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//===- BranchFolding.cpp - Fold machine code branch instructions ----------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// 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. It also must handle virtual registers for targets that emit virtual
// ISA (e.g. NVPTX).
#include "BranchFolding.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/CodeGen/Analysis.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineJumpTableInfo.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/MachineSizeOpts.h"
#include "llvm/CodeGen/MBFIWrapper.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Function.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/LaneBitmask.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/BlockFrequency.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetMachine.h"
#include <cassert>
#include <cstddef>
#include <iterator>
#include <numeric>
using namespace llvm;
#define DEBUG_TYPE "branch-folder"
STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
STATISTIC(NumBranchOpts, "Number of branches optimized");
STATISTIC(NumTailMerge , "Number of block tails merged");
STATISTIC(NumHoist , "Number of times common instructions are hoisted");
STATISTIC(NumTailCalls, "Number of tail calls optimized");
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>
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>
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 {
static char ID;
explicit BranchFolderPass(): MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
} // end anonymous namespace
char BranchFolderPass::ID = 0;
char &llvm::BranchFolderPassID = BranchFolderPass::ID;
"Control Flow Optimizer", false, false)
bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
// TailMerge can create jump into if branches that make CFG irreducible for
// HW that requires structurized CFG.
bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
MBFIWrapper MBBFreqInfo(
BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true, MBBFreqInfo,
return Folder.OptimizeFunction(MF, MF.getSubtarget().getInstrInfo(),
BranchFolder::BranchFolder(bool DefaultEnableTailMerge, bool CommonHoist,
MBFIWrapper &FreqInfo,
const MachineBranchProbabilityInfo &ProbInfo,
ProfileSummaryInfo *PSI, unsigned MinTailLength)
: EnableHoistCommonCode(CommonHoist), MinCommonTailLength(MinTailLength),
MBBFreqInfo(FreqInfo), MBPI(ProbInfo), PSI(PSI) {
if (MinCommonTailLength == 0)
MinCommonTailLength = TailMergeSize;
switch (FlagEnableTailMerge) {
case cl::BOU_UNSET:
EnableTailMerge = DefaultEnableTailMerge;
case cl::BOU_TRUE: EnableTailMerge = true; break;
case cl::BOU_FALSE: EnableTailMerge = false; break;
void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
assert(MBB->pred_empty() && "MBB must be dead!");
LLVM_DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
MachineFunction *MF = MBB->getParent();
// drop all successors.
while (!MBB->succ_empty())
// Avoid matching if this pointer gets reused.
// Update call site info.
std::for_each(MBB->begin(), MBB->end(), [MF](const MachineInstr &MI) {
if (MI.shouldUpdateCallSiteInfo())
// Remove the block.
if (MLI)
bool BranchFolder::OptimizeFunction(MachineFunction &MF,
const TargetInstrInfo *tii,
const TargetRegisterInfo *tri,
MachineLoopInfo *mli, bool AfterPlacement) {
if (!tii) return false;
MachineRegisterInfo &MRI = MF.getRegInfo();
AfterBlockPlacement = AfterPlacement;
TII = tii;
TRI = tri;
MLI = mli;
this->MRI = &MRI;
UpdateLiveIns = MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF);
if (!UpdateLiveIns)
bool MadeChange = false;
// Recalculate EH scope membership.
EHScopeMembership = getEHScopeMembership(MF);
bool MadeChangeThisIteration = true;
while (MadeChangeThisIteration) {
MadeChangeThisIteration = TailMergeBlocks(MF);
// No need to clean up if tail merging does not change anything after the
// block placement.
if (!AfterBlockPlacement || MadeChangeThisIteration)
MadeChangeThisIteration |= OptimizeBranches(MF);
if (EnableHoistCommonCode)
MadeChangeThisIteration |= HoistCommonCode(MF);
MadeChange |= MadeChangeThisIteration;
// See if any jump tables have become dead as the code generator
// did its thing.
MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
if (!JTI)
return MadeChange;
// Walk the function to find jump tables that are live.
BitVector JTIsLive(JTI->getJumpTables().size());
for (const MachineBasicBlock &BB : MF) {
for (const MachineInstr &I : BB)
for (const MachineOperand &Op : I.operands()) {
if (!Op.isJTI()) continue;
// Remember that this JT is live.
// Finally, remove dead jump tables. This happens when the
// indirect jump was unreachable (and thus deleted).
for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
if (!JTIsLive.test(i)) {
MadeChange = true;
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. We can't use MachineOperand's
// hash_code here because it's not deterministic and we sort by hash value
// later.
unsigned OperandHash = 0;
switch (Op.getType()) {
case MachineOperand::MO_Register:
OperandHash = Op.getReg();
case MachineOperand::MO_Immediate:
OperandHash = Op.getImm();
case MachineOperand::MO_MachineBasicBlock:
OperandHash = Op.getMBB()->getNumber();
case MachineOperand::MO_FrameIndex:
case MachineOperand::MO_ConstantPoolIndex:
case MachineOperand::MO_JumpTableIndex:
OperandHash = Op.getIndex();
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();
Hash += ((OperandHash << 3) | Op.getType()) << (i & 31);
return Hash;
/// HashEndOfMBB - Hash the last instruction in the MBB.
static unsigned HashEndOfMBB(const MachineBasicBlock &MBB) {
MachineBasicBlock::const_iterator I = MBB.getLastNonDebugInstr();
if (I == MBB.end())
return 0;
return HashMachineInstr(*I);
/// Whether MI should be counted as an instruction when calculating common tail.
static bool countsAsInstruction(const MachineInstr &MI) {
return !(MI.isDebugInstr() || MI.isCFIInstruction());
/// Iterate backwards from the given iterator \p I, towards the beginning of the
/// block. If a MI satisfying 'countsAsInstruction' is found, return an iterator
/// pointing to that MI. If no such MI is found, return the end iterator.
static MachineBasicBlock::iterator
skipBackwardPastNonInstructions(MachineBasicBlock::iterator I,
MachineBasicBlock *MBB) {
while (I != MBB->begin()) {
if (countsAsInstruction(*I))
return I;
return MBB->end();
/// Given two machine basic blocks, return the number of instructions they
/// actually have in common together at their end. If a common tail is found (at
/// least by one instruction), then iterators for the first shared instruction
/// in each block are returned as well.
