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//===- CSKYConstantIslandPass.cpp - Emit PC Relative loads ----------------===//
// 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
// Loading constants inline is expensive on CSKY and it's in general better
// to place the constant nearby in code space and then it can be loaded with a
// simple 16/32 bit load instruction like lrw.
// The constants can be not just numbers but addresses of functions and labels.
// This can be particularly helpful in static relocation mode for embedded
// non-linux targets.
#include "CSKY.h"
#include "CSKYConstantPoolValue.h"
#include "CSKYMachineFunctionInfo.h"
#include "CSKYSubtarget.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/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineConstantPool.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Type.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 <vector>
using namespace llvm;
#define DEBUG_TYPE "CSKY-constant-islands"
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");
namespace {
using Iter = MachineBasicBlock::iterator;
using ReverseIter = MachineBasicBlock::reverse_iterator;
/// CSKYConstantIslands - Due to limited PC-relative displacements, CSKY
/// 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 CSKYConstantIslands : public MachineFunctionPass {
/// BasicBlockInfo - Information about the offset and size of a single
/// basic block.
struct BasicBlockInfo {
/// Offset - Distance from the beginning of the function to the beginning
/// of this basic block.
/// Offsets are computed assuming worst case padding before an aligned
/// block. This means that subtracting basic block offsets always gives a
/// conservative estimate of the real distance which may be smaller.
/// Because worst case padding is used, the computed offset of an aligned
/// block may not actually be aligned.
unsigned Offset = 0;
/// Size - Size of the basic block in bytes. If the block contains
/// inline assembly, this is a worst case estimate.
/// The size does not include any alignment padding whether from the
/// beginning of the block, or from an aligned jump table at the end.
unsigned Size = 0;
BasicBlockInfo() = default;
unsigned postOffset() const { return Offset + Size; }
std::vector<BasicBlockInfo> BBInfo;
/// 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;
CPUser(MachineInstr *Mi, MachineInstr *Cpemi, unsigned Maxdisp, bool Neg)
: MI(Mi), CPEMI(Cpemi), MaxDisp(Maxdisp), NegOk(Neg) {
HighWaterMark = CPEMI->getParent();
/// getMaxDisp - Returns the maximum displacement supported by MI.
unsigned getMaxDisp() const { return MaxDisp - 16; }
void setMaxDisp(unsigned Val) { MaxDisp = Val; }
/// 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.
std::vector<std::vector<CPEntry>> CPEntries;
/// 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;
int UncondBr;
ImmBranch(MachineInstr *Mi, unsigned Maxdisp, bool Cond, int Ubr)
: MI(Mi), MaxDisp(Maxdisp), IsCond(Cond), UncondBr(Ubr) {}
/// ImmBranches - Keep track of all the immediate branch instructions.
std::vector<ImmBranch> ImmBranches;
const CSKYSubtarget *STI = nullptr;
const CSKYInstrInfo *TII;
CSKYMachineFunctionInfo *MFI;
MachineFunction *MF = nullptr;
MachineConstantPool *MCP = nullptr;
unsigned PICLabelUId;
void initPICLabelUId(unsigned UId) { PICLabelUId = UId; }
unsigned createPICLabelUId() { return PICLabelUId++; }
static char ID;
CSKYConstantIslands() : MachineFunctionPass(ID) {}
StringRef getPassName() const override { return "CSKY Constant Islands"; }
bool runOnMachineFunction(MachineFunction &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
void doInitialPlacement(std::vector<MachineInstr *> &CPEMIs);
CPEntry *findConstPoolEntry(unsigned CPI, const MachineInstr *CPEMI);
Align getCPEAlign(const MachineInstr &CPEMI);
void initializeFunctionInfo(const std::vector<MachineInstr *> &CPEMIs);
unsigned getOffsetOf(MachineInstr *MI) const;
unsigned getUserOffset(CPUser &) const;
void dumpBBs();
bool isOffsetInRange(unsigned UserOffset, unsigned TrialOffset, unsigned Disp,
bool NegativeOK);
bool isOffsetInRange(unsigned UserOffset, unsigned TrialOffset,
