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//===-- MipsLongBranch.cpp - Emit long branches ---------------------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This pass expands a branch or jump instruction into a long branch if its
// offset is too large to fit into its immediate field.
// FIXME: Fix pc-region jump instructions which cross 256MB segment boundaries.
#include "Mips.h"
#include "MCTargetDesc/MipsBaseInfo.h"
#include "MipsTargetMachine.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetRegisterInfo.h"
using namespace llvm;
#define DEBUG_TYPE "mips-long-branch"
STATISTIC(LongBranches, "Number of long branches.");
static cl::opt<bool> SkipLongBranch(
cl::desc("MIPS: Skip long branch pass."),
static cl::opt<bool> ForceLongBranch(
cl::desc("MIPS: Expand all branches to long format."),
namespace {
typedef MachineBasicBlock::iterator Iter;
typedef MachineBasicBlock::reverse_iterator ReverseIter;
struct MBBInfo {
uint64_t Size, Address;
bool HasLongBranch;
MachineInstr *Br;
MBBInfo() : Size(0), HasLongBranch(false), Br(nullptr) {}
class MipsLongBranch : public MachineFunctionPass {
static char ID;
MipsLongBranch(TargetMachine &tm)
: MachineFunctionPass(ID), TM(tm),
IsPIC(TM.getRelocationModel() == Reloc::PIC_),
LongBranchSeqSize(!IsPIC ? 2 : (ABI == MipsSubtarget::N64 ? 13 : 9)) {}
const char *getPassName() const override {
return "Mips Long Branch";
bool runOnMachineFunction(MachineFunction &F) override;
void splitMBB(MachineBasicBlock *MBB);
void initMBBInfo();
int64_t computeOffset(const MachineInstr *Br);
void replaceBranch(MachineBasicBlock &MBB, Iter Br, DebugLoc DL,
MachineBasicBlock *MBBOpnd);
void expandToLongBranch(MBBInfo &Info);
const TargetMachine &TM;
MachineFunction *MF;
SmallVector<MBBInfo, 16> MBBInfos;
bool IsPIC;
unsigned ABI;
unsigned LongBranchSeqSize;
char MipsLongBranch::ID = 0;
} // end of anonymous namespace
/// createMipsLongBranchPass - Returns a pass that converts branches to long
/// branches.
FunctionPass *llvm::createMipsLongBranchPass(MipsTargetMachine &tm) {
return new MipsLongBranch(tm);
/// Iterate over list of Br's operands and search for a MachineBasicBlock
/// operand.
static MachineBasicBlock *getTargetMBB(const MachineInstr &Br) {
for (unsigned I = 0, E = Br.getDesc().getNumOperands(); I < E; ++I) {
const MachineOperand &MO = Br.getOperand(I);
if (MO.isMBB())
return MO.getMBB();
assert(false && "This instruction does not have an MBB operand.");
return nullptr;
// Traverse the list of instructions backwards until a non-debug instruction is
// found or it reaches E.
static ReverseIter getNonDebugInstr(ReverseIter B, ReverseIter E) {
for (; B != E; ++B)
if (!B->isDebugValue())
return B;
return E;
// Split MBB if it has two direct jumps/branches.
void MipsLongBranch::splitMBB(MachineBasicBlock *MBB) {
ReverseIter End = MBB->rend();
ReverseIter LastBr = getNonDebugInstr(MBB->rbegin(), End);
// Return if MBB has no branch instructions.
if ((LastBr == End) ||
(!LastBr->isConditionalBranch() && !LastBr->isUnconditionalBranch()))
ReverseIter FirstBr = getNonDebugInstr(std::next(LastBr), End);
// MBB has only one branch instruction if FirstBr is not a branch
// instruction.
if ((FirstBr == End) ||
(!FirstBr->isConditionalBranch() && !FirstBr->isUnconditionalBranch()))
assert(!FirstBr->isIndirectBranch() && "Unexpected indirect branch found.");
// Create a new MBB. Move instructions in MBB to the newly created MBB.
MachineBasicBlock *NewMBB =
// Insert NewMBB and fix control flow.
