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Diffstat (limited to 'lib/CodeGen/BranchFolding.cpp')
| -rw-r--r-- | lib/CodeGen/BranchFolding.cpp | 1204 |
1 files changed, 1204 insertions, 0 deletions
diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp new file mode 100644 index 0000000..2635303 --- /dev/null +++ b/lib/CodeGen/BranchFolding.cpp @@ -0,0 +1,1204 @@ +//===-- 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 "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/ADT/SmallSet.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); + +namespace { + struct VISIBILITY_HIDDEN BranchFolder : public MachineFunctionPass { + static char ID; + explicit BranchFolder(bool defaultEnableTailMerge) : + MachineFunctionPass(&ID) { + switch (FlagEnableTailMerge) { + case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; + case cl::BOU_TRUE: EnableTailMerge = true; break; + case cl::BOU_FALSE: EnableTailMerge = false; break; + } + } + + 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 EnableTailMerge; + bool TailMergeBlocks(MachineFunction &MF); + bool TryMergeBlocks(MachineBasicBlock* SuccBB, + MachineBasicBlock* PredBB); + void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, + MachineBasicBlock *NewDest); + MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB, + MachineBasicBlock::iterator BBI1); + unsigned ComputeSameTails(unsigned CurHash, unsigned minCommonTailLength); + void RemoveBlocksWithHash(unsigned CurHash, MachineBasicBlock* SuccBB, + MachineBasicBlock* PredBB); + unsigned CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, + unsigned maxCommonTailLength); + + typedef std::pair<unsigned,MachineBasicBlock*> MergePotentialsElt; + typedef std::vector<MergePotentialsElt>::iterator MPIterator; + std::vector<MergePotentialsElt> MergePotentials; + + typedef std::pair<MPIterator, MachineBasicBlock::iterator> SameTailElt; + std::vector<SameTailElt> SameTails; + + const TargetRegisterInfo *RegInfo; + RegScavenger *RS; + // Branch optzn. + bool OptimizeBranches(MachineFunction &MF); + void OptimizeBlock(MachineBasicBlock *MBB); + void RemoveDeadBlock(MachineBasicBlock *MBB); + bool OptimizeImpDefsBlock(MachineBasicBlock *MBB); + + bool CanFallThrough(MachineBasicBlock *CurBB); + bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable, + MachineBasicBlock *TBB, MachineBasicBlock *FBB, + const SmallVectorImpl<MachineOperand> &Cond); + }; + char BranchFolder::ID = 0; +} + +FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) { + return new BranchFolder(DefaultEnableTailMerge); } + +/// 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!"); + DOUT << "\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->getOpcode() != TargetInstrInfo::IMPLICIT_DEF) + break; + unsigned Reg = I->getOperand(0).getReg(); + ImpDefRegs.insert(Reg); + for (const unsigned *SubRegs = RegInfo->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; +} + +bool BranchFolder::runOnMachineFunction(MachineFunction &MF) { + TII = MF.getTarget().getInstrInfo(); + if (!TII) return false; + + RegInfo = MF.getTarget().getRegisterInfo(); + + // Fix CFG. The later algorithms expect it to be right. + bool EverMadeChange = 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)) + EverMadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); + EverMadeChange |= OptimizeImpDefsBlock(MBB); + } + + RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; + + MMI = getAnalysisIfAvailable<MachineModuleInfo>(); + + 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<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); + if (!JTs.empty()) { + // 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) + 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); + EverMadeChange = true; + } + } + + delete RS; + 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.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->getOpcode() == TargetInstrInfo::INLINEASM) { + ++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(RegInfo->getNumRegs()); + RS->getRegsUsed(RegsLiveAtExit, false); + for (unsigned int i=0, e=RegInfo->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 = 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>()); +} + +static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p, + const std::pair<unsigned,MachineBasicBlock*> &q) { + if (p.first < q.first) + return true; + else if (p.first > q.first) + return false; + else if (p.second->getNumber() < q.second->getNumber()) + return true; + else if (p.second->getNumber() > q.second->getNumber()) + return false; + else { + // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing + // an object with itself. +#ifndef _GLIBCXX_DEBUG + assert(0 && "Predecessor appears twice"); +#endif + 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) { + 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->first==CurHash; + --CurMPIter) { + for (MPIterator I = prior(CurMPIter); I->first==CurHash ; --I) { + unsigned CommonTailLen = ComputeCommonTailLength( + CurMPIter->second, + I->second, + TrialBBI1, TrialBBI2); + // If we will have to split a block, there should be at least + // minCommonTailLength instructions in common; if not, at worst + // we will be replacing a fallthrough into the common tail with a + // branch, which at worst breaks even with falling through into + // the duplicated common tail, so 1 instruction in common is enough. + // We will always pick a block we do not have to split as the common + // tail if there is one. + // (Empty blocks will get forwarded and need not be considered.) + if (CommonTailLen >= minCommonTailLength || + (CommonTailLen > 0 && + (TrialBBI1==CurMPIter->second->begin() || + TrialBBI2==I->second->begin()))) { + if (CommonTailLen > maxCommonTailLength) { + SameTails.clear(); + maxCommonTailLength = CommonTailLen; + HighestMPIter = CurMPIter; + SameTails.push_back(std::make_pair(CurMPIter, TrialBBI1)); + } + if (HighestMPIter == CurMPIter && + CommonTailLen == maxCommonTailLength) + SameTails.push_back(std::make_pair(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->first==CurHash; + --CurMPIter) { + // Put the unconditional branch back, if we need one. + MachineBasicBlock *CurMBB = CurMPIter->second; + if (SuccBB && CurMBB != PredBB) + FixTail(CurMBB, SuccBB, TII); + if (CurMPIter==B) + break; + } + if (CurMPIter->first!