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//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
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// The LLVM Compiler Infrastructure
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//===----------------------------------------------------------------------===//
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// This pass forwards branches to unconditional branches to make them branch
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// directly to the target block. This pass often results in dead MBB's, which
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// Note that this pass must be run after register allocation, it cannot handle
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "branchfolding"
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#include "BranchFolding.h"
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#include "llvm/Function.h"
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/CodeGen/MachineModuleInfo.h"
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#include "llvm/CodeGen/MachineFunctionPass.h"
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#include "llvm/CodeGen/MachineJumpTableInfo.h"
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#include "llvm/CodeGen/RegisterScavenging.h"
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#include "llvm/Target/TargetInstrInfo.h"
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#include "llvm/Target/TargetMachine.h"
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#include "llvm/Target/TargetRegisterInfo.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/ErrorHandling.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/ADT/SmallSet.h"
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#include "llvm/ADT/SetVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/STLExtras.h"
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STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
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STATISTIC(NumBranchOpts, "Number of branches optimized");
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STATISTIC(NumTailMerge , "Number of block tails merged");
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static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
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cl::init(cl::BOU_UNSET), cl::Hidden);
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// Throttle for huge numbers of predecessors (compile speed problems)
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static cl::opt<unsigned>
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TailMergeThreshold("tail-merge-threshold",
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cl::desc("Max number of predecessors to consider tail merging"),
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cl::init(150), cl::Hidden);
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// Heuristic for tail merging (and, inversely, tail duplication).
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// TODO: This should be replaced with a target query.
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static cl::opt<unsigned>
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TailMergeSize("tail-merge-size",
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cl::desc("Min number of instructions to consider tail merging"),
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cl::init(3), cl::Hidden);
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/// BranchFolderPass - Wrap branch folder in a machine function pass.
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class BranchFolderPass : public MachineFunctionPass,
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explicit BranchFolderPass(bool defaultEnableTailMerge)
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: MachineFunctionPass(&ID), BranchFolder(defaultEnableTailMerge) {}
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virtual bool runOnMachineFunction(MachineFunction &MF);
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virtual const char *getPassName() const { return "Control Flow Optimizer"; }
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char BranchFolderPass::ID = 0;
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FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
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return new BranchFolderPass(DefaultEnableTailMerge);
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bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
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return OptimizeFunction(MF,
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MF.getTarget().getInstrInfo(),
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MF.getTarget().getRegisterInfo(),
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getAnalysisIfAvailable<MachineModuleInfo>());
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BranchFolder::BranchFolder(bool defaultEnableTailMerge) {
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switch (FlagEnableTailMerge) {
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case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
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case cl::BOU_TRUE: EnableTailMerge = true; break;
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case cl::BOU_FALSE: EnableTailMerge = false; break;
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/// RemoveDeadBlock - Remove the specified dead machine basic block from the
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/// function, updating the CFG.
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void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
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assert(MBB->pred_empty() && "MBB must be dead!");
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DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
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MachineFunction *MF = MBB->getParent();
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// drop all successors.
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while (!MBB->succ_empty())
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MBB->removeSuccessor(MBB->succ_end()-1);
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// If there are any labels in the basic block, unregister them from
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// MachineModuleInfo.
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if (MMI && !MBB->empty()) {
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for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
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// The label ID # is always operand #0, an immediate.
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MMI->InvalidateLabel(I->getOperand(0).getImm());
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/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
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/// followed by terminators, and if the implicitly defined registers are not
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/// used by the terminators, remove those implicit_def's. e.g.
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/// r0 = implicit_def
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/// r1 = implicit_def
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/// This block can be optimized away later if the implicit instructions are
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bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
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SmallSet<unsigned, 4> ImpDefRegs;
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MachineBasicBlock::iterator I = MBB->begin();
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while (I != MBB->end()) {
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if (!I->isImplicitDef())
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unsigned Reg = I->getOperand(0).getReg();
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ImpDefRegs.insert(Reg);
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for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
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unsigned SubReg = *SubRegs; ++SubRegs)
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ImpDefRegs.insert(SubReg);
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if (ImpDefRegs.empty())
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MachineBasicBlock::iterator FirstTerm = I;
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while (I != MBB->end()) {
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if (!TII->isUnpredicatedTerminator(I))
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// See if it uses any of the implicitly defined registers.
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
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MachineOperand &MO = I->getOperand(i);
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if (!MO.isReg() || !MO.isUse())
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unsigned Reg = MO.getReg();
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if (ImpDefRegs.count(Reg))
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while (I != FirstTerm) {
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MachineInstr *ImpDefMI = &*I;
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MBB->erase(ImpDefMI);
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/// OptimizeFunction - Perhaps branch folding, tail merging and other
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/// CFG optimizations on the given function.