const CPUser &U);
void computeBlockSize(MachineBasicBlock *MBB);
MachineBasicBlock *splitBlockBeforeInstr(MachineInstr &MI);
void updateForInsertedWaterBlock(MachineBasicBlock *NewBB);
void adjustBBOffsetsAfter(MachineBasicBlock *BB);
bool decrementCPEReferenceCount(unsigned CPI, MachineInstr *CPEMI);
int findInRangeCPEntry(CPUser &U, unsigned UserOffset);
bool findAvailableWater(CPUser &U, unsigned UserOffset,
water_iterator &WaterIter);
void createNewWater(unsigned CPUserIndex, unsigned UserOffset,
MachineBasicBlock *&NewMBB);
bool handleConstantPoolUser(unsigned CPUserIndex);
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 isBBInRange(MachineInstr *MI, MachineBasicBlock *BB, unsigned Disp);
bool fixupImmediateBr(ImmBranch &Br);
bool fixupConditionalBr(ImmBranch &Br);
bool fixupUnconditionalBr(ImmBranch &Br);
} // end anonymous namespace
char CSKYConstantIslands::ID = 0;
bool CSKYConstantIslands::isOffsetInRange(unsigned UserOffset,
unsigned TrialOffset,
const CPUser &U) {
return isOffsetInRange(UserOffset, TrialOffset, U.getMaxDisp(), U.NegOk);
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
/// print block size and offset information - debugging
LLVM_DUMP_METHOD void CSKYConstantIslands::dumpBBs() {
for (unsigned J = 0, E = BBInfo.size(); J != E; ++J) {
const BasicBlockInfo &BBI = BBInfo[J];
dbgs() << format("%08x %bb.%u\t", BBI.Offset, J)
<< format(" size=%#x\n", BBInfo[J].Size);
bool CSKYConstantIslands::runOnMachineFunction(MachineFunction &Mf) {
MF = &Mf;
MCP = Mf.getConstantPool();
STI = &Mf.getSubtarget<CSKYSubtarget>();
LLVM_DEBUG(dbgs() << "***** CSKYConstantIslands: "
<< MCP->getConstants().size() << " CP entries, aligned to "
<< MCP->getConstantPoolAlign().value() << " bytes *****\n");
TII = STI->getInstrInfo();
MFI = MF->getInfo<CSKYMachineFunctionInfo>();
// This pass invalidates liveness information when it splits basic blocks.
// Renumber all of the machine basic blocks in the function, guaranteeing that
// the numbers agree with the position of the block in the function.
bool MadeChange = false;
// 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())
/// 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.
/// 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)
CPChange |= handleConstantPoolUser(I);
if (CPChange && ++NoCPIters > 30)
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;
LLVM_DEBUG(dbgs() << '\n'; dumpBBs());
return MadeChange;
/// doInitialPlacement - Perform the initial placement of the constant pool
/// entries. To start with, we put them all at the end of the function.
void CSKYConstantIslands::doInitialPlacement(
std::vector<MachineInstr *> &CPEMIs) {
// Create the basic block to hold the CPE's.
MachineBasicBlock *BB = MF->CreateMachineBasicBlock();
// MachineConstantPool measures alignment in bytes. We measure in log2(bytes).
const Align MaxAlign = MCP->getConstantPoolAlign();
// 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.
// 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(Log2(MaxAlign) + 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);
assert(Size >= 4 && "Too small constant pool entry");
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(CSKY::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 <= Log2(MaxAlign); ++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');
/// BBHasFallthrough - Return true if the specified basic block can fallthrough
/// into the block immediately after it.
static bool 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);
for (MachineBasicBlock::succ_iterator I = MBB->succ_begin(),
E = MBB->succ_end();
I != E; ++I)
if (*I == NextBB)
return true;
return false;
/// findConstPoolEntry - Given the constpool index and CONSTPOOL_ENTRY MI,
/// look up the corresponding CPEntry.
CSKYConstantIslands::CPEntry *
CSKYConstantIslands::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 (unsigned I = 0, E = CPEs.size(); I != E; ++I) {
return &CPEs[I];
return nullptr;
/// getCPEAlign - Returns the required alignment of the constant pool entry
/// represented by CPEMI. Alignment is measured in log2(bytes) units.
