//
// The LLVM Compiler Infrastructure
//
-// This file was developed by the LLVM research group and is distributed under
-// the University of Illinois Open Source License. See LICENSE.TXT for details.
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineJumpTableInfo.h"
+#include "llvm/CodeGen/RegisterScavenging.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
+#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
+#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include <algorithm>
STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
STATISTIC(NumBranchOpts, "Number of branches optimized");
STATISTIC(NumTailMerge , "Number of block tails merged");
-static cl::opt<bool> EnableTailMerge("enable-tail-merge", cl::Hidden);
+static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
+ cl::init(cl::BOU_UNSET), cl::Hidden);
+// Throttle for huge numbers of predecessors (compile speed problems)
+static cl::opt<unsigned>
+TailMergeThreshold("tail-merge-threshold",
+ cl::desc("Max number of predecessors to consider tail merging"),
+ cl::init(150), cl::Hidden);
namespace {
- struct BranchFolder : public MachineFunctionPass {
+ struct VISIBILITY_HIDDEN BranchFolder : public MachineFunctionPass {
+ static char ID;
+ explicit BranchFolder(bool defaultEnableTailMerge) :
+ MachineFunctionPass(&ID) {
+ switch (FlagEnableTailMerge) {
+ case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
+ case cl::BOU_TRUE: EnableTailMerge = true; break;
+ case cl::BOU_FALSE: EnableTailMerge = false; break;
+ }
+ }
+
virtual bool runOnMachineFunction(MachineFunction &MF);
virtual const char *getPassName() const { return "Control Flow Optimizer"; }
const TargetInstrInfo *TII;
bool MadeChange;
private:
// Tail Merging.
+ bool EnableTailMerge;
bool TailMergeBlocks(MachineFunction &MF);
+ bool TryMergeBlocks(MachineBasicBlock* SuccBB,
+ MachineBasicBlock* PredBB);
void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
MachineBasicBlock *NewDest);
MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB,
MachineBasicBlock::iterator BBI1);
-
+ unsigned ComputeSameTails(unsigned CurHash, unsigned minCommonTailLength);
+ void RemoveBlocksWithHash(unsigned CurHash, MachineBasicBlock* SuccBB,
+ MachineBasicBlock* PredBB);
+ unsigned CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
+ unsigned maxCommonTailLength);
+
+ typedef std::pair<unsigned,MachineBasicBlock*> MergePotentialsElt;
+ typedef std::vector<MergePotentialsElt>::iterator MPIterator;
+ std::vector<MergePotentialsElt> MergePotentials;
+
+ typedef std::pair<MPIterator, MachineBasicBlock::iterator> SameTailElt;
+ std::vector<SameTailElt> SameTails;
+
+ const TargetRegisterInfo *RegInfo;
+ RegScavenger *RS;
// Branch optzn.
bool OptimizeBranches(MachineFunction &MF);
void OptimizeBlock(MachineBasicBlock *MBB);
void RemoveDeadBlock(MachineBasicBlock *MBB);
+ bool OptimizeImpDefsBlock(MachineBasicBlock *MBB);
bool CanFallThrough(MachineBasicBlock *CurBB);
bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable,
MachineBasicBlock *TBB, MachineBasicBlock *FBB,
- const std::vector<MachineOperand> &Cond);
+ const SmallVectorImpl<MachineOperand> &Cond);
};
+ char BranchFolder::ID = 0;
}
-FunctionPass *llvm::createBranchFoldingPass() { return new BranchFolder(); }
+FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
+ return new BranchFolder(DefaultEnableTailMerge); }
/// RemoveDeadBlock - Remove the specified dead machine basic block from the
/// function, updating the CFG.
while (!MBB->succ_empty())
MBB->removeSuccessor(MBB->succ_end()-1);
- // If there is DWARF info to active, check to see if there are any LABEL
- // records in the basic block. If so, unregister them from MachineModuleInfo.
+ // If there are any labels in the basic block, unregister them from
+ // MachineModuleInfo.
if (MMI && !MBB->empty()) {
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
I != E; ++I) {
- if ((unsigned)I->getOpcode() == TargetInstrInfo::LABEL) {
+ if (I->isLabel())
// The label ID # is always operand #0, an immediate.
MMI->InvalidateLabel(I->getOperand(0).getImm());
- }
}
}
// Remove the block.
- MF->getBasicBlockList().erase(MBB);
+ MF->erase(MBB);
+}
+
+/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
+/// followed by terminators, and if the implicitly defined registers are not
+/// used by the terminators, remove those implicit_def's. e.g.
+/// BB1:
+/// r0 = implicit_def
+/// r1 = implicit_def
+/// br
+/// This block can be optimized away later if the implicit instructions are
+/// removed.
+bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
+ SmallSet<unsigned, 4> ImpDefRegs;
+ MachineBasicBlock::iterator I = MBB->begin();
+ while (I != MBB->end()) {
+ if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF)
+ break;
+ unsigned Reg = I->getOperand(0).getReg();
+ ImpDefRegs.insert(Reg);
+ for (const unsigned *SubRegs = RegInfo->getSubRegisters(Reg);
+ unsigned SubReg = *SubRegs; ++SubRegs)
+ ImpDefRegs.insert(SubReg);
+ ++I;
+ }
+ if (ImpDefRegs.empty())
+ return false;
+
+ MachineBasicBlock::iterator FirstTerm = I;
+ while (I != MBB->end()) {
+ if (!TII->isUnpredicatedTerminator(I))
+ return false;
+ // See if it uses any of the implicitly defined registers.
+ for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
+ MachineOperand &MO = I->getOperand(i);
+ if (!MO.isReg() || !MO.isUse())
+ continue;
+ unsigned Reg = MO.getReg();
+ if (ImpDefRegs.count(Reg))
+ return false;
+ }
+ ++I;
+ }
+
+ I = MBB->begin();
+ while (I != FirstTerm) {
+ MachineInstr *ImpDefMI = &*I;
+ ++I;
+ MBB->erase(ImpDefMI);
+ }
+
+ return true;
}
bool BranchFolder::runOnMachineFunction(MachineFunction &MF) {
TII = MF.getTarget().getInstrInfo();
if (!TII) return false;
- MMI = getAnalysisToUpdate<MachineModuleInfo>();
-
+ RegInfo = MF.getTarget().getRegisterInfo();
+
+ // Fix CFG. The later algorithms expect it to be right.
bool EverMadeChange = false;
+ for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
+ MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
+ SmallVector<MachineOperand, 4> Cond;
+ if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true))
+ EverMadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
+ EverMadeChange |= OptimizeImpDefsBlock(MBB);
+ }
+
+ RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
+
+ MMI = getAnalysisIfAvailable<MachineModuleInfo>();
+
bool MadeChangeThisIteration = true;
while (MadeChangeThisIteration) {
MadeChangeThisIteration = false;
// If a jump table was merge with another one, walk the function rewriting
// references to jump tables to reference the new JT ID's. Keep track of
// whether we see a jump table idx, if not, we can delete the JT.
- std::vector<bool> JTIsLive;
- JTIsLive.resize(JTs.size());
+ BitVector JTIsLive(JTs.size());
for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
BB != E; ++BB) {
for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
I != E; ++I)
for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
MachineOperand &Op = I->getOperand(op);
- if (!Op.isJumpTableIndex()) continue;
- unsigned NewIdx = JTMapping[Op.getJumpTableIndex()];
- Op.setJumpTableIndex(NewIdx);
+ if (!Op.isJTI()) continue;
+ unsigned NewIdx = JTMapping[Op.getIndex()];
+ Op.setIndex(NewIdx);
// Remember that this JT is live.
- JTIsLive[NewIdx] = true;
+ JTIsLive.set(NewIdx);
}
}
// indirect jump was unreachable (and thus deleted) or because the jump
// table was merged with some other one.
for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
- if (!JTIsLive[i]) {
+ if (!JTIsLive.test(i)) {
JTI->RemoveJumpTable(i);
EverMadeChange = true;
}
}
+ delete RS;
return EverMadeChange;
}
case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
case MachineOperand::MO_MachineBasicBlock:
- OperandHash = Op.getMachineBasicBlock()->getNumber();
+ OperandHash = Op.getMBB()->getNumber();
break;
- case MachineOperand::MO_FrameIndex: OperandHash = Op.getFrameIndex(); break;
+ case MachineOperand::MO_FrameIndex:
case MachineOperand::MO_ConstantPoolIndex:
- OperandHash = Op.getConstantPoolIndex();
- break;
case MachineOperand::MO_JumpTableIndex:
- OperandHash = Op.getJumpTableIndex();
+ OperandHash = Op.getIndex();
break;
case MachineOperand::MO_GlobalAddress:
case MachineOperand::MO_ExternalSymbol:
return Hash;
}
-/// HashEndOfMBB - Hash the last two instructions in the MBB. We hash two
-/// instructions, because cross-jumping only saves code when at least two
-/// instructions are removed (since a branch must be inserted).
-static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) {
+/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks
+/// with no successors, we hash two instructions, because cross-jumping
+/// only saves code when at least two instructions are removed (since a
+/// branch must be inserted). For blocks with a successor, one of the
+/// two blocks to be tail-merged will end with a branch already, so
+/// it gains to cross-jump even for one instruction.
+
+static unsigned HashEndOfMBB(const MachineBasicBlock *MBB,
+ unsigned minCommonTailLength) {
MachineBasicBlock::const_iterator I = MBB->end();
if (I == MBB->begin())
return 0; // Empty MBB.
--I;
unsigned Hash = HashMachineInstr(I);
- if (I == MBB->begin())
+ if (I == MBB->begin() || minCommonTailLength == 1)
return Hash; // Single instr MBB.