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bool BranchFolder::OptimizeFunction(MachineFunction &MF,
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const TargetInstrInfo *tii,
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const TargetRegisterInfo *tri,
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MachineModuleInfo *mmi) {
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if (!tii) return false;
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RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
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// Fix CFG. The later algorithms expect it to be right.
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bool MadeChange = false;
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for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
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MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
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SmallVector<MachineOperand, 4> Cond;
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if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true))
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MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
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MadeChange |= OptimizeImpDefsBlock(MBB);
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bool MadeChangeThisIteration = true;
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while (MadeChangeThisIteration) {
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MadeChangeThisIteration = false;
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MadeChangeThisIteration |= TailMergeBlocks(MF);
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MadeChangeThisIteration |= OptimizeBranches(MF);
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MadeChange |= MadeChangeThisIteration;
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// See if any jump tables have become mergable or dead as the code generator
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MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
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const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables();
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// Figure out how these jump tables should be merged.
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std::vector<unsigned> JTMapping;
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JTMapping.reserve(JTs.size());
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// We always keep the 0th jump table.
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JTMapping.push_back(0);
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// Scan the jump tables, seeing if there are any duplicates. Note that this
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// is N^2, which should be fixed someday.
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for (unsigned i = 1, e = JTs.size(); i != e; ++i) {
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if (JTs[i].MBBs.empty())
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JTMapping.push_back(i);
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JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs));
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// If a jump table was merge with another one, walk the function rewriting
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// references to jump tables to reference the new JT ID's. Keep track of
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// whether we see a jump table idx, if not, we can delete the JT.
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BitVector JTIsLive(JTs.size());
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for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
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for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
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for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
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MachineOperand &Op = I->getOperand(op);
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if (!Op.isJTI()) continue;
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unsigned NewIdx = JTMapping[Op.getIndex()];
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// Remember that this JT is live.
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JTIsLive.set(NewIdx);
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// Finally, remove dead jump tables. This happens either because the
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// indirect jump was unreachable (and thus deleted) or because the jump
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// table was merged with some other one.
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for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
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if (!JTIsLive.test(i)) {
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JTI->RemoveJumpTable(i);
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//===----------------------------------------------------------------------===//
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// Tail Merging of Blocks
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//===----------------------------------------------------------------------===//
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/// HashMachineInstr - Compute a hash value for MI and its operands.
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static unsigned HashMachineInstr(const MachineInstr *MI) {
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unsigned Hash = MI->getOpcode();
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for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
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const MachineOperand &Op = MI->getOperand(i);
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// Merge in bits from the operand if easy.
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unsigned OperandHash = 0;
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switch (Op.getType()) {
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case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
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case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
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case MachineOperand::MO_MachineBasicBlock:
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OperandHash = Op.getMBB()->getNumber();
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case MachineOperand::MO_FrameIndex:
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case MachineOperand::MO_ConstantPoolIndex:
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case MachineOperand::MO_JumpTableIndex:
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OperandHash = Op.getIndex();
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case MachineOperand::MO_GlobalAddress:
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case MachineOperand::MO_ExternalSymbol:
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// Global address / external symbol are too hard, don't bother, but do
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// pull in the offset.
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OperandHash = Op.getOffset();
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Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
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/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks
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/// with no successors, we hash two instructions, because cross-jumping
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/// only saves code when at least two instructions are removed (since a
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/// branch must be inserted). For blocks with a successor, one of the
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/// two blocks to be tail-merged will end with a branch already, so
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/// it gains to cross-jump even for one instruction.
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static unsigned HashEndOfMBB(const MachineBasicBlock *MBB,
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unsigned minCommonTailLength) {
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MachineBasicBlock::const_iterator I = MBB->end();
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if (I == MBB->begin())
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return 0; // Empty MBB.
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unsigned Hash = HashMachineInstr(I);
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if (I == MBB->begin() || minCommonTailLength == 1)
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return Hash; // Single instr MBB.
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// Hash in the second-to-last instruction.
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Hash ^= HashMachineInstr(I) << 2;
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/// ComputeCommonTailLength - Given two machine basic blocks, compute the number
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/// of instructions they actually have in common together at their end. Return
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/// iterators for the first shared instruction in each block.
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static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
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MachineBasicBlock *MBB2,
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MachineBasicBlock::iterator &I1,
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MachineBasicBlock::iterator &I2) {
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unsigned TailLen = 0;
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while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
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// Don't merge debugging pseudos.
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if (I1->isDebugValue() || I2->isDebugValue() ||
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!I1->isIdenticalTo(I2) ||
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// FIXME: This check is dubious. It's used to get around a problem where
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// people incorrectly expect inline asm directives to remain in the same
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// relative order. This is untenable because normal compiler
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// optimizations (like this one) may reorder and/or merge these
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/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
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/// after it, replacing it with an unconditional branch to NewDest. This
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/// returns true if OldInst's block is modified, false if NewDest is modified.
