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551ccae044
Move include/Config and include/Support into include/llvm/Config, include/llvm/ADT and include/llvm/Support. From here on out, all LLVM public header files must be under include/llvm/. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16137 91177308-0d34-0410-b5e6-96231b3b80d8
205 lines
7.5 KiB
C++
205 lines
7.5 KiB
C++
//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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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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// it then removes.
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//
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// Note that this pass must be run after register allocation, it cannot handle
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// SSA form.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/CodeGen/MachineFunctionPass.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/ADT/STLExtras.h"
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using namespace llvm;
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namespace {
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struct BranchFolder : public MachineFunctionPass {
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virtual bool runOnMachineFunction(MachineFunction &MF);
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virtual const char *getPassName() const { return "Branch Folder"; }
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private:
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bool OptimizeBlock(MachineFunction::iterator MBB,
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const TargetInstrInfo &TII);
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bool isUncondBranch(const MachineInstr *MI, const TargetInstrInfo &TII) {
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return TII.isBarrier(MI->getOpcode()) && TII.isBranch(MI->getOpcode());
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}
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bool isCondBranch(const MachineInstr *MI, const TargetInstrInfo &TII) {
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return TII.isBranch(MI->getOpcode()) && !TII.isBarrier(MI->getOpcode());
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}
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};
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}
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FunctionPass *llvm::createBranchFoldingPass() { return new BranchFolder(); }
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bool BranchFolder::runOnMachineFunction(MachineFunction &MF) {
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bool EverMadeChange = false;
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bool MadeChange = true;
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const TargetInstrInfo &TII = *MF.getTarget().getInstrInfo();
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while (MadeChange) {
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MadeChange = false;
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for (MachineFunction::iterator MBB = ++MF.begin(), E = MF.end(); MBB != E;
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++MBB)
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MadeChange |= OptimizeBlock(MBB, TII);
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// If branches were folded away somehow, do a quick scan and delete any dead
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// blocks.
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if (MadeChange) {
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for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
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MachineBasicBlock *MBB = I++;
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// Is it dead?
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if (MBB->pred_empty()) {
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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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MF.getBasicBlockList().erase(MBB);
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}
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}
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}
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EverMadeChange |= MadeChange;
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}
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return EverMadeChange;
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}
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/// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to
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/// 'Old', change the code and CFG so that it branches to 'New' instead.
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static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB,
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MachineBasicBlock *Old,
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MachineBasicBlock *New,
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const TargetInstrInfo &TII) {
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assert(Old != New && "Cannot replace self with self!");
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MachineBasicBlock::iterator I = BB->end();
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while (I != BB->begin()) {
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--I;
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if (!TII.isTerminatorInstr(I->getOpcode())) break;
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// Scan the operands of this machine instruction, replacing any uses of Old
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// with New.
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
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if (I->getOperand(i).isMachineBasicBlock() &&
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I->getOperand(i).getMachineBasicBlock() == Old)
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I->getOperand(i).setMachineBasicBlock(New);
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}
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// If BB falls through into Old, insert an unconditional branch to New.
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MachineFunction::iterator BBSucc = BB; ++BBSucc;
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if (BBSucc != BB->getParent()->end() && &*BBSucc == Old)
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TII.insertGoto(*BB, *New);
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std::vector<MachineBasicBlock*> Succs(BB->succ_begin(), BB->succ_end());
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for (int i = Succs.size()-1; i >= 0; --i)
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if (Succs[i] == Old) {
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BB->removeSuccessor(Old);
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BB->addSuccessor(New);
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}
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}
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bool BranchFolder::OptimizeBlock(MachineFunction::iterator MBB,
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const TargetInstrInfo &TII) {
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// If this block is empty, make everyone use it's fall-through, not the block
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// explicitly.
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if (MBB->empty()) {
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if (MBB->pred_empty()) return false;
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MachineFunction::iterator FallThrough =next(MBB);
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assert(FallThrough != MBB->getParent()->end() &&
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"Fell off the end of the function!");
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while (!MBB->pred_empty()) {
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MachineBasicBlock *Pred = *(MBB->pred_end()-1);
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ReplaceUsesOfBlockWith(Pred, MBB, FallThrough, TII);
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}
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return true;
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}
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if (MBB->pred_size() == 1) {
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// If this block has a single predecessor, and if that block has a single
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// successor, merge this block into that block.
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MachineBasicBlock *Pred = *MBB->pred_begin();
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if (Pred->succ_size() == 1) {
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// Delete all of the terminators from end of the pred block. NOTE, this
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// assumes that terminators do not have side effects!
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while (!Pred->empty() && TII.isTerminatorInstr(Pred->back().getOpcode()))
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Pred->pop_back();
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// Splice the instructions over.
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Pred->splice(Pred->end(), MBB, MBB->begin(), MBB->end());
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// If MBB does not end with a barrier, add a goto instruction to the end.
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if (Pred->empty() || !TII.isBarrier(Pred->back().getOpcode()))
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TII.insertGoto(*Pred, *next(MBB));
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// Update the CFG now.
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Pred->removeSuccessor(Pred->succ_begin());
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while (!MBB->succ_empty()) {
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Pred->addSuccessor(*(MBB->succ_end()-1));
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MBB->removeSuccessor(MBB->succ_end()-1);
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}
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return true;
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}
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}
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// If the first instruction in this block is an unconditional branch, and if
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// there are predecessors, fold the branch into the predecessors.
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if (!MBB->pred_empty() && isUncondBranch(MBB->begin(), TII)) {
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MachineInstr *Br = MBB->begin();
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assert(Br->getNumOperands() == 1 && Br->getOperand(0).isMachineBasicBlock()
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&& "Uncond branch should take one MBB argument!");
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MachineBasicBlock *Dest = Br->getOperand(0).getMachineBasicBlock();
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while (!MBB->pred_empty()) {
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MachineBasicBlock *Pred = *(MBB->pred_end()-1);
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ReplaceUsesOfBlockWith(Pred, MBB, Dest, TII);
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}
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return true;
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}
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// If the last instruction is an unconditional branch and the fall through
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// block is the destination, just delete the branch.
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if (isUncondBranch(--MBB->end(), TII)) {
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MachineBasicBlock::iterator MI = --MBB->end();
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MachineInstr *UncondBr = MI;
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MachineFunction::iterator FallThrough = next(MBB);
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MachineFunction::iterator UncondDest =
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MI->getOperand(0).getMachineBasicBlock();
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if (UncondDest == FallThrough) {
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// Just delete the branch. This does not effect the CFG.
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MBB->erase(UncondBr);
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return true;
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}
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// Okay, so we don't have a fall-through. Check to see if we have an
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// conditional branch that would be a fall through if we reversed it. If
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// so, invert the condition and delete the uncond branch.
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if (MI != MBB->begin() && isCondBranch(--MI, TII)) {
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// We assume that conditional branches always have the branch dest as the
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// last operand. This could be generalized in the future if needed.
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unsigned LastOpnd = MI->getNumOperands()-1;
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if (MachineFunction::iterator(
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MI->getOperand(LastOpnd).getMachineBasicBlock()) == FallThrough) {
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// Change the cond branch to go to the uncond dest, nuke the uncond,
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// then reverse the condition.
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MI->getOperand(LastOpnd).setMachineBasicBlock(UncondDest);
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MBB->erase(UncondBr);
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TII.reverseBranchCondition(MI);
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return true;
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}
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}
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}
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return false;
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}
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