MachineBasicBlock *Tgt = getTargetMBB(*FirstBr);
MF->insert(std::next(MachineFunction::iterator(MBB)), NewMBB);
NewMBB->splice(NewMBB->end(), MBB, (++LastBr).base(), MBB->end());
// Fill MBBInfos.
void MipsLongBranch::initMBBInfo() {
// Split the MBBs if they have two branches. Each basic block should have at
// most one branch after this loop is executed.
for (MachineFunction::iterator I = MF->begin(), E = MF->end(); I != E;)
const MipsInstrInfo *TII =
static_cast<const MipsInstrInfo*>(TM.getInstrInfo());
for (unsigned I = 0, E = MBBInfos.size(); I < E; ++I) {
MachineBasicBlock *MBB = MF->getBlockNumbered(I);
// Compute size of MBB.
for (MachineBasicBlock::instr_iterator MI = MBB->instr_begin();
MI != MBB->instr_end(); ++MI)
MBBInfos[I].Size += TII->GetInstSizeInBytes(&*MI);
// Search for MBB's branch instruction.
ReverseIter End = MBB->rend();
ReverseIter Br = getNonDebugInstr(MBB->rbegin(), End);
if ((Br != End) && !Br->isIndirectBranch() &&
(Br->isConditionalBranch() ||
(Br->isUnconditionalBranch() &&
TM.getRelocationModel() == Reloc::PIC_)))
MBBInfos[I].Br = (++Br).base();
// Compute offset of branch in number of bytes.
int64_t MipsLongBranch::computeOffset(const MachineInstr *Br) {
int64_t Offset = 0;
int ThisMBB = Br->getParent()->getNumber();
int TargetMBB = getTargetMBB(*Br)->getNumber();
// Compute offset of a forward branch.
if (ThisMBB < TargetMBB) {
for (int N = ThisMBB + 1; N < TargetMBB; ++N)
Offset += MBBInfos[N].Size;
return Offset + 4;
// Compute offset of a backward branch.
for (int N = ThisMBB; N >= TargetMBB; --N)
Offset += MBBInfos[N].Size;
return -Offset + 4;
// Replace Br with a branch which has the opposite condition code and a
// MachineBasicBlock operand MBBOpnd.
void MipsLongBranch::replaceBranch(MachineBasicBlock &MBB, Iter Br,
DebugLoc DL, MachineBasicBlock *MBBOpnd) {
const MipsInstrInfo *TII =
static_cast<const MipsInstrInfo*>(TM.getInstrInfo());
unsigned NewOpc = TII->getOppositeBranchOpc(Br->getOpcode());
const MCInstrDesc &NewDesc = TII->get(NewOpc);
MachineInstrBuilder MIB = BuildMI(MBB, Br, DL, NewDesc);
for (unsigned I = 0, E = Br->getDesc().getNumOperands(); I < E; ++I) {
MachineOperand &MO = Br->getOperand(I);
if (!MO.isReg()) {
assert(MO.isMBB() && "MBB operand expected.");
// Bundle the instruction in the delay slot to the newly created branch
// and erase the original branch.
MachineBasicBlock::instr_iterator II(Br);
// Expand branch instructions to long branches.
void MipsLongBranch::expandToLongBranch(MBBInfo &I) {
MachineBasicBlock::iterator Pos;
MachineBasicBlock *MBB = I.Br->getParent(), *TgtMBB = getTargetMBB(*I.Br);
DebugLoc DL = I.Br->getDebugLoc();
const BasicBlock *BB = MBB->getBasicBlock();
MachineFunction::iterator FallThroughMBB = ++MachineFunction::iterator(MBB);
MachineBasicBlock *LongBrMBB = MF->CreateMachineBasicBlock(BB);
const MipsInstrInfo *TII =
static_cast<const MipsInstrInfo*>(TM.getInstrInfo());
MF->insert(FallThroughMBB, LongBrMBB);
if (IsPIC) {
MachineBasicBlock *BalTgtMBB = MF->CreateMachineBasicBlock(BB);
MF->insert(FallThroughMBB, BalTgtMBB);
if (ABI != MipsSubtarget::N64) {
// $longbr:
// addiu $sp, $sp, -8
// sw $ra, 0($sp)
// lui $at, %hi($tgt - $baltgt)
// bal $baltgt
// addiu $at, $at, %lo($tgt - $baltgt)
// $baltgt:
// addu $at, $ra, $at
// lw $ra, 0($sp)
// jr $at
// addiu $sp, $sp, 8
// $fallthrough:
Pos = LongBrMBB->begin();
BuildMI(*LongBrMBB, Pos, DL, TII->get(Mips::ADDiu), Mips::SP)
BuildMI(*LongBrMBB, Pos, DL, TII->get(Mips::SW)).addReg(Mips::RA)
// LUi and ADDiu instructions create 32-bit offset of the target basic
// block from the target of BAL instruction. We cannot use immediate
// value for this offset because it cannot be determined accurately when
// the program has inline assembly statements. We therefore use the
// relocation expressions %hi($tgt-$baltgt) and %lo($tgt-$baltgt) which
// are resolved during the fixup, so the values will always be correct.
