |
| MachineFunction::iterator(PriorTBB) == FallThrough && |
| !MBB->canFallThrough()) { |
| 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 == --MF.end() && |
| !IsBetterFallthrough(PriorTBB, MBB)) |
| 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() && |
| (!PriorTBB->canFallThrough() || PriorTBB->empty())) |
| DoTransform = false; |
| |
| |
| if (DoTransform) { |
| // Reverse the branch so we will fall through on the previous true cond. |
| SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); |
| if (!TII->ReverseBranchCondition(NewPriorCond)) { |
| DEBUG(dbgs() << "\nMoving MBB: " << *MBB |
| << "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(--MF.end()); |
| MadeChange = true; |
| ++NumBranchOpts; |
| return MadeChange; |
| } |
| } |
| } |
| } |
| |
| // Analyze the branch in the current block. |
| MachineBasicBlock *CurTBB = 0, *CurFBB = 0; |
| SmallVector<MachineOperand, 4> CurCond; |
| bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); |
| if (!CurUnAnalyzable) { |
| // If the CFG for the prior block has extra edges, remove them. |
| MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); |
| |
| // 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) { |
| SmallVector<MachineOperand, 4> NewCond(CurCond); |
| if (!TII->ReverseBranchCondition(NewCond)) { |
| TII->RemoveBranch(*MBB); |
| TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); |
| MadeChange = true; |
| ++NumBranchOpts; |
| goto ReoptimizeBlock; |
| } |
| } |
| |
| // 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 && |
| MBB->begin()->getDesc().isBranch() && CurTBB != MBB && |
| !MBB->hasAddressTaken()) { |
| // 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 = !PrevBB.canFallThrough(); |
| 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. |
| size_t PI = 0; |
| bool DidChange = false; |
| bool HasBranchToSelf = false; |
| while(PI != MBB->pred_size()) { |
| MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); |
| if (PMBB == MBB) { |
| // If this block has an uncond branch to itself, leave it. |
| ++PI; |
| HasBranchToSelf = true; |
| } else { |
| DidChange = true; |
| PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); |
| // If this change resulted in PMBB ending in a conditional |
| // branch where both conditions go to the same destination, |
| // change this to an unconditional branch (and fix the CFG). |
| MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0; |
| SmallVector<MachineOperand, 4> NewCurCond; |
| bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB, |
| NewCurFBB, NewCurCond, true); |
| if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) { |
| TII->RemoveBranch(*PMBB); |
| NewCurCond.clear(); |
| TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond); |
| MadeChange = true; |
| ++NumBranchOpts; |
| PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false); |
| } |
| } |
| } |
| |
| // Change any jumptables to go to the new MBB. |
| if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) |
| MJTI->ReplaceMBBInJumpTables(MBB, CurTBB); |
| if (DidChange) { |
| ++NumBranchOpts; |
| MadeChange = true; |
| if (!HasBranchToSelf) return MadeChange; |
| } |
| } |
| } |
| |
| // 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 (!PrevBB.canFallThrough()) { |
| |
| // 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 = MBB->canFallThrough(); |
| |
| if (!MBB->isLandingPad()) { |
| // 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; |
| MachineBasicBlock *PredTBB = 0, *PredFBB = 0; |
| SmallVector<MachineOperand, 4> PredCond; |
| if (PredBB != MBB && !PredBB->canFallThrough() && |
| !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) |
| && (!CurFallsThru || !CurTBB || !CurFBB) |
| && (!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. |
| // Also, if there are already 2 branches here, we cannot add a third; |
| // this means we have the case |
| // Bcc next |
| // B elsewhere |
| // next: |
| if (CurFallsThru) { |
| MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB)); |
| CurCond.clear(); |
| TII->InsertBranch(*MBB, NextBB, 0, CurCond); |
| } |
| MBB->moveAfter(PredBB); |
| MadeChange = true; |
| goto ReoptimizeBlock; |
| } |
| } |
| } |
| |
| 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; |
| |
| // If this block doesn't already fall-through to that successor, and if |
| // the succ doesn't already have a block that can fall through into it, |
| // and if the successor isn't an EH destination, we can arrange for the |
| // fallthrough to happen. |
| if (SuccBB != MBB && &*SuccPrev != MBB && |
| !SuccPrev->canFallThrough() && !CurUnAnalyzable && |
| !SuccBB->isLandingPad()) { |
| MBB->moveBefore(SuccBB); |
| MadeChange = true; |
| goto ReoptimizeBlock; |
| } |
| } |
| |
| // 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. |
| MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0; |
| SmallVector<MachineOperand, 4> PrevCond; |
| if (FallThrough != MF.end() && |
| !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) && |
| PrevBB.isSuccessor(FallThrough)) { |
| MBB->moveAfter(--MF.end()); |
| MadeChange = true; |
| return MadeChange; |
| } |
| } |
| } |
| |
| return MadeChange; |
| } |