/// Non-instructions according to countsAsInstruction are ignored.
static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
MachineBasicBlock *MBB2,
MachineBasicBlock::iterator &I1,
MachineBasicBlock::iterator &I2) {
MachineBasicBlock::iterator MBBI1 = MBB1->end();
MachineBasicBlock::iterator MBBI2 = MBB2->end();
unsigned TailLen = 0;
while (true) {
MBBI1 = skipBackwardPastNonInstructions(MBBI1, MBB1);
MBBI2 = skipBackwardPastNonInstructions(MBBI2, MBB2);
if (MBBI1 == MBB1->end() || MBBI2 == MBB2->end())
if (!MBBI1->isIdenticalTo(*MBBI2) ||
// 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.
MBBI1->isInlineAsm()) {
if (MBBI1->getFlag(MachineInstr::NoMerge) ||
I1 = MBBI1;
I2 = MBBI2;
return TailLen;
void BranchFolder::replaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
MachineBasicBlock &NewDest) {
if (UpdateLiveIns) {
// OldInst should always point to an instruction.
MachineBasicBlock &OldMBB = *OldInst->getParent();
// Move backward to the place where will insert the jump.
MachineBasicBlock::iterator I = OldMBB.end();
do {
} while (I != OldInst);
// Merging the tails may have switched some undef operand to non-undef ones.
// Add IMPLICIT_DEFS into OldMBB as necessary to have a definition of the
// register.
for (MachineBasicBlock::RegisterMaskPair P : NewDest.liveins()) {
// We computed the liveins with computeLiveIn earlier and should only see
// full registers:
assert(P.LaneMask == LaneBitmask::getAll() &&
"Can only handle full register.");
MCPhysReg Reg = P.PhysReg;
if (!LiveRegs.available(*MRI, Reg))
DebugLoc DL;
BuildMI(OldMBB, OldInst, DL, TII->get(TargetOpcode::IMPLICIT_DEF), Reg);
TII->ReplaceTailWithBranchTo(OldInst, &NewDest);
MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
MachineBasicBlock::iterator BBI1,
const BasicBlock *BB) {
if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
return nullptr;
MachineFunction &MF = *CurMBB.getParent();
// Create the fall-through block.
MachineFunction::iterator MBBI = CurMBB.getIterator();
MachineBasicBlock *NewMBB = MF.CreateMachineBasicBlock(BB);
CurMBB.getParent()->insert(++MBBI, NewMBB);
// Move all the successors of this block to the specified block.
// Add an edge from CurMBB to NewMBB for the fall-through.
// Splice the code over.
NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
// NewMBB belongs to the same loop as CurMBB.
if (MLI)
if (MachineLoop *ML = MLI->getLoopFor(&CurMBB))
ML->addBasicBlockToLoop(NewMBB, MLI->getBase());
// NewMBB inherits CurMBB's block frequency.
MBBFreqInfo.setBlockFreq(NewMBB, MBBFreqInfo.getBlockFreq(&CurMBB));
if (UpdateLiveIns)
computeAndAddLiveIns(LiveRegs, *NewMBB);
// Add the new block to the EH scope.
const auto &EHScopeI = EHScopeMembership.find(&CurMBB);
if (EHScopeI != EHScopeMembership.end()) {
auto n = EHScopeI->second;
EHScopeMembership[NewMBB] = n;
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) {
if (!countsAsInstruction(*I))
if (I->isCall())
Time += 10;
else if (I->mayLoadOrStore())
Time += 2;
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 = std::next(MachineFunction::iterator(CurMBB));
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 4> Cond;
DebugLoc dl = CurMBB->findBranchDebugLoc();
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->insertBranch(*CurMBB, SuccBB, nullptr, Cond, dl);
TII->insertBranch(*CurMBB, SuccBB, nullptr,
SmallVector<MachineOperand, 0>(), dl);
BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
if (getHash() < o.getHash())
return true;
if (getHash() > o.getHash())
return false;
if (getBlock()->getNumber() < o.getBlock()->getNumber())
return true;
if (getBlock()->getNumber() > o.getBlock()->getNumber())
return false;
// _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
// an object with itself.
llvm_unreachable("Predecessor appears twice");
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;
while (true) {
if (I == MBB->begin()) {
I = MBB->end();
if (!I->isTerminator()) break;
return NumTerms;
/// A no successor, non-return block probably ends in unreachable and is cold.
/// Also consider a block that ends in an indirect branch to be a return block,
/// since many targets use plain indirect branches to return.
static bool blockEndsInUnreachable(const MachineBasicBlock *MBB) {
if (!MBB->succ_empty())
return false;
if (MBB->empty())
return true;
return !(MBB->back().isReturn() || MBB->back().isIndirectBranch());
/// 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.
/// MBB1, MBB2 The blocks to check
/// MinCommonTailLength Minimum size of tail block to be merged.
/// CommonTailLen Out parameter to record the size of the shared tail between
/// MBB1 and MBB2
/// I1, I2 Iterator references that will be changed to point to the first
/// instruction in the common tail shared by MBB1,MBB2
/// SuccBB A common successor of MBB1, MBB2 which are in a canonical form
/// relative to SuccBB
/// PredBB The layout predecessor of SuccBB, if any.
/// EHScopeMembership map from block to EH scope #.
