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//===- ARMConstantIslandPass.cpp - ARM constant islands -------------------===//
// 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 file contains a pass that splits the constant pool up into 'islands'
// which are scattered through-out the function. This is required due to the
// limited pc-relative displacements that ARM has.
#include "ARM.h"
#include "ARMBaseInstrInfo.h"
#include "ARMBasicBlockInfo.h"
#include "ARMMachineFunctionInfo.h"
#include "ARMSubtarget.h"
#include "MCTargetDesc/ARMBaseInfo.h"
#include "MVETailPredUtils.h"
#include "Thumb2InstrInfo.h"
#include "Utils/ARMBaseInfo.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/CodeGen/LivePhysRegs.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineConstantPool.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineJumpTableInfo.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/Format.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <iterator>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "arm-cp-islands"
"ARM constant island placement and branch shortening pass"
STATISTIC(NumCPEs, "Number of constpool entries");
STATISTIC(NumSplit, "Number of uncond branches inserted");
STATISTIC(NumCBrFixed, "Number of cond branches fixed");
STATISTIC(NumUBrFixed, "Number of uncond branches fixed");
STATISTIC(NumTBs, "Number of table branches generated");
STATISTIC(NumT2CPShrunk, "Number of Thumb2 constantpool instructions shrunk");
STATISTIC(NumT2BrShrunk, "Number of Thumb2 immediate branches shrunk");
STATISTIC(NumCBZ, "Number of CBZ / CBNZ formed");
STATISTIC(NumJTMoved, "Number of jump table destination blocks moved");
STATISTIC(NumJTInserted, "Number of jump table intermediate blocks inserted");
STATISTIC(NumLEInserted, "Number of LE backwards branches inserted");
static cl::opt<bool>
AdjustJumpTableBlocks("arm-adjust-jump-tables", cl::Hidden, cl::init(true),
cl::desc("Adjust basic block layout to better use TB[BH]"));
static cl::opt<unsigned>
CPMaxIteration("arm-constant-island-max-iteration", cl::Hidden, cl::init(30),
cl::desc("The max number of iteration for converge"));
static cl::opt<bool> SynthesizeThumb1TBB(
"arm-synthesize-thumb-1-tbb", cl::Hidden, cl::init(true),
cl::desc("Use compressed jump tables in Thumb-1 by synthesizing an "
"equivalent to the TBB/TBH instructions"));
namespace {
/// ARMConstantIslands - Due to limited PC-relative displacements, ARM
/// requires constant pool entries to be scattered among the instructions
/// inside a function. To do this, it completely ignores the normal LLVM
/// constant pool; instead, it places constants wherever it feels like with
/// special instructions.
/// The terminology used in this pass includes:
/// Islands - Clumps of constants placed in the function.
/// Water - Potential places where an island could be formed.
/// CPE - A constant pool entry that has been placed somewhere, which
/// tracks a list of users.
class ARMConstantIslands : public MachineFunctionPass {
std::unique_ptr<ARMBasicBlockUtils> BBUtils = nullptr;
/// WaterList - A sorted list of basic blocks where islands could be placed
/// (i.e. blocks that don't fall through to the following block, due
/// to a return, unreachable, or unconditional branch).
std::vector<MachineBasicBlock*> WaterList;
/// NewWaterList - The subset of WaterList that was created since the
/// previous iteration by inserting unconditional branches.
SmallSet<MachineBasicBlock*, 4> NewWaterList;
using water_iterator = std::vector<MachineBasicBlock *>::iterator;
/// CPUser - One user of a constant pool, keeping the machine instruction
/// pointer, the constant pool being referenced, and the max displacement
/// allowed from the instruction to the CP. The HighWaterMark records the
/// highest basic block where a new CPEntry can be placed. To ensure this
/// pass terminates, the CP entries are initially placed at the end of the
/// function and then move monotonically to lower addresses. The
/// exception to this rule is when the current CP entry for a particular
/// CPUser is out of range, but there is another CP entry for the same
/// constant value in range. We want to use the existing in-range CP
/// entry, but if it later moves out of range, the search for new water
/// should resume where it left off. The HighWaterMark is used to record
/// that point.
struct CPUser {
MachineInstr *MI;
MachineInstr *CPEMI;
MachineBasicBlock *HighWaterMark;
unsigned MaxDisp;
bool NegOk;
bool IsSoImm;
bool KnownAlignment = false;
CPUser(MachineInstr *mi, MachineInstr *cpemi, unsigned maxdisp,
bool neg, bool soimm)
: MI(mi), CPEMI(cpemi), MaxDisp(maxdisp), NegOk(neg), IsSoImm(soimm) {
HighWaterMark = CPEMI->getParent();
/// getMaxDisp - Returns the maximum displacement supported by MI.
/// Correct for unknown alignment.
/// Conservatively subtract 2 bytes to handle weird alignment effects.
unsigned getMaxDisp() const {
return (KnownAlignment ? MaxDisp : MaxDisp - 2) - 2;
/// CPUsers - Keep track of all of the machine instructions that use various
/// constant pools and their max displacement.
std::vector<CPUser> CPUsers;
/// CPEntry - One per constant pool entry, keeping the machine instruction
/// pointer, the constpool index, and the number of CPUser's which
/// reference this entry.
struct CPEntry {
MachineInstr *CPEMI;
unsigned CPI;
unsigned RefCount;
CPEntry(MachineInstr *cpemi, unsigned cpi, unsigned rc = 0)
: CPEMI(cpemi), CPI(cpi), RefCount(rc) {}
/// CPEntries - Keep track of all of the constant pool entry machine
/// instructions. For each original constpool index (i.e. those that existed
/// upon entry to this pass), it keeps a vector of entries. Original
/// elements are cloned as we go along; the clones are put in the vector of
/// the original element, but have distinct CPIs.
/// The first half of CPEntries contains generic constants, the second half
/// contains jump tables. Use getCombinedIndex on a generic CPEMI to look up
/// which vector it will be in here.
std::vector<std::vector<CPEntry>> CPEntries;
/// Maps a JT index to the offset in CPEntries containing copies of that
/// table. The equivalent map for a CONSTPOOL_ENTRY is the identity.
DenseMap<int, int> JumpTableEntryIndices;
/// Maps a JT index to the LEA that actually uses the index to calculate its
/// base address.
DenseMap<int, int> JumpTableUserIndices;
// Maps a MachineBasicBlock to the number of jump tables entries.
DenseMap<const MachineBasicBlock *, int> BlockJumpTableRefCount;
/// ImmBranch - One per immediate branch, keeping the machine instruction
/// pointer, conditional or unconditional, the max displacement,
/// and (if isCond is true) the corresponding unconditional branch
/// opcode.
struct ImmBranch {
MachineInstr *MI;
unsigned MaxDisp : 31;
bool isCond : 1;
unsigned UncondBr;
ImmBranch(MachineInstr *mi, unsigned maxdisp, bool cond, unsigned ubr)
: MI(mi), MaxDisp(maxdisp), isCond(cond), UncondBr(ubr) {}
/// ImmBranches - Keep track of all the immediate branch instructions.
std::vector<ImmBranch> ImmBranches;
/// PushPopMIs - Keep track of all the Thumb push / pop instructions.
SmallVector<MachineInstr*, 4> PushPopMIs;
/// T2JumpTables - Keep track of all the Thumb2 jumptable instructions.
SmallVector<MachineInstr*, 4> T2JumpTables;
MachineFunction *MF;
MachineConstantPool *MCP;
const ARMBaseInstrInfo *TII;
const ARMSubtarget *STI;
ARMFunctionInfo *AFI;
MachineDominatorTree *DT = nullptr;
bool isThumb;
bool isThumb1;
bool isThumb2;
bool isPositionIndependentOrROPI;
static char ID;
ARMConstantIslands() : MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
StringRef getPassName() const override {
void doInitialConstPlacement(std::vector<MachineInstr *> &CPEMIs);
void doInitialJumpTablePlacement(std::vector<MachineInstr *> &CPEMIs);
bool BBHasFallthrough(MachineBasicBlock *MBB);
CPEntry *findConstPoolEntry(unsigned CPI, const MachineInstr *CPEMI);
Align getCPEAlign(const MachineInstr *CPEMI);
void scanFunctionJumpTables();
void initializeFunctionInfo(const std::vector<MachineInstr*> &CPEMIs);
MachineBasicBlock *splitBlockBeforeInstr(MachineInstr *MI);
void updateForInsertedWaterBlock(MachineBasicBlock *NewBB);
bool decrementCPEReferenceCount(unsigned CPI, MachineInstr* CPEMI);
unsigned getCombinedIndex(const MachineInstr *CPEMI);
int findInRangeCPEntry(CPUser& U, unsigned UserOffset);
bool findAvailableWater(CPUser&U, unsigned UserOffset,
water_iterator &WaterIter, bool CloserWater);
void createNewWater(unsigned CPUserIndex, unsigned UserOffset,
MachineBasicBlock *&NewMBB);
bool handleConstantPoolUser(unsigned CPUserIndex, bool CloserWater);
void removeDeadCPEMI(MachineInstr *CPEMI);
bool removeUnusedCPEntries();
bool isCPEntryInRange(MachineInstr *MI, unsigned UserOffset,
MachineInstr *CPEMI, unsigned Disp, bool NegOk,
bool DoDump = false);
bool isWaterInRange(unsigned UserOffset, MachineBasicBlock *Water,
CPUser &U, unsigned &Growth);
bool fixupImmediateBr(ImmBranch &Br);
bool fixupConditionalBr(ImmBranch &Br);
bool fixupUnconditionalBr(ImmBranch &Br);
bool optimizeThumb2Instructions();
bool optimizeThumb2Branches();
bool reorderThumb2JumpTables();
bool preserveBaseRegister(MachineInstr *JumpMI, MachineInstr *LEAMI,
unsigned &DeadSize, bool &CanDeleteLEA,
bool &BaseRegKill);
bool optimizeThumb2JumpTables();
MachineBasicBlock *adjustJTTargetBlockForward(unsigned JTI,
MachineBasicBlock *BB,
MachineBasicBlock *JTBB);
unsigned getUserOffset(CPUser&) const;
void dumpBBs();
void verify();
bool isOffsetInRange(unsigned UserOffset, unsigned TrialOffset,
unsigned Disp, bool NegativeOK, bool IsSoImm = false);
bool isOffsetInRange(unsigned UserOffset, unsigned TrialOffset,
const CPUser &U) {
return isOffsetInRange(UserOffset, TrialOffset,
U.getMaxDisp(), U.NegOk, U.IsSoImm);
} // end anonymous namespace
char ARMConstantIslands::ID = 0;
/// verify - check BBOffsets, BBSizes, alignment of islands
void ARMConstantIslands::verify() {
#ifndef NDEBUG
BBInfoVector &BBInfo = BBUtils->getBBInfo();
assert(is_sorted(*MF, [&BBInfo](const MachineBasicBlock &LHS,
const MachineBasicBlock &RHS) {
return BBInfo[LHS.getNumber()].postOffset() <
LLVM_DEBUG(dbgs() << "Verifying " << CPUsers.size() << " CP users.\n");
for (CPUser &U : CPUsers) {
unsigned UserOffset = getUserOffset(U);
// Verify offset using the real max displacement without the safety
// adjustment.
