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//===- PeepholeOptimizer.cpp - Peephole Optimizations ---------------------===//
// 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
// Perform peephole optimizations on the machine code:
// - Optimize Extensions
// Optimization of sign / zero extension instructions. It may be extended to
// handle other instructions with similar properties.
// On some targets, some instructions, e.g. X86 sign / zero extension, may
// leave the source value in the lower part of the result. This optimization
// will replace some uses of the pre-extension value with uses of the
// sub-register of the results.
// - Optimize Comparisons
// Optimization of comparison instructions. For instance, in this code:
// sub r1, 1
// cmp r1, 0
// bz L1
// If the "sub" instruction all ready sets (or could be modified to set) the
// same flag that the "cmp" instruction sets and that "bz" uses, then we can
// eliminate the "cmp" instruction.
// Another instance, in this code:
// sub r1, r3 | sub r1, imm
// cmp r3, r1 or cmp r1, r3 | cmp r1, imm
// bge L1
// If the branch instruction can use flag from "sub", then we can replace
// "sub" with "subs" and eliminate the "cmp" instruction.
// - Optimize Loads:
// Loads that can be folded into a later instruction. A load is foldable
// if it loads to virtual registers and the virtual register defined has
// a single use.
// - Optimize Copies and Bitcast (more generally, target specific copies):
// Rewrite copies and bitcasts to avoid cross register bank copies
// when possible.
// E.g., Consider the following example, where capital and lower
// letters denote different register file:
// b = copy A <-- cross-bank copy
// C = copy b <-- cross-bank copy
// =>
// b = copy A <-- cross-bank copy
// C = copy A <-- same-bank copy
// E.g., for bitcast:
// b = bitcast A <-- cross-bank copy
// C = bitcast b <-- cross-bank copy
// =>
// b = bitcast A <-- cross-bank copy
// C = copy A <-- same-bank copy
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/MC/LaneBitmask.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <cstdint>
#include <memory>
#include <utility>
using namespace llvm;
using RegSubRegPair = TargetInstrInfo::RegSubRegPair;
using RegSubRegPairAndIdx = TargetInstrInfo::RegSubRegPairAndIdx;
#define DEBUG_TYPE "peephole-opt"
// Optimize Extensions
static cl::opt<bool>
Aggressive("aggressive-ext-opt", cl::Hidden,
cl::desc("Aggressive extension optimization"));
static cl::opt<bool>
DisablePeephole("disable-peephole", cl::Hidden, cl::init(false),
cl::desc("Disable the peephole optimizer"));
/// Specifiy whether or not the value tracking looks through
/// complex instructions. When this is true, the value tracker
/// bails on everything that is not a copy or a bitcast.
static cl::opt<bool>
DisableAdvCopyOpt("disable-adv-copy-opt", cl::Hidden, cl::init(false),
cl::desc("Disable advanced copy optimization"));
static cl::opt<bool> DisableNAPhysCopyOpt(
"disable-non-allocatable-phys-copy-opt", cl::Hidden, cl::init(false),
cl::desc("Disable non-allocatable physical register copy optimization"));
// Limit the number of PHI instructions to process
// in PeepholeOptimizer::getNextSource.
static cl::opt<unsigned> RewritePHILimit(
"rewrite-phi-limit", cl::Hidden, cl::init(10),
cl::desc("Limit the length of PHI chains to lookup"));
// Limit the length of recurrence chain when evaluating the benefit of
// commuting operands.
static cl::opt<unsigned> MaxRecurrenceChain(
"recurrence-chain-limit", cl::Hidden, cl::init(3),
cl::desc("Maximum length of recurrence chain when evaluating the benefit "
"of commuting operands"));
STATISTIC(NumReuse, "Number of extension results reused");
STATISTIC(NumCmps, "Number of compares eliminated");
STATISTIC(NumImmFold, "Number of move immediate folded");
STATISTIC(NumLoadFold, "Number of loads folded");
STATISTIC(NumSelects, "Number of selects optimized");
STATISTIC(NumUncoalescableCopies, "Number of uncoalescable copies optimized");
STATISTIC(NumRewrittenCopies, "Number of copies rewritten");
STATISTIC(NumNAPhysCopies, "Number of non-allocatable physical copies removed");
namespace {
class ValueTrackerResult;
class RecurrenceInstr;
class PeepholeOptimizer : public MachineFunctionPass {
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
MachineRegisterInfo *MRI;
MachineDominatorTree *DT; // Machine dominator tree
MachineLoopInfo *MLI;
static char ID; // Pass identification
PeepholeOptimizer() : MachineFunctionPass(ID) {
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
if (Aggressive) {
/// Track Def -> Use info used for rewriting copies.
using RewriteMapTy = SmallDenseMap<RegSubRegPair, ValueTrackerResult>;
/// Sequence of instructions that formulate recurrence cycle.
using RecurrenceCycle = SmallVector<RecurrenceInstr, 4>;
bool optimizeCmpInstr(MachineInstr &MI);
bool optimizeExtInstr(MachineInstr &MI, MachineBasicBlock &MBB,
SmallPtrSetImpl<MachineInstr*> &LocalMIs);
bool optimizeSelect(MachineInstr &MI,
SmallPtrSetImpl<MachineInstr *> &LocalMIs);
bool optimizeCondBranch(MachineInstr &MI);
bool optimizeCoalescableCopy(MachineInstr &MI);
bool optimizeUncoalescableCopy(MachineInstr &MI,
SmallPtrSetImpl<MachineInstr *> &LocalMIs);
bool optimizeRecurrence(MachineInstr &PHI);
bool findNextSource(RegSubRegPair RegSubReg, RewriteMapTy &RewriteMap);
bool isMoveImmediate(MachineInstr &MI,
SmallSet<unsigned, 4> &ImmDefRegs,
DenseMap<unsigned, MachineInstr*> &ImmDefMIs);
bool foldImmediate(MachineInstr &MI, SmallSet<unsigned, 4> &ImmDefRegs,
DenseMap<unsigned, MachineInstr*> &ImmDefMIs);
/// Finds recurrence cycles, but only ones that formulated around
/// a def operand and a use operand that are tied. If there is a use
/// operand commutable with the tied use operand, find recurrence cycle
/// along that operand as well.
bool findTargetRecurrence(unsigned Reg,
const SmallSet<unsigned, 2> &TargetReg,
RecurrenceCycle &RC);
/// If copy instruction \p MI is a virtual register copy, track it in
/// the set \p CopySrcRegs and \p CopyMIs. If this virtual register was
/// previously seen as a copy, replace the uses of this copy with the
/// previously seen copy's destination register.
bool foldRedundantCopy(MachineInstr &MI,
SmallSet<unsigned, 4> &CopySrcRegs,
DenseMap<unsigned, MachineInstr *> &CopyMIs);
/// Is the register \p Reg a non-allocatable physical register?
bool isNAPhysCopy(unsigned Reg);
/// If copy instruction \p MI is a non-allocatable virtual<->physical
/// register copy, track it in the \p NAPhysToVirtMIs map. If this
/// non-allocatable physical register was previously copied to a virtual
/// registered and hasn't been clobbered, the virt->phys copy can be
/// deleted.
bool foldRedundantNAPhysCopy(MachineInstr &MI,
DenseMap<unsigned, MachineInstr *> &NAPhysToVirtMIs);
bool isLoadFoldable(MachineInstr &MI,
SmallSet<unsigned, 16> &FoldAsLoadDefCandidates);
/// Check whether \p MI is understood by the register coalescer
/// but may require some rewriting.
bool isCoalescableCopy(const MachineInstr &MI) {
// SubregToRegs are not interesting, because they are already register
// coalescer friendly.
return MI.isCopy() || (!DisableAdvCopyOpt &&
(MI.isRegSequence() || MI.isInsertSubreg() ||
/// Check whether \p MI is a copy like instruction that is
/// not recognized by the register coalescer.
bool isUncoalescableCopy(const MachineInstr &MI) {
return MI.isBitcast() ||
(!DisableAdvCopyOpt &&
(MI.isRegSequenceLike() || MI.isInsertSubregLike() ||
MachineInstr &rewriteSource(MachineInstr &CopyLike,
RegSubRegPair Def, RewriteMapTy &RewriteMap);
/// Helper class to hold instructions that are inside recurrence cycles.