--I;
unsigned TailLen = 0;
while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
--I1; --I2;
- if (!I1->isIdenticalTo(I2)) {
+ if (!I1->isIdenticalTo(I2) ||
+ // FIXME: This check is dubious. It's used to get around a problem where
+ // people incorrectly expect inline asm directives to remain in the same
+ // relative order. This is untenable because normal compiler
+ // optimizations (like this one) may reorder and/or merge these
+ // directives.
+ I1->getOpcode() == TargetInstrInfo::INLINEASM) {
++I1; ++I2;
break;
}
// If OldBB isn't immediately before OldBB, insert a branch to it.
if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest))
- TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>());
+ TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>());
OldBB->addSuccessor(NewDest);
++NumTailMerge;
}
/// 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 = new MachineBasicBlock(CurMBB.getBasicBlock());
- CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB);
+ MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock());
+ CurMBB.getParent()->insert(++MBBI, NewMBB);
// Move all the successors of this block to the specified block.
- while (!CurMBB.succ_empty()) {
- MachineBasicBlock *S = *(CurMBB.succ_end()-1);
- NewMBB->addSuccessor(S);
- CurMBB.removeSuccessor(S);
- }
+ NewMBB->transferSuccessors(&CurMBB);
// Add an edge from CurMBB to NewMBB for the fall-through.
CurMBB.addSuccessor(NewMBB);
// Splice the code over.
NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
+
+ // For targets that use the register scavenger, we must maintain LiveIns.
+ if (RS) {
+ RS->enterBasicBlock(&CurMBB);
+ if (!CurMBB.empty())
+ RS->forward(prior(CurMBB.end()));
+ BitVector RegsLiveAtExit(RegInfo->getNumRegs());
+ RS->getRegsUsed(RegsLiveAtExit, false);
+ for (unsigned int i=0, e=RegInfo->getNumRegs(); i!=e; i++)
+ if (RegsLiveAtExit[i])
+ NewMBB->addLiveIn(i);
+ }
+
return NewMBB;
}
/// EstimateRuntime - Make a rough estimate for how long it will take to run
/// the specified code.
static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
- MachineBasicBlock::iterator E,
- const TargetInstrInfo *TII) {
+ MachineBasicBlock::iterator E) {
unsigned Time = 0;
for (; I != E; ++I) {
- const TargetInstrDescriptor &TID = TII->get(I->getOpcode());
- if (TID.Flags & M_CALL_FLAG)
+ const TargetInstrDesc &TID = I->getDesc();
+ if (TID.isCall())
Time += 10;
- else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG))
+ else if (TID.mayLoad() || TID.mayStore())
Time += 2;
else
++Time;
return Time;
}
-/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at
-/// MBB2I and then insert an unconditional branch in the other block. Determine
-/// which is the best to split
-static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1,
- MachineBasicBlock::iterator MBB1I,
- MachineBasicBlock *MBB2,
- MachineBasicBlock::iterator MBB2I,
- const TargetInstrInfo *TII) {
- // TODO: if we had some notion of which block was hotter, we could split
- // the hot block, so it is the fall-through. Since we don't have profile info
- // make a decision based on which will hurt most to split.
- unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII);
- unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII);
-
- // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the
- // MBB1 block so it falls through. This will penalize the MBB2 path, but will
- // have a lower overall impact on the program execution.
- return MBB1Time < MBB2Time;
+// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
+// branches temporarily for tail merging). In the case where CurMBB ends
+// with a conditional branch to the next block, optimize by reversing the
+// test and conditionally branching to SuccMBB instead.
+
+static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB,
+ const TargetInstrInfo *TII) {
+ MachineFunction *MF = CurMBB->getParent();
+ MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB));
+ MachineBasicBlock *TBB = 0, *FBB = 0;
+ SmallVector<MachineOperand, 4> Cond;
+ if (I != MF->end() &&
+ !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
+ MachineBasicBlock *NextBB = I;
+ if (TBB == NextBB && !Cond.empty() && !FBB) {
+ if (!TII->ReverseBranchCondition(Cond)) {
+ TII->RemoveBranch(*CurMBB);
+ TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond);
+ return;
+ }
+ }
+ }
+ TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>());
}
-bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
- MadeChange = false;
-
- if (!EnableTailMerge) return false;
-
- // Find blocks with no successors.
- std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials;
- for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
- if (I->succ_empty())
- MergePotentials.push_back(std::make_pair(HashEndOfMBB(I), I));
+static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p,
+ const std::pair<unsigned,MachineBasicBlock*> &q) {
+ if (p.first < q.first)
+ return true;
+ else if (p.first > q.first)
+ return false;
+ else if (p.second->getNumber() < q.second->getNumber())
+ return true;
+ else if (p.second->getNumber() > q.second->getNumber())
+ return false;
+ else {
+ // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
+ // an object with itself.