if (isCPEntryInRange(U.MI, UserOffset, U.CPEMI, U.getMaxDisp()+2, U.NegOk,
/* DoDump = */ true)) {
LLVM_DEBUG(dbgs() << "OK\n");
LLVM_DEBUG(dbgs() << "Out of range.\n");
llvm_unreachable("Constant pool entry out of range!");
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
/// print block size and offset information - debugging
LLVM_DUMP_METHOD void ARMConstantIslands::dumpBBs() {
BBInfoVector &BBInfo = BBUtils->getBBInfo();
for (unsigned J = 0, E = BBInfo.size(); J !=E; ++J) {
const BasicBlockInfo &BBI = BBInfo[J];
dbgs() << format("%08x %bb.%u\t", BBI.Offset, J)
<< " kb=" << unsigned(BBI.KnownBits)
<< " ua=" << unsigned(BBI.Unalign) << " pa=" << Log2(BBI.PostAlign)
<< format(" size=%#x\n", BBInfo[J].Size);
// Align blocks where the previous block does not fall through. This may add
// extra NOP's but they will not be executed. It uses the PrefLoopAlignment as a
// measure of how much to align, and only runs at CodeGenOptLevel::Aggressive.
static bool AlignBlocks(MachineFunction *MF, const ARMSubtarget *STI) {
if (MF->getTarget().getOptLevel() != CodeGenOptLevel::Aggressive ||
return false;
auto *TLI = STI->getTargetLowering();
const Align Alignment = TLI->getPrefLoopAlignment();
if (Alignment < 4)
return false;
bool Changed = false;
bool PrevCanFallthough = true;
for (auto &MBB : *MF) {
if (!PrevCanFallthough) {
Changed = true;
PrevCanFallthough = MBB.canFallThrough();
// For LOB's, the ARMLowOverheadLoops pass may remove the unconditional
// branch later in the pipeline.
if (STI->hasLOB()) {
for (const auto &MI : reverse(MBB.terminators())) {
if (MI.getOpcode() == ARM::t2B &&
MI.getOperand(0).getMBB() == MBB.getNextNode())
if (isLoopStart(MI) || MI.getOpcode() == ARM::t2LoopEnd ||
MI.getOpcode() == ARM::t2LoopEndDec) {
PrevCanFallthough = true;
// Any other terminator - nothing to do
return Changed;
bool ARMConstantIslands::runOnMachineFunction(MachineFunction &mf) {
MF = &mf;
MCP = mf.getConstantPool();
BBUtils = std::unique_ptr<ARMBasicBlockUtils>(new ARMBasicBlockUtils(mf));
LLVM_DEBUG(dbgs() << "***** ARMConstantIslands: "
<< MCP->getConstants().size() << " CP entries, aligned to "
<< MCP->getConstantPoolAlign().value() << " bytes *****\n");
STI = &MF->getSubtarget<ARMSubtarget>();
TII = STI->getInstrInfo();
isPositionIndependentOrROPI =
STI->getTargetLowering()->isPositionIndependent() || STI->isROPI();
AFI = MF->getInfo<ARMFunctionInfo>();
DT = &getAnalysis<MachineDominatorTree>();
isThumb = AFI->isThumbFunction();
isThumb1 = AFI->isThumb1OnlyFunction();
isThumb2 = AFI->isThumb2Function();
bool GenerateTBB = isThumb2 || (isThumb1 && SynthesizeThumb1TBB);
// TBB generation code in this constant island pass has not been adapted to
// deal with speculation barriers.
if (STI->hardenSlsRetBr())
GenerateTBB = false;
// Renumber all of the machine basic blocks in the function, guaranteeing that
// the numbers agree with the position of the block in the function.
// Try to reorder and otherwise adjust the block layout to make good use
// of the TB[BH] instructions.
bool MadeChange = false;
if (GenerateTBB && AdjustJumpTableBlocks) {
MadeChange |= reorderThumb2JumpTables();
// Data is out of date, so clear it. It'll be re-computed later.
// Blocks may have shifted around. Keep the numbering up to date.
// Align any non-fallthrough blocks
MadeChange |= AlignBlocks(MF, STI);
// Perform the initial placement of the constant pool entries. To start with,
// we put them all at the end of the function.
std::vector<MachineInstr*> CPEMIs;
if (!MCP->isEmpty())
if (MF->getJumpTableInfo())
/// The next UID to take is the first unused one.
// Do the initial scan of the function, building up information about the
// sizes of each block, the location of all the water, and finding all of the
// constant pool users.
// Functions with jump tables need an alignment of 4 because they use the ADR
// instruction, which aligns the PC to 4 bytes before adding an offset.
if (!T2JumpTables.empty())
/// Remove dead constant pool entries.
MadeChange |= removeUnusedCPEntries();
// Iteratively place constant pool entries and fix up branches until there
// is no change.
unsigned NoCPIters = 0, NoBRIters = 0;
while (true) {
LLVM_DEBUG(dbgs() << "Beginning CP iteration #" << NoCPIters << '\n');
bool CPChange = false;
for (unsigned i = 0, e = CPUsers.size(); i != e; ++i)
// For most inputs, it converges in no more than 5 iterations.
// If it doesn't end in 10, the input may have huge BB or many CPEs.
// In this case, we will try different heuristics.
CPChange |= handleConstantPoolUser(i, NoCPIters >= CPMaxIteration / 2);
if (CPChange && ++NoCPIters > CPMaxIteration)
report_fatal_error("Constant Island pass failed to converge!");
// Clear NewWaterList now. If we split a block for branches, it should
// appear as "new water" for the next iteration of constant pool placement.
LLVM_DEBUG(dbgs() << "Beginning BR iteration #" << NoBRIters << '\n');
bool BRChange = false;
for (unsigned i = 0, e = ImmBranches.size(); i != e; ++i)
BRChange |= fixupImmediateBr(ImmBranches[i]);
if (BRChange && ++NoBRIters > 30)
report_fatal_error("Branch Fix Up pass failed to converge!");
if (!CPChange && !BRChange)
MadeChange = true;
// Shrink 32-bit Thumb2 load and store instructions.
if (isThumb2 && !STI->prefers32BitThumb())
MadeChange |= optimizeThumb2Instructions();
// Shrink 32-bit branch instructions.
if (isThumb && STI->hasV8MBaselineOps())
MadeChange |= optimizeThumb2Branches();
// Optimize jump tables using TBB / TBH.
if (GenerateTBB && !STI->genExecuteOnly())
MadeChange |= optimizeThumb2JumpTables();
// After a while, this might be made debug-only, but it is not expensive.
// Save the mapping between original and cloned constpool entries.
for (unsigned i = 0, e = CPEntries.size(); i != e; ++i) {
for (unsigned j = 0, je = CPEntries[i].size(); j != je; ++j) {
const CPEntry & CPE = CPEntries[i][j];
if (CPE.CPEMI && CPE.CPEMI->getOperand(1).isCPI())
AFI->recordCPEClone(i, CPE.CPI);
LLVM_DEBUG(dbgs() << '\n'; dumpBBs());
return MadeChange;
/// Perform the initial placement of the regular constant pool entries.
/// To start with, we put them all at the end of the function.
ARMConstantIslands::doInitialConstPlacement(std::vector<MachineInstr*> &CPEMIs) {
// Create the basic block to hold the CPE's.
MachineBasicBlock *BB = MF->CreateMachineBasicBlock();
// MachineConstantPool measures alignment in bytes.
const Align MaxAlign = MCP->getConstantPoolAlign();
const unsigned MaxLogAlign = Log2(MaxAlign);
// Mark the basic block as required by the const-pool.
// The function needs to be as aligned as the basic blocks. The linker may
// move functions around based on their alignment.
// Special case: halfword literals still need word alignment on the function.
Align FuncAlign = MaxAlign;
if (MaxAlign == 2)
FuncAlign = Align(4);
// Order the entries in BB by descending alignment. That ensures correct
// alignment of all entries as long as BB is sufficiently aligned. Keep
// track of the insertion point for each alignment. We are going to bucket
// sort the entries as they are created.
SmallVector<MachineBasicBlock::iterator, 8> InsPoint(MaxLogAlign + 1,
// Add all of the constants from the constant pool to the end block, use an
// identity mapping of CPI's to CPE's.
const std::vector<MachineConstantPoolEntry> &CPs = MCP->getConstants();
const DataLayout &TD = MF->getDataLayout();
for (unsigned i = 0, e = CPs.size(); i != e; ++i) {
unsigned Size = CPs[i].getSizeInBytes(TD);
Align Alignment = CPs[i].getAlign();
// Verify that all constant pool entries are a multiple of their alignment.
// If not, we would have to pad them out so that instructions stay aligned.
assert(isAligned(Alignment, Size) && "CP Entry not multiple of 4 bytes!");
// Insert CONSTPOOL_ENTRY before entries with a smaller alignment.
unsigned LogAlign = Log2(Alignment);
MachineBasicBlock::iterator InsAt = InsPoint[LogAlign];
MachineInstr *CPEMI =
BuildMI(*BB, InsAt, DebugLoc(), TII->get(ARM::CONSTPOOL_ENTRY))
// Ensure that future entries with higher alignment get inserted before
// CPEMI. This is bucket sort with iterators.
for (unsigned a = LogAlign + 1; a <= MaxLogAlign; ++a)
if (InsPoint[a] == InsAt)
InsPoint[a] = CPEMI;
// Add a new CPEntry, but no corresponding CPUser yet.
CPEntries.emplace_back(1, CPEntry(CPEMI, i));
LLVM_DEBUG(dbgs() << "Moved CPI#" << i << " to end of function, size = "
<< Size << ", align = " << Alignment.value() << '\n');
/// Do initial placement of the jump tables. Because Thumb2's TBB and TBH
/// instructions can be made more efficient if the jump table immediately
/// follows the instruction, it's best to place them immediately next to their
/// jumps to begin with. In almost all cases they'll never be moved from that
/// position.
void ARMConstantIslands::doInitialJumpTablePlacement(
std::vector<MachineInstr *> &CPEMIs) {
unsigned i = CPEntries.size();
auto MJTI = MF->getJumpTableInfo();
const std::vector<MachineJumpTableEntry> &JT = MJTI->getJumpTables();
// Only inline jump tables are placed in the function.
if (MJTI->getEntryKind() != MachineJumpTableInfo::EK_Inline)
MachineBasicBlock *LastCorrectlyNumberedBB = nullptr;
for (MachineBasicBlock &MBB : *MF) {
auto MI = MBB.getLastNonDebugInstr();
// Look past potential SpeculationBarriers at end of BB.
while (MI != MBB.end() &&
(isSpeculationBarrierEndBBOpcode(MI->getOpcode()) ||
if (MI == MBB.end())
unsigned JTOpcode;
switch (MI->getOpcode()) {
case ARM::BR_JTadd:
case ARM::BR_JTr:
case ARM::tBR_JTr:
case ARM::BR_JTm_i12:
case ARM::BR_JTm_rs:
// These instructions are emitted only in ARM or Thumb1 modes which do not
// support PACBTI. Hence we don't add BTI instructions in the destination
// blocks.
assert(!MF->getInfo<ARMFunctionInfo>()->branchTargetEnforcement() &&
"Branch protection must not be enabled for Arm or Thumb1 modes");
case ARM::t2BR_JT:
case ARM::tTBB_JT:
case ARM::t2TBB_JT:
case ARM::tTBH_JT:
case ARM::t2TBH_JT:
unsigned NumOps = MI->getDesc().getNumOperands();
MachineOperand JTOp =
MI->getOperand(NumOps - (MI->isPredicable() ? 2 : 1));
unsigned JTI = JTOp.getIndex();
unsigned Size = JT[JTI].MBBs.size() * sizeof(uint32_t);
MachineBasicBlock *JumpTableBB = MF->CreateMachineBasicBlock();
MF->insert(std::next(MachineFunction::iterator(MBB)), JumpTableBB);
MachineInstr *CPEMI = BuildMI(*JumpTableBB, JumpTableBB->begin(),
DebugLoc(), TII->get(JTOpcode))
CPEntries.emplace_back(1, CPEntry(CPEMI, JTI));
JumpTableEntryIndices.insert(std::make_pair(JTI, CPEntries.size() - 1));
if (!LastCorrectlyNumberedBB)
LastCorrectlyNumberedBB = &MBB;
// If we did anything then we need to renumber the subsequent blocks.
if (LastCorrectlyNumberedBB)
/// BBHasFallthrough - Return true if the specified basic block can fallthrough
/// into the block immediately after it.
bool ARMConstantIslands::BBHasFallthrough(MachineBasicBlock *MBB) {
// Get the next machine basic block in the function.