/// The recurrence cycle is formulated around 1) a def operand and its
/// tied use operand, or 2) a def operand and a use operand that is commutable
/// with another use operand which is tied to the def operand. In the latter
/// case, index of the tied use operand and the commutable use operand are
/// maintained with CommutePair.
class RecurrenceInstr {
using IndexPair = std::pair<unsigned, unsigned>;
RecurrenceInstr(MachineInstr *MI) : MI(MI) {}
RecurrenceInstr(MachineInstr *MI, unsigned Idx1, unsigned Idx2)
: MI(MI), CommutePair(std::make_pair(Idx1, Idx2)) {}
MachineInstr *getMI() const { return MI; }
Optional<IndexPair> getCommutePair() const { return CommutePair; }
MachineInstr *MI;
Optional<IndexPair> CommutePair;
/// Helper class to hold a reply for ValueTracker queries.
/// Contains the returned sources for a given search and the instructions
/// where the sources were tracked from.
class ValueTrackerResult {
/// Track all sources found by one ValueTracker query.
SmallVector<RegSubRegPair, 2> RegSrcs;
/// Instruction using the sources in 'RegSrcs'.
const MachineInstr *Inst = nullptr;
ValueTrackerResult() = default;
ValueTrackerResult(unsigned Reg, unsigned SubReg) {
addSource(Reg, SubReg);
bool isValid() const { return getNumSources() > 0; }
void setInst(const MachineInstr *I) { Inst = I; }
const MachineInstr *getInst() const { return Inst; }
void clear() {
Inst = nullptr;
void addSource(unsigned SrcReg, unsigned SrcSubReg) {
RegSrcs.push_back(RegSubRegPair(SrcReg, SrcSubReg));
void setSource(int Idx, unsigned SrcReg, unsigned SrcSubReg) {
assert(Idx < getNumSources() && "Reg pair source out of index");
RegSrcs[Idx] = RegSubRegPair(SrcReg, SrcSubReg);
int getNumSources() const { return RegSrcs.size(); }
RegSubRegPair getSrc(int Idx) const {
return RegSrcs[Idx];
unsigned getSrcReg(int Idx) const {
assert(Idx < getNumSources() && "Reg source out of index");
return RegSrcs[Idx].Reg;
unsigned getSrcSubReg(int Idx) const {
assert(Idx < getNumSources() && "SubReg source out of index");
return RegSrcs[Idx].SubReg;
bool operator==(const ValueTrackerResult &Other) {
if (Other.getInst() != getInst())
return false;
if (Other.getNumSources() != getNumSources())
return false;
for (int i = 0, e = Other.getNumSources(); i != e; ++i)
if (Other.getSrcReg(i) != getSrcReg(i) ||
Other.getSrcSubReg(i) != getSrcSubReg(i))
return false;
return true;
/// Helper class to track the possible sources of a value defined by
/// a (chain of) copy related instructions.
/// Given a definition (instruction and definition index), this class
/// follows the use-def chain to find successive suitable sources.
/// The given source can be used to rewrite the definition into
/// def = COPY src.
/// For instance, let us consider the following snippet:
/// v0 =
/// v2 = INSERT_SUBREG v1, v0, sub0
/// def = COPY v2.sub0
/// Using a ValueTracker for def = COPY v2.sub0 will give the following
/// suitable sources:
/// v2.sub0 and v0.
/// Then, def can be rewritten into def = COPY v0.
class ValueTracker {
/// The current point into the use-def chain.
const MachineInstr *Def = nullptr;
/// The index of the definition in Def.
unsigned DefIdx = 0;
/// The sub register index of the definition.
unsigned DefSubReg;
/// The register where the value can be found.
unsigned Reg;
/// MachineRegisterInfo used to perform tracking.
const MachineRegisterInfo &MRI;
/// Optional TargetInstrInfo used to perform some complex tracking.
const TargetInstrInfo *TII;
/// Dispatcher to the right underlying implementation of getNextSource.
ValueTrackerResult getNextSourceImpl();
/// Specialized version of getNextSource for Copy instructions.
ValueTrackerResult getNextSourceFromCopy();
/// Specialized version of getNextSource for Bitcast instructions.
ValueTrackerResult getNextSourceFromBitcast();
/// Specialized version of getNextSource for RegSequence instructions.
ValueTrackerResult getNextSourceFromRegSequence();
/// Specialized version of getNextSource for InsertSubreg instructions.
ValueTrackerResult getNextSourceFromInsertSubreg();
/// Specialized version of getNextSource for ExtractSubreg instructions.
ValueTrackerResult getNextSourceFromExtractSubreg();
/// Specialized version of getNextSource for SubregToReg instructions.
ValueTrackerResult getNextSourceFromSubregToReg();
/// Specialized version of getNextSource for PHI instructions.
ValueTrackerResult getNextSourceFromPHI();
/// Create a ValueTracker instance for the value defined by \p Reg.
/// \p DefSubReg represents the sub register index the value tracker will
/// track. It does not need to match the sub register index used in the
/// definition of \p Reg.
/// If \p Reg is a physical register, a value tracker constructed with
/// this constructor will not find any alternative source.
/// Indeed, when \p Reg is a physical register that constructor does not
/// know which definition of \p Reg it should track.
/// Use the next constructor to track a physical register.
ValueTracker(unsigned Reg, unsigned DefSubReg,
const MachineRegisterInfo &MRI,
const TargetInstrInfo *TII = nullptr)
: DefSubReg(DefSubReg), Reg(Reg), MRI(MRI), TII(TII) {
if (!TargetRegisterInfo::isPhysicalRegister(Reg)) {
Def = MRI.getVRegDef(Reg);
DefIdx = MRI.def_begin(Reg).getOperandNo();
/// Following the use-def chain, get the next available source
/// for the tracked value.
/// \return A ValueTrackerResult containing a set of registers
/// and sub registers with tracked values. A ValueTrackerResult with
/// an empty set of registers means no source was found.
ValueTrackerResult getNextSource();
} // end anonymous namespace
char PeepholeOptimizer::ID = 0;
char &llvm::PeepholeOptimizerID = PeepholeOptimizer::ID;
"Peephole Optimizations", false, false)
"Peephole Optimizations", false, false)
/// If instruction is a copy-like instruction, i.e. it reads a single register
/// and writes a single register and it does not modify the source, and if the
/// source value is preserved as a sub-register of the result, then replace all
/// reachable uses of the source with the subreg of the result.