// Since we cannot create %hi($tgt-$baltgt) and %lo($tgt-$baltgt)
// expressions at this point (it is possible only at the MC layer),
// we replace LUi and ADDiu with pseudo instructions
// LONG_BRANCH_LUi and LONG_BRANCH_ADDiu, and add both basic
// blocks as operands to these instructions. When lowering these pseudo
// instructions to LUi and ADDiu in the MC layer, we will create
// %hi($tgt-$baltgt) and %lo($tgt-$baltgt) expressions and add them as
// operands to lowered instructions.
BuildMI(*LongBrMBB, Pos, DL, TII->get(Mips::LONG_BRANCH_LUi), Mips::AT)
MIBundleBuilder(*LongBrMBB, Pos)
.append(BuildMI(*MF, DL, TII->get(Mips::BAL_BR)).addMBB(BalTgtMBB))
.append(BuildMI(*MF, DL, TII->get(Mips::LONG_BRANCH_ADDiu), Mips::AT)
Pos = BalTgtMBB->begin();
BuildMI(*BalTgtMBB, Pos, DL, TII->get(Mips::ADDu), Mips::AT)
BuildMI(*BalTgtMBB, Pos, DL, TII->get(Mips::LW), Mips::RA)
MIBundleBuilder(*BalTgtMBB, Pos)
.append(BuildMI(*MF, DL, TII->get(Mips::JR)).addReg(Mips::AT))
.append(BuildMI(*MF, DL, TII->get(Mips::ADDiu), Mips::SP)
} else {
// $longbr:
// daddiu $sp, $sp, -16
// sd $ra, 0($sp)
// lui64 $at, %highest($tgt - $baltgt)
// daddiu $at, $at, %higher($tgt - $baltgt)
// dsll $at, $at, 16
// daddiu $at, $at, %hi($tgt - $baltgt)
// dsll $at, $at, 16
// bal $baltgt
// daddiu $at, $at, %lo($tgt - $baltgt)
// $baltgt:
// daddu $at, $ra, $at
// ld $ra, 0($sp)
// jr64 $at
// daddiu $sp, $sp, 16
// $fallthrough:
// TODO: %highest and %higher can have non-zero values only when the
// offset is greater than 4GB, which is highly unlikely. Replace
// them (and the following instructon that shifts $at by 16) with the
// instruction that sets $at to zero.