MachineFunction::iterator MBBI = MBB->getIterator();
// Can't fall off end of function.
if (std::next(MBBI) == MBB->getParent()->end())
return false;
MachineBasicBlock *NextBB = &*std::next(MBBI);
if (!MBB->isSuccessor(NextBB))
return false;
// Try to analyze the end of the block. A potential fallthrough may already
// have an unconditional branch for whatever reason.
MachineBasicBlock *TBB, *FBB;
SmallVector<MachineOperand, 4> Cond;
bool TooDifficult = TII->analyzeBranch(*MBB, TBB, FBB, Cond);
return TooDifficult || FBB == nullptr;
/// findConstPoolEntry - Given the constpool index and CONSTPOOL_ENTRY MI,
/// look up the corresponding CPEntry.
ARMConstantIslands::CPEntry *
ARMConstantIslands::findConstPoolEntry(unsigned CPI,
const MachineInstr *CPEMI) {
std::vector<CPEntry> &CPEs = CPEntries[CPI];
// Number of entries per constpool index should be small, just do a
// linear search.
for (CPEntry &CPE : CPEs)
return &CPE;
return nullptr;
/// getCPEAlign - Returns the required alignment of the constant pool entry
/// represented by CPEMI.
Align ARMConstantIslands::getCPEAlign(const MachineInstr *CPEMI) {
switch (CPEMI->getOpcode()) {
return isThumb1 ? Align(4) : Align(1);
return isThumb1 ? Align(4) : Align(2);
return Align(2);
return Align(4);
llvm_unreachable("unknown constpool entry kind");
unsigned CPI = getCombinedIndex(CPEMI);
assert(CPI < MCP->getConstants().size() && "Invalid constant pool index.");
return MCP->getConstants()[CPI].getAlign();
// Exception landing pads, blocks that has their adress taken, and function
// entry blocks will always be (potential) indirect jump targets, regardless of
// whether they are referenced by or not by jump tables.
static bool isAlwaysIndirectTarget(const MachineBasicBlock &MBB) {
return MBB.isEHPad() || MBB.hasAddressTaken() ||
&MBB == &MBB.getParent()->front();
/// scanFunctionJumpTables - Do a scan of the function, building up
/// information about the sizes of each block and the locations of all
/// the jump tables.
void ARMConstantIslands::scanFunctionJumpTables() {
for (MachineBasicBlock &MBB : *MF) {
for (MachineInstr &I : MBB)
if (I.isBranch() &&
(I.getOpcode() == ARM::t2BR_JT || I.getOpcode() == ARM::tBR_JTr))
if (!MF->getInfo<ARMFunctionInfo>()->branchTargetEnforcement())
if (const MachineJumpTableInfo *JTI = MF->getJumpTableInfo())
for (const MachineJumpTableEntry &JTE : JTI->getJumpTables())
for (const MachineBasicBlock *MBB : JTE.MBBs) {
if (isAlwaysIndirectTarget(*MBB))
// Set the reference count essentially to infinity, it will never
// reach zero and the BTI Instruction will never be removed.
BlockJumpTableRefCount[MBB] = std::numeric_limits<int>::max();
/// initializeFunctionInfo - Do the initial scan of the function, building up
/// information about the sizes of each block, the location of all the water,
/// and finding all of the constant pool users.
void ARMConstantIslands::
initializeFunctionInfo(const std::vector<MachineInstr*> &CPEMIs) {
BBInfoVector &BBInfo = BBUtils->getBBInfo();
// The known bits of the entry block offset are determined by the function
// alignment.
BBInfo.front().KnownBits = Log2(MF->getAlignment());
// Compute block offsets and known bits.
// We only care about jump table instructions when jump tables are inline.
MachineJumpTableInfo *MJTI = MF->getJumpTableInfo();
bool InlineJumpTables =
MJTI && MJTI->getEntryKind() == MachineJumpTableInfo::EK_Inline;
// Now go back through the instructions and build up our data structures.
for (MachineBasicBlock &MBB : *MF) {
// If this block doesn't fall through into the next MBB, then this is
// 'water' that a constant pool island could be placed.
if (!BBHasFallthrough(&MBB))
for (MachineInstr &I : MBB) {
if (I.isDebugInstr())
unsigned Opc = I.getOpcode();
if (I.isBranch()) {
bool isCond = false;
unsigned Bits = 0;
unsigned Scale = 1;
int UOpc = Opc;
switch (Opc) {
continue; // Ignore other JT branches
case ARM::t2BR_JT:
case ARM::tBR_JTr:
if (InlineJumpTables)
continue; // Does not get an entry in ImmBranches
case ARM::Bcc:
isCond = true;
UOpc = ARM::B;
case ARM::B:
Bits = 24;
Scale = 4;
case ARM::tBcc:
isCond = true;
UOpc = ARM::tB;
Bits = 8;
Scale = 2;
case ARM::tB:
Bits = 11;
Scale = 2;
case ARM::t2Bcc:
isCond = true;
UOpc = ARM::t2B;
Bits = 20;
Scale = 2;
case ARM::t2B:
Bits = 24;
Scale = 2;
// Record this immediate branch.
unsigned MaxOffs = ((1 << (Bits-1))-1) * Scale;
ImmBranches.push_back(ImmBranch(&I, MaxOffs, isCond, UOpc));
if (Opc == ARM::tPUSH || Opc == ARM::tPOP_RET)
// Scan the instructions for constant pool operands.
for (unsigned op = 0, e = I.getNumOperands(); op != e; ++op)
if (I.getOperand(op).isCPI() ||
(I.getOperand(op).isJTI() && InlineJumpTables)) {
// We found one. The addressing mode tells us the max displacement
// from the PC that this instruction permits.
// Basic size info comes from the TSFlags field.
unsigned Bits = 0;
unsigned Scale = 1;
bool NegOk = false;
bool IsSoImm = false;
switch (Opc) {
llvm_unreachable("Unknown addressing mode for CP reference!");
// Taking the address of a CP entry.
case ARM::LEApcrel:
case ARM::LEApcrelJT: {
// This takes a SoImm, which is 8 bit immediate rotated. We'll
// pretend the maximum offset is 255 * 4. Since each instruction
// 4 byte wide, this is always correct. We'll check for other
// displacements that fits in a SoImm as well.
Bits = 8;
NegOk = true;
IsSoImm = true;
unsigned CPI = I.getOperand(op).getIndex();
assert(CPI < CPEMIs.size());
MachineInstr *CPEMI = CPEMIs[CPI];
const Align CPEAlign = getCPEAlign(CPEMI);
const unsigned LogCPEAlign = Log2(CPEAlign);
if (LogCPEAlign >= 2)
Scale = 4;
// For constants with less than 4-byte alignment,
// we'll pretend the maximum offset is 255 * 1.
Scale = 1;
case ARM::t2LEApcrel:
case ARM::t2LEApcrelJT:
Bits = 12;
NegOk = true;
case ARM::tLEApcrel:
case ARM::tLEApcrelJT:
Bits = 8;
Scale = 4;
case ARM::LDRBi12:
case ARM::LDRi12:
case ARM::LDRcp:
case ARM::t2LDRpci:
case ARM::t2LDRHpci:
case ARM::t2LDRSHpci:
case ARM::t2LDRBpci:
case ARM::t2LDRSBpci:
Bits = 12; // +-offset_12
NegOk = true;
case ARM::tLDRpci:
Bits = 8;
Scale = 4; // +(offset_8*4)
case ARM::VLDRD:
case ARM::VLDRS:
Bits = 8;
Scale = 4; // +-(offset_8*4)
NegOk = true;
case ARM::VLDRH:
Bits = 8;
Scale = 2; // +-(offset_8*2)
NegOk = true;
// Remember that this is a user of a CP entry.
unsigned CPI = I.getOperand(op).getIndex();
if (I.getOperand(op).isJTI()) {
JumpTableUserIndices.insert(std::make_pair(CPI, CPUsers.size()));
CPI = JumpTableEntryIndices[CPI];
MachineInstr *CPEMI = CPEMIs[CPI];
unsigned MaxOffs = ((1 << Bits)-1) * Scale;
CPUsers.push_back(CPUser(&I, CPEMI, MaxOffs, NegOk, IsSoImm));
// Increment corresponding CPEntry reference count.
CPEntry *CPE = findConstPoolEntry(CPI, CPEMI);
assert(CPE && "Cannot find a corresponding CPEntry!");
// Instructions can only use one CP entry, don't bother scanning the
// rest of the operands.
/// CompareMBBNumbers - Little predicate function to sort the WaterList by MBB
/// ID.
static bool CompareMBBNumbers(const MachineBasicBlock *LHS,
const MachineBasicBlock *RHS) {
return LHS->getNumber() < RHS->getNumber();
/// updateForInsertedWaterBlock - When a block is newly inserted into the
/// machine function, it upsets all of the block numbers. Renumber the blocks
/// and update the arrays that parallel this numbering.
void ARMConstantIslands::updateForInsertedWaterBlock(MachineBasicBlock *NewBB) {
// Renumber the MBB's to keep them consecutive.
// Insert an entry into BBInfo to align it properly with the (newly
// renumbered) block numbers.
BBUtils->insert(NewBB->getNumber(), BasicBlockInfo());
// Next, update WaterList. Specifically, we need to add NewMBB as having
// available water after it.
water_iterator IP = llvm::lower_bound(WaterList, NewBB, CompareMBBNumbers);
WaterList.insert(IP, NewBB);
/// Split the basic block containing MI into two blocks, which are joined by
/// an unconditional branch. Update data structures and renumber blocks to
/// account for this change and returns the newly created block.
MachineBasicBlock *ARMConstantIslands::splitBlockBeforeInstr(MachineInstr *MI) {
MachineBasicBlock *OrigBB = MI->getParent();
// Collect liveness information at MI.
LivePhysRegs LRs(*MF->getSubtarget().getRegisterInfo());
auto LivenessEnd = ++MachineBasicBlock::iterator(MI).getReverse();
for (MachineInstr &LiveMI : make_range(OrigBB->rbegin(), LivenessEnd))
// Create a new MBB for the code after the OrigBB.