Align CSKYConstantIslands::getCPEAlign(const MachineInstr &CPEMI) {
assert(CPEMI.getOpcode() == CSKY::CONSTPOOL_ENTRY);
unsigned CPI = CPEMI.getOperand(1).getIndex();
assert(CPI < MCP->getConstants().size() && "Invalid constant pool index.");
return MCP->getConstants()[CPI].getAlign();
/// 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 CSKYConstantIslands::initializeFunctionInfo(
const std::vector<MachineInstr *> &CPEMIs) {
// First thing, compute the size of all basic blocks, and see if the function
// has any inline assembly in it. If so, we have to be conservative about
// alignment assumptions, as we don't know for sure the size of any
// instructions in the inline assembly.
for (MachineFunction::iterator I = MF->begin(), E = MF->end(); I != E; ++I)
// Compute block offsets.
// 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 &MI : MBB) {
if (MI.isDebugInstr())
int Opc = MI.getOpcode();
if (MI.isBranch() && !MI.isIndirectBranch()) {
bool IsCond = MI.isConditionalBranch();
unsigned Bits = 0;
unsigned Scale = 1;
int UOpc = CSKY::BR32;
switch (MI.getOpcode()) {
case CSKY::BR16:
case CSKY::BF16:
case CSKY::BT16:
Bits = 10;
Scale = 2;
Bits = 16;
Scale = 2;
// Record this immediate branch.
unsigned MaxOffs = ((1 << (Bits - 1)) - 1) * Scale;
ImmBranches.push_back(ImmBranch(&MI, MaxOffs, IsCond, UOpc));
// Scan the instructions for constant pool operands.
for (unsigned Op = 0, E = MI.getNumOperands(); Op != E; ++Op)
if (MI.getOperand(Op).isCPI()) {
// 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;
switch (Opc) {
llvm_unreachable("Unknown addressing mode for CP reference!");
case CSKY::MOVIH32:
case CSKY::ORI32:
case CSKY::PseudoTLSLA32:
case CSKY::JSRI32:
case CSKY::JMPI32:
case CSKY::LRW32:
case CSKY::LRW32_Gen:
Bits = 16;
Scale = 4;
case CSKY::f2FLRW_S:
case CSKY::f2FLRW_D:
Bits = 8;
Scale = 4;
case CSKY::GRS32:
Bits = 17;
Scale = 2;
NegOk = true;
// Remember that this is a user of a CP entry.
unsigned CPI = MI.getOperand(Op).getIndex();
MachineInstr *CPEMI = CPEMIs[CPI];
unsigned MaxOffs = ((1 << Bits) - 1) * Scale;
CPUsers.push_back(CPUser(&MI, CPEMI, MaxOffs, NegOk));
// Increment corresponding CPEntry reference count.
CPEntry *CPE = findConstPoolEntry(CPI, CPEMI);
assert(CPE && "Cannot find a corresponding CPEntry!");
/// computeBlockSize - Compute the size and some alignment information for MBB.
/// This function updates BBInfo directly.
void CSKYConstantIslands::computeBlockSize(MachineBasicBlock *MBB) {
BasicBlockInfo &BBI = BBInfo[MBB->getNumber()];
BBI.Size = 0;
for (const MachineInstr &MI : *MBB)
BBI.Size += TII->getInstSizeInBytes(MI);
/// getOffsetOf - Return the current offset of the specified machine instruction
/// from the start of the function. This offset changes as stuff is moved
/// around inside the function.
unsigned CSKYConstantIslands::getOffsetOf(MachineInstr *MI) const {
MachineBasicBlock *MBB = MI->getParent();
// The offset is composed of two things: the sum of the sizes of all MBB's
// before this instruction's block, and the offset from the start of the block
// it is in.
unsigned Offset = BBInfo[MBB->getNumber()].Offset;
// Sum instructions before MI in MBB.
for (MachineBasicBlock::iterator I = MBB->begin(); &*I != MI; ++I) {
assert(I != MBB->end() && "Didn't find MI in its own basic block?");
Offset += TII->getInstSizeInBytes(*I);
return Offset;
/// 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 CSKYConstantIslands::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.
BBInfo.insert(BBInfo.begin() + 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);
unsigned CSKYConstantIslands::getUserOffset(CPUser &U) const {
unsigned UserOffset = getOffsetOf(U.MI);
UserOffset &= ~3u;
return UserOffset;
/// 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 *
CSKYConstantIslands::splitBlockBeforeInstr(MachineInstr &MI) {
MachineBasicBlock *OrigBB = MI.getParent();
// 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.