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void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
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MachineBasicBlock *NewDest) {
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MachineBasicBlock *OldBB = OldInst->getParent();
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// Remove all the old successors of OldBB from the CFG.
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while (!OldBB->succ_empty())
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OldBB->removeSuccessor(OldBB->succ_begin());
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// Remove all the dead instructions from the end of OldBB.
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OldBB->erase(OldInst, OldBB->end());
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// If OldBB isn't immediately before OldBB, insert a branch to it.
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if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest))
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TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>());
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OldBB->addSuccessor(NewDest);
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/// SplitMBBAt - Given a machine basic block and an iterator into it, split the
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/// MBB so that the part before the iterator falls into the part starting at the
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/// iterator. This returns the new MBB.
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MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
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MachineBasicBlock::iterator BBI1) {
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MachineFunction &MF = *CurMBB.getParent();
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// Create the fall-through block.
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MachineFunction::iterator MBBI = &CurMBB;
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MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock());
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CurMBB.getParent()->insert(++MBBI, NewMBB);
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// Move all the successors of this block to the specified block.
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NewMBB->transferSuccessors(&CurMBB);
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// Add an edge from CurMBB to NewMBB for the fall-through.
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CurMBB.addSuccessor(NewMBB);
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// Splice the code over.
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NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
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// For targets that use the register scavenger, we must maintain LiveIns.
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RS->enterBasicBlock(&CurMBB);
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RS->forward(prior(CurMBB.end()));
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BitVector RegsLiveAtExit(TRI->getNumRegs());
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RS->getRegsUsed(RegsLiveAtExit, false);
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for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++)
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if (RegsLiveAtExit[i])
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NewMBB->addLiveIn(i);
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/// EstimateRuntime - Make a rough estimate for how long it will take to run
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/// the specified code.
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static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
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MachineBasicBlock::iterator E) {
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for (; I != E; ++I) {
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if (I->isDebugValue())
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const TargetInstrDesc &TID = I->getDesc();
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else if (TID.mayLoad() || TID.mayStore())
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// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
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// branches temporarily for tail merging). In the case where CurMBB ends
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// with a conditional branch to the next block, optimize by reversing the
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// test and conditionally branching to SuccMBB instead.
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static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
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const TargetInstrInfo *TII) {
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MachineFunction *MF = CurMBB->getParent();
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MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB));
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MachineBasicBlock *TBB = 0, *FBB = 0;
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SmallVector<MachineOperand, 4> Cond;
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if (I != MF->end() &&
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!TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
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MachineBasicBlock *NextBB = I;
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if (TBB == NextBB && !Cond.empty() && !FBB) {
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if (!TII->ReverseBranchCondition(Cond)) {
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TII->RemoveBranch(*CurMBB);
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TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond);
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TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>());
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BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
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if (getHash() < o.getHash())
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else if (getHash() > o.getHash())
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else if (getBlock()->getNumber() < o.getBlock()->getNumber())
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else if (getBlock()->getNumber() > o.getBlock()->getNumber())
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// _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
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// an object with itself.
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#ifndef _GLIBCXX_DEBUG
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llvm_unreachable("Predecessor appears twice");
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/// CountTerminators - Count the number of terminators in the given
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/// block and set I to the position of the first non-terminator, if there
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/// is one, or MBB->end() otherwise.
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static unsigned CountTerminators(MachineBasicBlock *MBB,
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MachineBasicBlock::iterator &I) {
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unsigned NumTerms = 0;
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if (I == MBB->begin()) {
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if (!I->getDesc().isTerminator()) break;
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/// ProfitableToMerge - Check if two machine basic blocks have a common tail
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/// and decide if it would be profitable to merge those tails. Return the
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/// length of the common tail and iterators to the first common instruction
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static bool ProfitableToMerge(MachineBasicBlock *MBB1,
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MachineBasicBlock *MBB2,
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unsigned minCommonTailLength,
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unsigned &CommonTailLen,
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MachineBasicBlock::iterator &I1,
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MachineBasicBlock::iterator &I2,
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MachineBasicBlock *SuccBB,
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MachineBasicBlock *PredBB) {
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CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
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MachineFunction *MF = MBB1->getParent();
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if (CommonTailLen == 0)
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// It's almost always profitable to merge any number of non-terminator
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// instructions with the block that falls through into the common successor.
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if (MBB1 == PredBB || MBB2 == PredBB) {
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MachineBasicBlock::iterator I;
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unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
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if (CommonTailLen > NumTerms)
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// If one of the blocks can be completely merged and happens to be in
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// a position where the other could fall through into it, merge any number
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// of instructions, because it can be done without a branch.