/// AfterPlacement True if we are merging blocks after layout. Stricter
/// thresholds apply to prevent undoing tail-duplication.
static bool
ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
unsigned MinCommonTailLength, unsigned &CommonTailLen,
MachineBasicBlock::iterator &I1,
MachineBasicBlock::iterator &I2, MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB,
DenseMap<const MachineBasicBlock *, int> &EHScopeMembership,
bool AfterPlacement,
MBFIWrapper &MBBFreqInfo,
ProfileSummaryInfo *PSI) {
// It is never profitable to tail-merge blocks from two different EH scopes.
if (!EHScopeMembership.empty()) {
auto EHScope1 = EHScopeMembership.find(MBB1);
assert(EHScope1 != EHScopeMembership.end());
auto EHScope2 = EHScopeMembership.find(MBB2);
assert(EHScope2 != EHScopeMembership.end());
if (EHScope1->second != EHScope2->second)
return false;
CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
if (CommonTailLen == 0)
return false;
LLVM_DEBUG(dbgs() << "Common tail length of " << printMBBReference(*MBB1)
<< " and " << printMBBReference(*MBB2) << " is "
<< CommonTailLen << '\n');
// Move the iterators to the beginning of the MBB if we only got debug
// instructions before the tail. This is to avoid splitting a block when we
// only got debug instructions before the tail (to be invariant on -g).
if (skipDebugInstructionsForward(MBB1->begin(), MBB1->end()) == I1)
I1 = MBB1->begin();
if (skipDebugInstructionsForward(MBB2->begin(), MBB2->end()) == I2)
I2 = MBB2->begin();
bool FullBlockTail1 = I1 == MBB1->begin();
bool FullBlockTail2 = I2 == MBB2->begin();
// It's almost always profitable to merge any number of non-terminator
// instructions with the block that falls through into the common successor.
// This is true only for a single successor. For multiple successors, we are
// trading a conditional branch for an unconditional one.
// TODO: Re-visit successor size for non-layout tail merging.
if ((MBB1 == PredBB || MBB2 == PredBB) &&
(!AfterPlacement || MBB1->succ_size() == 1)) {
MachineBasicBlock::iterator I;
unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
if (CommonTailLen > NumTerms)
return true;
// If these are identical non-return blocks with no successors, merge them.
// Such blocks are typically cold calls to noreturn functions like abort, and
// are unlikely to become a fallthrough target after machine block placement.
// Tail merging these blocks is unlikely to create additional unconditional
// branches, and will reduce the size of this cold code.
if (FullBlockTail1 && FullBlockTail2 &&
blockEndsInUnreachable(MBB1) && blockEndsInUnreachable(MBB2))
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) && FullBlockTail2)
return true;
if (MBB2->isLayoutSuccessor(MBB1) && FullBlockTail1)
return true;
// If both blocks are identical and end in a branch, merge them unless they
// both have a fallthrough predecessor and successor.
// We can only do this after block placement because it depends on whether
// there are fallthroughs, and we don't know until after layout.
if (AfterPlacement && FullBlockTail1 && FullBlockTail2) {
auto BothFallThrough = [](MachineBasicBlock *MBB) {
if (MBB->succ_size() != 0 && !MBB->canFallThrough())
return false;
MachineFunction::iterator I(MBB);
MachineFunction *MF = MBB->getParent();
return (MBB != &*MF->begin()) && std::prev(I)->canFallThrough();
if (!BothFallThrough(MBB1) || !BothFallThrough(MBB2))
return true;
// If both blocks have an unconditional branch temporarily stripped out,
// count that as an additional common instruction for the following
// heuristics. This heuristic is only accurate for single-succ blocks, so to
// make sure that during layout merging and duplicating don't crash, we check
// for that when merging during layout.
unsigned EffectiveTailLen = CommonTailLen;
if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
(MBB1->succ_size() == 1 || !AfterPlacement) &&
!MBB1->back().isBarrier() &&
// 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.
MachineFunction *MF = MBB1->getParent();
bool OptForSize =
MF->getFunction().hasOptSize() ||
(llvm::shouldOptimizeForSize(MBB1, PSI, &MBBFreqInfo) &&
llvm::shouldOptimizeForSize(MBB2, PSI, &MBBFreqInfo));
return EffectiveTailLen >= 2 && OptForSize &&
(FullBlockTail1 || FullBlockTail2);
unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
unsigned MinCommonTailLength,
MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB) {
unsigned maxCommonTailLength = 0U;
MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
MPIterator HighestMPIter = std::prev(MergePotentials.end());
for (MPIterator CurMPIter = std::prev(MergePotentials.end()),
B = MergePotentials.begin();
CurMPIter != B && CurMPIter->getHash() == CurHash; --CurMPIter) {
for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) {
unsigned CommonTailLen;
if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
CommonTailLen, TrialBBI1, TrialBBI2,
SuccBB, PredBB,
AfterBlockPlacement, MBBFreqInfo, PSI)) {
if (CommonTailLen > maxCommonTailLength) {
maxCommonTailLength = CommonTailLen;
HighestMPIter = CurMPIter;
SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
if (HighestMPIter == CurMPIter &&
CommonTailLen == maxCommonTailLength)
SameTails.push_back(SameTailElt(I, TrialBBI2));
if (I == B)
return maxCommonTailLength;
void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB) {
MPIterator CurMPIter, B;
for (CurMPIter = std::prev(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)
if (CurMPIter->getHash() != CurHash)
MergePotentials.erase(CurMPIter, MergePotentials.end());
bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
MachineBasicBlock *SuccBB,
unsigned maxCommonTailLength,
unsigned &commonTailIndex) {
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;
// 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(),
if (t <= TimeEstimate) {
TimeEstimate = t;
commonTailIndex = i;
MachineBasicBlock::iterator BBI =
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
LLVM_DEBUG(dbgs() << "\nSplitting " << printMBBReference(*MBB) << ", size "
<< maxCommonTailLength);
// If the split block unconditionally falls-thru to SuccBB, it will be
// merged. In control flow terms it should then take SuccBB's name. e.g. If
// SuccBB is an inner loop, the common tail is still part of the inner loop.
const BasicBlock *BB = (SuccBB && MBB->succ_size() == 1) ?