/// Do not generate an EXTRACT that is used only in a debug use, as this changes
/// the code. Since this code does not currently share EXTRACTs, just ignore all
/// debug uses.
bool PeepholeOptimizer::
optimizeExtInstr(MachineInstr &MI, MachineBasicBlock &MBB,
SmallPtrSetImpl<MachineInstr*> &LocalMIs) {
unsigned SrcReg, DstReg, SubIdx;
if (!TII->isCoalescableExtInstr(MI, SrcReg, DstReg, SubIdx))
return false;
if (TargetRegisterInfo::isPhysicalRegister(DstReg) ||
return false;
if (MRI->hasOneNonDBGUse(SrcReg))
// No other uses.
return false;
// Ensure DstReg can get a register class that actually supports
// sub-registers. Don't change the class until we commit.
const TargetRegisterClass *DstRC = MRI->getRegClass(DstReg);
DstRC = TRI->getSubClassWithSubReg(DstRC, SubIdx);
if (!DstRC)
return false;
// The ext instr may be operating on a sub-register of SrcReg as well.
// PPC::EXTSW is a 32 -> 64-bit sign extension, but it reads a 64-bit
// register.
// If UseSrcSubIdx is Set, SubIdx also applies to SrcReg, and only uses of
// SrcReg:SubIdx should be replaced.
bool UseSrcSubIdx =
TRI->getSubClassWithSubReg(MRI->getRegClass(SrcReg), SubIdx) != nullptr;
// The source has other uses. See if we can replace the other uses with use of
// the result of the extension.
SmallPtrSet<MachineBasicBlock*, 4> ReachedBBs;
for (MachineInstr &UI : MRI->use_nodbg_instructions(DstReg))
// Uses that are in the same BB of uses of the result of the instruction.
SmallVector<MachineOperand*, 8> Uses;
// Uses that the result of the instruction can reach.
SmallVector<MachineOperand*, 8> ExtendedUses;
bool ExtendLife = true;
for (MachineOperand &UseMO : MRI->use_nodbg_operands(SrcReg)) {
MachineInstr *UseMI = UseMO.getParent();
if (UseMI == &MI)
if (UseMI->isPHI()) {
ExtendLife = false;
// Only accept uses of SrcReg:SubIdx.
if (UseSrcSubIdx && UseMO.getSubReg() != SubIdx)
// It's an error to translate this:
// %reg1025 = <sext> %reg1024
// ...
// %reg1026 = SUBREG_TO_REG 0, %reg1024, 4
// into this:
// %reg1025 = <sext> %reg1024
// ...
// %reg1027 = COPY %reg1025:4
// %reg1026 = SUBREG_TO_REG 0, %reg1027, 4
// The problem here is that SUBREG_TO_REG is there to assert that an
// implicit zext occurs. It doesn't insert a zext instruction. If we allow
// the COPY here, it will give us the value after the <sext>, not the
// original value of %reg1024 before <sext>.
if (UseMI->getOpcode() == TargetOpcode::SUBREG_TO_REG)
MachineBasicBlock *UseMBB = UseMI->getParent();
if (UseMBB == &MBB) {
// Local uses that come after the extension.
if (!LocalMIs.count(UseMI))
} else if (ReachedBBs.count(UseMBB)) {
// Non-local uses where the result of the extension is used. Always
// replace these unless it's a PHI.
} else if (Aggressive && DT->dominates(&MBB, UseMBB)) {
// We may want to extend the live range of the extension result in order
// to replace these uses.
} else {
// Both will be live out of the def MBB anyway. Don't extend live range of
// the extension result.
ExtendLife = false;
if (ExtendLife && !ExtendedUses.empty())
// Extend the liveness of the extension result.
Uses.append(ExtendedUses.begin(), ExtendedUses.end());
// Now replace all uses.
bool Changed = false;
if (!Uses.empty()) {
SmallPtrSet<MachineBasicBlock*, 4> PHIBBs;
// Look for PHI uses of the extended result, we don't want to extend the
// liveness of a PHI input. It breaks all kinds of assumptions down
// stream. A PHI use is expected to be the kill of its source values.
for (MachineInstr &UI : MRI->use_nodbg_instructions(DstReg))
if (UI.isPHI())
const TargetRegisterClass *RC = MRI->getRegClass(SrcReg);
for (unsigned i = 0, e = Uses.size(); i != e; ++i) {
MachineOperand *UseMO = Uses[i];
MachineInstr *UseMI = UseMO->getParent();
MachineBasicBlock *UseMBB = UseMI->getParent();
if (PHIBBs.count(UseMBB))
// About to add uses of DstReg, clear DstReg's kill flags.
if (!Changed) {
MRI->constrainRegClass(DstReg, DstRC);
unsigned NewVR = MRI->createVirtualRegister(RC);
MachineInstr *Copy = BuildMI(*UseMBB, UseMI, UseMI->getDebugLoc(),
TII->get(TargetOpcode::COPY), NewVR)
.addReg(DstReg, 0, SubIdx);
// SubIdx applies to both SrcReg and DstReg when UseSrcSubIdx is set.
if (UseSrcSubIdx) {
Changed = true;
return Changed;
/// If the instruction is a compare and the previous instruction it's comparing
/// against already sets (or could be modified to set) the same flag as the
/// compare, then we can remove the comparison and use the flag from the
/// previous instruction.
bool PeepholeOptimizer::optimizeCmpInstr(MachineInstr &MI) {
// If this instruction is a comparison against zero and isn't comparing a
// physical register, we can try to optimize it.
unsigned SrcReg, SrcReg2;
int CmpMask, CmpValue;
if (!TII->analyzeCompare(MI, SrcReg, SrcReg2, CmpMask, CmpValue) ||
TargetRegisterInfo::isPhysicalRegister(SrcReg) ||
(SrcReg2 != 0 && TargetRegisterInfo::isPhysicalRegister(SrcReg2)))
return false;
// Attempt to optimize the comparison instruction.
if (TII->optimizeCompareInstr(MI, SrcReg, SrcReg2, CmpMask, CmpValue, MRI)) {
return true;
return false;
/// Optimize a select instruction.
bool PeepholeOptimizer::optimizeSelect(MachineInstr &MI,
SmallPtrSetImpl<MachineInstr *> &LocalMIs) {
unsigned TrueOp = 0;
unsigned FalseOp = 0;
bool Optimizable = false;
SmallVector<MachineOperand, 4> Cond;
if (TII->analyzeSelect(MI, Cond, TrueOp, FalseOp, Optimizable))
return false;
if (!Optimizable)
return false;
if (!TII->optimizeSelect(MI, LocalMIs))
return false;
return true;
/// Check if a simpler conditional branch can be generated.
bool PeepholeOptimizer::optimizeCondBranch(MachineInstr &MI) {
return TII->optimizeCondBranch(MI);
/// Try to find the next source that share the same register file
/// for the value defined by \p Reg and \p SubReg.
/// When true is returned, the \p RewriteMap can be used by the client to
/// retrieve all Def -> Use along the way up to the next source. Any found
/// Use that is not itself a key for another entry, is the next source to
/// use. During the search for the next source, multiple sources can be found
/// given multiple incoming sources of a PHI instruction. In this case, we
/// look in each PHI source for the next source; all found next sources must
/// share the same register file as \p Reg and \p SubReg. The client should
/// then be capable to rewrite all intermediate PHIs to get the next source.