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// TODO: If the blocks are not adjacent, move one of them so that they are?
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if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
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if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
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// If both blocks have an unconditional branch temporarily stripped out,
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// count that as an additional common instruction for the following
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unsigned EffectiveTailLen = CommonTailLen;
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if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
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!MBB1->back().getDesc().isBarrier() &&
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!MBB2->back().getDesc().isBarrier())
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// Check if the common tail is long enough to be worthwhile.
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if (EffectiveTailLen >= minCommonTailLength)
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// If we are optimizing for code size, 2 instructions in common is enough if
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// we don't have to split a block. At worst we will be introducing 1 new
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// branch instruction, which is likely to be smaller than the 2
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// instructions that would be deleted in the merge.
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if (EffectiveTailLen >= 2 &&
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MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
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(I1 == MBB1->begin() || I2 == MBB2->begin()))
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/// ComputeSameTails - Look through all the blocks in MergePotentials that have
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/// hash CurHash (guaranteed to match the last element). Build the vector
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/// SameTails of all those that have the (same) largest number of instructions
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/// in common of any pair of these blocks. SameTails entries contain an
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/// iterator into MergePotentials (from which the MachineBasicBlock can be
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/// found) and a MachineBasicBlock::iterator into that MBB indicating the
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/// instruction where the matching code sequence begins.
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/// Order of elements in SameTails is the reverse of the order in which
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/// those blocks appear in MergePotentials (where they are not necessarily
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unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
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unsigned minCommonTailLength,
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MachineBasicBlock *SuccBB,
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MachineBasicBlock *PredBB) {
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unsigned maxCommonTailLength = 0U;
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MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
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MPIterator HighestMPIter = prior(MergePotentials.end());
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for (MPIterator CurMPIter = prior(MergePotentials.end()),
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B = MergePotentials.begin();
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CurMPIter != B && CurMPIter->getHash() == CurHash;
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for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) {
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unsigned CommonTailLen;
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if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
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CommonTailLen, TrialBBI1, TrialBBI2,
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if (CommonTailLen > maxCommonTailLength) {
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maxCommonTailLength = CommonTailLen;
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HighestMPIter = CurMPIter;
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SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
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if (HighestMPIter == CurMPIter &&
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CommonTailLen == maxCommonTailLength)
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SameTails.push_back(SameTailElt(I, TrialBBI2));
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return maxCommonTailLength;
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/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
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/// MergePotentials, restoring branches at ends of blocks as appropriate.
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void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
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MachineBasicBlock *SuccBB,
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MachineBasicBlock *PredBB) {
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MPIterator CurMPIter, B;
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for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin();
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CurMPIter->getHash() == CurHash;
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// Put the unconditional branch back, if we need one.
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MachineBasicBlock *CurMBB = CurMPIter->getBlock();
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if (SuccBB && CurMBB != PredBB)
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FixTail(CurMBB, SuccBB, TII);
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if (CurMPIter->getHash() != CurHash)
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MergePotentials.erase(CurMPIter, MergePotentials.end());
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/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
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/// only of the common tail. Create a block that does by splitting one.
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unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
623
unsigned maxCommonTailLength) {
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unsigned commonTailIndex = 0;
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unsigned TimeEstimate = ~0U;
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for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
627
// Use PredBB if possible; that doesn't require a new branch.
628
if (SameTails[i].getBlock() == PredBB) {
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// Otherwise, make a (fairly bogus) choice based on estimate of
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// how long it will take the various blocks to execute.
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unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
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SameTails[i].getTailStartPos());
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if (t <= TimeEstimate) {
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MachineBasicBlock::iterator BBI =
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SameTails[commonTailIndex].getTailStartPos();
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MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
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DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
647
<< maxCommonTailLength);
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MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
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SameTails[commonTailIndex].setBlock(newMBB);
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SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
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// If we split PredBB, newMBB is the new predecessor.
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return commonTailIndex;
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// See if any of the blocks in MergePotentials (which all have a common single
661
// successor, or all have no successor) can be tail-merged. If there is a
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// successor, any blocks in MergePotentials that are not tail-merged and
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// are not immediately before Succ must have an unconditional branch to
664
// Succ added (but the predecessor/successor lists need no adjustment).
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// The lone predecessor of Succ that falls through into Succ,
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// if any, is given in PredBB.
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bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
669
MachineBasicBlock *PredBB) {
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bool MadeChange = false;
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// Except for the special cases below, tail-merge if there are at least
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// this many instructions in common.
