llvm-6502/lib/CodeGen/BranchFolding.cpp
David Greene 465e2b950d Change errs() to dbgs().
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92097 91177308-0d34-0410-b5e6-96231b3b80d8
2009-12-24 00:34:21 +00:00

1291 lines
50 KiB
C++

//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass forwards branches to unconditional branches to make them branch
// directly to the target block. This pass often results in dead MBB's, which
// it then removes.
//
// Note that this pass must be run after register allocation, it cannot handle
// SSA form.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "branchfolding"
#include "BranchFolding.h"
#include "llvm/Function.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineJumpTableInfo.h"
#include "llvm/CodeGen/RegisterScavenging.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include <algorithm>
using namespace llvm;
STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
STATISTIC(NumBranchOpts, "Number of branches optimized");
STATISTIC(NumTailMerge , "Number of block tails merged");
static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
cl::init(cl::BOU_UNSET), cl::Hidden);
// Throttle for huge numbers of predecessors (compile speed problems)
static cl::opt<unsigned>
TailMergeThreshold("tail-merge-threshold",
cl::desc("Max number of predecessors to consider tail merging"),
cl::init(150), cl::Hidden);
// Heuristic for tail merging (and, inversely, tail duplication).
// TODO: This should be replaced with a target query.
static cl::opt<unsigned>
TailMergeSize("tail-merge-size",
cl::desc("Min number of instructions to consider tail merging"),
cl::init(3), cl::Hidden);
namespace {
/// BranchFolderPass - Wrap branch folder in a machine function pass.
class BranchFolderPass : public MachineFunctionPass,
public BranchFolder {
public:
static char ID;
explicit BranchFolderPass(bool defaultEnableTailMerge)
: MachineFunctionPass(&ID), BranchFolder(defaultEnableTailMerge) {}
virtual bool runOnMachineFunction(MachineFunction &MF);
virtual const char *getPassName() const { return "Control Flow Optimizer"; }
};
}
char BranchFolderPass::ID = 0;
FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
return new BranchFolderPass(DefaultEnableTailMerge);
}
bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
return OptimizeFunction(MF,
MF.getTarget().getInstrInfo(),
MF.getTarget().getRegisterInfo(),
getAnalysisIfAvailable<MachineModuleInfo>());
}
BranchFolder::BranchFolder(bool defaultEnableTailMerge) {
switch (FlagEnableTailMerge) {
case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
case cl::BOU_TRUE: EnableTailMerge = true; break;
case cl::BOU_FALSE: EnableTailMerge = false; break;
}
}
/// RemoveDeadBlock - Remove the specified dead machine basic block from the
/// function, updating the CFG.
void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
assert(MBB->pred_empty() && "MBB must be dead!");
DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
MachineFunction *MF = MBB->getParent();
// drop all successors.
while (!MBB->succ_empty())
MBB->removeSuccessor(MBB->succ_end()-1);
// If there are any labels in the basic block, unregister them from
// MachineModuleInfo.
if (MMI && !MBB->empty()) {
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
I != E; ++I) {
if (I->isLabel())
// The label ID # is always operand #0, an immediate.
MMI->InvalidateLabel(I->getOperand(0).getImm());
}
}
// Remove the block.
MF->erase(MBB);
}
/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
/// followed by terminators, and if the implicitly defined registers are not
/// used by the terminators, remove those implicit_def's. e.g.
/// BB1:
/// r0 = implicit_def
/// r1 = implicit_def
/// br
/// This block can be optimized away later if the implicit instructions are
/// removed.
bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
SmallSet<unsigned, 4> ImpDefRegs;
MachineBasicBlock::iterator I = MBB->begin();
while (I != MBB->end()) {
if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF)
break;
unsigned Reg = I->getOperand(0).getReg();
ImpDefRegs.insert(Reg);
for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
unsigned SubReg = *SubRegs; ++SubRegs)
ImpDefRegs.insert(SubReg);
++I;
}
if (ImpDefRegs.empty())
return false;
MachineBasicBlock::iterator FirstTerm = I;
while (I != MBB->end()) {
if (!TII->isUnpredicatedTerminator(I))
return false;
// See if it uses any of the implicitly defined registers.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
MachineOperand &MO = I->getOperand(i);
if (!MO.isReg() || !MO.isUse())
continue;
unsigned Reg = MO.getReg();
if (ImpDefRegs.count(Reg))
return false;
}
++I;
}
I = MBB->begin();
while (I != FirstTerm) {
MachineInstr *ImpDefMI = &*I;
++I;
MBB->erase(ImpDefMI);
}
return true;
}
/// OptimizeFunction - Perhaps branch folding, tail merging and other
/// CFG optimizations on the given function.