+#ifndef _GLIBCXX_DEBUG
+ assert(0 && "Predecessor appears twice");
+#endif
+ return false;
+ }
+}
+
+/// ComputeSameTails - Look through all the blocks in MergePotentials that have
+/// hash CurHash (guaranteed to match the last element). Build the vector
+/// SameTails of all those that have the (same) largest number of instructions
+/// in common of any pair of these blocks. SameTails entries contain an
+/// iterator into MergePotentials (from which the MachineBasicBlock can be
+/// found) and a MachineBasicBlock::iterator into that MBB indicating the
+/// instruction where the matching code sequence begins.
+/// Order of elements in SameTails is the reverse of the order in which
+/// those blocks appear in MergePotentials (where they are not necessarily
+/// consecutive).
+unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
+ unsigned minCommonTailLength) {
+ unsigned maxCommonTailLength = 0U;
+ SameTails.clear();
+ MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
+ MPIterator HighestMPIter = prior(MergePotentials.end());
+ for (MPIterator CurMPIter = prior(MergePotentials.end()),
+ B = MergePotentials.begin();
+ CurMPIter!=B && CurMPIter->first==CurHash;
+ --CurMPIter) {
+ for (MPIterator I = prior(CurMPIter); I->first==CurHash ; --I) {
+ unsigned CommonTailLen = ComputeCommonTailLength(
+ CurMPIter->second,
+ I->second,
+ TrialBBI1, TrialBBI2);
+ // If we will have to split a block, there should be at least
+ // minCommonTailLength instructions in common; if not, at worst
+ // we will be replacing a fallthrough into the common tail with a
+ // branch, which at worst breaks even with falling through into
+ // the duplicated common tail, so 1 instruction in common is enough.
+ // We will always pick a block we do not have to split as the common
+ // tail if there is one.
+ // (Empty blocks will get forwarded and need not be considered.)
+ if (CommonTailLen >= minCommonTailLength ||
+ (CommonTailLen > 0 &&
+ (TrialBBI1==CurMPIter->second->begin() ||
+ TrialBBI2==I->second->begin()))) {
+ if (CommonTailLen > maxCommonTailLength) {
+ SameTails.clear();
+ maxCommonTailLength = CommonTailLen;
+ HighestMPIter = CurMPIter;
+ SameTails.push_back(std::make_pair(CurMPIter, TrialBBI1));
+ }
+ if (HighestMPIter == CurMPIter &&
+ CommonTailLen == maxCommonTailLength)
+ SameTails.push_back(std::make_pair(I, TrialBBI2));
+ }
+ if (I==B)
+ break;
+ }
+ }
+ return maxCommonTailLength;
+}
+
+/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
+/// MergePotentials, restoring branches at ends of blocks as appropriate.
+void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
+ MachineBasicBlock* SuccBB,
+ MachineBasicBlock* PredBB) {
+ MPIterator CurMPIter, B;
+ for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin();
+ CurMPIter->first==CurHash;
+ --CurMPIter) {
+ // Put the unconditional branch back, if we need one.
+ MachineBasicBlock *CurMBB = CurMPIter->second;
+ if (SuccBB && CurMBB != PredBB)
+ FixTail(CurMBB, SuccBB, TII);
+ if (CurMPIter==B)
+ break;
+ }
+ if (CurMPIter->first!=CurHash)
+ CurMPIter++;
+ MergePotentials.erase(CurMPIter, MergePotentials.end());
+}
+
+/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
+/// only of the common tail. Create a block that does by splitting one.
+unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
+ unsigned maxCommonTailLength) {
+ unsigned i, commonTailIndex;
+ unsigned TimeEstimate = ~0U;
+ for (i=0, commonTailIndex=0; i<SameTails.size(); i++) {
+ // Use PredBB if possible; that doesn't require a new branch.
+ if (SameTails[i].first->second==PredBB) {
+ commonTailIndex = i;
+ break;
+ }
+ // Otherwise, make a (fairly bogus) choice based on estimate of
+ // how long it will take the various blocks to execute.
+ unsigned t = EstimateRuntime(SameTails[i].first->second->begin(),
+ SameTails[i].second);
+ if (t<=TimeEstimate) {
+ TimeEstimate = t;
+ commonTailIndex = i;
+ }
}
+
+ MachineBasicBlock::iterator BBI = SameTails[commonTailIndex].second;
+ MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second;
+
+ DOUT << "\nSplitting " << MBB->getNumber() << ", size " <<
+ maxCommonTailLength;
+
+ MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
+ SameTails[commonTailIndex].first->second = newMBB;
+ SameTails[commonTailIndex].second = newMBB->begin();
+ // If we split PredBB, newMBB is the new predecessor.
+ if (PredBB==MBB)
+ PredBB = newMBB;
+
+ return commonTailIndex;
+}
+
+// See if any of the blocks in MergePotentials (which all have a common single
+// successor, or all have no successor) can be tail-merged. If there is a
+// successor, any blocks in MergePotentials that are not tail-merged and
+// are not immediately before Succ must have an unconditional branch to
+// Succ added (but the predecessor/successor lists need no adjustment).
+// The lone predecessor of Succ that falls through into Succ,
+// if any, is given in PredBB.
+
+bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
+ MachineBasicBlock* PredBB) {
+ // It doesn't make sense to save a single instruction since tail merging
+ // will add a jump.