=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 i, commonTailIndex; + unsigned TimeEstimate = ~0U; + for (i=0, commonTailIndex=0; i<SameTails.size(); i++) { + // Use PredBB if possible; that doesn't require a new branch. + if (SameTails[i].first->second==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].first->second->begin(), + SameTails[i].second); + if (t<=TimeEstimate) { + TimeEstimate = t; + commonTailIndex = i; + } + } + + MachineBasicBlock::iterator BBI = SameTails[commonTailIndex].second; + MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second; + + DOUT << "\nSplitting " << MBB->getNumber() << ", size " << + maxCommonTailLength; + + MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI); + SameTails[commonTailIndex].first->second = newMBB; + SameTails[commonTailIndex].second = 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::TryMergeBlocks(MachineBasicBlock *SuccBB, + MachineBasicBlock* PredBB) { + // It doesn't make sense to save a single instruction since tail merging + // will add a jump. + // FIXME: Ask the target to provide the threshold? + unsigned minCommonTailLength = (SuccBB ? 1 : 2) + 1; + MadeChange = false; + + DOUT << "\nTryMergeBlocks " << MergePotentials.size() << '\n'; + + // Sort by hash value so that blocks with identical end sequences sort + // together. + std::stable_sort(MergePotentials.begin(), MergePotentials.end(),MergeCompare); + + // Walk through equivalence sets looking for actual exact matches. + while (MergePotentials.size() > 1) { + unsigned CurHash = prior(MergePotentials.end())->first; + + // 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); + + // 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()->second-> + getParent()->begin(); + unsigned int commonTailIndex, i; + for (commonTailIndex=SameTails.size(), i=0; i<SameTails.size(); i++) { + MachineBasicBlock *MBB = SameTails[i].first->second; + if (MBB->begin() == SameTails[i].second && MBB != EntryBB) { + commonTailIndex = i; + if (MBB==PredBB) + break; + } + } + + if (commonTailIndex==SameTails.size()) { + // 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].first->second; + // MBB is common tail. Adjust all other BB's to jump to this one. + // Traversal must be forwards so erases work. + DOUT << "\nUsing common tail " << MBB->getNumber() << " for "; + for (unsigned int i=0; i<SameTails.size(); ++i) { + if (commonTailIndex==i) + continue; + DOUT << SameTails[i].first->second->getNumber() << ","; + // Hack the end off BB i, making it jump to BB commonTailIndex instead. + ReplaceTailWithBranchTo(SameTails[i].second, MBB); + // BB i is no longer a predecessor of SuccBB; remove it from the worklist. + MergePotentials.erase(SameTails[i].first); + } + DOUT << "\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; + + 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(std::make_pair(HashEndOfMBB(I, 2U), I)); + } + // See if we can do any tail merging on those. + if (MergePotentials.size() < TailMergeThreshold && + MergePotentials.size() >= 2) + MadeChange |= TryMergeBlocks(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 = MF.begin(), E = MF.end(); I != E; ++I) { + if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) { + 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; + 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 = 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(std::make_pair(HashEndOfMBB(PBB, 1U), *P)); + } + } + if (MergePotentials.size() >= 2) + MadeChange |= TryMergeBlocks(I, PredBB); + // Reinsert an unconditional branch if needed. + // The 1 below can occur as a result of removing blocks in TryMergeBlocks. + PredBB = prior(I); // this may have been changed in TryMergeBlocks + if (MergePotentials.size()==1 && + MergePotentials.begin()->second != PredBB) + FixTail(MergePotentials.begin()->second, I, TII); + } + } + 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; +} + + +/// 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 SmallVectorImpl<MachineOperand> &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; + SmallVector<MachineOperand, 4> Cond; + bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond, true); + 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) { + // 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. +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. Landing pads should not do this since the landing-pad table + // points to this block. + if (MBB->empty() && !MBB->isLandingPad()) { + // 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); + Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); + } + + // 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; + 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; + 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) { + SmallVector<MachineOperand, 4> 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)) + 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. + SmallVector<MachineOperand, 4> 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; + 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; + 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 && + MBB->begin()->getDesc().isBranch() && 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. + 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, NewCurFBB, false); + } + } + } + + // 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); + + 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; + if (PredBB != MBB && !CanFallThrough(PredBB) + && (!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 = 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<MachineOperand> SuccPrevCond; + + // 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 && !CanFallThrough(SuccPrev) && + !SuccBB->isLandingPad()) { + 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; + } + } + } +} |