bool BranchFolder::OptimizeFunction(MachineFunction &MF,
const TargetInstrInfo *tii,
const TargetRegisterInfo *tri,
MachineModuleInfo *mmi) {
if (!tii) return false;
TII = tii;
TRI = tri;
MMI = mmi;
RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
// Fix CFG. The later algorithms expect it to be right.
bool MadeChange = false;
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
SmallVector<MachineOperand, 4> Cond;
if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true))
MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
MadeChange |= OptimizeImpDefsBlock(MBB);
}
bool MadeChangeThisIteration = true;
while (MadeChangeThisIteration) {
MadeChangeThisIteration = false;
MadeChangeThisIteration |= TailMergeBlocks(MF);
MadeChangeThisIteration |= OptimizeBranches(MF);
MadeChange |= MadeChangeThisIteration;
}
// See if any jump tables have become mergable or dead as the code generator
// did its thing.
MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables();
if (!JTs.empty()) {
// Figure out how these jump tables should be merged.
std::vector<unsigned> JTMapping;
JTMapping.reserve(JTs.size());
// We always keep the 0th jump table.
JTMapping.push_back(0);
// Scan the jump tables, seeing if there are any duplicates. Note that this
// is N^2, which should be fixed someday.
for (unsigned i = 1, e = JTs.size(); i != e; ++i) {
if (JTs[i].MBBs.empty())
JTMapping.push_back(i);
else
JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs));
}
// If a jump table was merge with another one, walk the function rewriting
// references to jump tables to reference the new JT ID's. Keep track of
// whether we see a jump table idx, if not, we can delete the JT.
BitVector JTIsLive(JTs.size());
for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
BB != E; ++BB) {
for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
I != E; ++I)
for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
MachineOperand &Op = I->getOperand(op);
if (!Op.isJTI()) continue;
unsigned NewIdx = JTMapping[Op.getIndex()];
Op.setIndex(NewIdx);
// Remember that this JT is live.
JTIsLive.set(NewIdx);
}
}
// Finally, remove dead jump tables. This happens either because the
// indirect jump was unreachable (and thus deleted) or because the jump
// table was merged with some other one.
for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
if (!JTIsLive.test(i)) {
JTI->RemoveJumpTable(i);
MadeChange = true;
}
}
delete RS;
return MadeChange;
}
//===----------------------------------------------------------------------===//
// Tail Merging of Blocks
//===----------------------------------------------------------------------===//
/// HashMachineInstr - Compute a hash value for MI and its operands.
static unsigned HashMachineInstr(const MachineInstr *MI) {
unsigned Hash = MI->getOpcode();
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &Op = MI->getOperand(i);
// Merge in bits from the operand if easy.
unsigned OperandHash = 0;
switch (Op.getType()) {
case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
case MachineOperand::MO_MachineBasicBlock:
OperandHash = Op.getMBB()->getNumber();
break;
case MachineOperand::MO_FrameIndex:
case MachineOperand::MO_ConstantPoolIndex:
case MachineOperand::MO_JumpTableIndex:
OperandHash = Op.getIndex();
break;
case MachineOperand::MO_GlobalAddress:
case MachineOperand::MO_ExternalSymbol:
// Global address / external symbol are too hard, don't bother, but do
// pull in the offset.
OperandHash = Op.getOffset();
break;
default: break;
}
Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
}
return Hash;
}
/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks
/// with no successors, we hash two instructions, because cross-jumping
/// only saves code when at least two instructions are removed (since a
/// branch must be inserted). For blocks with a successor, one of the
/// two blocks to be tail-merged will end with a branch already, so
/// it gains to cross-jump even for one instruction.
static unsigned HashEndOfMBB(const MachineBasicBlock *MBB,
unsigned minCommonTailLength) {
MachineBasicBlock::const_iterator I = MBB->end();
if (I == MBB->begin())
return 0; // Empty MBB.
--I;
unsigned Hash = HashMachineInstr(I);
if (I == MBB->begin() || minCommonTailLength == 1)
return Hash; // Single instr MBB.
--I;
// Hash in the second-to-last instruction.
Hash ^= HashMachineInstr(I) << 2;
return Hash;
}
/// ComputeCommonTailLength - Given two machine basic blocks, compute the number
/// of instructions they actually have in common together at their end. Return
/// iterators for the first shared instruction in each block.
static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
MachineBasicBlock *MBB2,
MachineBasicBlock::iterator &I1,
MachineBasicBlock::iterator &I2) {
I1 = MBB1->end();
I2 = MBB2->end();
unsigned TailLen = 0;
while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
--I1; --I2;
if (!I1->isIdenticalTo(I2) ||
// FIXME: This check is dubious. It's used to get around a problem where
// people incorrectly expect inline asm directives to remain in the same
// relative order. This is untenable because normal compiler
// optimizations (like this one) may reorder and/or merge these
// directives.
