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//===- HexagonGenMux.cpp --------------------------------------------------===//
// 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
// During instruction selection, MUX instructions are generated for
// conditional assignments. Since such assignments often present an
// opportunity to predicate instructions, HexagonExpandCondsets
// expands MUXes into pairs of conditional transfers, and then proceeds
// with predication of the producers/consumers of the registers involved.
// This happens after exiting from the SSA form, but before the machine
// instruction scheduler. After the scheduler and after the register
// allocation there can be cases of pairs of conditional transfers
// resulting from a MUX where neither of them was further predicated. If
// these transfers are now placed far enough from the instruction defining
// the predicate register, they cannot use the .new form. In such cases it
// is better to collapse them back to a single MUX instruction.
#include "HexagonInstrInfo.h"
#include "HexagonRegisterInfo.h"
#include "HexagonSubtarget.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/CodeGen/LivePhysRegs.h"
#include "llvm/CodeGen/MachineBasicBlock.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/IR/DebugLoc.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/MathExtras.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <limits>
#include <utility>
#define DEBUG_TYPE "hexmux"
using namespace llvm;
namespace llvm {
FunctionPass *createHexagonGenMux();
void initializeHexagonGenMuxPass(PassRegistry& Registry);
} // end namespace llvm
// Initialize this to 0 to always prefer generating mux by default.
static cl::opt<unsigned> MinPredDist("hexagon-gen-mux-threshold", cl::Hidden,
cl::init(0), cl::desc("Minimum distance between predicate definition and "
"farther of the two predicated uses"));
namespace {
class HexagonGenMux : public MachineFunctionPass {
static char ID;
HexagonGenMux() : MachineFunctionPass(ID) {}
StringRef getPassName() const override {
return "Hexagon generate mux instructions";
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnMachineFunction(MachineFunction &MF) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
const HexagonInstrInfo *HII = nullptr;
const HexagonRegisterInfo *HRI = nullptr;
struct CondsetInfo {
unsigned PredR = 0;
unsigned TrueX = std::numeric_limits<unsigned>::max();
unsigned FalseX = std::numeric_limits<unsigned>::max();
CondsetInfo() = default;
struct DefUseInfo {
BitVector Defs, Uses;
DefUseInfo() = default;
DefUseInfo(const BitVector &D, const BitVector &U) : Defs(D), Uses(U) {}
struct MuxInfo {
MachineBasicBlock::iterator At;
unsigned DefR, PredR;
MachineOperand *SrcT, *SrcF;
MachineInstr *Def1, *Def2;
MuxInfo(MachineBasicBlock::iterator It, unsigned DR, unsigned PR,
MachineOperand *TOp, MachineOperand *FOp, MachineInstr &D1,
MachineInstr &D2)
: At(It), DefR(DR), PredR(PR), SrcT(TOp), SrcF(FOp), Def1(&D1),
Def2(&D2) {}
using InstrIndexMap = DenseMap<MachineInstr *, unsigned>;
using DefUseInfoMap = DenseMap<unsigned, DefUseInfo>;
using MuxInfoList = SmallVector<MuxInfo, 4>;
bool isRegPair(unsigned Reg) const {
return Hexagon::DoubleRegsRegClass.contains(Reg);
void getSubRegs(unsigned Reg, BitVector &SRs) const;
void expandReg(unsigned Reg, BitVector &Set) const;
void getDefsUses(const MachineInstr *MI, BitVector &Defs,
BitVector &Uses) const;
void buildMaps(MachineBasicBlock &B, InstrIndexMap &I2X,
DefUseInfoMap &DUM);
bool isCondTransfer(unsigned Opc) const;
unsigned getMuxOpcode(const MachineOperand &Src1,
const MachineOperand &Src2) const;
bool genMuxInBlock(MachineBasicBlock &B);
} // end anonymous namespace
char HexagonGenMux::ID = 0;
INITIALIZE_PASS(HexagonGenMux, "hexagon-gen-mux",
"Hexagon generate mux instructions", false, false)
void HexagonGenMux::getSubRegs(unsigned Reg, BitVector &SRs) const {
for (MCSubRegIterator I(Reg, HRI); I.isValid(); ++I)
SRs[*I] = true;
void HexagonGenMux::expandReg(unsigned Reg, BitVector &Set) const {
if (isRegPair(Reg))
getSubRegs(Reg, Set);
Set[Reg] = true;
void HexagonGenMux::getDefsUses(const MachineInstr *MI, BitVector &Defs,
BitVector &Uses) const {
// First, get the implicit defs and uses for this instruction.