MachineBasicBlock *NewBB =
MachineFunction::iterator MBBI = ++OrigBB->getIterator();
MF->insert(MBBI, NewBB);
// Splice the instructions starting with MI over to NewBB.
NewBB->splice(NewBB->end(), OrigBB, MI, OrigBB->end());
// Add an unconditional branch from OrigBB to NewBB.
// Note the new unconditional branch is not being recorded.
// There doesn't seem to be meaningful DebugInfo available; this doesn't
// correspond to anything in the source.
unsigned Opc = isThumb ? (isThumb2 ? ARM::t2B : ARM::tB) : ARM::B;
if (!isThumb)
BuildMI(OrigBB, DebugLoc(), TII->get(Opc)).addMBB(NewBB);
BuildMI(OrigBB, DebugLoc(), TII->get(Opc))
// Update the CFG. All succs of OrigBB are now succs of NewBB.
// OrigBB branches to NewBB.
// Update live-in information in the new block.
MachineRegisterInfo &MRI = MF->getRegInfo();
for (MCPhysReg L : LRs)
if (!MRI.isReserved(L))
// Update internal data structures to account for the newly inserted MBB.
// This is almost the same as updateForInsertedWaterBlock, except that
// the Water goes after OrigBB, not NewBB.
// Insert an entry into BBInfo to align it properly with the (newly
// renumbered) block numbers.
BBUtils->insert(NewBB->getNumber(), BasicBlockInfo());
// Next, update WaterList. Specifically, we need to add OrigMBB as having
// available water after it (but not if it's already there, which happens
// when splitting before a conditional branch that is followed by an
// unconditional branch - in that case we want to insert NewBB).
water_iterator IP = llvm::lower_bound(WaterList, OrigBB, CompareMBBNumbers);
MachineBasicBlock* WaterBB = *IP;
if (WaterBB == OrigBB)
WaterList.insert(std::next(IP), NewBB);
WaterList.insert(IP, OrigBB);
// Figure out how large the OrigBB is. As the first half of the original
// block, it cannot contain a tablejump. The size includes
// the new jump we added. (It should be possible to do this without
// recounting everything, but it's very confusing, and this is rarely
// executed.)
// Figure out how large the NewMBB is. As the second half of the original
// block, it may contain a tablejump.
// All BBOffsets following these blocks must be modified.
return NewBB;
/// getUserOffset - Compute the offset of U.MI as seen by the hardware
/// displacement computation. Update U.KnownAlignment to match its current
/// basic block location.
unsigned ARMConstantIslands::getUserOffset(CPUser &U) const {
unsigned UserOffset = BBUtils->getOffsetOf(U.MI);
SmallVectorImpl<BasicBlockInfo> &BBInfo = BBUtils->getBBInfo();
const BasicBlockInfo &BBI = BBInfo[U.MI->getParent()->getNumber()];
unsigned KnownBits = BBI.internalKnownBits();
// The value read from PC is offset from the actual instruction address.
UserOffset += (isThumb ? 4 : 8);
// Because of inline assembly, we may not know the alignment (mod 4) of U.MI.
// Make sure U.getMaxDisp() returns a constrained range.
U.KnownAlignment = (KnownBits >= 2);
// On Thumb, offsets==2 mod 4 are rounded down by the hardware for
// purposes of the displacement computation; compensate for that here.
// For unknown alignments, getMaxDisp() constrains the range instead.
if (isThumb && U.KnownAlignment)
UserOffset &= ~3u;
return UserOffset;
/// isOffsetInRange - Checks whether UserOffset (the location of a constant pool
/// reference) is within MaxDisp of TrialOffset (a proposed location of a
/// constant pool entry).
/// UserOffset is computed by getUserOffset above to include PC adjustments. If
/// the mod 4 alignment of UserOffset is not known, the uncertainty must be
/// subtracted from MaxDisp instead. CPUser::getMaxDisp() does that.
bool ARMConstantIslands::isOffsetInRange(unsigned UserOffset,
unsigned TrialOffset, unsigned MaxDisp,
bool NegativeOK, bool IsSoImm) {
if (UserOffset <= TrialOffset) {
// User before the Trial.
if (TrialOffset - UserOffset <= MaxDisp)
return true;
// FIXME: Make use full range of soimm values.
} else if (NegativeOK) {
if (UserOffset - TrialOffset <= MaxDisp)
return true;
// FIXME: Make use full range of soimm values.
return false;
/// isWaterInRange - Returns true if a CPE placed after the specified
/// Water (a basic block) will be in range for the specific MI.
/// Compute how much the function will grow by inserting a CPE after Water.
bool ARMConstantIslands::isWaterInRange(unsigned UserOffset,
MachineBasicBlock* Water, CPUser &U,
unsigned &Growth) {
BBInfoVector &BBInfo = BBUtils->getBBInfo();
const Align CPEAlign = getCPEAlign(U.CPEMI);
const unsigned CPEOffset = BBInfo[Water->getNumber()].postOffset(CPEAlign);
unsigned NextBlockOffset;
Align NextBlockAlignment;
MachineFunction::const_iterator NextBlock = Water->getIterator();
if (++NextBlock == MF->end()) {
NextBlockOffset = BBInfo[Water->getNumber()].postOffset();
} else {
NextBlockOffset = BBInfo[NextBlock->getNumber()].Offset;
NextBlockAlignment = NextBlock->getAlignment();
unsigned Size = U.CPEMI->getOperand(2).getImm();
unsigned CPEEnd = CPEOffset + Size;
// The CPE may be able to hide in the alignment padding before the next
// block. It may also cause more padding to be required if it is more aligned
// that the next block.
if (CPEEnd > NextBlockOffset) {
Growth = CPEEnd - NextBlockOffset;
// Compute the padding that would go at the end of the CPE to align the next
// block.
Growth += offsetToAlignment(CPEEnd, NextBlockAlignment);
// If the CPE is to be inserted before the instruction, that will raise
// the offset of the instruction. Also account for unknown alignment padding
// in blocks between CPE and the user.
if (CPEOffset < UserOffset)
UserOffset += Growth + UnknownPadding(MF->getAlignment(), Log2(CPEAlign));
} else
// CPE fits in existing padding.
Growth = 0;
return isOffsetInRange(UserOffset, CPEOffset, U);
/// isCPEntryInRange - Returns true if the distance between specific MI and
/// specific ConstPool entry instruction can fit in MI's displacement field.
bool ARMConstantIslands::isCPEntryInRange(MachineInstr *MI, unsigned UserOffset,
MachineInstr *CPEMI, unsigned MaxDisp,
bool NegOk, bool DoDump) {
unsigned CPEOffset = BBUtils->getOffsetOf(CPEMI);
if (DoDump) {
BBInfoVector &BBInfo = BBUtils->getBBInfo();
unsigned Block = MI->getParent()->getNumber();
const BasicBlockInfo &BBI = BBInfo[Block];
dbgs() << "User of CPE#" << CPEMI->getOperand(0).getImm()
<< " max delta=" << MaxDisp
<< format(" insn address=%#x", UserOffset) << " in "
<< printMBBReference(*MI->getParent()) << ": "
<< format("%#x-%x\t", BBI.Offset, BBI.postOffset()) << *MI
<< format("CPE address=%#x offset=%+d: ", CPEOffset,
int(CPEOffset - UserOffset));
return isOffsetInRange(UserOffset, CPEOffset, MaxDisp, NegOk);
#ifndef NDEBUG
/// BBIsJumpedOver - Return true of the specified basic block's only predecessor
/// unconditionally branches to its only successor.
static bool BBIsJumpedOver(MachineBasicBlock *MBB) {
if (MBB->pred_size() != 1 || MBB->succ_size() != 1)
return false;
MachineBasicBlock *Succ = *MBB->succ_begin();
MachineBasicBlock *Pred = *MBB->pred_begin();
MachineInstr *PredMI = &Pred->back();
if (PredMI->getOpcode() == ARM::B || PredMI->getOpcode() == ARM::tB
|| PredMI->getOpcode() == ARM::t2B)
return PredMI->getOperand(0).getMBB() == Succ;
return false;
#endif // NDEBUG
/// decrementCPEReferenceCount - find the constant pool entry with index CPI
/// and instruction CPEMI, and decrement its refcount. If the refcount
/// becomes 0 remove the entry and instruction. Returns true if we removed
/// the entry, false if we didn't.
bool ARMConstantIslands::decrementCPEReferenceCount(unsigned CPI,
MachineInstr *CPEMI) {
// Find the old entry. Eliminate it if it is no longer used.
CPEntry *CPE = findConstPoolEntry(CPI, CPEMI);
assert(CPE && "Unexpected!");
if (--CPE->RefCount == 0) {
CPE->CPEMI = nullptr;
return true;
return false;
unsigned ARMConstantIslands::getCombinedIndex(const MachineInstr *CPEMI) {
if (CPEMI->getOperand(1).isCPI())
return CPEMI->getOperand(1).getIndex();
return JumpTableEntryIndices[CPEMI->getOperand(1).getIndex()];
/// LookForCPEntryInRange - see if the currently referenced CPE is in range;
/// if not, see if an in-range clone of the CPE is in range, and if so,
/// change the data structures so the user references the clone. Returns:
/// 0 = no existing entry found
/// 1 = entry found, and there were no code insertions or deletions
/// 2 = entry found, and there were code insertions or deletions
int ARMConstantIslands::findInRangeCPEntry(CPUser& U, unsigned UserOffset) {
MachineInstr *UserMI = U.MI;
MachineInstr *CPEMI = U.CPEMI;
// Check to see if the CPE is already in-range.
if (isCPEntryInRange(UserMI, UserOffset, CPEMI, U.getMaxDisp(), U.NegOk,
true)) {
LLVM_DEBUG(dbgs() << "In range\n");
return 1;
// No. Look for previously created clones of the CPE that are in range.
unsigned CPI = getCombinedIndex(CPEMI);
std::vector<CPEntry> &CPEs = CPEntries[CPI];
for (CPEntry &CPE : CPEs) {
// We already tried this one
// Removing CPEs can leave empty entries, skip
if (CPE.CPEMI == nullptr)
if (isCPEntryInRange(UserMI, UserOffset, CPE.CPEMI, U.getMaxDisp(),
U.NegOk)) {
LLVM_DEBUG(dbgs() << "Replacing CPE#" << CPI << " with CPE#" << CPE.CPI
<< "\n");
// Point the CPUser node to the replacement
// Change the CPI in the instruction operand to refer to the clone.
for (MachineOperand &MO : UserMI->operands())
if (MO.isCPI()) {
// Adjust the refcount of the clone...
// ...and the original. If we didn't remove the old entry, none of the
// addresses changed, so we don't need another pass.
return decrementCPEReferenceCount(CPI, CPEMI) ? 2 : 1;
return 0;
/// getUnconditionalBrDisp - Returns the maximum displacement that can fit in
/// the specific unconditional branch instruction.
static inline unsigned getUnconditionalBrDisp(int Opc) {
switch (Opc) {
case ARM::tB:
return ((1<<10)-1)*2;
case ARM::t2B:
return ((1<<23)-1)*2;
return ((1<<23)-1)*4;
/// findAvailableWater - Look for an existing entry in the WaterList in which
/// we can place the CPE referenced from U so it's within range of U's MI.