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unsigned minCommonTailLength = TailMergeSize;
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DEBUG(dbgs() << "\nTryTailMergeBlocks: ";
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for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
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dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
679
<< (i == e-1 ? "" : ", ");
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dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n';
684
dbgs() << " which has fall-through from BB#"
685
<< PredBB->getNumber() << "\n";
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dbgs() << "Looking for common tails of at least "
688
<< minCommonTailLength << " instruction"
689
<< (minCommonTailLength == 1 ? "" : "s") << '\n';
692
// Sort by hash value so that blocks with identical end sequences sort
694
std::stable_sort(MergePotentials.begin(), MergePotentials.end());
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// Walk through equivalence sets looking for actual exact matches.
697
while (MergePotentials.size() > 1) {
698
unsigned CurHash = MergePotentials.back().getHash();
700
// Build SameTails, identifying the set of blocks with this hash code
701
// and with the maximum number of instructions in common.
702
unsigned maxCommonTailLength = ComputeSameTails(CurHash,
706
// If we didn't find any pair that has at least minCommonTailLength
707
// instructions in common, remove all blocks with this hash code and retry.
708
if (SameTails.empty()) {
709
RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
713
// If one of the blocks is the entire common tail (and not the entry
714
// block, which we can't jump to), we can treat all blocks with this same
715
// tail at once. Use PredBB if that is one of the possibilities, as that
716
// will not introduce any extra branches.
717
MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
718
getParent()->begin();
719
unsigned commonTailIndex = SameTails.size();
720
// If there are two blocks, check to see if one can be made to fall through
722
if (SameTails.size() == 2 &&
723
SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
724
SameTails[1].tailIsWholeBlock())
726
else if (SameTails.size() == 2 &&
727
SameTails[1].getBlock()->isLayoutSuccessor(
728
SameTails[0].getBlock()) &&
729
SameTails[0].tailIsWholeBlock())
732
// Otherwise just pick one, favoring the fall-through predecessor if
734
for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
735
MachineBasicBlock *MBB = SameTails[i].getBlock();
736
if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
742
if (SameTails[i].tailIsWholeBlock())
747
if (commonTailIndex == SameTails.size() ||
748
(SameTails[commonTailIndex].getBlock() == PredBB &&
749
!SameTails[commonTailIndex].tailIsWholeBlock())) {
750
// None of the blocks consist entirely of the common tail.
751
// Split a block so that one does.
752
commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength);
755
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
756
// MBB is common tail. Adjust all other BB's to jump to this one.
757
// Traversal must be forwards so erases work.
758
DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber()
760
for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
761
if (commonTailIndex == i)
763
DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
764
<< (i == e-1 ? "" : ", "));
765
// Hack the end off BB i, making it jump to BB commonTailIndex instead.
766
ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
767
// BB i is no longer a predecessor of SuccBB; remove it from the worklist.
768
MergePotentials.erase(SameTails[i].getMPIter());
770
DEBUG(dbgs() << "\n");
771
// We leave commonTailIndex in the worklist in case there are other blocks
772
// that match it with a smaller number of instructions.
778
bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
780
if (!EnableTailMerge) return false;
782
bool MadeChange = false;
784
// First find blocks with no successors.
785
MergePotentials.clear();
786
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
788
MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I, 2U), I));
791
// See if we can do any tail merging on those.
792
if (MergePotentials.size() < TailMergeThreshold &&
793
MergePotentials.size() >= 2)
794
MadeChange |= TryTailMergeBlocks(NULL, NULL);
796
// Look at blocks (IBB) with multiple predecessors (PBB).
797
// We change each predecessor to a canonical form, by
798
// (1) temporarily removing any unconditional branch from the predecessor
800
// (2) alter conditional branches so they branch to the other block
801
// not IBB; this may require adding back an unconditional branch to IBB
802
// later, where there wasn't one coming in. E.g.
804
// fallthrough to QBB
807
// with a conceptual B to IBB after that, which never actually exists.
808
// With those changes, we see whether the predecessors' tails match,
809
// and merge them if so. We change things out of canonical form and
810
// back to the way they were later in the process. (OptimizeBranches
811
// would undo some of this, but we can't use it, because we'd get into
812
// a compile-time infinite loop repeatedly doing and undoing the same
815
for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
817
if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) {
818
SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
819
MachineBasicBlock *IBB = I;
820
MachineBasicBlock *PredBB = prior(I);
821
MergePotentials.clear();
822
for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
825
MachineBasicBlock *PBB = *P;
826
// Skip blocks that loop to themselves, can't tail merge these.
829
// Visit each predecessor only once.
830
if (!UniquePreds.insert(PBB))
832
MachineBasicBlock *TBB = 0, *FBB = 0;
833
SmallVector<MachineOperand, 4> Cond;
834
if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
835
// Failing case: IBB is the target of a cbr, and
836
// we cannot reverse the branch.