SuccBB->getBasicBlock() : MBB->getBasicBlock();
MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI, BB);
if (!newMBB) {
LLVM_DEBUG(dbgs() << "... failed!");
return false;
// If we split PredBB, newMBB is the new predecessor.
if (PredBB == MBB)
PredBB = newMBB;
return true;
static void
mergeOperations(MachineBasicBlock::iterator MBBIStartPos,
MachineBasicBlock &MBBCommon) {
MachineBasicBlock *MBB = MBBIStartPos->getParent();
// Note CommonTailLen does not necessarily matches the size of
// the common BB nor all its instructions because of debug
// instructions differences.
unsigned CommonTailLen = 0;
for (auto E = MBB->end(); MBBIStartPos != E; ++MBBIStartPos)
MachineBasicBlock::reverse_iterator MBBI = MBB->rbegin();
MachineBasicBlock::reverse_iterator MBBIE = MBB->rend();
MachineBasicBlock::reverse_iterator MBBICommon = MBBCommon.rbegin();
MachineBasicBlock::reverse_iterator MBBIECommon = MBBCommon.rend();
while (CommonTailLen--) {
assert(MBBI != MBBIE && "Reached BB end within common tail length!");
if (!countsAsInstruction(*MBBI)) {
while ((MBBICommon != MBBIECommon) && !countsAsInstruction(*MBBICommon))
assert(MBBICommon != MBBIECommon &&
"Reached BB end within common tail length!");
assert(MBBICommon->isIdenticalTo(*MBBI) && "Expected matching MIIs!");
// Merge MMOs from memory operations in the common block.
if (MBBICommon->mayLoadOrStore())
MBBICommon->cloneMergedMemRefs(*MBB->getParent(), {&*MBBICommon, &*MBBI});
// Drop undef flags if they aren't present in all merged instructions.
for (unsigned I = 0, E = MBBICommon->getNumOperands(); I != E; ++I) {
MachineOperand &MO = MBBICommon->getOperand(I);
if (MO.isReg() && MO.isUndef()) {
const MachineOperand &OtherMO = MBBI->getOperand(I);
if (!OtherMO.isUndef())
void BranchFolder::mergeCommonTails(unsigned commonTailIndex) {
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
std::vector<MachineBasicBlock::iterator> NextCommonInsts(SameTails.size());
for (unsigned int i = 0 ; i != SameTails.size() ; ++i) {
if (i != commonTailIndex) {
NextCommonInsts[i] = SameTails[i].getTailStartPos();
mergeOperations(SameTails[i].getTailStartPos(), *MBB);
} else {
assert(SameTails[i].getTailStartPos() == MBB->begin() &&
"MBB is not a common tail only block");
for (auto &MI : *MBB) {
if (!countsAsInstruction(MI))
DebugLoc DL = MI.getDebugLoc();
for (unsigned int i = 0 ; i < NextCommonInsts.size() ; i++) {
if (i == commonTailIndex)
auto &Pos = NextCommonInsts[i];
assert(Pos != SameTails[i].getBlock()->end() &&
"Reached BB end within common tail");
while (!countsAsInstruction(*Pos)) {
assert(Pos != SameTails[i].getBlock()->end() &&
"Reached BB end within common tail");
assert(MI.isIdenticalTo(*Pos) && "Expected matching MIIs!");
DL = DILocation::getMergedLocation(DL, Pos->getDebugLoc());
NextCommonInsts[i] = ++Pos;
if (UpdateLiveIns) {
LivePhysRegs NewLiveIns(*TRI);
computeLiveIns(NewLiveIns, *MBB);
// The flag merging may lead to some register uses no longer using the
// <undef> flag, add IMPLICIT_DEFs in the predecessors as necessary.
for (MachineBasicBlock *Pred : MBB->predecessors()) {
MachineBasicBlock::iterator InsertBefore = Pred->getFirstTerminator();
for (Register Reg : NewLiveIns) {
if (!LiveRegs.available(*MRI, Reg))
DebugLoc DL;
BuildMI(*Pred, InsertBefore, DL, TII->get(TargetOpcode::IMPLICIT_DEF),
addLiveIns(*MBB, NewLiveIns);
// See if any of the blocks in MergePotentials (which all have SuccBB as a
// successor, or all have no successor if it is null) 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.
// MinCommonTailLength - Except for the special cases below, tail-merge if
// there are at least this many instructions in common.
bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB,
unsigned MinCommonTailLength) {
bool MadeChange = false;
dbgs() << "\nTryTailMergeBlocks: ";
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) dbgs()
<< printMBBReference(*MergePotentials[i].getBlock())
<< (i == e - 1 ? "" : ", ");
dbgs() << "\n"; if (SuccBB) {
dbgs() << " with successor " << printMBBReference(*SuccBB) << '\n';
if (PredBB)
dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\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.
array_pod_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,
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);
// If one of the blocks is the entire common tail (and is not the entry
// block/an EH pad, 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 =
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() && !SameTails[1].getBlock()->isEHPad())
commonTailIndex = 1;
else if (SameTails.size() == 2 &&
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 || MBB->isEHPad()) &&
if (MBB == PredBB) {
commonTailIndex = i;
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.
if (!CreateCommonTailOnlyBlock(PredBB, SuccBB,
maxCommonTailLength, commonTailIndex)) {
RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
// Recompute common tail MBB's edge weights and block frequency.
// Merge debug locations, MMOs and undef flags across identical instructions
// for common tail.
// MBB is common tail. Adjust all other BB's to jump to this one.
// Traversal must be forwards so erases work.
LLVM_DEBUG(dbgs() << "\nUsing common tail in " << printMBBReference(*MBB)
<< " for ");
for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
if (commonTailIndex == i)
LLVM_DEBUG(dbgs() << printMBBReference(*SameTails[i].getBlock())
<< (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.
LLVM_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) {
bool MadeChange = false;
if (!EnableTailMerge)
return MadeChange;
// First find blocks with no successors.
// Block placement may create new tail merging opportunities for these blocks.
for (MachineBasicBlock &MBB : MF) {
if (MergePotentials.size() == TailMergeThreshold)
if (!TriedMerging.count(&MBB) && MBB.succ_empty())
MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(MBB), &MBB));
// If this is a large problem, avoid visiting the same basic blocks
// multiple times.
if (MergePotentials.size() == TailMergeThreshold)
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
// See if we can do any tail merging on those.
if (MergePotentials.size() >= 2)
MadeChange |= TryTailMergeBlocks(nullptr, nullptr, MinCommonTailLength);
// 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 = std::next(MF.begin()), E = MF.end();
I != E; ++I) {
if (I->pred_size() < 2) continue;
SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
MachineBasicBlock *IBB = &*I;
MachineBasicBlock *PredBB = &*std::prev(I);
MachineLoop *ML;
// Bail if merging after placement and IBB is the loop header because
// -- If merging predecessors that belong to the same loop as IBB, the
// common tail of merged predecessors may become the loop top if block
// placement is called again and the predecessors may branch to this common
// tail and require more branches. This can be relaxed if
// MachineBlockPlacement::findBestLoopTop is more flexible.