unsigned Opc = MI->getOpcode();
const MCInstrDesc &D = HII->get(Opc);
if (const MCPhysReg *R = D.ImplicitDefs)
while (*R)
expandReg(*R++, Defs);
if (const MCPhysReg *R = D.ImplicitUses)
while (*R)
expandReg(*R++, Uses);
// Look over all operands, and collect explicit defs and uses.
for (const MachineOperand &MO : MI->operands()) {
if (!MO.isReg() || MO.isImplicit())
Register R = MO.getReg();
BitVector &Set = MO.isDef() ? Defs : Uses;
expandReg(R, Set);
void HexagonGenMux::buildMaps(MachineBasicBlock &B, InstrIndexMap &I2X,
DefUseInfoMap &DUM) {
unsigned Index = 0;
unsigned NR = HRI->getNumRegs();
BitVector Defs(NR), Uses(NR);
for (MachineInstr &MI : B) {
I2X.insert(std::make_pair(&MI, Index));
getDefsUses(&MI, Defs, Uses);
DUM.insert(std::make_pair(Index, DefUseInfo(Defs, Uses)));
bool HexagonGenMux::isCondTransfer(unsigned Opc) const {
switch (Opc) {
case Hexagon::A2_tfrt:
case Hexagon::A2_tfrf:
case Hexagon::C2_cmoveit:
case Hexagon::C2_cmoveif:
return true;
return false;
unsigned HexagonGenMux::getMuxOpcode(const MachineOperand &Src1,
const MachineOperand &Src2) const {
bool IsReg1 = Src1.isReg(), IsReg2 = Src2.isReg();
if (IsReg1)
return IsReg2 ? Hexagon::C2_mux : Hexagon::C2_muxir;
if (IsReg2)
return Hexagon::C2_muxri;
// Neither is a register. The first source is extendable, but the second
// is not (s8).
if (Src2.isImm() && isInt<8>(Src2.getImm()))
return Hexagon::C2_muxii;
return 0;
bool HexagonGenMux::genMuxInBlock(MachineBasicBlock &B) {
bool Changed = false;
InstrIndexMap I2X;
DefUseInfoMap DUM;
buildMaps(B, I2X, DUM);
using CondsetMap = DenseMap<unsigned, CondsetInfo>;
CondsetMap CM;
MuxInfoList ML;
for (MachineInstr &MI : llvm::make_early_inc_range(B)) {
unsigned Opc = MI.getOpcode();
if (!isCondTransfer(Opc))
Register DR = MI.getOperand(0).getReg();
if (isRegPair(DR))
MachineOperand &PredOp = MI.getOperand(1);
if (PredOp.isUndef())
Register PR = PredOp.getReg();
unsigned Idx = I2X.lookup(&MI);
CondsetMap::iterator F = CM.find(DR);
bool IfTrue = HII->isPredicatedTrue(Opc);
// If there is no record of a conditional transfer for this register,
// or the predicate register differs, create a new record for it.
if (F != CM.end() && F->second.PredR != PR) {
F = CM.end();
if (F == CM.end()) {
auto It = CM.insert(std::make_pair(DR, CondsetInfo()));
F = It.first;
F->second.PredR = PR;
CondsetInfo &CI = F->second;
if (IfTrue)
CI.TrueX = Idx;
CI.FalseX = Idx;
if (CI.TrueX == std::numeric_limits<unsigned>::max() ||
CI.FalseX == std::numeric_limits<unsigned>::max())
// There is now a complete definition of DR, i.e. we have the predicate
// register, the definition if-true, and definition if-false.