// TODO: Add support for 16bit instr.
BuildMI(OrigBB, DebugLoc(), TII->get(CSKY::BR32)).addMBB(NewBB);
// Update the CFG. All succs of OrigBB are now succs of NewBB.
// OrigBB branches to NewBB.
// 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.
BBInfo.insert(BBInfo.begin() + 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;
/// isOffsetInRange - Checks whether UserOffset (the location of a constant pool
/// reference) is within MaxDisp of TrialOffset (a proposed location of a
/// constant pool entry).
bool CSKYConstantIslands::isOffsetInRange(unsigned UserOffset,
unsigned TrialOffset,
unsigned MaxDisp, bool NegativeOK) {
if (UserOffset <= TrialOffset) {
// User before the Trial.
if (TrialOffset - UserOffset <= MaxDisp)
return true;
} else if (NegativeOK) {
if (UserOffset - TrialOffset <= MaxDisp)
return true;
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 CSKYConstantIslands::isWaterInRange(unsigned UserOffset,
MachineBasicBlock *Water, CPUser &U,
unsigned &Growth) {
unsigned CPEOffset = BBInfo[Water->getNumber()].postOffset();
unsigned NextBlockOffset;
Align NextBlockAlignment;
MachineFunction::const_iterator NextBlock = ++Water->getIterator();
if (NextBlock == MF->end()) {
NextBlockOffset = BBInfo[Water->getNumber()].postOffset();
NextBlockAlignment = Align(4);
} 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;
} 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 CSKYConstantIslands::isCPEntryInRange(MachineInstr *MI,
unsigned UserOffset,
MachineInstr *CPEMI,
unsigned MaxDisp, bool NegOk,
bool DoDump) {
unsigned CPEOffset = getOffsetOf(CPEMI);
if (DoDump) {
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() == CSKY::BR32 /*TODO: change to 16bit instr. */)
return PredMI->getOperand(0).getMBB() == Succ;
return false;
void CSKYConstantIslands::adjustBBOffsetsAfter(MachineBasicBlock *BB) {
unsigned BBNum = BB->getNumber();
for (unsigned I = BBNum + 1, E = MF->getNumBlockIDs(); I < E; ++I) {
// Get the offset and known bits at the end of the layout predecessor.
// Include the alignment of the current block.
unsigned Offset = BBInfo[I - 1].Offset + BBInfo[I - 1].Size;
BBInfo[I].Offset = Offset;
/// 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 CSKYConstantIslands::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;
/// 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 CSKYConstantIslands::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 = CPEMI->getOperand(1).getIndex();
std::vector<CPEntry> &CPEs = CPEntries[CPI];
for (unsigned I = 0, E = CPEs.size(); I != E; ++I) {
// We already tried this one
// Removing CPEs can leave empty entries, skip
if (CPEs[I].CPEMI == nullptr)
if (isCPEntryInRange(UserMI, UserOffset, CPEs[I].CPEMI, U.getMaxDisp(),
U.NegOk)) {
LLVM_DEBUG(dbgs() << "Replacing CPE#" << CPI << " with CPE#"
<< CPEs[I].CPI << "\n");
// Point the CPUser node to the replacement
// Change the CPI in the instruction operand to refer to the clone.
for (unsigned J = 0, E = UserMI->getNumOperands(); J != E; ++J)
if (UserMI->getOperand(J).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) {
unsigned Bits, Scale;
switch (Opc) {
case CSKY::BR16:
Bits = 10;
Scale = 2;
case CSKY::BR32:
Bits = 16;
Scale = 2;
unsigned MaxOffs = ((1 << (Bits - 1)) - 1) * Scale;
return MaxOffs;
/// 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.