// --If merging predecessors that do not belong to the same loop as IBB, the
// loop info of IBB's loop and the other loops may be affected. Calling the
// block placement again may make big change to the layout and eliminate the
// reason to do tail merging here.
if (AfterBlockPlacement && MLI) {
ML = MLI->getLoopFor(IBB);
if (ML && IBB == ML->getHeader())
for (MachineBasicBlock *PBB : I->predecessors()) {
if (MergePotentials.size() == TailMergeThreshold)
if (TriedMerging.count(PBB))
// Skip blocks that loop to themselves, can't tail merge these.
if (PBB == IBB)
// Visit each predecessor only once.
if (!UniquePreds.insert(PBB).second)
// Skip blocks which may jump to a landing pad or jump from an asm blob.
// Can't tail merge these.
if (PBB->hasEHPadSuccessor() || PBB->mayHaveInlineAsmBr())
// After block placement, only consider predecessors that belong to the
// same loop as IBB. The reason is the same as above when skipping loop
// header.
if (AfterBlockPlacement && MLI)
if (ML != MLI->getLoopFor(PBB))
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
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))
// This is the QBB case described above
if (!FBB) {
auto Next = ++PBB->getIterator();
if (Next != MF.end())
FBB = &*Next;
// Remove the unconditional branch at the end, if any.
if (TBB && (Cond.empty() || FBB)) {
DebugLoc dl = PBB->findBranchDebugLoc();
if (!Cond.empty())
// reinsert conditional branch only, for now
TII->insertBranch(*PBB, (TBB == IBB) ? FBB : TBB, nullptr,
NewCond, dl);
MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(*PBB), PBB));
// If this is a large problem, avoid visiting the same basic blocks multiple
// times.
if (MergePotentials.size() == TailMergeThreshold)
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
if (MergePotentials.size() >= 2)
MadeChange |= TryTailMergeBlocks(IBB, PredBB, MinCommonTailLength);
// Reinsert an unconditional branch if needed. The 1 below can occur as a
// result of removing blocks in TryTailMergeBlocks.
PredBB = &*std::prev(I); // this may have been changed in TryTailMergeBlocks
if (MergePotentials.size() == 1 &&
MergePotentials.begin()->getBlock() != PredBB)
FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
return MadeChange;
void BranchFolder::setCommonTailEdgeWeights(MachineBasicBlock &TailMBB) {
SmallVector<BlockFrequency, 2> EdgeFreqLs(TailMBB.succ_size());
BlockFrequency AccumulatedMBBFreq;
// Aggregate edge frequency of successor edge j:
// edgeFreq(j) = sum (freq(bb) * edgeProb(bb, j)),
// where bb is a basic block that is in SameTails.
for (const auto &Src : SameTails) {
const MachineBasicBlock *SrcMBB = Src.getBlock();
BlockFrequency BlockFreq = MBBFreqInfo.getBlockFreq(SrcMBB);
AccumulatedMBBFreq += BlockFreq;
// It is not necessary to recompute edge weights if TailBB has less than two
// successors.
if (TailMBB.succ_size() <= 1)
auto EdgeFreq = EdgeFreqLs.begin();
for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
SuccI != SuccE; ++SuccI, ++EdgeFreq)
*EdgeFreq += BlockFreq * MBPI.getEdgeProbability(SrcMBB, *SuccI);
MBBFreqInfo.setBlockFreq(&TailMBB, AccumulatedMBBFreq);
if (TailMBB.succ_size() <= 1)
auto SumEdgeFreq =
std::accumulate(EdgeFreqLs.begin(), EdgeFreqLs.end(), BlockFrequency(0))
auto EdgeFreq = EdgeFreqLs.begin();
if (SumEdgeFreq > 0) {
for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
SuccI != SuccE; ++SuccI, ++EdgeFreq) {
auto Prob = BranchProbability::getBranchProbability(
EdgeFreq->getFrequency(), SumEdgeFreq);
TailMBB.setSuccProbability(SuccI, Prob);
// Branch Optimization
bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
bool MadeChange = false;
// Make sure blocks are numbered in order
// Renumbering blocks alters EH scope membership, recalculate it.
EHScopeMembership = getEHScopeMembership(MF);
for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
I != E; ) {
MachineBasicBlock *MBB = &*I++;
MadeChange |= OptimizeBlock(MBB);
// If it is dead, remove it.
if (MBB->pred_empty()) {
MadeChange = true;
return MadeChange;
// Blocks should be considered empty if they contain only debug info;
// else the debug info would affect codegen.
static bool IsEmptyBlock(MachineBasicBlock *MBB) {
return MBB->getFirstNonDebugInstr() == MBB->end();
// Blocks with only debug info and branches should be considered the same
// as blocks with only branches.
static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
MachineBasicBlock::iterator I = MBB->getFirstNonDebugInstr();
assert(I != MBB->end() && "empty block!");
return I->isBranch();
/// 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) {
assert(MBB1 && MBB2 && "Unknown MachineBasicBlock");
// 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.
MachineBasicBlock::iterator MBB1I = MBB1->getLastNonDebugInstr();
MachineBasicBlock::iterator MBB2I = MBB2->getLastNonDebugInstr();
if (MBB1I == MBB1->end() || MBB2I == MBB2->end())
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;
return MBB2I->isCall() && !MBB1I->isCall();
/// getBranchDebugLoc - Find and return, if any, the DebugLoc of the branch
/// instructions on the block.
static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) {
MachineBasicBlock::iterator I = MBB.getLastNonDebugInstr();
if (I != MBB.end() && I->isBranch())
return I->getDebugLoc();
return DebugLoc();
static void copyDebugInfoToPredecessor(const TargetInstrInfo *TII,
MachineBasicBlock &MBB,
MachineBasicBlock &PredMBB) {
auto InsertBefore = PredMBB.getFirstTerminator();
for (MachineInstr &MI : MBB.instrs())
if (MI.isDebugInstr()) {
TII->duplicate(PredMBB, InsertBefore, MI);
LLVM_DEBUG(dbgs() << "Copied debug entity from empty block to pred: "
<< MI);
static void copyDebugInfoToSuccessor(const TargetInstrInfo *TII,
MachineBasicBlock &MBB,
MachineBasicBlock &SuccMBB) {
auto InsertBefore = SuccMBB.SkipPHIsAndLabels(SuccMBB.begin());
for (MachineInstr &MI : MBB.instrs())
if (MI.isDebugInstr()) {
TII->duplicate(SuccMBB, InsertBefore, MI);
LLVM_DEBUG(dbgs() << "Copied debug entity from empty block to succ: "
<< MI);
// Try to salvage DBG_VALUE instructions from an otherwise empty block. If such
// a basic block is removed we would lose the debug information unless we have
// copied the information to a predecessor/successor.