/// \return False if no alternative sources are available. True otherwise.
bool PeepholeOptimizer::findNextSource(RegSubRegPair RegSubReg,
RewriteMapTy &RewriteMap) {
// Do not try to find a new source for a physical register.
// So far we do not have any motivating example for doing that.
// Thus, instead of maintaining untested code, we will revisit that if
// that changes at some point.
unsigned Reg = RegSubReg.Reg;
if (TargetRegisterInfo::isPhysicalRegister(Reg))
return false;
const TargetRegisterClass *DefRC = MRI->getRegClass(Reg);
SmallVector<RegSubRegPair, 4> SrcToLook;
RegSubRegPair CurSrcPair = RegSubReg;
unsigned PHICount = 0;
do {
CurSrcPair = SrcToLook.pop_back_val();
// As explained above, do not handle physical registers
if (TargetRegisterInfo::isPhysicalRegister(CurSrcPair.Reg))
return false;
ValueTracker ValTracker(CurSrcPair.Reg, CurSrcPair.SubReg, *MRI, TII);
// Follow the chain of copies until we find a more suitable source, a phi
// or have to abort.
while (true) {
ValueTrackerResult Res = ValTracker.getNextSource();
// Abort at the end of a chain (without finding a suitable source).
if (!Res.isValid())
return false;
// Insert the Def -> Use entry for the recently found source.
ValueTrackerResult CurSrcRes = RewriteMap.lookup(CurSrcPair);
if (CurSrcRes.isValid()) {
assert(CurSrcRes == Res && "ValueTrackerResult found must match");
// An existent entry with multiple sources is a PHI cycle we must avoid.
// Otherwise it's an entry with a valid next source we already found.
if (CurSrcRes.getNumSources() > 1) {
<< "findNextSource: found PHI cycle, aborting...\n");
return false;
RewriteMap.insert(std::make_pair(CurSrcPair, Res));
// ValueTrackerResult usually have one source unless it's the result from
// a PHI instruction. Add the found PHI edges to be looked up further.
unsigned NumSrcs = Res.getNumSources();
if (NumSrcs > 1) {
if (PHICount >= RewritePHILimit) {
LLVM_DEBUG(dbgs() << "findNextSource: PHI limit reached\n");
return false;
for (unsigned i = 0; i < NumSrcs; ++i)
CurSrcPair = Res.getSrc(0);
// Do not extend the live-ranges of physical registers as they add
// constraints to the register allocator. Moreover, if we want to extend
// the live-range of a physical register, unlike SSA virtual register,
// we will have to check that they aren't redefine before the related use.
if (TargetRegisterInfo::isPhysicalRegister(CurSrcPair.Reg))
return false;
// Keep following the chain if the value isn't any better yet.
const TargetRegisterClass *SrcRC = MRI->getRegClass(CurSrcPair.Reg);
if (!TRI->shouldRewriteCopySrc(DefRC, RegSubReg.SubReg, SrcRC,
// We currently cannot deal with subreg operands on PHI instructions
// (see insertPHI()).
if (PHICount > 0 && CurSrcPair.SubReg != 0)
// We found a suitable source, and are done with this chain.
} while (!SrcToLook.empty());
// If we did not find a more suitable source, there is nothing to optimize.
return CurSrcPair.Reg != Reg;
/// Insert a PHI instruction with incoming edges \p SrcRegs that are
/// guaranteed to have the same register class. This is necessary whenever we
/// successfully traverse a PHI instruction and find suitable sources coming
/// from its edges. By inserting a new PHI, we provide a rewritten PHI def
/// suitable to be used in a new COPY instruction.
static MachineInstr &
insertPHI(MachineRegisterInfo &MRI, const TargetInstrInfo &TII,
const SmallVectorImpl<RegSubRegPair> &SrcRegs,
MachineInstr &OrigPHI) {
assert(!SrcRegs.empty() && "No sources to create a PHI instruction?");
const TargetRegisterClass *NewRC = MRI.getRegClass(SrcRegs[0].Reg);
// NewRC is only correct if no subregisters are involved. findNextSource()
// should have rejected those cases already.
assert(SrcRegs[0].SubReg == 0 && "should not have subreg operand");
unsigned NewVR = MRI.createVirtualRegister(NewRC);
MachineBasicBlock *MBB = OrigPHI.getParent();
MachineInstrBuilder MIB = BuildMI(*MBB, &OrigPHI, OrigPHI.getDebugLoc(),
TII.get(TargetOpcode::PHI), NewVR);
unsigned MBBOpIdx = 2;
for (const RegSubRegPair &RegPair : SrcRegs) {
MIB.addReg(RegPair.Reg, 0, RegPair.SubReg);
// Since we're extended the lifetime of RegPair.Reg, clear the
// kill flags to account for that and make RegPair.Reg reaches
// the new PHI.
MBBOpIdx += 2;
return *MIB;
namespace {
/// Interface to query instructions amenable to copy rewriting.
class Rewriter {
MachineInstr &CopyLike;
unsigned CurrentSrcIdx = 0; ///< The index of the source being rewritten.
Rewriter(MachineInstr &CopyLike) : CopyLike(CopyLike) {}
virtual ~Rewriter() {}
/// Get the next rewritable source (SrcReg, SrcSubReg) and
/// the related value that it affects (DstReg, DstSubReg).
/// A source is considered rewritable if its register class and the
/// register class of the related DstReg may not be register
/// coalescer friendly. In other words, given a copy-like instruction
/// not all the arguments may be returned at rewritable source, since
/// some arguments are none to be register coalescer friendly.
/// Each call of this method moves the current source to the next
/// rewritable source.
/// For instance, let CopyLike be the instruction to rewrite.
/// CopyLike has one definition and one source:
/// dst.dstSubIdx = CopyLike src.srcSubIdx.
/// The first call will give the first rewritable source, i.e.,
/// the only source this instruction has:
/// (SrcReg, SrcSubReg) = (src, srcSubIdx).
/// This source defines the whole definition, i.e.,
/// (DstReg, DstSubReg) = (dst, dstSubIdx).
/// The second and subsequent calls will return false, as there is only one
/// rewritable source.
/// \return True if a rewritable source has been found, false otherwise.
/// The output arguments are valid if and only if true is returned.
virtual bool getNextRewritableSource(RegSubRegPair &Src,
RegSubRegPair &Dst) = 0;
/// Rewrite the current source with \p NewReg and \p NewSubReg if possible.
/// \return True if the rewriting was possible, false otherwise.
virtual bool RewriteCurrentSource(unsigned NewReg, unsigned NewSubReg) = 0;
/// Rewriter for COPY instructions.
class CopyRewriter : public Rewriter {
CopyRewriter(MachineInstr &MI) : Rewriter(MI) {
assert(MI.isCopy() && "Expected copy instruction");
virtual ~CopyRewriter() = default;
bool getNextRewritableSource(RegSubRegPair &Src,
RegSubRegPair &Dst) override {
// CurrentSrcIdx > 0 means this function has already been called.
if (CurrentSrcIdx > 0)
return false;
// This is the first call to getNextRewritableSource.
// Move the CurrentSrcIdx to remember that we made that call.