/// Returns true if found, false if not. If it returns true, WaterIter
/// is set to the WaterList entry. For Thumb, prefer water that will not
/// introduce padding to water that will. To ensure that this pass
/// terminates, the CPE location for a particular CPUser is only allowed to
/// move to a lower address, so search backward from the end of the list and
/// prefer the first water that is in range.
bool ARMConstantIslands::findAvailableWater(CPUser &U, unsigned UserOffset,
water_iterator &WaterIter,
bool CloserWater) {
if (WaterList.empty())
return false;
unsigned BestGrowth = ~0u;
// The nearest water without splitting the UserBB is right after it.
// If the distance is still large (we have a big BB), then we need to split it
// if we don't converge after certain iterations. This helps the following
// situation to converge:
// BB0:
// Big BB
// BB1:
// Constant Pool
// When a CP access is out of range, BB0 may be used as water. However,
// inserting islands between BB0 and BB1 makes other accesses out of range.
MachineBasicBlock *UserBB = U.MI->getParent();
BBInfoVector &BBInfo = BBUtils->getBBInfo();
const Align CPEAlign = getCPEAlign(U.CPEMI);
unsigned MinNoSplitDisp = BBInfo[UserBB->getNumber()].postOffset(CPEAlign);
if (CloserWater && MinNoSplitDisp > U.getMaxDisp() / 2)
return false;
for (water_iterator IP = std::prev(WaterList.end()), B = WaterList.begin();;
--IP) {
MachineBasicBlock* WaterBB = *IP;
// Check if water is in range and is either at a lower address than the
// current "high water mark" or a new water block that was created since
// the previous iteration by inserting an unconditional branch. In the
// latter case, we want to allow resetting the high water mark back to
// this new water since we haven't seen it before. Inserting branches
// should be relatively uncommon and when it does happen, we want to be
// sure to take advantage of it for all the CPEs near that block, so that
// we don't insert more branches than necessary.
// When CloserWater is true, we try to find the lowest address after (or
// equal to) user MI's BB no matter of padding growth.
unsigned Growth;
if (isWaterInRange(UserOffset, WaterBB, U, Growth) &&
(WaterBB->getNumber() < U.HighWaterMark->getNumber() ||
NewWaterList.count(WaterBB) || WaterBB == U.MI->getParent()) &&
Growth < BestGrowth) {
// This is the least amount of required padding seen so far.
BestGrowth = Growth;
WaterIter = IP;
LLVM_DEBUG(dbgs() << "Found water after " << printMBBReference(*WaterBB)
<< " Growth=" << Growth << '\n');
if (CloserWater && WaterBB == U.MI->getParent())
return true;
// Keep looking unless it is perfect and we're not looking for the lowest
// possible address.
if (!CloserWater && BestGrowth == 0)
return true;
if (IP == B)
return BestGrowth != ~0u;
/// createNewWater - No existing WaterList entry will work for
/// CPUsers[CPUserIndex], so create a place to put the CPE. The end of the
/// block is used if in range, and the conditional branch munged so control
/// flow is correct. Otherwise the block is split to create a hole with an
/// unconditional branch around it. In either case NewMBB is set to a
/// block following which the new island can be inserted (the WaterList
/// is not adjusted).
void ARMConstantIslands::createNewWater(unsigned CPUserIndex,
unsigned UserOffset,
MachineBasicBlock *&NewMBB) {
CPUser &U = CPUsers[CPUserIndex];
MachineInstr *UserMI = U.MI;
MachineInstr *CPEMI = U.CPEMI;
const Align CPEAlign = getCPEAlign(CPEMI);
MachineBasicBlock *UserMBB = UserMI->getParent();
BBInfoVector &BBInfo = BBUtils->getBBInfo();
const BasicBlockInfo &UserBBI = BBInfo[UserMBB->getNumber()];
// If the block does not end in an unconditional branch already, and if the
// end of the block is within range, make new water there. (The addition
// below is for the unconditional branch we will be adding: 4 bytes on ARM +
// Thumb2, 2 on Thumb1.
if (BBHasFallthrough(UserMBB)) {
// Size of branch to insert.
unsigned Delta = isThumb1 ? 2 : 4;
// Compute the offset where the CPE will begin.
unsigned CPEOffset = UserBBI.postOffset(CPEAlign) + Delta;
if (isOffsetInRange(UserOffset, CPEOffset, U)) {
LLVM_DEBUG(dbgs() << "Split at end of " << printMBBReference(*UserMBB)
<< format(", expected CPE offset %#x\n", CPEOffset));
NewMBB = &*++UserMBB->getIterator();
// Add an unconditional branch from UserMBB to fallthrough block. Record
// it for branch lengthening; this new branch will not get out of range,
// but if the preceding conditional branch is out of range, the targets
// will be exchanged, and the altered branch may be out of range, so the
// machinery has to know about it.
int UncondBr = isThumb ? ((isThumb2) ? ARM::t2B : ARM::tB) : ARM::B;
if (!isThumb)
BuildMI(UserMBB, DebugLoc(), TII->get(UncondBr)).addMBB(NewMBB);
BuildMI(UserMBB, DebugLoc(), TII->get(UncondBr))
unsigned MaxDisp = getUnconditionalBrDisp(UncondBr);
MaxDisp, false, UncondBr));
// What a big block. Find a place within the block to split it. This is a
// little tricky on Thumb1 since instructions are 2 bytes and constant pool
// entries are 4 bytes: if instruction I references island CPE, and
// instruction I+1 references CPE', it will not work well to put CPE as far
// forward as possible, since then CPE' cannot immediately follow it (that
// location is 2 bytes farther away from I+1 than CPE was from I) and we'd
// need to create a new island. So, we make a first guess, then walk through
// the instructions between the one currently being looked at and the
// possible insertion point, and make sure any other instructions that
// reference CPEs will be able to use the same island area; if not, we back
// up the insertion point.
// Try to split the block so it's fully aligned. Compute the latest split
// point where we can add a 4-byte branch instruction, and then align to
// Align which is the largest possible alignment in the function.
const Align Align = MF->getAlignment();
assert(Align >= CPEAlign && "Over-aligned constant pool entry");
unsigned KnownBits = UserBBI.internalKnownBits();
unsigned UPad = UnknownPadding(Align, KnownBits);
unsigned BaseInsertOffset = UserOffset + U.getMaxDisp() - UPad;
LLVM_DEBUG(dbgs() << format("Split in middle of big block before %#x",
// The 4 in the following is for the unconditional branch we'll be inserting
// (allows for long branch on Thumb1). Alignment of the island is handled
// inside isOffsetInRange.
BaseInsertOffset -= 4;
LLVM_DEBUG(dbgs() << format(", adjusted to %#x", BaseInsertOffset)
<< " la=" << Log2(Align) << " kb=" << KnownBits
<< " up=" << UPad << '\n');
// This could point off the end of the block if we've already got constant
// pool entries following this block; only the last one is in the water list.
// Back past any possible branches (allow for a conditional and a maximally
// long unconditional).
if (BaseInsertOffset + 8 >= UserBBI.postOffset()) {
// Ensure BaseInsertOffset is larger than the offset of the instruction
// following UserMI so that the loop which searches for the split point
// iterates at least once.
BaseInsertOffset =
std::max(UserBBI.postOffset() - UPad - 8,
UserOffset + TII->getInstSizeInBytes(*UserMI) + 1);
// If the CP is referenced(ie, UserOffset) is in first four instructions
// after IT, this recalculated BaseInsertOffset could be in the middle of
// an IT block. If it is, change the BaseInsertOffset to just after the
// IT block. This still make the CP Entry is in range becuase of the
// following reasons.
// 1. The initial BaseseInsertOffset calculated is (UserOffset +
// U.getMaxDisp() - UPad).
// 2. An IT block is only at most 4 instructions plus the "it" itself (18
// bytes).
// 3. All the relevant instructions support much larger Maximum
// displacement.
MachineBasicBlock::iterator I = UserMI;
Register PredReg;
for (unsigned Offset = UserOffset + TII->getInstSizeInBytes(*UserMI);
I->getOpcode() != ARM::t2IT &&
getITInstrPredicate(*I, PredReg) != ARMCC::AL;
Offset += TII->getInstSizeInBytes(*I), I = std::next(I)) {
BaseInsertOffset =
std::max(BaseInsertOffset, Offset + TII->getInstSizeInBytes(*I) + 1);
assert(I != UserMBB->end() && "Fell off end of block");
LLVM_DEBUG(dbgs() << format("Move inside block: %#x\n", BaseInsertOffset));
unsigned EndInsertOffset = BaseInsertOffset + 4 + UPad +
MachineBasicBlock::iterator MI = UserMI;
unsigned CPUIndex = CPUserIndex+1;
unsigned NumCPUsers = CPUsers.size();
MachineInstr *LastIT = nullptr;
for (unsigned Offset = UserOffset + TII->getInstSizeInBytes(*UserMI);
Offset < BaseInsertOffset;
Offset += TII->getInstSizeInBytes(*MI), MI = std::next(MI)) {
assert(MI != UserMBB->end() && "Fell off end of block");
if (CPUIndex < NumCPUsers && CPUsers[CPUIndex].MI == &*MI) {
CPUser &U = CPUsers[CPUIndex];
if (!isOffsetInRange(Offset, EndInsertOffset, U)) {
// Shift intertion point by one unit of alignment so it is within reach.
BaseInsertOffset -= Align.value();
EndInsertOffset -= Align.value();
// This is overly conservative, as we don't account for CPEMIs being
// reused within the block, but it doesn't matter much. Also assume CPEs
// are added in order with alignment padding. We may eventually be able
// to pack the aligned CPEs better.
EndInsertOffset += U.CPEMI->getOperand(2).getImm();
// Remember the last IT instruction.
if (MI->getOpcode() == ARM::t2IT)
LastIT = &*MI;
// Avoid splitting an IT block.
if (LastIT) {
Register PredReg;
ARMCC::CondCodes CC = getITInstrPredicate(*MI, PredReg);
if (CC != ARMCC::AL)
MI = LastIT;
// Avoid splitting a MOVW+MOVT pair with a relocation on Windows.
// On Windows, this instruction pair is covered by one single
// IMAGE_REL_ARM_MOV32T relocation which covers both instructions. If a
// constant island is injected inbetween them, the relocation will clobber
// the instruction and fail to update the MOVT instruction.
// (These instructions are bundled up until right before the ConstantIslands
// pass.)
if (STI->isTargetWindows() && isThumb && MI->getOpcode() == ARM::t2MOVTi16 &&
(MI->getOperand(2).getTargetFlags() & ARMII::MO_OPTION_MASK) ==
assert(MI->getOpcode() == ARM::t2MOVi16 &&
(MI->getOperand(1).getTargetFlags() & ARMII::MO_OPTION_MASK) ==
// We really must not split an IT block.