// First, check if the definitions are far enough from the definition
// of the predicate register.
unsigned MinX = std::min(CI.TrueX, CI.FalseX);
unsigned MaxX = std::max(CI.TrueX, CI.FalseX);
// Specifically, check if the predicate definition is within a prescribed
// distance from the farther of the two predicated instructions.
unsigned SearchX = (MaxX >= MinPredDist) ? MaxX-MinPredDist : 0;
bool NearDef = false;
for (unsigned X = SearchX; X < MaxX; ++X) {
const DefUseInfo &DU = DUM.lookup(X);
if (!DU.Defs[PR])
NearDef = true;
if (NearDef)
// The predicate register is not defined in the last few instructions.
// Check if the conversion to MUX is possible (either "up", i.e. at the
// place of the earlier partial definition, or "down", where the later
// definition is located). Examine all defs and uses between these two
// definitions.
// SR1, SR2 - source registers from the first and the second definition.
MachineBasicBlock::iterator It1 = B.begin(), It2 = B.begin();
std::advance(It1, MinX);
std::advance(It2, MaxX);
MachineInstr &Def1 = *It1, &Def2 = *It2;
MachineOperand *Src1 = &Def1.getOperand(2), *Src2 = &Def2.getOperand(2);
Register SR1 = Src1->isReg() ? Src1->getReg() : Register();
Register SR2 = Src2->isReg() ? Src2->getReg() : Register();
bool Failure = false, CanUp = true, CanDown = true;
for (unsigned X = MinX+1; X < MaxX; X++) {
const DefUseInfo &DU = DUM.lookup(X);
if (DU.Defs[PR] || DU.Defs[DR] || DU.Uses[DR]) {
Failure = true;
if (CanDown && DU.Defs[SR1])
CanDown = false;
if (CanUp && DU.Defs[SR2])
CanUp = false;
if (Failure || (!CanUp && !CanDown))
MachineOperand *SrcT = (MinX == CI.TrueX) ? Src1 : Src2;
MachineOperand *SrcF = (MinX == CI.FalseX) ? Src1 : Src2;
// Prefer "down", since this will move the MUX farther away from the
// predicate definition.
MachineBasicBlock::iterator At = CanDown ? Def2 : Def1;
ML.push_back(MuxInfo(At, DR, PR, SrcT, SrcF, Def1, Def2));
for (MuxInfo &MX : ML) {
unsigned MxOpc = getMuxOpcode(*MX.SrcT, *MX.SrcF);
if (!MxOpc)
// Basic correctness check: since we are deleting instructions, validate the
// iterators. There is a possibility that one of Def1 or Def2 is translated
// to "mux" and being considered for other "mux" instructions.
if (!MX.At->getParent() || !MX.Def1->getParent() || !MX.Def2->getParent())
MachineBasicBlock &B = *MX.At->getParent();
const DebugLoc &DL = B.findDebugLoc(MX.At);
auto NewMux = BuildMI(B, MX.At, DL, HII->get(MxOpc), MX.DefR)
Changed = true;
// Fix up kill flags.
LivePhysRegs LPR(*HRI);
auto IsLive = [&LPR,this] (unsigned Reg) -> bool {
for (MCSubRegIterator S(Reg, HRI, true); S.isValid(); ++S)
if (LPR.contains(*S))
return true;
return false;
for (MachineInstr &I : llvm::reverse(B)) {
if (I.isDebugInstr())
// This isn't 100% accurate, but it's safe.
// It won't detect (as a kill) a case like this
// r0 = add r0, 1 <-- r0 should be "killed"
// ... = r0
for (MachineOperand &Op : I.operands()) {
if (!Op.isReg() || !Op.isUse())
assert(Op.getSubReg() == 0 && "Should have physical registers only");
bool Live = IsLive(Op.getReg());
return Changed;
bool HexagonGenMux::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
HII = MF.getSubtarget<HexagonSubtarget>().getInstrInfo();
HRI = MF.getSubtarget<HexagonSubtarget>().getRegisterInfo();
bool Changed = false;
for (auto &I : MF)
Changed |= genMuxInBlock(I);
return Changed;
FunctionPass *llvm::createHexagonGenMux() {
return new HexagonGenMux();