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Diffstat (limited to 'contrib/llvm/lib/CodeGen/BranchFolding.cpp')
| -rw-r--r-- | contrib/llvm/lib/CodeGen/BranchFolding.cpp | 1335 |
1 files changed, 1335 insertions, 0 deletions
diff --git a/contrib/llvm/lib/CodeGen/BranchFolding.cpp b/contrib/llvm/lib/CodeGen/BranchFolding.cpp new file mode 100644 index 0000000..9dec22e --- /dev/null +++ b/contrib/llvm/lib/CodeGen/BranchFolding.cpp @@ -0,0 +1,1335 @@ +//===-- 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); + + // 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 dead as the code generator + // did its thing. + MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); + if (JTI == 0) { + delete RS; + return MadeChange; + } + + // Walk the function to find jump tables that are live. + BitVector JTIsLive(JTI->getJumpTables().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; + + // Remember that this JT is live. + JTIsLive.set(Op.getIndex()); + } + } + + // Finally, remove dead jump tables. This happens when the + // indirect jump was unreachable (and thus deleted). + 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 instruction in the MBB. +static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) { + MachineBasicBlock::const_iterator I = MBB->end(); + if (I == MBB->begin()) + return 0; // Empty MBB. + + --I; + // Skip debug info so it will not affect codegen. + while (I->isDebugValue()) { + if (I==MBB->begin()) + return 0; // MBB empty except for debug info. + --I; + } + + return HashMachineInstr(I); +} + +/// 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; + // Skip debugging pseudos; necessary to avoid changing the code. + while (I1->isDebugValue()) { + if (I1==MBB1->begin()) { + while (I2->isDebugValue()) { + if (I2==MBB2->begin()) + // I1==DBG at begin; I2==DBG at begin + return TailLen; + --I2; + } + ++I2; + // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin + return TailLen; + } + --I1; + } + // I1==first (untested) non-DBG preceding known match + while (I2->isDebugValue()) { + if (I2==MBB2->begin()) { + ++I1; + // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin + return TailLen; + } + --I2; + } + // I1, I2==first (untested) non-DBGs preceding known match + 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; + } + // Back past possible debugging pseudos at beginning of block. This matters + // when one block differs from the other only by whether debugging pseudos + // are present at the beginning. (This way, the various checks later for + // I1==MBB1->begin() work as expected.) + if (I1 == MBB1->begin() && I2 != MBB2->begin()) { + --I2; + while (I2->isDebugValue()) { + if (I2 == MBB2->begin()) { + return TailLen; + } + --I2; + } + ++I2; + } + if (I2 == MBB2->begin() && I1 != MBB1->begin()) { + --I1; + while (I1->isDebugValue()) { + if (I1 == MBB1->begin()) + return TailLen; + --I1; + } + ++I1; + } + 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) { + if (I->isDebugValue()) + continue; + 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(); + + // If the common tail includes any debug info we will take it pretty + // randomly from one of the inputs. Might be better to remove it? + 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), 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), *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; +} + +// Blocks should be considered empty if they contain only debug info; +// else the debug info would affect codegen. +static bool IsEmptyBlock(MachineBasicBlock *MBB) { + if (MBB->empty()) + return true; + for (MachineBasicBlock::iterator MBBI = MBB->begin(), MBBE = MBB->end(); + MBBI!=MBBE; ++MBBI) { + if (!MBBI->isDebugValue()) + return false; + } + return true; +} + +// Blocks with only debug info and branches should be considered the same +// as blocks with only branches. +static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) { + MachineBasicBlock::iterator MBBI, MBBE; + for (MBBI = MBB->begin(), MBBE = MBB->end(); MBBI!=MBBE; ++MBBI) { + if (!MBBI->isDebugValue()) + break; + } + return (MBBI->getDesc().isBranch()); +} + +/// 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 (IsEmptyBlock(MBB1) || IsEmptyBlock(MBB2)) 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; + + // Neither block consists entirely of debug info (per IsEmptyBlock check), + // so we needn't test for falling off the beginning here. + MachineBasicBlock::iterator MBB1I = --MBB1->end(); + while (MBB1I->isDebugValue()) + --MBB1I; + MachineBasicBlock::iterator MBB2I = --MBB2->end(); + while (MBB2I->isDebugValue()) + --MBB2I; + 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 (IsEmptyBlock(MBB) && !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; + + 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 && + IsBranchOnlyBlock(MBB) && 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 the only things remaining in the block are debug info, remove these + // as well, so this will behave the same as an empty block in non-debug + // mode. + if (!MBB->empty()) { + bool NonDebugInfoFound = false; + for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); + I != E; ++I) { + if (!I->isDebugValue()) { + NonDebugInfoFound = true; + break; + } + } + if (!NonDebugInfoFound) + // Make the block empty, losing the debug info (we could probably + // improve this in some cases.) + MBB->erase(MBB->begin(), MBB->end()); + } + // 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; +} |
