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llvm-6502/lib/CodeGen/BranchFolding.cpp
Bob Wilson 15acadde5f Split tail duplication into a separate pass. This is needed to avoid 
running tail duplication when doing branch folding for if-conversion, and
we also want to be able to run tail duplication earlier to fix some
reg alloc problems.  Move the CanFallThrough function from BranchFolding
to MachineBasicBlock so that it can be shared by TailDuplication.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89904 91177308-0d34-0410-b5e6-96231b3b80d8
2009-11-26 00:32:21 +00:00

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//===-- 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(errs() << "\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 = 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(errs() << "\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(errs() << "\nTryTailMergeBlocks: ";
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
errs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
<< (i == e-1 ? "" : ", ");
errs() << "\n";
if (SuccBB) {
errs() << " with successor BB#" << SuccBB->getNumber() << '\n';
if (PredBB)
errs() << " which has fall-through from BB#"
<< PredBB->getNumber() << "\n";
}
errs() << "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(errs() << "\nUsing common tail in BB#" << MBB->getNumber()
<< " for ");
for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
if (commonTailIndex == i)
continue;
DEBUG(errs() << "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(errs() << "\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 = 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 = 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(errs() << "\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(errs() << "\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 = TII->BlockHasNoFallThrough(PrevBB);
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, *PredFBB;
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 = 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, *PrevFBB;
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;
}
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