+ // FIXME: Ask the target to provide the threshold?
+ unsigned minCommonTailLength = (SuccBB ? 1 : 2) + 1;
+ MadeChange = false;
+ DOUT << "\nTryMergeBlocks " << MergePotentials.size() << '\n';
+
// Sort by hash value so that blocks with identical end sequences sort
// together.
- std::stable_sort(MergePotentials.begin(), MergePotentials.end());
+ std::stable_sort(MergePotentials.begin(), MergePotentials.end(),MergeCompare);
// Walk through equivalence sets looking for actual exact matches.
while (MergePotentials.size() > 1) {
- unsigned CurHash = (MergePotentials.end()-1)->first;
- unsigned PrevHash = (MergePotentials.end()-2)->first;
- MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second;
+ unsigned CurHash = prior(MergePotentials.end())->first;
- // If there is nothing that matches the hash of the current basic block,
- // give up.
- if (CurHash != PrevHash) {
- MergePotentials.pop_back();
+ // Build SameTails, identifying the set of blocks with this hash code
+ // and with the maximum number of instructions in common.
+ unsigned maxCommonTailLength = ComputeSameTails(CurHash,
+ minCommonTailLength);
+
+ // If we didn't find any pair that has at least minCommonTailLength
+ // instructions in common, remove all blocks with this hash code and retry.
+ if (SameTails.empty()) {
+ RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
continue;
}
-
- // Determine the actual length of the shared tail between these two basic
- // blocks. Because the hash can have collisions, it's possible that this is
- // less than 2.
- MachineBasicBlock::iterator BBI1, BBI2;
- unsigned CommonTailLen =
- ComputeCommonTailLength(CurMBB, (MergePotentials.end()-2)->second,
- BBI1, BBI2);
-
- // If the tails don't have at least two instructions in common, see if there
- // is anything else in the equivalence class that does match.
- if (CommonTailLen < 2) {
- unsigned FoundMatch = ~0U;
- for (int i = MergePotentials.size()-2;
- i != -1 && MergePotentials[i].first == CurHash; --i) {
- CommonTailLen = ComputeCommonTailLength(CurMBB,
- MergePotentials[i].second,
- BBI1, BBI2);
- if (CommonTailLen >= 2) {
- FoundMatch = i;
+
+ // If one of the blocks is the entire common tail (and not the entry
+ // block, which we can't jump to), we can treat all blocks with this same
+ // tail at once. Use PredBB if that is one of the possibilities, as that
+ // will not introduce any extra branches.
+ MachineBasicBlock *EntryBB = MergePotentials.begin()->second->
+ getParent()->begin();
+ unsigned int commonTailIndex, i;
+ for (commonTailIndex=SameTails.size(), i=0; i<SameTails.size(); i++) {
+ MachineBasicBlock *MBB = SameTails[i].first->second;
+ if (MBB->begin() == SameTails[i].second && MBB != EntryBB) {
+ commonTailIndex = i;
+ if (MBB==PredBB)
break;
- }
- }
-
- // If we didn't find anything that has at least two instructions matching
- // this one, bail out.
- if (FoundMatch == ~0U) {
- MergePotentials.pop_back();
- continue;
}
-
- // Otherwise, move the matching block to the right position.
- std::swap(MergePotentials[FoundMatch], *(MergePotentials.end()-2));
}
- MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second;
+ if (commonTailIndex==SameTails.size()) {
+ // None of the blocks consist entirely of the common tail.
+ // Split a block so that one does.
+ commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength);
+ }
- // If neither block is the entire common tail, split the tail of one block
- // to make it redundant with the other tail.
- if (CurMBB->begin() != BBI1 && MBB2->begin() != BBI2) {
- if (0) { // Enable this to disable partial tail merges.
- MergePotentials.pop_back();
+ MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second;
+ // MBB is common tail. Adjust all other BB's to jump to this one.
+ // Traversal must be forwards so erases work.
+ DOUT << "\nUsing common tail " << MBB->getNumber() << " for ";
+ for (unsigned int i=0; i<SameTails.size(); ++i) {
+ if (commonTailIndex==i)
continue;
- }
-
- // Decide whether we want to split CurMBB or MBB2.
- if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII)) {
- CurMBB = SplitMBBAt(*CurMBB, BBI1);
- BBI1 = CurMBB->begin();
- MergePotentials.back().second = CurMBB;
- } else {
- MBB2 = SplitMBBAt(*MBB2, BBI2);
- BBI2 = MBB2->begin();
- (MergePotentials.end()-2)->second = MBB2;
- }
- }
-
- if (MBB2->begin() == BBI2) {
- // Hack the end off CurMBB, making it jump to MBBI@ instead.
- ReplaceTailWithBranchTo(BBI1, MBB2);
- // This modifies CurMBB, so remove it from the worklist.
- MergePotentials.pop_back();
- } else {
- assert(CurMBB->begin() == BBI1 && "Didn't split block correctly?");
- // Hack the end off MBB2, making it jump to CurMBB instead.