837
SmallVector<MachineOperand, 4> NewCond(Cond);
838
if (!Cond.empty() && TBB == IBB) {
839
if (TII->ReverseBranchCondition(NewCond))
841
// This is the QBB case described above
843
FBB = llvm::next(MachineFunction::iterator(PBB));
845
// Failing case: the only way IBB can be reached from PBB is via
846
// exception handling. Happens for landing pads. Would be nice
847
// to have a bit in the edge so we didn't have to do all this.
848
if (IBB->isLandingPad()) {
849
MachineFunction::iterator IP = PBB; IP++;
850
MachineBasicBlock *PredNextBB = NULL;
854
if (IBB != PredNextBB) // fallthrough
857
if (TBB != IBB && FBB != IBB) // cbr then ubr
859
} else if (Cond.empty()) {
860
if (TBB != IBB) // ubr
863
if (TBB != IBB && IBB != PredNextBB) // cbr
867
// Remove the unconditional branch at the end, if any.
868
if (TBB && (Cond.empty() || FBB)) {
869
TII->RemoveBranch(*PBB);
871
// reinsert conditional branch only, for now
872
TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond);
874
MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB, 1U),
878
if (MergePotentials.size() >= 2)
879
MadeChange |= TryTailMergeBlocks(IBB, PredBB);
880
// Reinsert an unconditional branch if needed.
881
// The 1 below can occur as a result of removing blocks in TryTailMergeBlocks.
882
PredBB = prior(I); // this may have been changed in TryTailMergeBlocks
883
if (MergePotentials.size() == 1 &&
884
MergePotentials.begin()->getBlock() != PredBB)
885
FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
891
//===----------------------------------------------------------------------===//
892
// Branch Optimization
893
//===----------------------------------------------------------------------===//
895
bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
896
bool MadeChange = false;
898
// Make sure blocks are numbered in order
901
for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
902
MachineBasicBlock *MBB = I++;
903
MadeChange |= OptimizeBlock(MBB);
905
// If it is dead, remove it.
906
if (MBB->pred_empty()) {
907
RemoveDeadBlock(MBB);
916
/// IsBetterFallthrough - Return true if it would be clearly better to
917
/// fall-through to MBB1 than to fall through into MBB2. This has to return
918
/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
919
/// result in infinite loops.
920
static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
921
MachineBasicBlock *MBB2) {
922
// Right now, we use a simple heuristic. If MBB2 ends with a call, and
923
// MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
924
// optimize branches that branch to either a return block or an assert block
925
// into a fallthrough to the return.
926
if (MBB1->empty() || MBB2->empty()) return false;
928
// If there is a clear successor ordering we make sure that one block
929
// will fall through to the next
930
if (MBB1->isSuccessor(MBB2)) return true;
931
if (MBB2->isSuccessor(MBB1)) return false;
933
MachineInstr *MBB1I = --MBB1->end();
934
MachineInstr *MBB2I = --MBB2->end();
935
return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
938
/// OptimizeBlock - Analyze and optimize control flow related to the specified
939
/// block. This is never called on the entry block.
940
bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
941
bool MadeChange = false;
942
MachineFunction &MF = *MBB->getParent();
945
MachineFunction::iterator FallThrough = MBB;
948
// If this block is empty, make everyone use its fall-through, not the block
949
// explicitly. Landing pads should not do this since the landing-pad table
950
// points to this block. Blocks with their addresses taken shouldn't be
952
if (MBB->empty() && !MBB->isLandingPad() && !MBB->hasAddressTaken()) {
953
// Dead block? Leave for cleanup later.
954
if (MBB->pred_empty()) return MadeChange;
956
if (FallThrough == MF.end()) {
957
// TODO: Simplify preds to not branch here if possible!
959
// Rewrite all predecessors of the old block to go to the fallthrough
961
while (!MBB->pred_empty()) {
962
MachineBasicBlock *Pred = *(MBB->pred_end()-1);
963
Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
965
// If MBB was the target of a jump table, update jump tables to go to the
966
// fallthrough instead.
967
if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
968
MJTI->ReplaceMBBInJumpTables(MBB, FallThrough);
974
// Check to see if we can simplify the terminator of the block before this
976
MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
978
MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
979
SmallVector<MachineOperand, 4> PriorCond;
980
bool PriorUnAnalyzable =
981
TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
982
if (!PriorUnAnalyzable) {
983
// If the CFG for the prior block has extra edges, remove them.
984
MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
987
// If the previous branch is conditional and both conditions go to the same
988
// destination, remove the branch, replacing it with an unconditional one or
990
if (PriorTBB && PriorTBB == PriorFBB) {
991
TII->RemoveBranch(PrevBB);
994
TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
997
goto ReoptimizeBlock;
1000
// If the previous block unconditionally falls through to this block and
1001
// this block has no other predecessors, move the contents of this block
1002
// into the prior block. This doesn't usually happen when SimplifyCFG
1003
// has been used, but it can happen if tail merging splits a fall-through
1004
// predecessor of a block.