// TODO: This function only handles some simple cases. An alternative would be
// to run a heavier analysis, such as the LiveDebugValues pass, before we do
// branch folding.
static void salvageDebugInfoFromEmptyBlock(const TargetInstrInfo *TII,
MachineBasicBlock &MBB) {
assert(IsEmptyBlock(&MBB) && "Expected an empty block (except debug info).");
// If this MBB is the only predecessor of a successor it is legal to copy
// DBG_VALUE instructions to the beginning of the successor.
for (MachineBasicBlock *SuccBB : MBB.successors())
if (SuccBB->pred_size() == 1)
copyDebugInfoToSuccessor(TII, MBB, *SuccBB);
// If this MBB is the only successor of a predecessor it is legal to copy the
// DBG_VALUE instructions to the end of the predecessor (just before the
// terminators, assuming that the terminator isn't affecting the DBG_VALUE).
for (MachineBasicBlock *PredBB : MBB.predecessors())
if (PredBB->succ_size() == 1)
copyDebugInfoToPredecessor(TII, MBB, *PredBB);
bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
bool MadeChange = false;
MachineFunction &MF = *MBB->getParent();
MachineFunction::iterator FallThrough = MBB->getIterator();
// Make sure MBB and FallThrough belong to the same EH scope.
bool SameEHScope = true;
if (!EHScopeMembership.empty() && FallThrough != MF.end()) {
auto MBBEHScope = EHScopeMembership.find(MBB);
assert(MBBEHScope != EHScopeMembership.end());
auto FallThroughEHScope = EHScopeMembership.find(&*FallThrough);
assert(FallThroughEHScope != EHScopeMembership.end());
SameEHScope = MBBEHScope->second == FallThroughEHScope->second;
// Analyze the branch in the current block. As a side-effect, this may cause
// the block to become empty.
MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr;
SmallVector<MachineOperand, 4> CurCond;
bool CurUnAnalyzable =
TII->analyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
// 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 (IsEmptyBlock(MBB) && !MBB->isEHPad() && !MBB->hasAddressTaken() &&
SameEHScope) {
salvageDebugInfoFromEmptyBlock(TII, *MBB);
// 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 if (FallThrough->isEHPad()) {
// Don't rewrite to a landing pad fallthough. That could lead to the case
// where a BB jumps to more than one landing pad.
// TODO: Is it ever worth rewriting predecessors which don't already
// jump to a landing pad, and so can safely jump to the fallthrough?
} else if (MBB->isSuccessor(&*FallThrough)) {
// 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.
if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
MJTI->ReplaceMBBInJumpTables(MBB, &*FallThrough);
MadeChange = true;
return MadeChange;
// Check to see if we can simplify the terminator of the block before this
// one.
MachineBasicBlock &PrevBB = *std::prev(MachineFunction::iterator(MBB));
MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
SmallVector<MachineOperand, 4> PriorCond;
bool PriorUnAnalyzable =
TII->analyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
if (!PriorUnAnalyzable) {
// 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) {
DebugLoc dl = getBranchDebugLoc(PrevBB);
if (PriorTBB != MBB)
TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
MadeChange = true;
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() && !MBB->isEHPad()) {
LLVM_DEBUG(dbgs() << "\nMerging into block: " << PrevBB
<< "From MBB: " << *MBB);
// Remove redundant DBG_VALUEs first.
if (PrevBB.begin() != PrevBB.end()) {
MachineBasicBlock::iterator PrevBBIter = PrevBB.end();
MachineBasicBlock::iterator MBBIter = MBB->begin();
// Check if DBG_VALUE at the end of PrevBB is identical to the
// DBG_VALUE at the beginning of MBB.
while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end()
&& PrevBBIter->isDebugInstr() && MBBIter->isDebugInstr()) {
if (!MBBIter->isIdenticalTo(*PrevBBIter))
MachineInstr &DuplicateDbg = *MBBIter;
++MBBIter; -- PrevBBIter;
PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
MadeChange = true;
return MadeChange;
// If the previous branch *only* branches to *this* block (conditional or
// not) remove the branch.
if (PriorTBB == MBB && !PriorFBB) {
MadeChange = true;
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) {
DebugLoc dl = getBranchDebugLoc(PrevBB);
TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
MadeChange = true;
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)) {
DebugLoc dl = getBranchDebugLoc(PrevBB);
TII->insertBranch(PrevBB, PriorFBB, nullptr, NewPriorCond, dl);
MadeChange = true;
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 &&
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;
if (DoTransform) {
// Reverse the branch so we will fall through on the previous true cond.
SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
if (!TII->reverseBranchCondition(NewPriorCond)) {
LLVM_DEBUG(dbgs() << "\nMoving MBB: " << *MBB
<< "To make fallthrough to: " << *PriorTBB << "\n");
DebugLoc dl = getBranchDebugLoc(PrevBB);
TII->insertBranch(PrevBB, MBB, nullptr, NewPriorCond, dl);
// Move this block to the end of the function.
MadeChange = true;
return MadeChange;
bool OptForSize =
MF.getFunction().hasOptSize() ||
llvm::shouldOptimizeForSize(MBB, PSI, &MBBFreqInfo);
if (!IsEmptyBlock(MBB) && MBB->pred_size() == 1 && OptForSize) {
// Changing "Jcc foo; foo: jmp bar;" into "Jcc bar;" might change the branch
// direction, thereby defeating careful block placement and regressing
// performance. Therefore, only consider this for optsize functions.