- ReplaceTailWithBranchTo(BBI2, CurMBB);
- // This modifies MBB2, so remove it from the worklist.
- MergePotentials.erase(MergePotentials.end()-2);
+ DOUT << SameTails[i].first->second->getNumber() << ",";
+ // Hack the end off BB i, making it jump to BB commonTailIndex instead.
+ ReplaceTailWithBranchTo(SameTails[i].second, MBB);
+ // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
+ MergePotentials.erase(SameTails[i].first);
}
+ DOUT << "\n";
+ // We leave commonTailIndex in the worklist in case there are other blocks
+ // that match it with a smaller number of instructions.
MadeChange = true;
}
-
return MadeChange;
}
+bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
+
+ if (!EnableTailMerge) return false;
+
+ MadeChange = false;
+
+ // First find blocks with no successors.
+ MergePotentials.clear();
+ for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
+ if (I->succ_empty())
+ MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I));
+ }
+ // See if we can do any tail merging on those.
+ if (MergePotentials.size() < TailMergeThreshold &&
+ MergePotentials.size() >= 2)
+ MadeChange |= TryMergeBlocks(NULL, NULL);
+
+ // Look at blocks (IBB) with multiple predecessors (PBB).
+ // We change each predecessor to a canonical form, by
+ // (1) temporarily removing any unconditional branch from the predecessor
+ // to IBB, and
+ // (2) alter conditional branches so they branch to the other block
+ // not IBB; this may require adding back an unconditional branch to IBB
+ // later, where there wasn't one coming in. E.g.
+ // Bcc IBB
+ // fallthrough to QBB
+ // here becomes
+ // Bncc QBB
+ // with a conceptual B to IBB after that, which never actually exists.
+ // With those changes, we see whether the predecessors' tails match,
+ // and merge them if so. We change things out of canonical form and
+ // back to the way they were later in the process. (OptimizeBranches
+ // would undo some of this, but we can't use it, because we'd get into
+ // a compile-time infinite loop repeatedly doing and undoing the same
+ // transformations.)
+
+ for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
+ if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) {
+ MachineBasicBlock *IBB = I;
+ MachineBasicBlock *PredBB = prior(I);
+ MergePotentials.clear();
+ for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
+ E2 = I->pred_end();
+ P != E2; ++P) {
+ MachineBasicBlock* PBB = *P;
+ // Skip blocks that loop to themselves, can't tail merge these.
+ if (PBB==IBB)
+ continue;
+ MachineBasicBlock *TBB = 0, *FBB = 0;
+ SmallVector<MachineOperand, 4> Cond;
+ if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
+ // Failing case: IBB is the target of a cbr, and
+ // we cannot reverse the branch.
+ SmallVector<MachineOperand, 4> NewCond(Cond);
+ if (!Cond.empty() && TBB==IBB) {
+ if (TII->ReverseBranchCondition(NewCond))
+ continue;
+ // This is the QBB case described above
+ if (!FBB)
+ FBB = next(MachineFunction::iterator(PBB));
+ }
+ // Failing case: the only way IBB can be reached from PBB is via
+ // exception handling. Happens for landing pads. Would be nice
+ // to have a bit in the edge so we didn't have to do all this.
+ if (IBB->isLandingPad()) {
+ MachineFunction::iterator IP = PBB; IP++;
+ MachineBasicBlock* PredNextBB = NULL;
+ if (IP!=MF.end())
+ PredNextBB = IP;
+ if (TBB==NULL) {
+ if (IBB!=PredNextBB) // fallthrough
+ continue;
+ } else if (FBB) {
+ if (TBB!=IBB && FBB!=IBB) // cbr then ubr
+ continue;
+ } else if (Cond.empty()) {
+ if (TBB!=IBB) // ubr
+ continue;
+ } else {
+ if (TBB!=IBB && IBB!=PredNextBB) // cbr
+ continue;
+ }
+ }
+ // Remove the unconditional branch at the end, if any.
+ if (TBB && (Cond.empty() || FBB)) {
+ TII->RemoveBranch(*PBB);
+ if (!Cond.empty())
+ // reinsert conditional branch only, for now
+ TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond);
+ }
+ MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P));
+ }
+ }
+ if (MergePotentials.size() >= 2)
+ MadeChange |= TryMergeBlocks(I, PredBB);
+ // Reinsert an unconditional branch if needed.
+ // The 1 below can occur as a result of removing blocks in TryMergeBlocks.
+ PredBB = prior(I); // this may have been changed in TryMergeBlocks
+ if (MergePotentials.size()==1 &&
+ MergePotentials.begin()->second != PredBB)
+ FixTail(MergePotentials.begin()->second, I, TII);
+ }
+ }
+ return MadeChange;
+}
//===----------------------------------------------------------------------===//
// Branch Optimization
}
-/// CorrectExtraCFGEdges - Various pieces of code can cause excess edges in the
-/// CFG to be inserted. If we have proven that MBB can only branch to DestA and
-/// DestB, remove any other MBB successors from the CFG. DestA and DestB can
-/// be null.