CurrentSrcIdx = 1;
// The rewritable source is the argument.
const MachineOperand &MOSrc = CopyLike.getOperand(1);
Src = RegSubRegPair(MOSrc.getReg(), MOSrc.getSubReg());
// What we track are the alternative sources of the definition.
const MachineOperand &MODef = CopyLike.getOperand(0);
Dst = RegSubRegPair(MODef.getReg(), MODef.getSubReg());
return true;
bool RewriteCurrentSource(unsigned NewReg, unsigned NewSubReg) override {
if (CurrentSrcIdx != 1)
return false;
MachineOperand &MOSrc = CopyLike.getOperand(CurrentSrcIdx);
return true;
/// Helper class to rewrite uncoalescable copy like instructions
/// into new COPY (coalescable friendly) instructions.
class UncoalescableRewriter : public Rewriter {
unsigned NumDefs; ///< Number of defs in the bitcast.
UncoalescableRewriter(MachineInstr &MI) : Rewriter(MI) {
NumDefs = MI.getDesc().getNumDefs();
/// \see See Rewriter::getNextRewritableSource()
/// All such sources need to be considered rewritable in order to
/// rewrite a uncoalescable copy-like instruction. This method return
/// each definition that must be checked if rewritable.
bool getNextRewritableSource(RegSubRegPair &Src,
RegSubRegPair &Dst) override {
// Find the next non-dead definition and continue from there.
if (CurrentSrcIdx == NumDefs)
return false;
while (CopyLike.getOperand(CurrentSrcIdx).isDead()) {
if (CurrentSrcIdx == NumDefs)
return false;
// What we track are the alternative sources of the definition.
Src = RegSubRegPair(0, 0);
const MachineOperand &MODef = CopyLike.getOperand(CurrentSrcIdx);
Dst = RegSubRegPair(MODef.getReg(), MODef.getSubReg());
return true;
bool RewriteCurrentSource(unsigned NewReg, unsigned NewSubReg) override {
return false;
/// Specialized rewriter for INSERT_SUBREG instruction.
class InsertSubregRewriter : public Rewriter {
InsertSubregRewriter(MachineInstr &MI) : Rewriter(MI) {
assert(MI.isInsertSubreg() && "Invalid instruction");
/// \see See Rewriter::getNextRewritableSource()
/// Here CopyLike has the following form:
/// dst = INSERT_SUBREG Src1, Src2.src2SubIdx, subIdx.
/// Src1 has the same register class has dst, hence, there is
/// nothing to rewrite.
/// Src2.src2SubIdx, may not be register coalescer friendly.
/// Therefore, the first call to this method returns:
/// (SrcReg, SrcSubReg) = (Src2, src2SubIdx).
/// (DstReg, DstSubReg) = (dst, subIdx).
/// Subsequence calls will return false.
bool getNextRewritableSource(RegSubRegPair &Src,
RegSubRegPair &Dst) override {
// If we already get the only source we can rewrite, return false.
if (CurrentSrcIdx == 2)
return false;
// We are looking at v2 = INSERT_SUBREG v0, v1, sub0.
CurrentSrcIdx = 2;
const MachineOperand &MOInsertedReg = CopyLike.getOperand(2);
Src = RegSubRegPair(MOInsertedReg.getReg(), MOInsertedReg.getSubReg());
const MachineOperand &MODef = CopyLike.getOperand(0);
// We want to track something that is compatible with the
// partial definition.
if (MODef.getSubReg())
// Bail if we have to compose sub-register indices.
return false;
Dst = RegSubRegPair(MODef.getReg(),
return true;
bool RewriteCurrentSource(unsigned NewReg, unsigned NewSubReg) override {
if (CurrentSrcIdx != 2)
return false;
// We are rewriting the inserted reg.
MachineOperand &MO = CopyLike.getOperand(CurrentSrcIdx);
return true;
/// Specialized rewriter for EXTRACT_SUBREG instruction.
class ExtractSubregRewriter : public Rewriter {
const TargetInstrInfo &TII;
ExtractSubregRewriter(MachineInstr &MI, const TargetInstrInfo &TII)
: Rewriter(MI), TII(TII) {
assert(MI.isExtractSubreg() && "Invalid instruction");
/// \see Rewriter::getNextRewritableSource()
/// Here CopyLike has the following form:
/// dst.dstSubIdx = EXTRACT_SUBREG Src, subIdx.
/// There is only one rewritable source: Src.subIdx,
/// which defines dst.dstSubIdx.
bool getNextRewritableSource(RegSubRegPair &Src,
RegSubRegPair &Dst) override {
// If we already get the only source we can rewrite, return false.
if (CurrentSrcIdx == 1)
return false;
// We are looking at v1 = EXTRACT_SUBREG v0, sub0.
CurrentSrcIdx = 1;
const MachineOperand &MOExtractedReg = CopyLike.getOperand(1);
// If we have to compose sub-register indices, bail out.
if (MOExtractedReg.getSubReg())
return false;
Src = RegSubRegPair(MOExtractedReg.getReg(),
// We want to track something that is compatible with the definition.
const MachineOperand &MODef = CopyLike.getOperand(0);
Dst = RegSubRegPair(MODef.getReg(), MODef.getSubReg());
return true;
bool RewriteCurrentSource(unsigned NewReg, unsigned NewSubReg) override {
// The only source we can rewrite is the input register.
if (CurrentSrcIdx != 1)
return false;
// If we find a source that does not require to extract something,
// rewrite the operation with a copy.
if (!NewSubReg) {
// Move the current index to an invalid position.
// We do not want another call to this method to be able
// to do any change.
CurrentSrcIdx = -1;
// Rewrite the operation as a COPY.
// Get rid of the sub-register index.
// Morph the operation into a COPY.
return true;
CopyLike.getOperand(CurrentSrcIdx + 1).setImm(NewSubReg);
return true;
/// Specialized rewriter for REG_SEQUENCE instruction.
class RegSequenceRewriter : public Rewriter {
RegSequenceRewriter(MachineInstr &MI) : Rewriter(MI) {
assert(MI.isRegSequence() && "Invalid instruction");
/// \see Rewriter::getNextRewritableSource()
/// Here CopyLike has the following form:
/// dst = REG_SEQUENCE Src1.src1SubIdx, subIdx1, Src2.src2SubIdx, subIdx2.
/// Each call will return a different source, walking all the available
/// source.
/// The first call returns:
/// (SrcReg, SrcSubReg) = (Src1, src1SubIdx).
/// (DstReg, DstSubReg) = (dst, subIdx1).
/// The second call returns:
/// (SrcReg, SrcSubReg) = (Src2, src2SubIdx).
/// (DstReg, DstSubReg) = (dst, subIdx2).
/// And so on, until all the sources have been traversed, then
/// it returns false.
bool getNextRewritableSource(RegSubRegPair &Src,
RegSubRegPair &Dst) override {
// We are looking at v0 = REG_SEQUENCE v1, sub1, v2, sub2, etc.
// If this is the first call, move to the first argument.
if (CurrentSrcIdx == 0) {
CurrentSrcIdx = 1;
} else {
// Otherwise, move to the next argument and check that it is valid.
CurrentSrcIdx += 2;
if (CurrentSrcIdx >= CopyLike.getNumOperands())
return false;
const MachineOperand &MOInsertedReg = CopyLike.getOperand(CurrentSrcIdx);
Src.Reg = MOInsertedReg.getReg();
// If we have to compose sub-register indices, bail out.
if ((Src.SubReg = MOInsertedReg.getSubReg()))
return false;
// We want to track something that is compatible with the related
// partial definition.