1005
// This has to check PrevBB->succ_size() because EH edges are ignored by
1007
if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1008
PrevBB.succ_size() == 1 &&
1009
!MBB->hasAddressTaken()) {
1010
DEBUG(dbgs() << "\nMerging into block: " << PrevBB
1011
<< "From MBB: " << *MBB);
1012
PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1013
PrevBB.removeSuccessor(PrevBB.succ_begin());;
1014
assert(PrevBB.succ_empty());
1015
PrevBB.transferSuccessors(MBB);
1020
// If the previous branch *only* branches to *this* block (conditional or
1021
// not) remove the branch.
1022
if (PriorTBB == MBB && PriorFBB == 0) {
1023
TII->RemoveBranch(PrevBB);
1026
goto ReoptimizeBlock;
1029
// If the prior block branches somewhere else on the condition and here if
1030
// the condition is false, remove the uncond second branch.
1031
if (PriorFBB == MBB) {
1032
TII->RemoveBranch(PrevBB);
1033
TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
1036
goto ReoptimizeBlock;
1039
// If the prior block branches here on true and somewhere else on false, and
1040
// if the branch condition is reversible, reverse the branch to create a
1042
if (PriorTBB == MBB) {
1043
SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1044
if (!TII->ReverseBranchCondition(NewPriorCond)) {
1045
TII->RemoveBranch(PrevBB);
1046
TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
1049
goto ReoptimizeBlock;
1053
// If this block has no successors (e.g. it is a return block or ends with
1054
// a call to a no-return function like abort or __cxa_throw) and if the pred
1055
// falls through into this block, and if it would otherwise fall through
1056
// into the block after this, move this block to the end of the function.
1058
// We consider it more likely that execution will stay in the function (e.g.
1059
// due to loops) than it is to exit it. This asserts in loops etc, moving
1060
// the assert condition out of the loop body.
1061
if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
1062
MachineFunction::iterator(PriorTBB) == FallThrough &&
1063
!MBB->canFallThrough()) {
1064
bool DoTransform = true;
1066
// We have to be careful that the succs of PredBB aren't both no-successor
1067
// blocks. If neither have successors and if PredBB is the second from
1068
// last block in the function, we'd just keep swapping the two blocks for
1069
// last. Only do the swap if one is clearly better to fall through than
1071
if (FallThrough == --MF.end() &&
1072
!IsBetterFallthrough(PriorTBB, MBB))
1073
DoTransform = false;
1075
// We don't want to do this transformation if we have control flow like:
1084
// In this case, we could actually be moving the return block *into* a
1086
if (DoTransform && !MBB->succ_empty() &&
1087
(!PriorTBB->canFallThrough() || PriorTBB->empty()))
1088
DoTransform = false;
1092
// Reverse the branch so we will fall through on the previous true cond.
1093
SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1094
if (!TII->ReverseBranchCondition(NewPriorCond)) {
1095
DEBUG(dbgs() << "\nMoving MBB: " << *MBB
1096
<< "To make fallthrough to: " << *PriorTBB << "\n");
1098
TII->RemoveBranch(PrevBB);
1099
TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond);
1101
// Move this block to the end of the function.
1102
MBB->moveAfter(--MF.end());
1111
// Analyze the branch in the current block.
1112
MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
1113
SmallVector<MachineOperand, 4> CurCond;
1114
bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1115
if (!CurUnAnalyzable) {
1116
// If the CFG for the prior block has extra edges, remove them.
1117
MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1119
// If this is a two-way branch, and the FBB branches to this block, reverse
1120
// the condition so the single-basic-block loop is faster. Instead of:
1121
// Loop: xxx; jcc Out; jmp Loop
1123
// Loop: xxx; jncc Loop; jmp Out
1124
if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1125
SmallVector<MachineOperand, 4> NewCond(CurCond);
1126
if (!TII->ReverseBranchCondition(NewCond)) {
1127
TII->RemoveBranch(*MBB);
1128
TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
1131
goto ReoptimizeBlock;
1135
// If this branch is the only thing in its block, see if we can forward
1136
// other blocks across it.
1137
if (CurTBB && CurCond.empty() && CurFBB == 0 &&
1138
MBB->begin()->getDesc().isBranch() && CurTBB != MBB &&
1139
!MBB->hasAddressTaken()) {
1140
// This block may contain just an unconditional branch. Because there can
1141
// be 'non-branch terminators' in the block, try removing the branch and
1142
// then seeing if the block is empty.