I1->getOpcode() == TargetInstrInfo::INLINEASM) {
++I1; ++I2;
break;
}
++TailLen;
}
return TailLen;
}
/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
/// after it, replacing it with an unconditional branch to NewDest. This
/// returns true if OldInst's block is modified, false if NewDest is modified.
void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
MachineBasicBlock *NewDest) {
MachineBasicBlock *OldBB = OldInst->getParent();
// Remove all the old successors of OldBB from the CFG.
while (!OldBB->succ_empty())
OldBB->removeSuccessor(OldBB->succ_begin());
// Remove all the dead instructions from the end of OldBB.
OldBB->erase(OldInst, OldBB->end());
// If OldBB isn't immediately before OldBB, insert a branch to it.
if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest))
TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>());
OldBB->addSuccessor(NewDest);
++NumTailMerge;
}
/// SplitMBBAt - Given a machine basic block and an iterator into it, split the
/// MBB so that the part before the iterator falls into the part starting at the
/// iterator. This returns the new MBB.
MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
MachineBasicBlock::iterator BBI1) {
MachineFunction &MF = *CurMBB.getParent();
// Create the fall-through block.
MachineFunction::iterator MBBI = &CurMBB;
MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock());
CurMBB.getParent()->insert(++MBBI, NewMBB);
// Move all the successors of this block to the specified block.
NewMBB->transferSuccessors(&CurMBB);
// Add an edge from CurMBB to NewMBB for the fall-through.
CurMBB.addSuccessor(NewMBB);
// Splice the code over.
NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
// For targets that use the register scavenger, we must maintain LiveIns.
if (RS) {
RS->enterBasicBlock(&CurMBB);
if (!CurMBB.empty())
RS->forward(prior(CurMBB.end()));
BitVector RegsLiveAtExit(TRI->getNumRegs());
RS->getRegsUsed(RegsLiveAtExit, false);
for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++)
if (RegsLiveAtExit[i])
NewMBB->addLiveIn(i);
}
return NewMBB;
}
/// EstimateRuntime - Make a rough estimate for how long it will take to run
/// the specified code.
static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
MachineBasicBlock::iterator E) {
unsigned Time = 0;
for (; I != E; ++I) {
const TargetInstrDesc &TID = I->getDesc();
if (TID.isCall())
Time += 10;
else if (TID.mayLoad() || TID.mayStore())
Time += 2;
else
++Time;
}
return Time;
}
// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
// branches temporarily for tail merging). In the case where CurMBB ends
// with a conditional branch to the next block, optimize by reversing the
// test and conditionally branching to SuccMBB instead.
static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
const TargetInstrInfo *TII) {
MachineFunction *MF = CurMBB->getParent();
MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB));
MachineBasicBlock *TBB = 0, *FBB = 0;
SmallVector<MachineOperand, 4> Cond;
if (I != MF->end() &&
!TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
MachineBasicBlock *NextBB = I;
if (TBB == NextBB && !Cond.empty() && !FBB) {
if (!TII->ReverseBranchCondition(Cond)) {
TII->RemoveBranch(*CurMBB);
TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond);
return;
}
}
}
TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>());
}
bool
BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
if (getHash() < o.getHash())
return true;
else if (getHash() > o.getHash())
return false;
else if (getBlock()->getNumber() < o.getBlock()->getNumber())
return true;
else if (getBlock()->getNumber() > o.getBlock()->getNumber())
return false;
else {
// _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
// an object with itself.
#ifndef _GLIBCXX_DEBUG
llvm_unreachable("Predecessor appears twice");
#endif
return false;
}
}
/// CountTerminators - Count the number of terminators in the given
/// block and set I to the position of the first non-terminator, if there
/// is one, or MBB->end() otherwise.
static unsigned CountTerminators(MachineBasicBlock *MBB,
MachineBasicBlock::iterator &I) {
I = MBB->end();
unsigned NumTerms = 0;
for (;;) {
if (I == MBB->begin()) {
I = MBB->end();
break;
}
--I;
if (!I->getDesc().isTerminator()) break;
++NumTerms;
}
return NumTerms;
}
/// ProfitableToMerge - Check if two machine basic blocks have a common tail
/// and decide if it would be profitable to merge those tails. Return the
/// length of the common tail and iterators to the first common instruction
/// in each block.
static bool ProfitableToMerge(MachineBasicBlock *MBB1,
MachineBasicBlock *MBB2,
unsigned minCommonTailLength,
unsigned &CommonTailLen,
MachineBasicBlock::iterator &I1,
MachineBasicBlock::iterator &I2,
MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB) {
CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
MachineFunction *MF = MBB1->getParent();
if (CommonTailLen == 0)
return false;
// It's almost always profitable to merge any number of non-terminator
// instructions with the block that falls through into the common successor.
if (MBB1 == PredBB || MBB2 == PredBB) {
MachineBasicBlock::iterator I;
unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
if (CommonTailLen > NumTerms)
return true;
}
// If one of the blocks can be completely merged and happens to be in
// a position where the other could fall through into it, merge any number
// of instructions, because it can be done without a branch.