Dst.SubReg = CopyLike.getOperand(CurrentSrcIdx + 1).getImm();
const MachineOperand &MODef = CopyLike.getOperand(0);
Dst.Reg = MODef.getReg();
// If we have to compose sub-registers, bail.
return MODef.getSubReg() == 0;
bool RewriteCurrentSource(unsigned NewReg, unsigned NewSubReg) override {
// We cannot rewrite out of bound operands.
// Moreover, rewritable sources are at odd positions.
if ((CurrentSrcIdx & 1) != 1 || CurrentSrcIdx > CopyLike.getNumOperands())
return false;
MachineOperand &MO = CopyLike.getOperand(CurrentSrcIdx);
return true;
} // end anonymous namespace
/// Get the appropriated Rewriter for \p MI.
/// \return A pointer to a dynamically allocated Rewriter or nullptr if no
/// rewriter works for \p MI.
static Rewriter *getCopyRewriter(MachineInstr &MI, const TargetInstrInfo &TII) {
// Handle uncoalescable copy-like instructions.
if (MI.isBitcast() || MI.isRegSequenceLike() || MI.isInsertSubregLike() ||
return new UncoalescableRewriter(MI);
switch (MI.getOpcode()) {
return nullptr;
case TargetOpcode::COPY:
return new CopyRewriter(MI);
case TargetOpcode::INSERT_SUBREG:
return new InsertSubregRewriter(MI);
case TargetOpcode::EXTRACT_SUBREG:
return new ExtractSubregRewriter(MI, TII);
case TargetOpcode::REG_SEQUENCE:
return new RegSequenceRewriter(MI);
/// Given a \p Def.Reg and Def.SubReg pair, use \p RewriteMap to find
/// the new source to use for rewrite. If \p HandleMultipleSources is true and
/// multiple sources for a given \p Def are found along the way, we found a
/// PHI instructions that needs to be rewritten.
/// TODO: HandleMultipleSources should be removed once we test PHI handling
/// with coalescable copies.
static RegSubRegPair
getNewSource(MachineRegisterInfo *MRI, const TargetInstrInfo *TII,
RegSubRegPair Def,
const PeepholeOptimizer::RewriteMapTy &RewriteMap,
bool HandleMultipleSources = true) {
RegSubRegPair LookupSrc(Def.Reg, Def.SubReg);
while (true) {
ValueTrackerResult Res = RewriteMap.lookup(LookupSrc);
// If there are no entries on the map, LookupSrc is the new source.
if (!Res.isValid())
return LookupSrc;
// There's only one source for this definition, keep searching...
unsigned NumSrcs = Res.getNumSources();
if (NumSrcs == 1) {
LookupSrc.Reg = Res.getSrcReg(0);
LookupSrc.SubReg = Res.getSrcSubReg(0);
// TODO: Remove once multiple srcs w/ coalescable copies are supported.
if (!HandleMultipleSources)
// Multiple sources, recurse into each source to find a new source
// for it. Then, rewrite the PHI accordingly to its new edges.
SmallVector<RegSubRegPair, 4> NewPHISrcs;
for (unsigned i = 0; i < NumSrcs; ++i) {
RegSubRegPair PHISrc(Res.getSrcReg(i), Res.getSrcSubReg(i));
getNewSource(MRI, TII, PHISrc, RewriteMap, HandleMultipleSources));
// Build the new PHI node and return its def register as the new source.
MachineInstr &OrigPHI = const_cast<MachineInstr &>(*Res.getInst());
MachineInstr &NewPHI = insertPHI(*MRI, *TII, NewPHISrcs, OrigPHI);
LLVM_DEBUG(dbgs() << "-- getNewSource\n");
LLVM_DEBUG(dbgs() << " Replacing: " << OrigPHI);
LLVM_DEBUG(dbgs() << " With: " << NewPHI);
const MachineOperand &MODef = NewPHI.getOperand(0);
return RegSubRegPair(MODef.getReg(), MODef.getSubReg());
return RegSubRegPair(0, 0);
/// Optimize generic copy instructions to avoid cross register bank copy.
/// The optimization looks through a chain of copies and tries to find a source
/// that has a compatible register class.
/// Two register classes are considered to be compatible if they share the same
/// register bank.
/// New copies issued by this optimization are register allocator
/// friendly. This optimization does not remove any copy as it may
/// overconstrain the register allocator, but replaces some operands
/// when possible.
/// \pre isCoalescableCopy(*MI) is true.
/// \return True, when \p MI has been rewritten. False otherwise.
bool PeepholeOptimizer::optimizeCoalescableCopy(MachineInstr &MI) {
assert(isCoalescableCopy(MI) && "Invalid argument");
assert(MI.getDesc().getNumDefs() == 1 &&
"Coalescer can understand multiple defs?!");
const MachineOperand &MODef = MI.getOperand(0);
// Do not rewrite physical definitions.
if (TargetRegisterInfo::isPhysicalRegister(MODef.getReg()))
return false;
bool Changed = false;
// Get the right rewriter for the current copy.
std::unique_ptr<Rewriter> CpyRewriter(getCopyRewriter(MI, *TII));
// If none exists, bail out.
if (!CpyRewriter)
return false;
// Rewrite each rewritable source.
RegSubRegPair Src;
RegSubRegPair TrackPair;
while (CpyRewriter->getNextRewritableSource(Src, TrackPair)) {
// Keep track of PHI nodes and its incoming edges when looking for sources.
RewriteMapTy RewriteMap;
// Try to find a more suitable source. If we failed to do so, or get the
// actual source, move to the next source.
if (!findNextSource(TrackPair, RewriteMap))
// Get the new source to rewrite. TODO: Only enable handling of multiple
// sources (PHIs) once we have a motivating example and testcases for it.
RegSubRegPair NewSrc = getNewSource(MRI, TII, TrackPair, RewriteMap,
if (Src.Reg == NewSrc.Reg || NewSrc.Reg == 0)
// Rewrite source.
if (CpyRewriter->RewriteCurrentSource(NewSrc.Reg, NewSrc.SubReg)) {
// We may have extended the live-range of NewSrc, account for that.
Changed = true;
// TODO: We could have a clean-up method to tidy the instruction.
// E.g., v0 = INSERT_SUBREG v1, v1.sub0, sub0
// => v0 = COPY v1
// Currently we haven't seen motivating example for that and we
// want to avoid untested code.
NumRewrittenCopies += Changed;
return Changed;
/// Rewrite the source found through \p Def, by using the \p RewriteMap
/// and create a new COPY instruction. More info about RewriteMap in
/// PeepholeOptimizer::findNextSource. Right now this is only used to handle
/// Uncoalescable copies, since they are copy like instructions that aren't
/// recognized by the register allocator.
MachineInstr &
PeepholeOptimizer::rewriteSource(MachineInstr &CopyLike,
RegSubRegPair Def, RewriteMapTy &RewriteMap) {
assert(!TargetRegisterInfo::isPhysicalRegister(Def.Reg) &&
"We do not rewrite physical registers");
// Find the new source to use in the COPY rewrite.