1143
TII->RemoveBranch(*MBB);
1145
// If this block is just an unconditional branch to CurTBB, we can
1146
// usually completely eliminate the block. The only case we cannot
1147
// completely eliminate the block is when the block before this one
1148
// falls through into MBB and we can't understand the prior block's branch
1151
bool PredHasNoFallThrough = !PrevBB.canFallThrough();
1152
if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1153
!PrevBB.isSuccessor(MBB)) {
1154
// If the prior block falls through into us, turn it into an
1155
// explicit branch to us to make updates simpler.
1156
if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1157
PriorTBB != MBB && PriorFBB != MBB) {
1158
if (PriorTBB == 0) {
1159
assert(PriorCond.empty() && PriorFBB == 0 &&
1160
"Bad branch analysis");
1163
assert(PriorFBB == 0 && "Machine CFG out of date!");
1166
TII->RemoveBranch(PrevBB);
1167
TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
1170
// Iterate through all the predecessors, revectoring each in-turn.
1172
bool DidChange = false;
1173
bool HasBranchToSelf = false;
1174
while(PI != MBB->pred_size()) {
1175
MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1177
// If this block has an uncond branch to itself, leave it.
1179
HasBranchToSelf = true;
1182
PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1183
// If this change resulted in PMBB ending in a conditional
1184
// branch where both conditions go to the same destination,
1185
// change this to an unconditional branch (and fix the CFG).
1186
MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
1187
SmallVector<MachineOperand, 4> NewCurCond;
1188
bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
1189
NewCurFBB, NewCurCond, true);
1190
if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1191
TII->RemoveBranch(*PMBB);
1193
TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond);
1196
PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
1201
// Change any jumptables to go to the new MBB.
1202
if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1203
MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
1207
if (!HasBranchToSelf) return MadeChange;
1212
// Add the branch back if the block is more than just an uncond branch.
1213
TII->InsertBranch(*MBB, CurTBB, 0, CurCond);
1217
// If the prior block doesn't fall through into this block, and if this
1218
// block doesn't fall through into some other block, see if we can find a
1219
// place to move this block where a fall-through will happen.
1220
if (!PrevBB.canFallThrough()) {
1222
// Now we know that there was no fall-through into this block, check to
1223
// see if it has a fall-through into its successor.
1224
bool CurFallsThru = MBB->canFallThrough();
1226
if (!MBB->isLandingPad()) {
1227
// Check all the predecessors of this block. If one of them has no fall
1228
// throughs, move this block right after it.
1229
for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
1230
E = MBB->pred_end(); PI != E; ++PI) {
1231
// Analyze the branch at the end of the pred.
1232
MachineBasicBlock *PredBB = *PI;
1233
MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
1234
MachineBasicBlock *PredTBB = 0, *PredFBB = 0;
1235
SmallVector<MachineOperand, 4> PredCond;
1236
if (PredBB != MBB && !PredBB->canFallThrough() &&
1237
!TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
1238
&& (!CurFallsThru || !CurTBB || !CurFBB)
1239
&& (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1240
// If the current block doesn't fall through, just move it.
1241
// If the current block can fall through and does not end with a
1242
// conditional branch, we need to append an unconditional jump to
1243
// the (current) next block. To avoid a possible compile-time
1244
// infinite loop, move blocks only backward in this case.
1245
// Also, if there are already 2 branches here, we cannot add a third;
1246
// this means we have the case
1251
MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB));
1253
TII->InsertBranch(*MBB, NextBB, 0, CurCond);
1255
MBB->moveAfter(PredBB);
1257
goto ReoptimizeBlock;
1262
if (!CurFallsThru) {
1263
// Check all successors to see if we can move this block before it.
1264
for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
1265
E = MBB->succ_end(); SI != E; ++SI) {
1266
// Analyze the branch at the end of the block before the succ.
1267
MachineBasicBlock *SuccBB = *SI;
1268
MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
1270
// If this block doesn't already fall-through to that successor, and if
1271
// the succ doesn't already have a block that can fall through into it,
1272
// and if the successor isn't an EH destination, we can arrange for the
1273
// fallthrough to happen.
1274
if (SuccBB != MBB && &*SuccPrev != MBB &&
1275
!SuccPrev->canFallThrough() && !CurUnAnalyzable &&
1276
!SuccBB->isLandingPad()) {
1277
MBB->moveBefore(SuccBB);
1279
goto ReoptimizeBlock;
1283
// Okay, there is no really great place to put this block. If, however,
1284
// the block before this one would be a fall-through if this block were
1285
// removed, move this block to the end of the function.
1286
MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0;
1287
SmallVector<MachineOperand, 4> PrevCond;
1288
if (FallThrough != MF.end() &&
1289
!TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1290
PrevBB.isSuccessor(FallThrough)) {
1291
MBB->moveAfter(--MF.end());