-static bool CorrectExtraCFGEdges(MachineBasicBlock &MBB,
- MachineBasicBlock *DestA,
- MachineBasicBlock *DestB,
- bool isCond,
- MachineFunction::iterator FallThru) {
- bool MadeChange = false;
- bool AddedFallThrough = false;
-
- // If this block ends with a conditional branch that falls through to its
- // successor, set DestB as the successor.
- if (isCond) {
- if (DestB == 0 && FallThru != MBB.getParent()->end()) {
- DestB = FallThru;
- AddedFallThrough = true;
- }
- } else {
- // If this is an unconditional branch with no explicit dest, it must just be
- // a fallthrough into DestB.
- if (DestA == 0 && FallThru != MBB.getParent()->end()) {
- DestA = FallThru;
- AddedFallThrough = true;
- }
- }
-
- MachineBasicBlock::pred_iterator SI = MBB.succ_begin();
- while (SI != MBB.succ_end()) {
- if (*SI == DestA) {
- DestA = 0;
- ++SI;
- } else if (*SI == DestB) {
- DestB = 0;
- ++SI;
- } else if ((*SI)->isLandingPad()) {
- ++SI;
- } else {
- // Otherwise, this is a superfluous edge, remove it.
- MBB.removeSuccessor(SI);
- MadeChange = true;
- }
- }
- if (!AddedFallThrough) {
- assert(DestA == 0 && DestB == 0 &&
- "MachineCFG is missing edges!");
- } else if (isCond) {
- assert(DestA == 0 && "MachineCFG is missing edges!");
- }
- return MadeChange;
-}
-
-
-/// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to
-/// 'Old', change the code and CFG so that it branches to 'New' instead.
-static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB,
- MachineBasicBlock *Old,
- MachineBasicBlock *New,
- const TargetInstrInfo *TII) {
- assert(Old != New && "Cannot replace self with self!");
-
- MachineBasicBlock::iterator I = BB->end();
- while (I != BB->begin()) {
- --I;
- if (!TII->isTerminatorInstr(I->getOpcode())) break;
-
- // Scan the operands of this machine instruction, replacing any uses of Old
- // with New.
- for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
- if (I->getOperand(i).isMachineBasicBlock() &&
- I->getOperand(i).getMachineBasicBlock() == Old)
- I->getOperand(i).setMachineBasicBlock(New);
- }
-
- // Update the successor information.
- std::vector<MachineBasicBlock*> Succs(BB->succ_begin(), BB->succ_end());
- for (int i = Succs.size()-1; i >= 0; --i)
- if (Succs[i] == Old) {
- BB->removeSuccessor(Old);
- BB->addSuccessor(New);
- }
-}
-
/// CanFallThrough - Return true if the specified block (with the specified
/// branch condition) can implicitly transfer control to the block after it by
/// falling off the end of it. This should return false if it can reach the
///
bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB,
bool BranchUnAnalyzable,
- MachineBasicBlock *TBB, MachineBasicBlock *FBB,
- const std::vector<MachineOperand> &Cond) {
+ MachineBasicBlock *TBB,
+ MachineBasicBlock *FBB,
+ const SmallVectorImpl<MachineOperand> &Cond) {
MachineFunction::iterator Fallthrough = CurBB;
++Fallthrough;
// If FallthroughBlock is off the end of the function, it can't fall through.
///
bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) {
MachineBasicBlock *TBB = 0, *FBB = 0;
- std::vector<MachineOperand> Cond;
- bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond);
+ SmallVector<MachineOperand, 4> Cond;
+ bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond, true);
return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond);
}
/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
/// result in infinite loops.
static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
- MachineBasicBlock *MBB2,
- const TargetInstrInfo &TII) {
+ MachineBasicBlock *MBB2) {
// Right now, we use a simple heuristic. If MBB2 ends with a call, and
// MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
// optimize branches that branch to either a return block or an assert block
// into a fallthrough to the return.
if (MBB1->empty() || MBB2->empty()) return false;
+
+ // If there is a clear successor ordering we make sure that one block
+ // will fall through to the next
+ if (MBB1->isSuccessor(MBB2)) return true;
+ if (MBB2->isSuccessor(MBB1)) return false;
MachineInstr *MBB1I = --MBB1->end();
MachineInstr *MBB2I = --MBB2->end();
- return TII.isCall(MBB2I->getOpcode()) && !TII.isCall(MBB1I->getOpcode());
+ return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
}
/// OptimizeBlock - Analyze and optimize control flow related to the specified
++FallThrough;
// If this block is empty, make everyone use its fall-through, not the block
- // explicitly.
- if (MBB->empty()) {
+ // explicitly. Landing pads should not do this since the landing-pad table
+ // points to this block.