RegSubRegPair NewSrc = getNewSource(MRI, TII, Def, RewriteMap);
// Insert the COPY.
const TargetRegisterClass *DefRC = MRI->getRegClass(Def.Reg);
unsigned NewVReg = MRI->createVirtualRegister(DefRC);
MachineInstr *NewCopy =
BuildMI(*CopyLike.getParent(), &CopyLike, CopyLike.getDebugLoc(),
TII->get(TargetOpcode::COPY), NewVReg)
.addReg(NewSrc.Reg, 0, NewSrc.SubReg);
if (Def.SubReg) {
LLVM_DEBUG(dbgs() << "-- RewriteSource\n");
LLVM_DEBUG(dbgs() << " Replacing: " << CopyLike);
LLVM_DEBUG(dbgs() << " With: " << *NewCopy);
MRI->replaceRegWith(Def.Reg, NewVReg);
// We extended the lifetime of NewSrc.Reg, clear the kill flags to
// account for that.
return *NewCopy;
/// Optimize copy-like instructions to create
/// register coalescer friendly instruction.
/// The optimization tries to kill-off the \p MI by looking
/// through a chain of copies to find a source that has a compatible
/// register class.
/// If such a source is found, it replace \p MI by a generic COPY
/// operation.
/// \pre isUncoalescableCopy(*MI) is true.
/// \return True, when \p MI has been optimized. In that case, \p MI has
/// been removed from its parent.
/// All COPY instructions created, are inserted in \p LocalMIs.
bool PeepholeOptimizer::optimizeUncoalescableCopy(
MachineInstr &MI, SmallPtrSetImpl<MachineInstr *> &LocalMIs) {
assert(isUncoalescableCopy(MI) && "Invalid argument");
UncoalescableRewriter CpyRewriter(MI);
// Rewrite each rewritable source by generating new COPYs. This works
// differently from optimizeCoalescableCopy since it first makes sure that all
// definitions can be rewritten.
RewriteMapTy RewriteMap;
RegSubRegPair Src;
RegSubRegPair Def;
SmallVector<RegSubRegPair, 4> RewritePairs;
while (CpyRewriter.getNextRewritableSource(Src, Def)) {
// If a physical register is here, this is probably for a good reason.
// Do not rewrite that.
if (TargetRegisterInfo::isPhysicalRegister(Def.Reg))
return false;
// If we do not know how to rewrite this definition, there is no point
// in trying to kill this instruction.
if (!findNextSource(Def, RewriteMap))
return false;
// The change is possible for all defs, do it.
for (const RegSubRegPair &Def : RewritePairs) {
// Rewrite the "copy" in a way the register coalescer understands.
MachineInstr &NewCopy = rewriteSource(MI, Def, RewriteMap);
// MI is now dead.
return true;
/// Check whether MI is a candidate for folding into a later instruction.
/// We only fold loads to virtual registers and the virtual register defined
/// has a single use.
bool PeepholeOptimizer::isLoadFoldable(
MachineInstr &MI, SmallSet<unsigned, 16> &FoldAsLoadDefCandidates) {
if (!MI.canFoldAsLoad() || !MI.mayLoad())
return false;
const MCInstrDesc &MCID = MI.getDesc();
if (MCID.getNumDefs() != 1)
return false;
unsigned Reg = MI.getOperand(0).getReg();
// To reduce compilation time, we check MRI->hasOneNonDBGUse when inserting
// loads. It should be checked when processing uses of the load, since
// uses can be removed during peephole.
if (!MI.getOperand(0).getSubReg() &&
TargetRegisterInfo::isVirtualRegister(Reg) &&
MRI->hasOneNonDBGUse(Reg)) {
return true;
return false;
bool PeepholeOptimizer::isMoveImmediate(
MachineInstr &MI, SmallSet<unsigned, 4> &ImmDefRegs,
DenseMap<unsigned, MachineInstr *> &ImmDefMIs) {
const MCInstrDesc &MCID = MI.getDesc();
if (!MI.isMoveImmediate())
return false;
if (MCID.getNumDefs() != 1)
return false;
unsigned Reg = MI.getOperand(0).getReg();
if (TargetRegisterInfo::isVirtualRegister(Reg)) {
ImmDefMIs.insert(std::make_pair(Reg, &MI));
return true;
return false;
/// Try folding register operands that are defined by move immediate
/// instructions, i.e. a trivial constant folding optimization, if
/// and only if the def and use are in the same BB.
bool PeepholeOptimizer::foldImmediate(MachineInstr &MI,
SmallSet<unsigned, 4> &ImmDefRegs,
DenseMap<unsigned, MachineInstr *> &ImmDefMIs) {
for (unsigned i = 0, e = MI.getDesc().getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI.getOperand(i);
if (!MO.isReg() || MO.isDef())
// Ignore dead implicit defs.
if (MO.isImplicit() && MO.isDead())
unsigned Reg = MO.getReg();
if (!TargetRegisterInfo::isVirtualRegister(Reg))
if (ImmDefRegs.count(Reg) == 0)
DenseMap<unsigned, MachineInstr*>::iterator II = ImmDefMIs.find(Reg);
assert(II != ImmDefMIs.end() && "couldn't find immediate definition");
if (TII->FoldImmediate(MI, *II->second, Reg, MRI)) {
return true;
return false;
// FIXME: This is very simple and misses some cases which should be handled when
// motivating examples are found.
// The copy rewriting logic should look at uses as well as defs and be able to
// eliminate copies across blocks.
// Later copies that are subregister extracts will also not be eliminated since
// only the first copy is considered.
// e.g.
// %1 = COPY %0
// %2 = COPY %0:sub1
// Should replace %2 uses with %1:sub1
bool PeepholeOptimizer::foldRedundantCopy(MachineInstr &MI,
SmallSet<unsigned, 4> &CopySrcRegs,
DenseMap<unsigned, MachineInstr *> &CopyMIs) {
assert(MI.isCopy() && "expected a COPY machine instruction");
unsigned SrcReg = MI.getOperand(1).getReg();
if (!TargetRegisterInfo::isVirtualRegister(SrcReg))
return false;
unsigned DstReg = MI.getOperand(0).getReg();
if (!TargetRegisterInfo::isVirtualRegister(DstReg))
return false;
if (CopySrcRegs.insert(SrcReg).second) {
// First copy of this reg seen.
CopyMIs.insert(std::make_pair(SrcReg, &MI));
return false;
MachineInstr *PrevCopy = CopyMIs.find(SrcReg)->second;
unsigned SrcSubReg = MI.getOperand(1).getSubReg();
unsigned PrevSrcSubReg = PrevCopy->getOperand(1).getSubReg();
// Can't replace different subregister extracts.
if (SrcSubReg != PrevSrcSubReg)
return false;
unsigned PrevDstReg = PrevCopy->getOperand(0).getReg();
// Only replace if the copy register class is the same.
// TODO: If we have multiple copies to different register classes, we may want
// to track multiple copies of the same source register.
if (MRI->getRegClass(DstReg) != MRI->getRegClass(PrevDstReg))
return false;
MRI->replaceRegWith(DstReg, PrevDstReg);
// Lifetime of the previous copy has been extended.
return true;
bool PeepholeOptimizer::isNAPhysCopy(unsigned Reg) {
return TargetRegisterInfo::isPhysicalRegister(Reg) &&
bool PeepholeOptimizer::foldRedundantNAPhysCopy(
MachineInstr &MI, DenseMap<unsigned, MachineInstr *> &NAPhysToVirtMIs) {
assert(MI.isCopy() && "expected a COPY machine instruction");
if (DisableNAPhysCopyOpt)
return false;
unsigned DstReg = MI.getOperand(0).getReg();
unsigned SrcReg = MI.getOperand(1).getReg();
if (isNAPhysCopy(SrcReg) && TargetRegisterInfo::isVirtualRegister(DstReg)) {
// %vreg = COPY %physreg
// Avoid using a datastructure which can track multiple live non-allocatable
// phys->virt copies since LLVM doesn't seem to do this.