MachineInstr &TailCall = *MBB->getFirstNonDebugInstr();
if (TII->isUnconditionalTailCall(TailCall)) {
MachineBasicBlock *Pred = *MBB->pred_begin();
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
bool PredAnalyzable =
!TII->analyzeBranch(*Pred, PredTBB, PredFBB, PredCond, true);
if (PredAnalyzable && !PredCond.empty() && PredTBB == MBB &&
PredTBB != PredFBB) {
// The predecessor has a conditional branch to this block which consists
// of only a tail call. Try to fold the tail call into the conditional
// branch.
if (TII->canMakeTailCallConditional(PredCond, TailCall)) {
// TODO: It would be nice if analyzeBranch() could provide a pointer
// to the branch instruction so replaceBranchWithTailCall() doesn't
// have to search for it.
TII->replaceBranchWithTailCall(*Pred, PredCond, TailCall);
MadeChange = true;
return MadeChange;
// If the predecessor is falling through to this block, we could reverse
// the branch condition and fold the tail call into that. However, after
// that we might have to re-arrange the CFG to fall through to the other
// block and there is a high risk of regressing code size rather than
// improving it.
if (!CurUnAnalyzable) {
// 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)) {
DebugLoc dl = getBranchDebugLoc(*MBB);
TII->insertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
MadeChange = true;
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 &&
IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
!MBB->hasAddressTaken() && !MBB->isEHPad()) {
DebugLoc dl = getBranchDebugLoc(*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 the only things remaining in the block are debug info, remove these
// as well, so this will behave the same as an empty block in non-debug
// mode.
if (IsEmptyBlock(MBB)) {
// Make the block empty, losing the debug info (we could probably
// improve this in some cases.)
MBB->erase(MBB->begin(), MBB->end());
// 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) {
assert(PriorCond.empty() && !PriorFBB &&
"Bad branch analysis");
PriorTBB = MBB;
} else {
assert(!PriorFBB && "Machine CFG out of date!");
PriorFBB = MBB;
DebugLoc pdl = getBranchDebugLoc(PrevBB);
TII->insertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, pdl);
// 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);
// 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.
MachineBasicBlock *NewCurTBB = nullptr, *NewCurFBB = nullptr;
SmallVector<MachineOperand, 4> NewCurCond;
bool NewCurUnAnalyzable = TII->analyzeBranch(
*PMBB, NewCurTBB, NewCurFBB, NewCurCond, true);
if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
DebugLoc pdl = getBranchDebugLoc(*PMBB);
TII->insertBranch(*PMBB, NewCurTBB, nullptr, NewCurCond, pdl);
MadeChange = true;
// Change any jumptables to go to the new MBB.
if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
if (DidChange) {
MadeChange = true;
if (!HasBranchToSelf) return MadeChange;
// Add the branch back if the block is more than just an uncond branch.
TII->insertBranch(*MBB, CurTBB, nullptr, CurCond, dl);
// 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->isEHPad()) {
// Check all the predecessors of this block. If one of them has no fall
// throughs, and analyzeBranch thinks it _could_ fallthrough to this
// block, move this block right after it.
for (MachineBasicBlock *PredBB : MBB->predecessors()) {
// Analyze the branch at the end of the pred.
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
if (PredBB != MBB && !PredBB->canFallThrough() &&
!TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) &&
(PredTBB == MBB || PredFBB == MBB) &&
(!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 = &*std::next(MBB->getIterator());
TII->insertBranch(*MBB, NextBB, nullptr, CurCond, DebugLoc());
MadeChange = true;
goto ReoptimizeBlock;
if (!CurFallsThru) {
// Check analyzable branch-successors to see if we can move this block
// before one.
if (!CurUnAnalyzable) {
for (MachineBasicBlock *SuccBB : {CurFBB, CurTBB}) {
if (!SuccBB)
// Analyze the branch at the end of the block before the succ.
MachineFunction::iterator SuccPrev = --SuccBB->getIterator();
// 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, we can arrange for the fallthrough to happen.
if (SuccBB != MBB && &*SuccPrev != MBB &&
!SuccPrev->canFallThrough()) {
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. There is no real
// advantage in "falling through" to an EH block, so we don't want to
// perform this transformation for that case.
// Also, Windows EH introduced the possibility of an arbitrary number of
// successors to a given block. The analyzeBranch call does not consider
// exception handling and so we can get in a state where a block
// containing a call is followed by multiple EH blocks that would be
// rotated infinitely at the end of the function if the transformation
// below were performed for EH "FallThrough" blocks. Therefore, even if
// that appears not to be happening anymore, we should assume that it is
// possible and not remove the "!FallThrough()->isEHPad" condition below.
MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr;
SmallVector<MachineOperand, 4> PrevCond;
if (FallThrough != MF.end() &&
!FallThrough->isEHPad() &&
!TII->analyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
PrevBB.isSuccessor(&*FallThrough)) {
MadeChange = true;
return MadeChange;
return MadeChange;
// Hoist Common Code
bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
bool MadeChange = false;
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) {
MachineBasicBlock *MBB = &*I++;
MadeChange |= HoistCommonCodeInSuccs(MBB);
return MadeChange;
/// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
/// its 'true' successor.
static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
MachineBasicBlock *TrueBB) {
for (MachineBasicBlock *SuccBB : BB->successors())
if (SuccBB != TrueBB)
return SuccBB;
return nullptr;
template <class Container>
static void addRegAndItsAliases(Register Reg, const TargetRegisterInfo *TRI,
Container &Set) {
if (Reg.isPhysical()) {
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
} else {
/// findHoistingInsertPosAndDeps - Find the location to move common instructions
/// in successors to. The location is usually just before the terminator,
/// however if the terminator is a conditional branch and its previous
/// instruction is the flag setting instruction, the previous instruction is
/// the preferred location. This function also gathers uses and defs of the
/// instructions from the insertion point to the end of the block. The data is
/// used by HoistCommonCodeInSuccs to ensure safety.
MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
const TargetInstrInfo *TII,
const TargetRegisterInfo *TRI,
SmallSet<Register, 4> &Uses,
SmallSet<Register, 4> &Defs) {
MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
if (!TII->isUnpredicatedTerminator(*Loc))
return MBB->end();
for (const MachineOperand &MO : Loc->operands()) {
if (!MO.isReg())
Register Reg = MO.getReg();
if (!Reg)
if (MO.isUse()) {
addRegAndItsAliases(Reg, TRI, Uses);
} else {
if (!MO.isDead())
// Don't try to hoist code in the rare case the terminator defines a
// register that is later used.
return MBB->end();
// If the terminator defines a register, make sure we don't hoist
// the instruction whose def might be clobbered by the terminator.
addRegAndItsAliases(Reg, TRI, Defs);
if (Uses.empty())
return Loc;
// If the terminator is the only instruction in the block and Uses is not
// empty (or we would have returned above), we can still safely hoist
// instructions just before the terminator as long as the Defs/Uses are not
// violated (which is checked in HoistCommonCodeInSuccs).
if (Loc == MBB->begin())
return Loc;
// The terminator is probably a conditional branch, try not to separate the
// branch from condition setting instruction.
MachineBasicBlock::iterator PI = prev_nodbg(Loc, MBB->begin());
bool IsDef = false;
for (const MachineOperand &MO : PI->operands()) {
// If PI has a regmask operand, it is probably a call. Separate away.
if (MO.isRegMask())
return Loc;
if (!MO.isReg() || MO.isUse())
Register Reg = MO.getReg();
if (!Reg)
if (Uses.count(Reg)) {
IsDef = true;
if (!IsDef)
// The condition setting instruction is not just before the conditional
// branch.
return Loc;
// Be conservative, don't insert instruction above something that may have
// side-effects. And since it's potentially bad to separate flag setting
// instruction from the conditional branch, just abort the optimization
// completely.
// Also avoid moving code above predicated instruction since it's hard to
// reason about register liveness with predicated instruction.
bool DontMoveAcrossStore = true;
if (!PI->isSafeToMove(nullptr, DontMoveAcrossStore) || TII->isPredicated(*PI))
return MBB->end();
// Find out what registers are live. Note this routine is ignoring other live
// registers which are only used by instructions in successor blocks.
for (const MachineOperand &MO : PI->operands()) {
if (!MO.isReg())
Register Reg = MO.getReg();
if (!Reg)
if (MO.isUse()) {
addRegAndItsAliases(Reg, TRI, Uses);
} else {
if (Uses.erase(Reg)) {
if (Register::isPhysicalRegister(Reg)) {
for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs)
Uses.erase(*SubRegs); // Use sub-registers to be conservative
addRegAndItsAliases(Reg, TRI, Defs);
return PI;
bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 4> Cond;
if (TII->analyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
return false;
if (!FBB) FBB = findFalseBlock(MBB, TBB);
if (!FBB)
// Malformed bcc? True and false blocks are the same?
return false;
// Restrict the optimization to cases where MBB is the only predecessor,
// it is an obvious win.
if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
return false;
// Find a suitable position to hoist the common instructions to. Also figure
// out which registers are used or defined by instructions from the insertion
// point to the end of the block.
SmallSet<Register, 4> Uses, Defs;
MachineBasicBlock::iterator Loc =
findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
if (Loc == MBB->end())
return false;
bool HasDups = false;
SmallSet<Register, 4> ActiveDefsSet, AllDefsSet;
MachineBasicBlock::iterator TIB = TBB->begin();
MachineBasicBlock::iterator FIB = FBB->begin();
MachineBasicBlock::iterator TIE = TBB->end();
MachineBasicBlock::iterator FIE = FBB->end();
while (TIB != TIE && FIB != FIE) {
// Skip dbg_value instructions. These do not count.
TIB = skipDebugInstructionsForward(TIB, TIE);
FIB = skipDebugInstructionsForward(FIB, FIE);
if (TIB == TIE || FIB == FIE)
if (!TIB->isIdenticalTo(*FIB, MachineInstr::CheckKillDead))
if (TII->isPredicated(*TIB))
// Hard to reason about register liveness with predicated instruction.
bool IsSafe = true;
for (MachineOperand &MO : TIB->operands()) {
// Don't attempt to hoist instructions with register masks.
if (MO.isRegMask()) {
IsSafe = false;
if (!MO.isReg())
Register Reg = MO.getReg();
if (!Reg)
if (MO.isDef()) {
if (Uses.count(Reg)) {
// Avoid clobbering a register that's used by the instruction at
// the point of insertion.
IsSafe = false;
if (Defs.count(Reg) && !MO.isDead()) {
// Don't hoist the instruction if the def would be clobber by the
// instruction at the point insertion. FIXME: This is overly
// conservative. It should be possible to hoist the instructions
// in BB2 in the following example:
// BB1:
// r1, eflag = op1 r2, r3
// brcc eflag
// BB2:
// r1 = op2, ...
// = op3, killed r1
IsSafe = false;
} else if (!ActiveDefsSet.count(Reg)) {
if (Defs.count(Reg)) {
// Use is defined by the instruction at the point of insertion.
IsSafe = false;
if (MO.isKill() && Uses.count(Reg))
// Kills a register that's read by the instruction at the point of
// insertion. Remove the kill marker.
if (!IsSafe)
bool DontMoveAcrossStore = true;
if (!TIB->isSafeToMove(nullptr, DontMoveAcrossStore))
// Remove kills from ActiveDefsSet, these registers had short live ranges.
for (const MachineOperand &MO : TIB->operands()) {
if (!MO.isReg() || !MO.isUse() || !MO.isKill())
Register Reg = MO.getReg();
if (!Reg)
if (!AllDefsSet.count(Reg)) {
if (Reg.isPhysical()) {
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
} else {
// Track local defs so we can update liveins.
for (const MachineOperand &MO : TIB->operands()) {
if (!MO.isReg() || !MO.isDef() || MO.isDead())
Register Reg = MO.getReg();
if (!Reg || Reg.isVirtual())
addRegAndItsAliases(Reg, TRI, ActiveDefsSet);
addRegAndItsAliases(Reg, TRI, AllDefsSet);
HasDups = true;
if (!HasDups)
return false;
MBB->splice(Loc, TBB, TBB->begin(), TIB);
FBB->erase(FBB->begin(), FIB);
if (UpdateLiveIns) {
return true;