#ifndef NDEBUG
Register PredReg;
assert(!isThumb || getITInstrPredicate(*MI, PredReg) == ARMCC::AL);
NewMBB = splitBlockBeforeInstr(&*MI);
/// handleConstantPoolUser - Analyze the specified user, checking to see if it
/// is out-of-range. If so, pick up the constant pool value and move it some
/// place in-range. Return true if we changed any addresses (thus must run
/// another pass of branch lengthening), false otherwise.
bool ARMConstantIslands::handleConstantPoolUser(unsigned CPUserIndex,
bool CloserWater) {
CPUser &U = CPUsers[CPUserIndex];
MachineInstr *UserMI = U.MI;
MachineInstr *CPEMI = U.CPEMI;
unsigned CPI = getCombinedIndex(CPEMI);
unsigned Size = CPEMI->getOperand(2).getImm();
// Compute this only once, it's expensive.
unsigned UserOffset = getUserOffset(U);
// See if the current entry is within range, or there is a clone of it
// in range.
int result = findInRangeCPEntry(U, UserOffset);
if (result==1) return false;
else if (result==2) return true;
// No existing clone of this CPE is within range.
// We will be generating a new clone. Get a UID for it.
unsigned ID = AFI->createPICLabelUId();
// Look for water where we can place this CPE.
MachineBasicBlock *NewIsland = MF->CreateMachineBasicBlock();
MachineBasicBlock *NewMBB;
water_iterator IP;
if (findAvailableWater(U, UserOffset, IP, CloserWater)) {
LLVM_DEBUG(dbgs() << "Found water in range\n");
MachineBasicBlock *WaterBB = *IP;
// If the original WaterList entry was "new water" on this iteration,
// propagate that to the new island. This is just keeping NewWaterList
// updated to match the WaterList, which will be updated below.
if (NewWaterList.erase(WaterBB))
// The new CPE goes before the following block (NewMBB).
NewMBB = &*++WaterBB->getIterator();
} else {
// No water found.
LLVM_DEBUG(dbgs() << "No water found\n");
createNewWater(CPUserIndex, UserOffset, NewMBB);
// splitBlockBeforeInstr adds to WaterList, which is important when it is
// called while handling branches so that the water will be seen on the
// next iteration for constant pools, but in this context, we don't want
// it. Check for this so it will be removed from the WaterList.
// Also remove any entry from NewWaterList.
MachineBasicBlock *WaterBB = &*--NewMBB->getIterator();
IP = find(WaterList, WaterBB);
if (IP != WaterList.end())
// We are adding new water. Update NewWaterList.
// Always align the new block because CP entries can be smaller than 4
// bytes. Be careful not to decrease the existing alignment, e.g. NewMBB may
// be an already aligned constant pool block.
const Align Alignment = isThumb ? Align(2) : Align(4);
if (NewMBB->getAlignment() < Alignment)
// Remove the original WaterList entry; we want subsequent insertions in
// this vicinity to go after the one we're about to insert. This
// considerably reduces the number of times we have to move the same CPE
// more than once and is also important to ensure the algorithm terminates.
if (IP != WaterList.end())
// Okay, we know we can put an island before NewMBB now, do it!
MF->insert(NewMBB->getIterator(), NewIsland);
// Update internal data structures to account for the newly inserted MBB.
// Now that we have an island to add the CPE to, clone the original CPE and
// add it to the island.
U.HighWaterMark = NewIsland;
U.CPEMI = BuildMI(NewIsland, DebugLoc(), CPEMI->getDesc())
CPEntries[CPI].push_back(CPEntry(U.CPEMI, ID, 1));
// Decrement the old entry, and remove it if refcount becomes 0.
decrementCPEReferenceCount(CPI, CPEMI);
// Mark the basic block as aligned as required by the const-pool entry.
// Increase the size of the island block to account for the new entry.
BBUtils->adjustBBSize(NewIsland, Size);
// Finally, change the CPI in the instruction operand to be ID.
for (MachineOperand &MO : UserMI->operands())
if (MO.isCPI()) {
dbgs() << " Moved CPE to #" << ID << " CPI=" << CPI
<< format(" offset=%#x\n",
return true;
/// removeDeadCPEMI - Remove a dead constant pool entry instruction. Update
/// sizes and offsets of impacted basic blocks.
void ARMConstantIslands::removeDeadCPEMI(MachineInstr *CPEMI) {
MachineBasicBlock *CPEBB = CPEMI->getParent();
unsigned Size = CPEMI->getOperand(2).getImm();
BBInfoVector &BBInfo = BBUtils->getBBInfo();
BBUtils->adjustBBSize(CPEBB, -Size);
// All succeeding offsets have the current size value added in, fix this.
if (CPEBB->empty()) {
BBInfo[CPEBB->getNumber()].Size = 0;
// This block no longer needs to be aligned.
} else {
// Entries are sorted by descending alignment, so realign from the front.
// An island has only one predecessor BB and one successor BB. Check if
// this BB's predecessor jumps directly to this BB's successor. This
// shouldn't happen currently.
assert(!BBIsJumpedOver(CPEBB) && "How did this happen?");
// FIXME: remove the empty blocks after all the work is done?
/// removeUnusedCPEntries - Remove constant pool entries whose refcounts
/// are zero.
bool ARMConstantIslands::removeUnusedCPEntries() {
unsigned MadeChange = false;
for (std::vector<CPEntry> &CPEs : CPEntries) {
for (CPEntry &CPE : CPEs) {
if (CPE.RefCount == 0 && CPE.CPEMI) {
CPE.CPEMI = nullptr;
MadeChange = true;
return MadeChange;
/// fixupImmediateBr - Fix up an immediate branch whose destination is too far
/// away to fit in its displacement field.
bool ARMConstantIslands::fixupImmediateBr(ImmBranch &Br) {
MachineInstr *MI = Br.MI;
MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();
// Check to see if the DestBB is already in-range.
if (BBUtils->isBBInRange(MI, DestBB, Br.MaxDisp))
return false;
if (!Br.isCond)
return fixupUnconditionalBr(Br);
return fixupConditionalBr(Br);
/// fixupUnconditionalBr - Fix up an unconditional branch whose destination is
/// too far away to fit in its displacement field. If the LR register has been
/// spilled in the epilogue, then we can use BL to implement a far jump.
/// Otherwise, add an intermediate branch instruction to a branch.
ARMConstantIslands::fixupUnconditionalBr(ImmBranch &Br) {
MachineInstr *MI = Br.MI;
MachineBasicBlock *MBB = MI->getParent();
if (!isThumb1)
llvm_unreachable("fixupUnconditionalBr is Thumb1 only!");
if (!AFI->isLRSpilled())
report_fatal_error("underestimated function size");
// Use BL to implement far jump.
Br.MaxDisp = (1 << 21) * 2;
BBInfoVector &BBInfo = BBUtils->getBBInfo();
BBInfo[MBB->getNumber()].Size += 2;
LLVM_DEBUG(dbgs() << " Changed B to long jump " << *MI);
return true;
/// fixupConditionalBr - Fix up a conditional branch whose destination is too
/// far away to fit in its displacement field. It is converted to an inverse
/// conditional branch + an unconditional branch to the destination.
ARMConstantIslands::fixupConditionalBr(ImmBranch &Br) {
MachineInstr *MI = Br.MI;
MachineBasicBlock *DestBB = MI->getOperand(0).getMBB();
// Add an unconditional branch to the destination and invert the branch
// condition to jump over it:
// blt L1
// =>
// bge L2
// b L1
// L2:
ARMCC::CondCodes CC = (ARMCC::CondCodes)MI->getOperand(1).getImm();
CC = ARMCC::getOppositeCondition(CC);
Register CCReg = MI->getOperand(2).getReg();
// If the branch is at the end of its MBB and that has a fall-through block,
// direct the updated conditional branch to the fall-through block. Otherwise,
// split the MBB before the next instruction.
MachineBasicBlock *MBB = MI->getParent();
MachineInstr *BMI = &MBB->back();
bool NeedSplit = (BMI != MI) || !BBHasFallthrough(MBB);
if (BMI != MI) {
if (std::next(MachineBasicBlock::iterator(MI)) == std::prev(MBB->end()) &&
BMI->getOpcode() == Br.UncondBr) {
// Last MI in the BB is an unconditional branch. Can we simply invert the
// condition and swap destinations:
// beq L1
// b L2
// =>
// bne L2
// b L1
MachineBasicBlock *NewDest = BMI->getOperand(0).getMBB();
if (BBUtils->isBBInRange(MI, NewDest, Br.MaxDisp)) {
dbgs() << " Invert Bcc condition and swap its destination with "
<< *BMI);
return true;
if (NeedSplit) {
// No need for the branch to the next block. We're adding an unconditional
// branch to the destination.
int delta = TII->getInstSizeInBytes(MBB->back());
BBUtils->adjustBBSize(MBB, -delta);
// The conditional successor will be swapped between the BBs after this, so
// update CFG.
// BBInfo[SplitBB].Offset is wrong temporarily, fixed below
MachineBasicBlock *NextBB = &*++MBB->getIterator();
LLVM_DEBUG(dbgs() << " Insert B to " << printMBBReference(*DestBB)
<< " also invert condition and change dest. to "
<< printMBBReference(*NextBB) << "\n");
// Insert a new conditional branch and a new unconditional branch.
// Also update the ImmBranch as well as adding a new entry for the new branch.
BuildMI(MBB, DebugLoc(), TII->get(MI->getOpcode()))
Br.MI = &MBB->back();
BBUtils->adjustBBSize(MBB, TII->getInstSizeInBytes(MBB->back()));
if (isThumb)
BuildMI(MBB, DebugLoc(), TII->get(Br.UncondBr))
BuildMI(MBB, DebugLoc(), TII->get(Br.UncondBr)).addMBB(DestBB);
BBUtils->adjustBBSize(MBB, TII->getInstSizeInBytes(MBB->back()));
unsigned MaxDisp = getUnconditionalBrDisp(Br.UncondBr);
ImmBranches.push_back(ImmBranch(&MBB->back(), MaxDisp, false, Br.UncondBr));
// Remove the old conditional branch. It may or may not still be in MBB.