Pos = LongBrMBB->begin();
BuildMI(*LongBrMBB, Pos, DL, TII->get(Mips::DADDiu), Mips::SP_64)
BuildMI(*LongBrMBB, Pos, DL, TII->get(Mips::SD)).addReg(Mips::RA_64)
BuildMI(*LongBrMBB, Pos, DL, TII->get(Mips::LONG_BRANCH_LUi64),
BuildMI(*LongBrMBB, Pos, DL, TII->get(Mips::LONG_BRANCH_DADDiu),
Mips::AT_64).addReg(Mips::AT_64).addMBB(TgtMBB, MipsII::MO_HIGHER)
BuildMI(*LongBrMBB, Pos, DL, TII->get(Mips::DSLL), Mips::AT_64)
BuildMI(*LongBrMBB, Pos, DL, TII->get(Mips::LONG_BRANCH_DADDiu),
Mips::AT_64).addReg(Mips::AT_64).addMBB(TgtMBB, MipsII::MO_ABS_HI)
BuildMI(*LongBrMBB, Pos, DL, TII->get(Mips::DSLL), Mips::AT_64)
MIBundleBuilder(*LongBrMBB, Pos)
.append(BuildMI(*MF, DL, TII->get(Mips::BAL_BR)).addMBB(BalTgtMBB))
.append(BuildMI(*MF, DL, TII->get(Mips::LONG_BRANCH_DADDiu),
.addMBB(TgtMBB, MipsII::MO_ABS_LO)
Pos = BalTgtMBB->begin();
BuildMI(*BalTgtMBB, Pos, DL, TII->get(Mips::DADDu), Mips::AT_64)
BuildMI(*BalTgtMBB, Pos, DL, TII->get(Mips::LD), Mips::RA_64)
MIBundleBuilder(*BalTgtMBB, Pos)
.append(BuildMI(*MF, DL, TII->get(Mips::JR64)).addReg(Mips::AT_64))
.append(BuildMI(*MF, DL, TII->get(Mips::DADDiu), Mips::SP_64)
assert(LongBrMBB->size() + BalTgtMBB->size() == LongBranchSeqSize);
} else {
// $longbr:
// j $tgt
// nop
// $fallthrough:
Pos = LongBrMBB->begin();
MIBundleBuilder(*LongBrMBB, Pos)
.append(BuildMI(*MF, DL, TII->get(Mips::J)).addMBB(TgtMBB))
.append(BuildMI(*MF, DL, TII->get(Mips::NOP)));
assert(LongBrMBB->size() == LongBranchSeqSize);
if (I.Br->isUnconditionalBranch()) {
// Change branch destination.
assert(I.Br->getDesc().getNumOperands() == 1);
} else
// Change branch destination and reverse condition.
replaceBranch(*MBB, I.Br, DL, FallThroughMBB);
static void emitGPDisp(MachineFunction &F, const MipsInstrInfo *TII) {
MachineBasicBlock &MBB = F.front();
MachineBasicBlock::iterator I = MBB.begin();
DebugLoc DL = MBB.findDebugLoc(MBB.begin());
BuildMI(MBB, I, DL, TII->get(Mips::LUi), Mips::V0)
.addExternalSymbol("_gp_disp", MipsII::MO_ABS_HI);
BuildMI(MBB, I, DL, TII->get(Mips::ADDiu), Mips::V0)
.addReg(Mips::V0).addExternalSymbol("_gp_disp", MipsII::MO_ABS_LO);
bool MipsLongBranch::runOnMachineFunction(MachineFunction &F) {
const MipsInstrInfo *TII =
static_cast<const MipsInstrInfo*>(TM.getInstrInfo());
if (TM.getSubtarget<MipsSubtarget>().inMips16Mode())
return false;
if ((TM.getRelocationModel() == Reloc::PIC_) &&
TM.getSubtarget<MipsSubtarget>().isABI_O32() &&
emitGPDisp(F, TII);
if (SkipLongBranch)
return true;
MF = &F;
SmallVectorImpl<MBBInfo>::iterator I, E = MBBInfos.end();
bool EverMadeChange = false, MadeChange = true;
while (MadeChange) {
MadeChange = false;
for (I = MBBInfos.begin(); I != E; ++I) {
// Skip if this MBB doesn't have a branch or the branch has already been
// converted to a long branch.
if (!I->Br || I->HasLongBranch)
int ShVal = TM.getSubtarget<MipsSubtarget>().inMicroMipsMode() ? 2 : 4;
// Check if offset fits into 16-bit immediate field of branches.
if (!ForceLongBranch && isInt<16>(computeOffset(I->Br) / ShVal))
I->HasLongBranch = true;
I->Size += LongBranchSeqSize * 4;
EverMadeChange = MadeChange = true;
if (!EverMadeChange)
return true;
// Compute basic block addresses.
if (TM.getRelocationModel() == Reloc::PIC_) {
uint64_t Address = 0;
for (I = MBBInfos.begin(); I != E; Address += I->Size, ++I)
I->Address = Address;
// Do the expansion.
for (I = MBBInfos.begin(); I != E; ++I)
if (I->HasLongBranch)
return true;