// TODO: If the blocks are not adjacent, move one of them so that they are?
if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
return true;
if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
return true;
// If both blocks have an unconditional branch temporarily stripped out,
// count that as an additional common instruction for the following
// heuristics.
unsigned EffectiveTailLen = CommonTailLen;
if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
!MBB1->back().getDesc().isBarrier() &&
!MBB2->back().getDesc().isBarrier())
++EffectiveTailLen;
// Check if the common tail is long enough to be worthwhile.
if (EffectiveTailLen >= minCommonTailLength)
return true;
// If we are optimizing for code size, 2 instructions in common is enough if
// we don't have to split a block. At worst we will be introducing 1 new
// branch instruction, which is likely to be smaller than the 2
// instructions that would be deleted in the merge.
if (EffectiveTailLen >= 2 &&
MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
(I1 == MBB1->begin() || I2 == MBB2->begin()))
return true;
return false;
}
/// ComputeSameTails - Look through all the blocks in MergePotentials that have
/// hash CurHash (guaranteed to match the last element). Build the vector
/// SameTails of all those that have the (same) largest number of instructions
/// in common of any pair of these blocks. SameTails entries contain an
/// iterator into MergePotentials (from which the MachineBasicBlock can be
/// found) and a MachineBasicBlock::iterator into that MBB indicating the
/// instruction where the matching code sequence begins.
/// Order of elements in SameTails is the reverse of the order in which
/// those blocks appear in MergePotentials (where they are not necessarily
/// consecutive).
unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
unsigned minCommonTailLength,
MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB) {
unsigned maxCommonTailLength = 0U;
SameTails.clear();
MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
MPIterator HighestMPIter = prior(MergePotentials.end());
for (MPIterator CurMPIter = prior(MergePotentials.end()),
B = MergePotentials.begin();
CurMPIter != B && CurMPIter->getHash() == CurHash;
--CurMPIter) {
for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) {
unsigned CommonTailLen;
if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
minCommonTailLength,
CommonTailLen, TrialBBI1, TrialBBI2,
SuccBB, PredBB)) {
if (CommonTailLen > maxCommonTailLength) {
SameTails.clear();
maxCommonTailLength = CommonTailLen;
HighestMPIter = CurMPIter;
SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
}
if (HighestMPIter == CurMPIter &&
CommonTailLen == maxCommonTailLength)
SameTails.push_back(SameTailElt(I, TrialBBI2));
}
if (I == B)
break;
}
}
return maxCommonTailLength;
}
/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
/// MergePotentials, restoring branches at ends of blocks as appropriate.
void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB) {
MPIterator CurMPIter, B;
for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin();
CurMPIter->getHash() == CurHash;
--CurMPIter) {
// Put the unconditional branch back, if we need one.
MachineBasicBlock *CurMBB = CurMPIter->getBlock();
if (SuccBB && CurMBB != PredBB)
FixTail(CurMBB, SuccBB, TII);
if (CurMPIter == B)
break;
}
if (CurMPIter->getHash() != CurHash)
CurMPIter++;
MergePotentials.erase(CurMPIter, MergePotentials.end());
}
/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
/// only of the common tail. Create a block that does by splitting one.
unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
unsigned maxCommonTailLength) {
unsigned commonTailIndex = 0;
unsigned TimeEstimate = ~0U;
for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
// Use PredBB if possible; that doesn't require a new branch.
if (SameTails[i].getBlock() == PredBB) {
commonTailIndex = i;
break;
}
// Otherwise, make a (fairly bogus) choice based on estimate of
// how long it will take the various blocks to execute.
unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
SameTails[i].getTailStartPos());
if (t <= TimeEstimate) {
TimeEstimate = t;
commonTailIndex = i;
}
}
MachineBasicBlock::iterator BBI =
SameTails[commonTailIndex].getTailStartPos();
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
<< maxCommonTailLength);
MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
SameTails[commonTailIndex].setBlock(newMBB);
SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
// If we split PredBB, newMBB is the new predecessor.
if (PredBB == MBB)
PredBB = newMBB;
return commonTailIndex;
}
// See if any of the blocks in MergePotentials (which all have a common single
// successor, or all have no successor) can be tail-merged. If there is a
// successor, any blocks in MergePotentials that are not tail-merged and
// are not immediately before Succ must have an unconditional branch to
// Succ added (but the predecessor/successor lists need no adjustment).