/// 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 CSKYConstantIslands::findAvailableWater(CPUser &U, unsigned UserOffset,
water_iterator &WaterIter) {
if (WaterList.empty())
return false;
unsigned BestGrowth = ~0u;
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.
unsigned Growth;
if (isWaterInRange(UserOffset, WaterBB, U, Growth) &&
(WaterBB->getNumber() < U.HighWaterMark->getNumber() ||
NewWaterList.count(WaterBB)) &&
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');
// Keep looking unless it is perfect.
if (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 CSKYConstantIslands::createNewWater(unsigned CPUserIndex,
unsigned UserOffset,
MachineBasicBlock *&NewMBB) {
CPUser &U = CPUsers[CPUserIndex];
MachineInstr *UserMI = U.MI;
MachineInstr *CPEMI = U.CPEMI;
MachineBasicBlock *UserMBB = UserMI->getParent();
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.
if (bbHasFallthrough(UserMBB)) {
// Size of branch to insert.
unsigned Delta = 4;
// Compute the offset where the CPE will begin.
unsigned CPEOffset = UserBBI.postOffset() + 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.
// TODO: Add support for 16bit instr.
int UncondBr = CSKY::BR32;
auto *NewMI = BuildMI(UserMBB, DebugLoc(), TII->get(UncondBr))
unsigned MaxDisp = getUnconditionalBrDisp(UncondBr);
ImmBranch(&UserMBB->back(), MaxDisp, false, UncondBr));
BBInfo[UserMBB->getNumber()].Size += TII->getInstSizeInBytes(*NewMI);
// What a big block. Find a place within the block to split it.
// 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();
unsigned BaseInsertOffset = UserOffset + U.getMaxDisp();
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
// Alignment of the island is handled
// inside isOffsetInRange.
BaseInsertOffset -= 4;
LLVM_DEBUG(dbgs() << format(", adjusted to %#x", BaseInsertOffset)
<< " la=" << Log2(Align) << '\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()) {
BaseInsertOffset = UserBBI.postOffset() - 8;
LLVM_DEBUG(dbgs() << format("Move inside block: %#x\n", BaseInsertOffset));
unsigned EndInsertOffset =
BaseInsertOffset + 4 + CPEMI->getOperand(2).getImm();
MachineBasicBlock::iterator MI = UserMI;
unsigned CPUIndex = CPUserIndex + 1;
unsigned NumCPUsers = CPUsers.size();
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();
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 CSKYConstantIslands::handleConstantPoolUser(unsigned CPUserIndex) {
CPUser &U = CPUsers[CPUserIndex];
MachineInstr *UserMI = U.MI;
MachineInstr *CPEMI = U.CPEMI;
unsigned CPI = CPEMI->getOperand(1).getIndex();
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;
if (result == 2)
return true;
// Look for water where we can place this CPE.
MachineBasicBlock *NewIsland = MF->CreateMachineBasicBlock();
MachineBasicBlock *NewMBB;
water_iterator IP;
if (findAvailableWater(U, UserOffset, IP)) {
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 {
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 = llvm::find(WaterList, WaterBB);
if (IP != WaterList.end())
// We are adding new water. Update NewWaterList.
// 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.
// Decrement the old entry, and remove it if refcount becomes 0.
decrementCPEReferenceCount(CPI, CPEMI);
// No existing clone of this CPE is within range.
// We will be generating a new clone. Get a UID for it.
unsigned ID = createPICLabelUId();
// 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(), TII->get(CSKY::CONSTPOOL_ENTRY))
CPEntries[CPI].push_back(CPEntry(U.CPEMI, ID, 1));
// 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.
BBInfo[NewIsland->getNumber()].Size += Size;
// Finally, change the CPI in the instruction operand to be ID.
for (unsigned I = 0, E = UserMI->getNumOperands(); I != E; ++I)
if (UserMI->getOperand(I).isCPI()) {
dbgs() << " Moved CPE to #" << ID << " CPI=" << CPI
<< format(" offset=%#x\n", BBInfo[NewIsland->getNumber()].Offset));
return true;
/// removeDeadCPEMI - Remove a dead constant pool entry instruction. Update
/// sizes and offsets of impacted basic blocks.
