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//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
// 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 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 "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/MRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.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);
namespace {
// Throttle for huge numbers of predecessors (compile speed problems)
cl::desc("Max number of predecessors to consider tail merging"),
cl::init(100), cl::Hidden);
struct BranchFolder : public MachineFunctionPass {
static char ID;
explicit BranchFolder(bool defaultEnableTailMerge) :
MachineFunctionPass((intptr_t)&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;
MachineModuleInfo *MMI;
bool MadeChange;
// 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);
std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials;
const MRegisterInfo *RegInfo;
RegScavenger *RS;
// Branch optzn.
bool OptimizeBranches(MachineFunction &MF);
void OptimizeBlock(MachineBasicBlock *MBB);
void RemoveDeadBlock(MachineBasicBlock *MBB);
bool CanFallThrough(MachineBasicBlock *CurBB);
bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable,
MachineBasicBlock *TBB, MachineBasicBlock *FBB,
const std::vector<MachineOperand> &Cond);
char BranchFolder::ID = 0;
FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
return new BranchFolder(DefaultEnableTailMerge); }
/// 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!");
DOUT << "\nRemoving MBB: " << *MBB;
MachineFunction *MF = MBB->getParent();
// drop all successors.
while (!MBB->succ_empty())
// 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 (MMI && !MBB->empty()) {
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
I != E; ++I) {
if ((unsigned)I->getOpcode() == TargetInstrInfo::LABEL) {
// The label ID # is always operand #0, an immediate.
// Remove the block.
bool BranchFolder::runOnMachineFunction(MachineFunction &MF) {
TII = MF.getTarget().getInstrInfo();
if (!TII) return false;
// 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;
std::vector<MachineOperand> Cond;
if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond))
EverMadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
RegInfo = MF.getTarget().getRegisterInfo();
RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
MMI = getAnalysisToUpdate<MachineModuleInfo>();
bool MadeChangeThisIteration = true;
while (MadeChangeThisIteration) {
MadeChangeThisIteration = false;
MadeChangeThisIteration |= TailMergeBlocks(MF);
MadeChangeThisIteration |= OptimizeBranches(MF);
EverMadeChange |= 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;
// We always keep the 0th jump table.
// 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 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;
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()];
// Remember that this JT is live.
JTIsLive[NewIdx] = true;
// 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[i]) {
EverMadeChange = true;
delete RS;
return EverMadeChange;
// 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.getMachineBasicBlock()->getNumber();
case MachineOperand::MO_FrameIndex: OperandHash = Op.getFrameIndex(); break;
case MachineOperand::MO_ConstantPoolIndex:
OperandHash = Op.getConstantPoolIndex();
case MachineOperand::MO_JumpTableIndex:
OperandHash = Op.getJumpTableIndex();
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();
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.
unsigned Hash = HashMachineInstr(I);
if (I == MBB->begin() || minCommonTailLength == 1)
return Hash; // Single instr MBB.
// 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)) {
++I1; ++I2;
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())
// 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, std::vector<MachineOperand>());
/// 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) {
// Create the fall-through block.
MachineFunction::iterator MBBI = &CurMBB;
MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock());
CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB);
// Move all the successors of this block to the specified block.
while (!CurMBB.succ_empty()) {
MachineBasicBlock *S = *(CurMBB.succ_end()-1);
// Add an edge from CurMBB to NewMBB for the fall-through.
// 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) {
if (!CurMBB.empty())
BitVector RegsLiveAtExit(RegInfo->getNumRegs());
RS->getRegsUsed(RegsLiveAtExit, false);
for (unsigned int i=0, e=RegInfo->getNumRegs(); i!=e; i++)
if (RegsLiveAtExit[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) {
unsigned Time = 0;
for (; I != E; ++I) {
const TargetInstrDescriptor &TID = TII->get(I->getOpcode());
if (TID.Flags & M_CALL_FLAG)
Time += 10;
else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG))
Time += 2;
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,
MachineBasicBlock *PredBB) {
// If one block is the entry block, split the other one; we can't generate
// a branch to the entry block, as its label is not emitted.
MachineBasicBlock *Entry = MBB1->getParent()->begin();
if (MBB1 == Entry)
return false;
if (MBB2 == Entry)
return true;
// If one block falls through into the common successor, choose that
// one to split; it is one instruction less to do that.
if (PredBB) {
if (MBB1 == PredBB)
return true;
else if (MBB2 == PredBB)
return false;
// 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;
std::vector<MachineOperand> Cond;
if (I != MF->end() &&
!TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond)) {
MachineBasicBlock *NextBB = I;
if (TBB == NextBB && Cond.size() && !FBB) {
if (!TII->ReverseBranchCondition(Cond)) {
TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond);
TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector<MachineOperand>());
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.
assert(0 && "Predecessor appears twice");
// 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) {
unsigned minCommonTailLength = (SuccBB ? 1 : 2);
MadeChange = false;
// Sort by hash value so that blocks with identical end sequences sort
// together.