// The lone predecessor of Succ that falls through into Succ,
// if any, is given in PredBB.
bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB) {
bool MadeChange = false;
// Except for the special cases below, tail-merge if there are at least
// this many instructions in common.
unsigned minCommonTailLength = TailMergeSize;
DEBUG(dbgs() << "\nTryTailMergeBlocks: ";
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
<< (i == e-1 ? "" : ", ");
dbgs() << "\n";
if (SuccBB) {
dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n';
if (PredBB)
dbgs() << " which has fall-through from BB#"
<< PredBB->getNumber() << "\n";
}
dbgs() << "Looking for common tails of at least "
<< minCommonTailLength << " instruction"
<< (minCommonTailLength == 1 ? "" : "s") << '\n';
);
// Sort by hash value so that blocks with identical end sequences sort
// together.
std::stable_sort(MergePotentials.begin(), MergePotentials.end());
// Walk through equivalence sets looking for actual exact matches.
while (MergePotentials.size() > 1) {
unsigned CurHash = MergePotentials.back().getHash();
// Build SameTails, identifying the set of blocks with this hash code
// and with the maximum number of instructions in common.
unsigned maxCommonTailLength = ComputeSameTails(CurHash,
minCommonTailLength,
SuccBB, PredBB);
// If we didn't find any pair that has at least minCommonTailLength
// instructions in common, remove all blocks with this hash code and retry.
if (SameTails.empty()) {
RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
continue;
}
// If one of the blocks is the entire common tail (and not the entry
// block, which we can't jump to), we can treat all blocks with this same
// tail at once. Use PredBB if that is one of the possibilities, as that
// will not introduce any extra branches.
MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
getParent()->begin();
unsigned commonTailIndex = SameTails.size();
// If there are two blocks, check to see if one can be made to fall through
// into the other.
if (SameTails.size() == 2 &&
SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
SameTails[1].tailIsWholeBlock())
commonTailIndex = 1;
else if (SameTails.size() == 2 &&
SameTails[1].getBlock()->isLayoutSuccessor(
SameTails[0].getBlock()) &&
SameTails[0].tailIsWholeBlock())
commonTailIndex = 0;
else {
// Otherwise just pick one, favoring the fall-through predecessor if
// there is one.
for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
MachineBasicBlock *MBB = SameTails[i].getBlock();
if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
continue;
if (MBB == PredBB) {
commonTailIndex = i;
break;
}
if (SameTails[i].tailIsWholeBlock())
commonTailIndex = i;
}
}
if (commonTailIndex == SameTails.size() ||
(SameTails[commonTailIndex].getBlock() == PredBB &&
!SameTails[commonTailIndex].tailIsWholeBlock())) {
// None of the blocks consist entirely of the common tail.
// Split a block so that one does.
commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength);
}
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
// MBB is common tail. Adjust all other BB's to jump to this one.
// Traversal must be forwards so erases work.
DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber()
<< " for ");
for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
if (commonTailIndex == i)
continue;
DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
<< (i == e-1 ? "" : ", "));
// Hack the end off BB i, making it jump to BB commonTailIndex instead.
ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
// BB i is no longer a predecessor of SuccBB; remove it from the worklist.
MergePotentials.erase(SameTails[i].getMPIter());
}
DEBUG(dbgs() << "\n");
// We leave commonTailIndex in the worklist in case there are other blocks
// that match it with a smaller number of instructions.
MadeChange = true;
}
return MadeChange;
}
bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
if (!EnableTailMerge) return false;
bool MadeChange = false;
// First find blocks with no successors.
MergePotentials.clear();
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
if (I->succ_empty())
MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I, 2U), I));
}
// See if we can do any tail merging on those.
if (MergePotentials.size() < TailMergeThreshold &&
MergePotentials.size() >= 2)
MadeChange |= TryTailMergeBlocks(NULL, NULL);
// Look at blocks (IBB) with multiple predecessors (PBB).
// We change each predecessor to a canonical form, by
// (1) temporarily removing any unconditional branch from the predecessor
// to IBB, and
// (2) alter conditional branches so they branch to the other block
// not IBB; this may require adding back an unconditional branch to IBB
// later, where there wasn't one coming in. E.g.
// Bcc IBB
// fallthrough to QBB
// here becomes
// Bncc QBB
// with a conceptual B to IBB after that, which never actually exists.
// With those changes, we see whether the predecessors' tails match,
// and merge them if so. We change things out of canonical form and
// back to the way they were later in the process. (OptimizeBranches
// would undo some of this, but we can't use it, because we'd get into
// a compile-time infinite loop repeatedly doing and undoing the same
// transformations.)
for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
I != E; ++I) {
if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) {
SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
MachineBasicBlock *IBB = I;
MachineBasicBlock *PredBB = prior(I);
MergePotentials.clear();
for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
E2 = I->pred_end();
P != E2; ++P) {
MachineBasicBlock *PBB = *P;
// Skip blocks that loop to themselves, can't tail merge these.
if (PBB == IBB)
continue;
// Visit each predecessor only once.
if (!UniquePreds.insert(PBB))
continue;
MachineBasicBlock *TBB = 0, *FBB = 0;
SmallVector<MachineOperand, 4> Cond;
if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
// Failing case: IBB is the target of a cbr, and
// we cannot reverse the branch.