BBUtils->adjustBBSize(MI->getParent(), -TII->getInstSizeInBytes(*MI));
return true;
bool ARMConstantIslands::optimizeThumb2Instructions() {
bool MadeChange = false;
// Shrink ADR and LDR from constantpool.
for (CPUser &U : CPUsers) {
unsigned Opcode = U.MI->getOpcode();
unsigned NewOpc = 0;
unsigned Scale = 1;
unsigned Bits = 0;
switch (Opcode) {
default: break;
case ARM::t2LEApcrel:
if (isARMLowRegister(U.MI->getOperand(0).getReg())) {
NewOpc = ARM::tLEApcrel;
Bits = 8;
Scale = 4;
case ARM::t2LDRpci:
if (isARMLowRegister(U.MI->getOperand(0).getReg())) {
NewOpc = ARM::tLDRpci;
Bits = 8;
Scale = 4;
if (!NewOpc)
unsigned UserOffset = getUserOffset(U);
unsigned MaxOffs = ((1 << Bits) - 1) * Scale;
// Be conservative with inline asm.
if (!U.KnownAlignment)
MaxOffs -= 2;
// FIXME: Check if offset is multiple of scale if scale is not 4.
if (isCPEntryInRange(U.MI, UserOffset, U.CPEMI, MaxOffs, false, true)) {
LLVM_DEBUG(dbgs() << "Shrink: " << *U.MI);
MachineBasicBlock *MBB = U.MI->getParent();
BBUtils->adjustBBSize(MBB, -2);
MadeChange = true;
return MadeChange;
bool ARMConstantIslands::optimizeThumb2Branches() {
auto TryShrinkBranch = [this](ImmBranch &Br) {
unsigned Opcode = Br.MI->getOpcode();
unsigned NewOpc = 0;
unsigned Scale = 1;
unsigned Bits = 0;
switch (Opcode) {
default: break;
case ARM::t2B:
NewOpc = ARM::tB;
Bits = 11;
Scale = 2;
case ARM::t2Bcc:
NewOpc = ARM::tBcc;
Bits = 8;
Scale = 2;
if (NewOpc) {
unsigned MaxOffs = ((1 << (Bits-1))-1) * Scale;
MachineBasicBlock *DestBB = Br.MI->getOperand(0).getMBB();
if (BBUtils->isBBInRange(Br.MI, DestBB, MaxOffs)) {
LLVM_DEBUG(dbgs() << "Shrink branch: " << *Br.MI);
MachineBasicBlock *MBB = Br.MI->getParent();
BBUtils->adjustBBSize(MBB, -2);
return true;
return false;
struct ImmCompare {
MachineInstr* MI = nullptr;
unsigned NewOpc = 0;
auto FindCmpForCBZ = [this](ImmBranch &Br, ImmCompare &ImmCmp,
MachineBasicBlock *DestBB) {
ImmCmp.MI = nullptr;
ImmCmp.NewOpc = 0;
// If the conditional branch doesn't kill CPSR, then CPSR can be liveout
// so this transformation is not safe.
if (!Br.MI->killsRegister(ARM::CPSR))
return false;
Register PredReg;
unsigned NewOpc = 0;
ARMCC::CondCodes Pred = getInstrPredicate(*Br.MI, PredReg);
if (Pred == ARMCC::EQ)
NewOpc = ARM::tCBZ;
else if (Pred == ARMCC::NE)
NewOpc = ARM::tCBNZ;
return false;
// Check if the distance is within 126. Subtract starting offset by 2
// because the cmp will be eliminated.
unsigned BrOffset = BBUtils->getOffsetOf(Br.MI) + 4 - 2;
BBInfoVector &BBInfo = BBUtils->getBBInfo();
unsigned DestOffset = BBInfo[DestBB->getNumber()].Offset;
if (BrOffset >= DestOffset || (DestOffset - BrOffset) > 126)
return false;
// Search backwards to find a tCMPi8
auto *TRI = STI->getRegisterInfo();
MachineInstr *CmpMI = findCMPToFoldIntoCBZ(Br.MI, TRI);
if (!CmpMI || CmpMI->getOpcode() != ARM::tCMPi8)
return false;
ImmCmp.MI = CmpMI;
ImmCmp.NewOpc = NewOpc;
return true;
auto TryConvertToLE = [this](ImmBranch &Br, ImmCompare &Cmp) {
if (Br.MI->getOpcode() != ARM::t2Bcc || !STI->hasLOB() ||
return false;
MachineBasicBlock *MBB = Br.MI->getParent();
MachineBasicBlock *DestBB = Br.MI->getOperand(0).getMBB();
if (BBUtils->getOffsetOf(MBB) < BBUtils->getOffsetOf(DestBB) ||
!BBUtils->isBBInRange(Br.MI, DestBB, 4094))
return false;
if (!DT->dominates(DestBB, MBB))
return false;
// We queried for the CBN?Z opcode based upon the 'ExitBB', the opposite
// target of Br. So now we need to reverse the condition.
Cmp.NewOpc = Cmp.NewOpc == ARM::tCBZ ? ARM::tCBNZ : ARM::tCBZ;
MachineInstrBuilder MIB = BuildMI(*MBB, Br.MI, Br.MI->getDebugLoc(),
// Swapped a t2Bcc for a t2LE, so no need to update the size of the block.
Br.MI = MIB;
return true;
bool MadeChange = false;
// The order in which branches appear in ImmBranches is approximately their
// order within the function body. By visiting later branches first, we reduce
// the distance between earlier forward branches and their targets, making it
// more likely that the cbn?z optimization, which can only apply to forward
// branches, will succeed.
for (ImmBranch &Br : reverse(ImmBranches)) {
MachineBasicBlock *DestBB = Br.MI->getOperand(0).getMBB();
MachineBasicBlock *MBB = Br.MI->getParent();
MachineBasicBlock *ExitBB = &MBB->back() == Br.MI ?
MBB->getFallThrough() :
ImmCompare Cmp;
if (FindCmpForCBZ(Br, Cmp, ExitBB) && TryConvertToLE(Br, Cmp)) {
DestBB = ExitBB;
MadeChange = true;
} else {
FindCmpForCBZ(Br, Cmp, DestBB);
MadeChange |= TryShrinkBranch(Br);
unsigned Opcode = Br.MI->getOpcode();
if ((Opcode != ARM::tBcc && Opcode != ARM::t2LE) || !Cmp.NewOpc)
Register Reg = Cmp.MI->getOperand(0).getReg();
// Check for Kill flags on Reg. If they are present remove them and set kill
// on the new CBZ.
auto *TRI = STI->getRegisterInfo();
MachineBasicBlock::iterator KillMI = Br.MI;
bool RegKilled = false;
do {
if (KillMI->killsRegister(Reg, TRI)) {
KillMI->clearRegisterKills(Reg, TRI);
RegKilled = true;
} while (KillMI != Cmp.MI);
// Create the new CBZ/CBNZ
LLVM_DEBUG(dbgs() << "Fold: " << *Cmp.MI << " and: " << *Br.MI);
MachineInstr *NewBR =
BuildMI(*MBB, Br.MI, Br.MI->getDebugLoc(), TII->get(Cmp.NewOpc))
.addReg(Reg, getKillRegState(RegKilled) |
.addMBB(DestBB, Br.MI->getOperand(0).getTargetFlags());
if (Br.MI->getOpcode() == ARM::tBcc) {
Br.MI = NewBR;
BBUtils->adjustBBSize(MBB, -2);
} else if (MBB->back().getOpcode() != ARM::t2LE) {
// An LE has been generated, but it's not the terminator - that is an
// unconditional branch. However, the logic has now been reversed with the
// CBN?Z being the conditional branch and the LE being the unconditional
// branch. So this means we can remove the redundant unconditional branch
// at the end of the block.
MachineInstr *LastMI = &MBB->back();
BBUtils->adjustBBSize(MBB, -LastMI->getDesc().getSize());
MadeChange = true;
return MadeChange;
static bool isSimpleIndexCalc(MachineInstr &I, unsigned EntryReg,
unsigned BaseReg) {
if (I.getOpcode() != ARM::t2ADDrs)
return false;
if (I.getOperand(0).getReg() != EntryReg)
return false;
if (I.getOperand(1).getReg() != BaseReg)
return false;
// FIXME: what about CC and IdxReg?
return true;
/// While trying to form a TBB/TBH instruction, we may (if the table
/// doesn't immediately follow the BR_JT) need access to the start of the
/// jump-table. We know one instruction that produces such a register; this
/// function works out whether that definition can be preserved to the BR_JT,
/// possibly by removing an intervening addition (which is usually needed to
/// calculate the actual entry to jump to).
bool ARMConstantIslands::preserveBaseRegister(MachineInstr *JumpMI,
MachineInstr *LEAMI,
unsigned &DeadSize,
bool &CanDeleteLEA,
bool &BaseRegKill) {
if (JumpMI->getParent() != LEAMI->getParent())
return false;
// Now we hope that we have at least these instructions in the basic block:
// BaseReg = t2LEA ...
// [...]
// EntryReg = t2ADDrs BaseReg, ...
// [...]
// t2BR_JT EntryReg
// We have to be very conservative about what we recognise here though. The
// main perturbing factors to watch out for are:
// + Spills at any point in the chain: not direct problems but we would
// expect a blocking Def of the spilled register so in practice what we
// can do is limited.
// + EntryReg == BaseReg: this is the one situation we should allow a Def
// of BaseReg, but only if the t2ADDrs can be removed.
// + Some instruction other than t2ADDrs computing the entry. Not seen in
// the wild, but we should be careful.
Register EntryReg = JumpMI->getOperand(0).getReg();
Register BaseReg = LEAMI->getOperand(0).getReg();
CanDeleteLEA = true;
BaseRegKill = false;
MachineInstr *RemovableAdd = nullptr;
MachineBasicBlock::iterator I(LEAMI);
for (++I; &*I != JumpMI; ++I) {
if (isSimpleIndexCalc(*I, EntryReg, BaseReg)) {
RemovableAdd = &*I;
for (const MachineOperand &MO : I->operands()) {
if (!MO.isReg() || !MO.getReg())
if (MO.isDef() && MO.getReg() == BaseReg)
return false;
if (MO.isUse() && MO.getReg() == BaseReg) {
BaseRegKill = BaseRegKill || MO.isKill();
CanDeleteLEA = false;
if (!RemovableAdd)
return true;
// Check the add really is removable, and that nothing else in the block
// clobbers BaseReg.
for (++I; &*I != JumpMI; ++I) {
for (const MachineOperand &MO : I->operands()) {
if (!MO.isReg() || !MO.getReg())
if (MO.isDef() && MO.getReg() == BaseReg)
return false;
if (MO.isUse() && MO.getReg() == EntryReg)
RemovableAdd = nullptr;
if (RemovableAdd) {
DeadSize += isThumb2 ? 4 : 2;
} else if (BaseReg == EntryReg) {
// The add wasn't removable, but clobbered the base for the TBB. So we can't
// preserve it.
return false;
// We reached the end of the block without seeing another definition of
// BaseReg (except, possibly the t2ADDrs, which was removed). BaseReg can be
// used in the TBB/TBH if necessary.
return true;
/// Returns whether CPEMI is the first instruction in the block
/// immediately following JTMI (assumed to be a TBB or TBH terminator). If so,
/// we can switch the first register to PC and usually remove the address
/// calculation that preceded it.
static bool jumpTableFollowsTB(MachineInstr *JTMI, MachineInstr *CPEMI) {
MachineFunction::iterator MBB = JTMI->getParent()->getIterator();
MachineFunction *MF = MBB->getParent();
return MBB != MF->end() && !MBB->empty() && &*MBB->begin() == CPEMI;
static void RemoveDeadAddBetweenLEAAndJT(MachineInstr *LEAMI,
MachineInstr *JumpMI,
unsigned &DeadSize) {
// Remove a dead add between the LEA and JT, which used to compute EntryReg,
// but the JT now uses PC. Finds the last ADD (if any) that def's EntryReg
// and is not clobbered / used.
MachineInstr *RemovableAdd = nullptr;
Register EntryReg = JumpMI->getOperand(0).getReg();
// Find the last ADD to set EntryReg
MachineBasicBlock::iterator I(LEAMI);
for (++I; &*I != JumpMI; ++I) {
if (I->getOpcode() == ARM::t2ADDrs && I->getOperand(0).getReg() == EntryReg)
RemovableAdd = &*I;
if (!RemovableAdd)
// Ensure EntryReg is not clobbered or used.