void CSKYConstantIslands::removeDeadCPEMI(MachineInstr *CPEMI) {
MachineBasicBlock *CPEBB = CPEMI->getParent();
unsigned Size = CPEMI->getOperand(2).getImm();
BBInfo[CPEBB->getNumber()].Size -= 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 CSKYConstantIslands::removeUnusedCPEntries() {
unsigned MadeChange = false;
for (unsigned I = 0, E = CPEntries.size(); I != E; ++I) {
std::vector<CPEntry> &CPEs = CPEntries[I];
for (unsigned J = 0, Ee = CPEs.size(); J != Ee; ++J) {
if (CPEs[J].RefCount == 0 && CPEs[J].CPEMI) {
CPEs[J].CPEMI = nullptr;
MadeChange = true;
return MadeChange;
/// isBBInRange - Returns true if the distance between specific MI and
/// specific BB can fit in MI's displacement field.
bool CSKYConstantIslands::isBBInRange(MachineInstr *MI,
MachineBasicBlock *DestBB,
unsigned MaxDisp) {
unsigned BrOffset = getOffsetOf(MI);
unsigned DestOffset = BBInfo[DestBB->getNumber()].Offset;
LLVM_DEBUG(dbgs() << "Branch of destination " << printMBBReference(*DestBB)
<< " from " << printMBBReference(*MI->getParent())
<< " max delta=" << MaxDisp << " from " << getOffsetOf(MI)
<< " to " << DestOffset << " offset "
<< int(DestOffset - BrOffset) << "\t" << *MI);
if (BrOffset <= DestOffset) {
// Branch before the Dest.
if (DestOffset - BrOffset <= MaxDisp)
return true;
} else {
if (BrOffset - DestOffset <= MaxDisp)
return true;
return false;
/// fixupImmediateBr - Fix up an immediate branch whose destination is too far
/// away to fit in its displacement field.
bool CSKYConstantIslands::fixupImmediateBr(ImmBranch &Br) {
MachineInstr *MI = Br.MI;
MachineBasicBlock *DestBB = TII->getBranchDestBlock(*MI);
// Check to see if the DestBB is already in-range.
if (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 BSR to implement a far jump.
/// Otherwise, add an intermediate branch instruction to a branch.
bool CSKYConstantIslands::fixupUnconditionalBr(ImmBranch &Br) {
MachineInstr *MI = Br.MI;
MachineBasicBlock *MBB = MI->getParent();
if (!MFI->isLRSpilled())
report_fatal_error("underestimated function size");
// Use BSR to implement far jump.
Br.MaxDisp = ((1 << (26 - 1)) - 1) * 2;
BBInfo[MBB->getNumber()].Size += 4;
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.
bool CSKYConstantIslands::fixupConditionalBr(ImmBranch &Br) {
MachineInstr *MI = Br.MI;
MachineBasicBlock *DestBB = TII->getBranchDestBlock(*MI);
SmallVector<MachineOperand, 4> Cond;
// Add an unconditional branch to the destination and invert the branch
// condition to jump over it:
// bteqz L1
// =>
// bnez L2
// b L1
// L2:
// 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->isUnconditionalBranch()) {
// Last MI in the BB is an unconditional branch. Can we simply invert the
// condition and swap destinations:
// beqz L1
// b L2
// =>
// bnez L2
// b L1
MachineBasicBlock *NewDest = TII->getBranchDestBlock(*BMI);
if (isBBInRange(MI, NewDest, Br.MaxDisp)) {
dbgs() << " Invert Bcc condition and swap its destination with "
<< *BMI);
BMI->getOperand(BMI->getNumExplicitOperands() - 1).setMBB(DestBB);
MI->getOperand(MI->getNumExplicitOperands() - 1).setMBB(NewDest);
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());
BBInfo[MBB->getNumber()].Size -= Delta;
// BBInfo[SplitBB].Offset is wrong temporarily, fixed below
// The conditional successor will be swapped between the BBs after this, so
// update CFG.
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(Cond[0].getImm()))
Br.MI = &MBB->back();
BBInfo[MBB->getNumber()].Size += TII->getInstSizeInBytes(MBB->back());
BuildMI(MBB, DebugLoc(), TII->get(Br.UncondBr)).addMBB(DestBB);
BBInfo[MBB->getNumber()].Size += 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.
BBInfo[MI->getParent()->getNumber()].Size -= TII->getInstSizeInBytes(*MI);
return true;
/// Returns a pass that converts branches to long branches.
FunctionPass *llvm::createCSKYConstantIslandPass() {
return new CSKYConstantIslands();
"CSKY constant island placement and branch shortening pass",
false, false)