SmallVector<MachineOperand, 4> NewCond(Cond);
if (!Cond.empty() && TBB == IBB) {
if (TII->ReverseBranchCondition(NewCond))
continue;
// This is the QBB case described above
if (!FBB)
FBB = llvm::next(MachineFunction::iterator(PBB));
}
// Failing case: the only way IBB can be reached from PBB is via
// exception handling. Happens for landing pads. Would be nice
// to have a bit in the edge so we didn't have to do all this.
if (IBB->isLandingPad()) {
MachineFunction::iterator IP = PBB; IP++;
MachineBasicBlock *PredNextBB = NULL;
if (IP != MF.end())
PredNextBB = IP;
if (TBB == NULL) {
if (IBB != PredNextBB) // fallthrough
continue;
} else if (FBB) {
if (TBB != IBB && FBB != IBB) // cbr then ubr
continue;
} else if (Cond.empty()) {
if (TBB != IBB) // ubr
continue;
} else {
if (TBB != IBB && IBB != PredNextBB) // cbr
continue;
}
}
// Remove the unconditional branch at the end, if any.
if (TBB && (Cond.empty() || FBB)) {
TII->RemoveBranch(*PBB);
if (!Cond.empty())
// reinsert conditional branch only, for now
TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond);
}
MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB, 1U),
*P));
}
}
if (MergePotentials.size() >= 2)
MadeChange |= TryTailMergeBlocks(IBB, PredBB);
// Reinsert an unconditional branch if needed.
// The 1 below can occur as a result of removing blocks in TryTailMergeBlocks.
PredBB = prior(I); // this may have been changed in TryTailMergeBlocks
if (MergePotentials.size() == 1 &&
MergePotentials.begin()->getBlock() != PredBB)
FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
}
}
return MadeChange;
}
//===----------------------------------------------------------------------===//
// Branch Optimization
//===----------------------------------------------------------------------===//
bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
bool MadeChange = false;
// Make sure blocks are numbered in order
MF.RenumberBlocks();
for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
MachineBasicBlock *MBB = I++;
MadeChange |= OptimizeBlock(MBB);
// If it is dead, remove it.
if (MBB->pred_empty()) {
RemoveDeadBlock(MBB);
MadeChange = true;
++NumDeadBlocks;
}
}
return MadeChange;
}
/// IsBetterFallthrough - Return true if it would be clearly better to
/// fall-through to MBB1 than to fall through into MBB2. This has to return
/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
/// result in infinite loops.
static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
MachineBasicBlock *MBB2) {
// Right now, we use a simple heuristic. If MBB2 ends with a call, and
// MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
// optimize branches that branch to either a return block or an assert block
// into a fallthrough to the return.
if (MBB1->empty() || MBB2->empty()) return false;
// If there is a clear successor ordering we make sure that one block
// will fall through to the next
if (MBB1->isSuccessor(MBB2)) return true;
if (MBB2->isSuccessor(MBB1)) return false;
MachineInstr *MBB1I = --MBB1->end();
MachineInstr *MBB2I = --MBB2->end();
return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
}
/// OptimizeBlock - Analyze and optimize control flow related to the specified
/// block. This is never called on the entry block.
bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
bool MadeChange = false;
MachineFunction &MF = *MBB->getParent();
ReoptimizeBlock:
MachineFunction::iterator FallThrough = MBB;
++FallThrough;
// If this block is empty, make everyone use its fall-through, not the block
// explicitly. Landing pads should not do this since the landing-pad table
// points to this block. Blocks with their addresses taken shouldn't be
// optimized away.
if (MBB->empty() && !MBB->isLandingPad() && !MBB->hasAddressTaken()) {
// Dead block? Leave for cleanup later.
if (MBB->pred_empty()) return MadeChange;
if (FallThrough == MF.end()) {
// TODO: Simplify preds to not branch here if possible!
} else {
// Rewrite all predecessors of the old block to go to the fallthrough
// instead.
while (!MBB->pred_empty()) {
MachineBasicBlock *Pred = *(MBB->pred_end()-1);
Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
}
// If MBB was the target of a jump table, update jump tables to go to the
// fallthrough instead.
MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, FallThrough);
MadeChange = true;
}
return MadeChange;
}
// Check to see if we can simplify the terminator of the block before this
// one.
MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
SmallVector<MachineOperand, 4> PriorCond;
bool PriorUnAnalyzable =
TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
if (!PriorUnAnalyzable) {
// If the CFG for the prior block has extra edges, remove them.
MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
!PriorCond.empty());
// If the previous branch is conditional and both conditions go to the same
// destination, remove the branch, replacing it with an unconditional one or
// a fall-through.
if (PriorTBB && PriorTBB == PriorFBB) {
TII->RemoveBranch(PrevBB);
PriorCond.clear();
if (PriorTBB != MBB)
TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
MadeChange = true;
++NumBranchOpts;
goto ReoptimizeBlock;
}
// If the previous block unconditionally falls through to this block and
// this block has no other predecessors, move the contents of this block
// into the prior block. This doesn't usually happen when SimplifyCFG
// has been used, but it can happen if tail merging splits a fall-through
// predecessor of a block.
// This has to check PrevBB->succ_size() because EH edges are ignored by
// AnalyzeBranch.
if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
PrevBB.succ_size() == 1 &&
!MBB->hasAddressTaken()) {
DEBUG(dbgs() << "\nMerging into block: " << PrevBB
<< "From MBB: " << *MBB);
PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
PrevBB.removeSuccessor(PrevBB.succ_begin());;
assert(PrevBB.succ_empty());
PrevBB.transferSuccessors(MBB);
MadeChange = true;
return MadeChange;
}
// If the previous branch *only* branches to *this* block (conditional or
// not) remove the branch.
if (PriorTBB == MBB && PriorFBB == 0) {
TII->RemoveBranch(PrevBB);
MadeChange = true;
++NumBranchOpts;
goto ReoptimizeBlock;
}
// If the prior block branches somewhere else on the condition and here if
// the condition is false, remove the uncond second branch.
if (PriorFBB == MBB) {
TII->RemoveBranch(PrevBB);
TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
MadeChange = true;
++NumBranchOpts;
goto ReoptimizeBlock;
}
// If the prior block branches here on true and somewhere else on false, and
// if the branch condition is reversible, reverse the branch to create a
// fall-through.
if (PriorTBB == MBB) {
SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
if (!TII->ReverseBranchCondition(NewPriorCond)) {
TII->RemoveBranch(PrevBB);
TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
MadeChange = true;
++NumBranchOpts;
goto ReoptimizeBlock;
}
}
// If this block has no successors (e.g. it is a return block or ends with
// a call to a no-return function like abort or __cxa_throw) and if the pred
// falls through into this block, and if it would otherwise fall through
// into the block after this, move this block to the end of the function.
//
// We consider it more likely that execution will stay in the function (e.g.
// due to loops) than it is to exit it. This asserts in loops etc, moving
// the assert condition out of the loop body.
if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
MachineFunction::iterator(PriorTBB) == FallThrough &&
!MBB->canFallThrough()) {
bool DoTransform = true;
// We have to be careful that the succs of PredBB aren't both no-successor
// blocks. If neither have successors and if PredBB is the second from
// last block in the function, we'd just keep swapping the two blocks for
// last. Only do the swap if one is clearly better to fall through than
// the other.
if (FallThrough == --MF.end() &&
!IsBetterFallthrough(PriorTBB, MBB))
DoTransform = false;
// We don't want to do this transformation if we have control flow like:
// br cond BB2
// BB1:
// ..
// jmp BBX
// BB2:
// ..
// ret
//
// In this case, we could actually be moving the return block *into* a
// loop!
if (DoTransform && !MBB->succ_empty() &&
(!PriorTBB->canFallThrough() || PriorTBB->empty()))
DoTransform = false;
if (DoTransform) {
// Reverse the branch so we will fall through on the previous true cond.
SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
if (!TII->ReverseBranchCondition(NewPriorCond)) {
DEBUG(dbgs() << "\nMoving MBB: " << *MBB
<< "To make fallthrough to: " << *PriorTBB << "\n");
TII->RemoveBranch(PrevBB);
TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond);
// Move this block to the end of the function.
MBB->moveAfter(--MF.end());
MadeChange = true;
++NumBranchOpts;
return MadeChange;
}
}
}
}
// Analyze the branch in the current block.
MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
SmallVector<MachineOperand, 4> CurCond;
bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
if (!CurUnAnalyzable) {
// If the CFG for the prior block has extra edges, remove them.
MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
// If this is a two-way branch, and the FBB branches to this block, reverse
// the condition so the single-basic-block loop is faster. Instead of:
// Loop: xxx; jcc Out; jmp Loop
// we want:
// Loop: xxx; jncc Loop; jmp Out
if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
SmallVector<MachineOperand, 4> NewCond(CurCond);
if (!TII->ReverseBranchCondition(NewCond)) {
TII->RemoveBranch(*MBB);
TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
MadeChange = true;
++NumBranchOpts;
goto ReoptimizeBlock;
}
}
// If this branch is the only thing in its block, see if we can forward
// other blocks across it.
if (CurTBB && CurCond.empty() && CurFBB == 0 &&
MBB->begin()->getDesc().isBranch() && CurTBB != MBB &&
!MBB->hasAddressTaken()) {
// This block may contain just an unconditional branch. Because there can
// be 'non-branch terminators' in the block, try removing the branch and
// then seeing if the block is empty.