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;
// If there is nothing that matches the hash of the current basic block,
// give up.
if (CurHash != PrevHash) {
if (SuccBB && CurMBB != PredBB)
FixTail(CurMBB, SuccBB, TII);
// Look through all the pairs of blocks that have the same hash as this
// one, and find the pair that has the largest number of instructions in
// common.
// Since instructions may get combined later (e.g. single stores into
// store multiple) this measure is not particularly accurate.
MachineBasicBlock::iterator BBI1, BBI2;
unsigned FoundI = ~0U, FoundJ = ~0U;
unsigned maxCommonTailLength = 0U;
for (int i = MergePotentials.size()-1;
i != -1 && MergePotentials[i].first == CurHash; --i) {
for (int j = i-1;
j != -1 && MergePotentials[j].first == CurHash; --j) {
MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
unsigned CommonTailLen = ComputeCommonTailLength(
TrialBBI1, TrialBBI2);
if (CommonTailLen >= minCommonTailLength &&
CommonTailLen > maxCommonTailLength) {
FoundI = i;
FoundJ = j;
maxCommonTailLength = CommonTailLen;
BBI1 = TrialBBI1;
BBI2 = TrialBBI2;
// If we didn't find any pair that has at least minCommonTailLength
// instructions in common, bail out. All entries with this
// hash code can go away now.
if (FoundI == ~0U) {
for (int i = MergePotentials.size()-1;
i != -1 && MergePotentials[i].first == CurHash; --i) {
// Put the unconditional branch back, if we need one.
CurMBB = MergePotentials[i].second;
if (SuccBB && CurMBB != PredBB)
FixTail(CurMBB, SuccBB, TII);
// Otherwise, move the block(s) to the right position(s). So that
// BBI1/2 will be valid, the last must be I and the next-to-last J.
if (FoundI != MergePotentials.size()-1)
std::swap(MergePotentials[FoundI], *(MergePotentials.end()-1));
if (FoundJ != MergePotentials.size()-2)
std::swap(MergePotentials[FoundJ], *(MergePotentials.end()-2));
CurMBB = (MergePotentials.end()-1)->second;
MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second;
// If neither block is the entire common tail, split the tail of one block
// to make it redundant with the other tail. Also, we cannot jump to the
// entry block, so if one block is the entry block, split the other one.
MachineBasicBlock *Entry = CurMBB->getParent()->begin();
if (CurMBB->begin() == BBI1 && CurMBB != Entry)
; // CurMBB is common tail
else if (MBB2->begin() == BBI2 && MBB2 != Entry)
; // MBB2 is common tail
else {
if (0) { // Enable this to disable partial tail merges.
// Decide whether we want to split CurMBB or MBB2.
if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII, PredBB)) {
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 && MBB2 != Entry) {
// Hack the end off CurMBB, making it jump to MBBI@ instead.
ReplaceTailWithBranchTo(BBI1, MBB2);
// This modifies CurMBB, so remove it from the worklist.
} else {
assert(CurMBB->begin() == BBI1 && CurMBB != Entry &&
"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.
MadeChange = true;
return MadeChange;
bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
if (!EnableTailMerge) return false;
MadeChange = false;
// First find blocks with no successors.
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)
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->succ_empty() && I->pred_size() >= 2 &&
I->pred_size() < TailMergeThreshold) {
MachineBasicBlock *IBB = I;
MachineBasicBlock *PredBB = prior(I);
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)
MachineBasicBlock *TBB = 0, *FBB = 0;
std::vector<MachineOperand> Cond;
if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond)) {
// Failing case: IBB is the target of a cbr, and
// we cannot reverse the branch.
std::vector<MachineOperand> NewCond(Cond);
if (Cond.size() && TBB==IBB) {
if (TII->ReverseBranchCondition(NewCond))
// 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
} else if (FBB) {
if (TBB!=IBB && FBB!=IBB) // cbr then ubr
} else if (Cond.size() == 0) {
if (TBB!=IBB) // ubr
} else {
if (TBB!=IBB && IBB!=PredNextBB) // cbr
// Remove the unconditional branch at the end, if any.
if (TBB && (Cond.size()==0 || FBB)) {
if (Cond.size())
// 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 be either an original single predecessor, or 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
bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
MadeChange = false;
// Make sure blocks are numbered in order
for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
MachineBasicBlock *MBB = I++;
// If it is dead, remove it.
if (MBB->pred_empty()) {
MadeChange = true;
return MadeChange;
/// 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
/// block after it, but it uses an explicit branch to do so (e.g. a table jump).