+ if (MBB->empty() && !MBB->isLandingPad()) {
// Dead block? Leave for cleanup later.
if (MBB->pred_empty()) return;
// instead.
while (!MBB->pred_empty()) {
MachineBasicBlock *Pred = *(MBB->pred_end()-1);
- ReplaceUsesOfBlockWith(Pred, MBB, FallThrough, TII);
+ Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
}
// If MBB was the target of a jump table, update jump tables to go to the
MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
- std::vector<MachineOperand> PriorCond;
+ SmallVector<MachineOperand, 4> PriorCond;
bool PriorUnAnalyzable =
- TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
+ TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
if (!PriorUnAnalyzable) {
// If the CFG for the prior block has extra edges, remove them.
- MadeChange |= CorrectExtraCFGEdges(PrevBB, PriorTBB, PriorFBB,
- !PriorCond.empty(), MBB);
+ 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
// if the branch condition is reversible, reverse the branch to create a
// fall-through.
if (PriorTBB == MBB) {
- std::vector<MachineOperand> NewPriorCond(PriorCond);
+ SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
if (!TII->ReverseBranchCondition(NewPriorCond)) {
TII->RemoveBranch(PrevBB);
TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
// last. Only do the swap if one is clearly better to fall through than
// the other.
if (FallThrough == --MBB->getParent()->end() &&
- !IsBetterFallthrough(PriorTBB, MBB, *TII))
+ !IsBetterFallthrough(PriorTBB, MBB))
DoTransform = false;
// We don't want to do this transformation if we have control flow like:
if (DoTransform) {
// Reverse the branch so we will fall through on the previous true cond.
- std::vector<MachineOperand> NewPriorCond(PriorCond);
+ SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
if (!TII->ReverseBranchCondition(NewPriorCond)) {
DOUT << "\nMoving MBB: " << *MBB;
DOUT << "To make fallthrough to: " << *PriorTBB << "\n";
// Analyze the branch in the current block.
MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
- std::vector<MachineOperand> CurCond;
- bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond);
+ 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 |= CorrectExtraCFGEdges(*MBB, CurTBB, CurFBB,
- !CurCond.empty(),
- ++MachineFunction::iterator(MBB));
+ 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:
// we want:
// Loop: xxx; jncc Loop; jmp Out
if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
- std::vector<MachineOperand> NewCond(CurCond);
+ SmallVector<MachineOperand, 4> NewCond(CurCond);
if (!TII->ReverseBranchCondition(NewCond)) {
TII->RemoveBranch(*MBB);
TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
// 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 &&
- TII->isBranch(MBB->begin()->getOpcode()) && CurTBB != MBB) {
+ MBB->begin()->getDesc().isBranch() && CurTBB != MBB) {
// This block may contain just an unconditional branch. Because there can
// be 'non-branch terminators' in the block, try removing the branch and
// then seeing if the block is empty.
}
// Iterate through all the predecessors, revectoring each in-turn.
- MachineBasicBlock::pred_iterator PI = MBB->pred_begin();
+ size_t PI = 0;
bool DidChange = false;
bool HasBranchToSelf = false;
- while (PI != MBB->pred_end()) {
- if (*PI == MBB) {
+ 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;
- ReplaceUsesOfBlockWith(*PI, MBB, CurTBB, TII);
+ PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
+ // If this change resulted in PMBB ending in a conditional
+ // branch where both conditions go to the same destination,
+ // change this to an unconditional branch (and fix the CFG).
+ MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
+ SmallVector<MachineOperand, 4> NewCurCond;
+ bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
+ NewCurFBB, NewCurCond, true);
+ if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
+ TII->RemoveBranch(*PMBB);
+ NewCurCond.clear();
+ TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond);
+ MadeChange = true;
+ ++NumBranchOpts;
+ PMBB->CorrectExtraCFGEdges(NewCurTBB, NewCurFBB, false);
+ }
}
}
// Now we know that there was no fall-through into this block, check to
// see if it has a fall-through into its successor.
bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB,
- CurCond);
+ CurCond);
if (!MBB->isLandingPad()) {
// Check all the predecessors of this block. If one of them has no fall
MachineBasicBlock *PredBB = *PI;
MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
if (PredBB != MBB && !CanFallThrough(PredBB)
+ && (!CurFallsThru || !CurTBB || !CurFBB)
&& (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
// If the current block doesn't fall through, just move it.
// If the current block can fall through and does not end with a
// conditional branch, we need to append an unconditional jump to
// the (current) next block. To avoid a possible compile-time
// infinite loop, move blocks only backward in this case.
+ // Also, if there are already 2 branches here, we cannot add a third;
+ // this means we have the case
+ // Bcc next
+ // B elsewhere
+ // next:
if (CurFallsThru) {
MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB));
CurCond.clear();
MachineBasicBlock *SuccBB = *SI;
MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
std::vector<MachineOperand> SuccPrevCond;
- if (SuccBB != MBB && !CanFallThrough(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 && !CanFallThrough(SuccPrev) &&
+ !SuccBB->isLandingPad()) {
MBB->moveBefore(SuccBB);
MadeChange = true;
return OptimizeBlock(MBB);
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