TII->RemoveBranch(*MBB);
// If this block is just an unconditional branch to CurTBB, we can
// usually completely eliminate the block. The only case we cannot
// completely eliminate the block is when the block before this one
// falls through into MBB and we can't understand the prior block's branch
// condition.
if (MBB->empty()) {
bool PredHasNoFallThrough = !PrevBB.canFallThrough();
if (PredHasNoFallThrough || !PriorUnAnalyzable ||
!PrevBB.isSuccessor(MBB)) {
// If the prior block falls through into us, turn it into an
// explicit branch to us to make updates simpler.
if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
PriorTBB != MBB && PriorFBB != MBB) {
if (PriorTBB == 0) {
assert(PriorCond.empty() && PriorFBB == 0 &&
"Bad branch analysis");
PriorTBB = MBB;
} else {
assert(PriorFBB == 0 && "Machine CFG out of date!");
PriorFBB = MBB;
}
TII->RemoveBranch(PrevBB);
TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
}
// Iterate through all the predecessors, revectoring each in-turn.
size_t PI = 0;
bool DidChange = false;
bool HasBranchToSelf = false;
while(PI != MBB->pred_size()) {
MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
if (PMBB == MBB) {
// If this block has an uncond branch to itself, leave it.
++PI;
HasBranchToSelf = true;
} else {
DidChange = true;
PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
// If this change resulted in PMBB ending in a conditional
// branch where both conditions go to the same destination,
// change this to an unconditional branch (and fix the CFG).
MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
SmallVector<MachineOperand, 4> NewCurCond;
bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
NewCurFBB, NewCurCond, true);
if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
TII->RemoveBranch(*PMBB);
NewCurCond.clear();
TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond);
MadeChange = true;
++NumBranchOpts;
PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
}
}
}
// Change any jumptables to go to the new MBB.
MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, CurTBB);
if (DidChange) {
++NumBranchOpts;
MadeChange = true;
if (!HasBranchToSelf) return MadeChange;
}
}
}
// Add the branch back if the block is more than just an uncond branch.
TII->InsertBranch(*MBB, CurTBB, 0, CurCond);
}
}
// If the prior block doesn't fall through into this block, and if this
// block doesn't fall through into some other block, see if we can find a
// place to move this block where a fall-through will happen.
if (!PrevBB.canFallThrough()) {
// Now we know that there was no fall-through into this block, check to
// see if it has a fall-through into its successor.
bool CurFallsThru = MBB->canFallThrough();
if (!MBB->isLandingPad()) {
// Check all the predecessors of this block. If one of them has no fall
// throughs, move this block right after it.
for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
E = MBB->pred_end(); PI != E; ++PI) {
// Analyze the branch at the end of the pred.
MachineBasicBlock *PredBB = *PI;
MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
MachineBasicBlock *PredTBB = 0, *PredFBB = 0;
SmallVector<MachineOperand, 4> PredCond;
if (PredBB != MBB && !PredBB->canFallThrough() &&
!TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
&& (!CurFallsThru || !CurTBB || !CurFBB)
&& (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
// If the current block doesn't fall through, just move it.
// If the current block can fall through and does not end with a
// conditional branch, we need to append an unconditional jump to
// the (current) next block. To avoid a possible compile-time
// infinite loop, move blocks only backward in this case.
// Also, if there are already 2 branches here, we cannot add a third;
// this means we have the case
// Bcc next
// B elsewhere
// next:
if (CurFallsThru) {
MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB));
CurCond.clear();
TII->InsertBranch(*MBB, NextBB, 0, CurCond);
}
MBB->moveAfter(PredBB);
MadeChange = true;
goto ReoptimizeBlock;
}
}
}
if (!CurFallsThru) {
// Check all successors to see if we can move this block before it.
for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
E = MBB->succ_end(); SI != E; ++SI) {
// Analyze the branch at the end of the block before the succ.
MachineBasicBlock *SuccBB = *SI;
MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
// If this block doesn't already fall-through to that successor, and if
// the succ doesn't already have a block that can fall through into it,
// and if the successor isn't an EH destination, we can arrange for the
// fallthrough to happen.
if (SuccBB != MBB && &*SuccPrev != MBB &&
!SuccPrev->canFallThrough() && !CurUnAnalyzable &&
!SuccBB->isLandingPad()) {
MBB->moveBefore(SuccBB);
MadeChange = true;
goto ReoptimizeBlock;
}
}
// Okay, there is no really great place to put this block. If, however,
// the block before this one would be a fall-through if this block were
// removed, move this block to the end of the function.
MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0;
SmallVector<MachineOperand, 4> PrevCond;
if (FallThrough != MF.end() &&
!TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
PrevBB.isSuccessor(FallThrough)) {
MBB->moveAfter(--MF.end());
MadeChange = true;
return MadeChange;
}
}
}
return MadeChange;
}