/// True is a conservative answer.
bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB,
bool BranchUnAnalyzable,
MachineBasicBlock *TBB, MachineBasicBlock *FBB,
const std::vector<MachineOperand> &Cond) {
MachineFunction::iterator Fallthrough = CurBB;
// If FallthroughBlock is off the end of the function, it can't fall through.
if (Fallthrough == CurBB->getParent()->end())
return false;
// If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible.
if (!CurBB->isSuccessor(Fallthrough))
return false;
// If we couldn't analyze the branch, assume it could fall through.
if (BranchUnAnalyzable) return true;
// If there is no branch, control always falls through.
if (TBB == 0) return true;
// If there is some explicit branch to the fallthrough block, it can obviously
// reach, even though the branch should get folded to fall through implicitly.
if (MachineFunction::iterator(TBB) == Fallthrough ||
MachineFunction::iterator(FBB) == Fallthrough)
return true;
// If it's an unconditional branch to some block not the fall through, it
// doesn't fall through.
if (Cond.empty()) return false;
// Otherwise, if it is conditional and has no explicit false block, it falls
// through.
return FBB == 0;
/// CanFallThrough - Return true if the specified 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 block after it, but it uses an explicit
/// branch to do so (e.g. a table jump).
/// True is a conservative answer.
bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) {
MachineBasicBlock *TBB = 0, *FBB = 0;
std::vector<MachineOperand> Cond;
bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond);
return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond);
/// 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,
const TargetInstrInfo &TII) {
// 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;
MachineInstr *MBB1I = --MBB1->end();
MachineInstr *MBB2I = --MBB2->end();
return TII.isCall(MBB2I->getOpcode()) && !TII.isCall(MBB1I->getOpcode());
/// OptimizeBlock - Analyze and optimize control flow related to the specified
/// block. This is never called on the entry block.
void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
MachineFunction::iterator FallThrough = MBB;
// 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.
if (MBB->empty() && !MBB->isLandingPad()) {
// Dead block? Leave for cleanup later.
if (MBB->pred_empty()) return;
if (FallThrough == MBB->getParent()->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.
ReplaceMBBInJumpTables(MBB, FallThrough);
MadeChange = true;
// 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;
std::vector<MachineOperand> PriorCond;
bool PriorUnAnalyzable =
TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
if (!PriorUnAnalyzable) {
// If the CFG for the prior block has extra edges, remove them.
MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
// 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) {
if (PriorTBB != MBB)
TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
MadeChange = true;
return OptimizeBlock(MBB);
// If the previous branch *only* branches to *this* block (conditional or
// not) remove the branch.
if (PriorTBB == MBB && PriorFBB == 0) {
MadeChange = true;
return OptimizeBlock(MBB);
// 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->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
MadeChange = true;
return OptimizeBlock(MBB);
// 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) {
std::vector<MachineOperand> NewPriorCond(PriorCond);
if (!TII->ReverseBranchCondition(NewPriorCond)) {
TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
MadeChange = true;
return OptimizeBlock(MBB);
// If this block doesn't fall through (e.g. it ends with an uncond branch or
// has no successors) 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 (!PriorCond.empty() && PriorFBB == 0 &&
MachineFunction::iterator(PriorTBB) == FallThrough &&
!CanFallThrough(MBB)) {
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 == --MBB->getParent()->end() &&
!IsBetterFallthrough(PriorTBB, MBB, *TII))
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() &&
(!CanFallThrough(PriorTBB) || PriorTBB->empty()))
DoTransform = false;
if (DoTransform) {
// Reverse the branch so we will fall through on the previous true cond.
std::vector<MachineOperand> NewPriorCond(PriorCond);
if (!TII->ReverseBranchCondition(NewPriorCond)) {
DOUT << "\nMoving MBB: " << *MBB;
DOUT << "To make fallthrough to: " << *PriorTBB << "\n";
TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond);
// Move this block to the end of the function.
MadeChange = true;
// Analyze the branch in the current block.
MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
std::vector<MachineOperand> CurCond;
bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond);
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) {
std::vector<MachineOperand> NewCond(CurCond);
if (!TII->ReverseBranchCondition(NewCond)) {
TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
MadeChange = true;
return OptimizeBlock(MBB);
// 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) {
// 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.
// 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->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.
HasBranchToSelf = true;
} else {
DidChange = true;
PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
// Change any jumptables to go to the new MBB.
ReplaceMBBInJumpTables(MBB, CurTBB);
if (DidChange) {
MadeChange = true;
if (!HasBranchToSelf) return;
// 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 (!CanFallThrough(&PrevBB, PriorUnAnalyzable,
PriorTBB, PriorFBB, PriorCond)) {
// 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,
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;
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));
TII->InsertBranch(*MBB, NextBB, 0, CurCond);
MadeChange = true;
return OptimizeBlock(MBB);
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;
std::vector<MachineOperand> SuccPrevCond;
// 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()) {
MadeChange = true;
return OptimizeBlock(MBB);
// 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.
if (FallThrough != MBB->getParent()->end() &&
PrevBB.isSuccessor(FallThrough)) {
MadeChange = true;