MachineBasicBlock::iterator J(RemovableAdd);
for (++J; &*J != JumpMI; ++J) {
for (const MachineOperand &MO : J->operands()) {
if (!MO.isReg() || !MO.getReg())
if (MO.isDef() && MO.getReg() == EntryReg)
if (MO.isUse() && MO.getReg() == EntryReg)
LLVM_DEBUG(dbgs() << "Removing Dead Add: " << *RemovableAdd);
DeadSize += 4;
/// optimizeThumb2JumpTables - Use tbb / tbh instructions to generate smaller
/// jumptables when it's possible.
bool ARMConstantIslands::optimizeThumb2JumpTables() {
bool MadeChange = false;
// FIXME: After the tables are shrunk, can we get rid some of the
// constantpool tables?
MachineJumpTableInfo *MJTI = MF->getJumpTableInfo();
if (!MJTI) return false;
const std::vector<MachineJumpTableEntry> &JT = MJTI->getJumpTables();
for (unsigned i = 0, e = T2JumpTables.size(); i != e; ++i) {
MachineInstr *MI = T2JumpTables[i];
const MCInstrDesc &MCID = MI->getDesc();
unsigned NumOps = MCID.getNumOperands();
unsigned JTOpIdx = NumOps - (MI->isPredicable() ? 2 : 1);
MachineOperand JTOP = MI->getOperand(JTOpIdx);
unsigned JTI = JTOP.getIndex();
assert(JTI < JT.size());
bool ByteOk = true;
bool HalfWordOk = true;
unsigned JTOffset = BBUtils->getOffsetOf(MI) + 4;
const std::vector<MachineBasicBlock*> &JTBBs = JT[JTI].MBBs;
BBInfoVector &BBInfo = BBUtils->getBBInfo();
for (MachineBasicBlock *MBB : JTBBs) {
unsigned DstOffset = BBInfo[MBB->getNumber()].Offset;
// Negative offset is not ok. FIXME: We should change BB layout to make
// sure all the branches are forward.
if (ByteOk && (DstOffset - JTOffset) > ((1<<8)-1)*2)
ByteOk = false;
unsigned TBHLimit = ((1<<16)-1)*2;
if (HalfWordOk && (DstOffset - JTOffset) > TBHLimit)
HalfWordOk = false;
if (!ByteOk && !HalfWordOk)
if (!ByteOk && !HalfWordOk)
CPUser &User = CPUsers[JumpTableUserIndices[JTI]];
MachineBasicBlock *MBB = MI->getParent();
if (!MI->getOperand(0).isKill()) // FIXME: needed now?
unsigned DeadSize = 0;
bool CanDeleteLEA = false;
bool BaseRegKill = false;
unsigned IdxReg = ~0U;
bool IdxRegKill = true;
if (isThumb2) {
IdxReg = MI->getOperand(1).getReg();
IdxRegKill = MI->getOperand(1).isKill();
bool PreservedBaseReg =
preserveBaseRegister(MI, User.MI, DeadSize, CanDeleteLEA, BaseRegKill);
if (!jumpTableFollowsTB(MI, User.CPEMI) && !PreservedBaseReg)
} else {
// We're in thumb-1 mode, so we must have something like:
// %idx = tLSLri %idx, 2
// %base = tLEApcrelJT
// %t = tLDRr %base, %idx
Register BaseReg = User.MI->getOperand(0).getReg();
MachineBasicBlock *UserMBB = User.MI->getParent();
MachineBasicBlock::iterator Shift = User.MI->getIterator();
if (Shift == UserMBB->begin())
Shift = prev_nodbg(Shift, UserMBB->begin());
if (Shift->getOpcode() != ARM::tLSLri ||
Shift->getOperand(3).getImm() != 2 ||
IdxReg = Shift->getOperand(2).getReg();
Register ShiftedIdxReg = Shift->getOperand(0).getReg();
// It's important that IdxReg is live until the actual TBB/TBH. Most of
// the range is checked later, but the LEA might still clobber it and not
// actually get removed.
if (BaseReg == IdxReg && !jumpTableFollowsTB(MI, User.CPEMI))
MachineInstr *Load = User.MI->getNextNode();
if (Load->getOpcode() != ARM::tLDRr)
if (Load->getOperand(1).getReg() != BaseReg ||
Load->getOperand(2).getReg() != ShiftedIdxReg ||
// If we're in PIC mode, there should be another ADD following.
auto *TRI = STI->getRegisterInfo();
// %base cannot be redefined after the load as it will appear before
// TBB/TBH like:
// %base =
// %base =
// tBB %base, %idx
if (registerDefinedBetween(BaseReg, Load->getNextNode(), MBB->end(), TRI))
if (isPositionIndependentOrROPI) {
MachineInstr *Add = Load->getNextNode();
if (Add->getOpcode() != ARM::tADDrr ||
Add->getOperand(2).getReg() != BaseReg ||
Add->getOperand(3).getReg() != Load->getOperand(0).getReg() ||
if (Add->getOperand(0).getReg() != MI->getOperand(0).getReg())
if (registerDefinedBetween(IdxReg, Add->getNextNode(), MI, TRI))
// IdxReg gets redefined in the middle of the sequence.
DeadSize += 2;
} else {
if (Load->getOperand(0).getReg() != MI->getOperand(0).getReg())
if (registerDefinedBetween(IdxReg, Load->getNextNode(), MI, TRI))
// IdxReg gets redefined in the middle of the sequence.
// Now safe to delete the load and lsl. The LEA will be removed later.
CanDeleteLEA = true;
DeadSize += 4;
LLVM_DEBUG(dbgs() << "Shrink JT: " << *MI);
MachineInstr *CPEMI = User.CPEMI;
unsigned Opc = ByteOk ? ARM::t2TBB_JT : ARM::t2TBH_JT;
if (!isThumb2)
Opc = ByteOk ? ARM::tTBB_JT : ARM::tTBH_JT;
MachineBasicBlock::iterator MI_JT = MI;
MachineInstr *NewJTMI =
BuildMI(*MBB, MI_JT, MI->getDebugLoc(), TII->get(Opc))
.addReg(IdxReg, getKillRegState(IdxRegKill))
.addJumpTableIndex(JTI, JTOP.getTargetFlags())
LLVM_DEBUG(dbgs() << printMBBReference(*MBB) << ": " << *NewJTMI);
if (jumpTableFollowsTB(MI, User.CPEMI)) {
if (CanDeleteLEA) {
if (isThumb2)
RemoveDeadAddBetweenLEAAndJT(User.MI, MI, DeadSize);
DeadSize += isThumb2 ? 4 : 2;
// The LEA was eliminated, the TBB instruction becomes the only new user
// of the jump table.
User.MI = NewJTMI;
User.MaxDisp = 4;
User.NegOk = false;
User.IsSoImm = false;
User.KnownAlignment = false;
} else {
// The LEA couldn't be eliminated, so we must add another CPUser to
// record the TBB or TBH use.
int CPEntryIdx = JumpTableEntryIndices[JTI];
auto &CPEs = CPEntries[CPEntryIdx];
auto Entry =
find_if(CPEs, [&](CPEntry &E) { return E.CPEMI == User.CPEMI; });
CPUsers.emplace_back(CPUser(NewJTMI, User.CPEMI, 4, false, false));
unsigned NewSize = TII->getInstSizeInBytes(*NewJTMI);
unsigned OrigSize = TII->getInstSizeInBytes(*MI);
int Delta = OrigSize - NewSize + DeadSize;
BBInfo[MBB->getNumber()].Size -= Delta;
MadeChange = true;
return MadeChange;
/// reorderThumb2JumpTables - Adjust the function's block layout to ensure that
/// jump tables always branch forwards, since that's what tbb and tbh need.
bool ARMConstantIslands::reorderThumb2JumpTables() {
bool MadeChange = false;
MachineJumpTableInfo *MJTI = MF->getJumpTableInfo();
if (!MJTI) return false;
const std::vector<MachineJumpTableEntry> &JT = MJTI->getJumpTables();
for (unsigned i = 0, e = T2JumpTables.size(); i != e; ++i) {
MachineInstr *MI = T2JumpTables[i];
const MCInstrDesc &MCID = MI->getDesc();
unsigned NumOps = MCID.getNumOperands();
unsigned JTOpIdx = NumOps - (MI->isPredicable() ? 2 : 1);
MachineOperand JTOP = MI->getOperand(JTOpIdx);
unsigned JTI = JTOP.getIndex();
assert(JTI < JT.size());
// We prefer if target blocks for the jump table come after the jump
// instruction so we can use TB[BH]. Loop through the target blocks
// and try to adjust them such that that's true.
int JTNumber = MI->getParent()->getNumber();
const std::vector<MachineBasicBlock*> &JTBBs = JT[JTI].MBBs;
for (MachineBasicBlock *MBB : JTBBs) {
int DTNumber = MBB->getNumber();
if (DTNumber < JTNumber) {
// The destination precedes the switch. Try to move the block forward
// so we have a positive offset.
MachineBasicBlock *NewBB =
adjustJTTargetBlockForward(JTI, MBB, MI->getParent());
if (NewBB)
MJTI->ReplaceMBBInJumpTable(JTI, MBB, NewBB);
MadeChange = true;
return MadeChange;
MachineBasicBlock *ARMConstantIslands::adjustJTTargetBlockForward(
unsigned JTI, MachineBasicBlock *BB, MachineBasicBlock *JTBB) {
// If the destination block is terminated by an unconditional branch,
// try to move it; otherwise, create a new block following the jump
// table that branches back to the actual target. This is a very simple
// heuristic. FIXME: We can definitely improve it.
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 4> Cond;
SmallVector<MachineOperand, 4> CondPrior;
MachineFunction::iterator BBi = BB->getIterator();
MachineFunction::iterator OldPrior = std::prev(BBi);
MachineFunction::iterator OldNext = std::next(BBi);
// If the block terminator isn't analyzable, don't try to move the block
bool B = TII->analyzeBranch(*BB, TBB, FBB, Cond);
// If the block ends in an unconditional branch, move it. The prior block
// has to have an analyzable terminator for us to move this one. Be paranoid
// and make sure we're not trying to move the entry block of the function.
if (!B && Cond.empty() && BB != &MF->front() &&
!TII->analyzeBranch(*OldPrior, TBB, FBB, CondPrior)) {
BB->updateTerminator(OldNext != MF->end() ? &*OldNext : nullptr);
// Update numbering to account for the block being moved.
return nullptr;
// Create a new MBB for the code after the jump BB.
MachineBasicBlock *NewBB =
MachineFunction::iterator MBBI = ++JTBB->getIterator();
MF->insert(MBBI, NewBB);
// Copy live-in information to new block.
for (const MachineBasicBlock::RegisterMaskPair &RegMaskPair : BB->liveins())
// Add an unconditional branch from NewBB to BB.
// There doesn't seem to be meaningful DebugInfo available; this doesn't
// correspond directly to anything in the source.
if (isThumb2)
BuildMI(NewBB, DebugLoc(), TII->get(ARM::t2B))
BuildMI(NewBB, DebugLoc(), TII->get(ARM::tB))
// Update internal data structures to account for the newly inserted MBB.
// Update the CFG.
JTBB->replaceSuccessor(BB, NewBB);
return NewBB;
/// createARMConstantIslandPass - returns an instance of the constpool
/// island pass.
FunctionPass *llvm::createARMConstantIslandPass() {
return new ARMConstantIslands();
INITIALIZE_PASS(ARMConstantIslands, "arm-cp-islands", ARM_CP_ISLANDS_OPT_NAME,
false, false)