NAPhysToVirtMIs.insert({SrcReg, &MI});
return false;
if (!(TargetRegisterInfo::isVirtualRegister(SrcReg) && isNAPhysCopy(DstReg)))
return false;
// %physreg = COPY %vreg
auto PrevCopy = NAPhysToVirtMIs.find(DstReg);
if (PrevCopy == NAPhysToVirtMIs.end()) {
// We can't remove the copy: there was an intervening clobber of the
// non-allocatable physical register after the copy to virtual.
LLVM_DEBUG(dbgs() << "NAPhysCopy: intervening clobber forbids erasing "
<< MI);
return false;
unsigned PrevDstReg = PrevCopy->second->getOperand(0).getReg();
if (PrevDstReg == SrcReg) {
// Remove the virt->phys copy: we saw the virtual register definition, and
// the non-allocatable physical register's state hasn't changed since then.
LLVM_DEBUG(dbgs() << "NAPhysCopy: erasing " << MI);
return true;
// Potential missed optimization opportunity: we saw a different virtual
// register get a copy of the non-allocatable physical register, and we only
// track one such copy. Avoid getting confused by this new non-allocatable
// physical register definition, and remove it from the tracked copies.
LLVM_DEBUG(dbgs() << "NAPhysCopy: missed opportunity " << MI);
return false;
/// \bried Returns true if \p MO is a virtual register operand.
static bool isVirtualRegisterOperand(MachineOperand &MO) {
if (!MO.isReg())
return false;
return TargetRegisterInfo::isVirtualRegister(MO.getReg());
bool PeepholeOptimizer::findTargetRecurrence(
unsigned Reg, const SmallSet<unsigned, 2> &TargetRegs,
RecurrenceCycle &RC) {
// Recurrence found if Reg is in TargetRegs.
if (TargetRegs.count(Reg))
return true;
// TODO: Curerntly, we only allow the last instruction of the recurrence
// cycle (the instruction that feeds the PHI instruction) to have more than
// one uses to guarantee that commuting operands does not tie registers
// with overlapping live range. Once we have actual live range info of
// each register, this constraint can be relaxed.
if (!MRI->hasOneNonDBGUse(Reg))
return false;
// Give up if the reccurrence chain length is longer than the limit.
if (RC.size() >= MaxRecurrenceChain)
return false;
MachineInstr &MI = *(MRI->use_instr_nodbg_begin(Reg));
unsigned Idx = MI.findRegisterUseOperandIdx(Reg);
// Only interested in recurrences whose instructions have only one def, which
// is a virtual register.
if (MI.getDesc().getNumDefs() != 1)
return false;
MachineOperand &DefOp = MI.getOperand(0);
if (!isVirtualRegisterOperand(DefOp))
return false;
// Check if def operand of MI is tied to any use operand. We are only
// interested in the case that all the instructions in the recurrence chain
// have there def operand tied with one of the use operand.
unsigned TiedUseIdx;
if (!MI.isRegTiedToUseOperand(0, &TiedUseIdx))
return false;
if (Idx == TiedUseIdx) {
return findTargetRecurrence(DefOp.getReg(), TargetRegs, RC);
} else {
// If Idx is not TiedUseIdx, check if Idx is commutable with TiedUseIdx.
unsigned CommIdx = TargetInstrInfo::CommuteAnyOperandIndex;
if (TII->findCommutedOpIndices(MI, Idx, CommIdx) && CommIdx == TiedUseIdx) {
RC.push_back(RecurrenceInstr(&MI, Idx, CommIdx));
return findTargetRecurrence(DefOp.getReg(), TargetRegs, RC);
return false;
/// Phi instructions will eventually be lowered to copy instructions.
/// If phi is in a loop header, a recurrence may formulated around the source
/// and destination of the phi. For such case commuting operands of the
/// instructions in the recurrence may enable coalescing of the copy instruction
/// generated from the phi. For example, if there is a recurrence of
/// LoopHeader:
/// %1 = phi(%0, %100)
/// LoopLatch:
/// %0<def, tied1> = ADD %2<def, tied0>, %1
/// , the fact that %0 and %2 are in the same tied operands set makes
/// the coalescing of copy instruction generated from the phi in
/// LoopHeader(i.e. %1 = COPY %0) impossible, because %1 and
/// %2 have overlapping live range. This introduces additional move
/// instruction to the final assembly. However, if we commute %2 and
/// %1 of ADD instruction, the redundant move instruction can be
/// avoided.
bool PeepholeOptimizer::optimizeRecurrence(MachineInstr &PHI) {
SmallSet<unsigned, 2> TargetRegs;
for (unsigned Idx = 1; Idx < PHI.getNumOperands(); Idx += 2) {
MachineOperand &MO = PHI.getOperand(Idx);
assert(isVirtualRegisterOperand(MO) && "Invalid PHI instruction");
bool Changed = false;
RecurrenceCycle RC;
if (findTargetRecurrence(PHI.getOperand(0).getReg(), TargetRegs, RC)) {
// Commutes operands of instructions in RC if necessary so that the copy to
// be generated from PHI can be coalesced.
LLVM_DEBUG(dbgs() << "Optimize recurrence chain from " << PHI);
for (auto &RI : RC) {
LLVM_DEBUG(dbgs() << "\tInst: " << *(RI.getMI()));
auto CP = RI.getCommutePair();
if (CP) {
Changed = true;
TII->commuteInstruction(*(RI.getMI()), false, (*CP).first,
LLVM_DEBUG(dbgs() << "\t\tCommuted: " << *(RI.getMI()));
return Changed;
bool PeepholeOptimizer::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
LLVM_DEBUG(dbgs() << "********** PEEPHOLE OPTIMIZER **********\n");
LLVM_DEBUG(dbgs() << "********** Function: " << MF.getName() << '\n');
if (DisablePeephole)
return false;
TII = MF.getSubtarget().getInstrInfo();
TRI = MF.getSubtarget().getRegisterInfo();
MRI = &MF.getRegInfo();
DT = Aggressive ? &getAnalysis<MachineDominatorTree>() : nullptr;
MLI = &getAnalysis<MachineLoopInfo>();
bool Changed = false;
for (MachineBasicBlock &MBB : MF) {
bool SeenMoveImm = false;
// During this forward scan, at some point it needs to answer the question
// "given a pointer to an MI in the current BB, is it located before or
// after the current instruction".
// To perform this, the following set keeps track of the MIs already seen
// during the scan, if a MI is not in the set, it is assumed to be located
// after. Newly created MIs have to be inserted in the set as well.
SmallPtrSet<MachineInstr*, 16> LocalMIs;
SmallSet<unsigned, 4> ImmDefRegs;
DenseMap<unsigned, MachineInstr*> ImmDefMIs;
SmallSet<unsigned, 16> FoldAsLoadDefCandidates;
// Track when a non-allocatable physical register is copied to a virtual
// register so that useless moves can be removed.
// %physreg is the map index; MI is the last valid `%vreg = COPY %physreg`
// without any intervening re-definition of %physreg.
DenseMap<unsigned, MachineInstr *> NAPhysToVirtMIs;
// Set of virtual registers that are copied from.
SmallSet<unsigned, 4> CopySrcRegs;
DenseMap<unsigned, MachineInstr *> CopySrcMIs;
bool IsLoopHeader = MLI->isLoopHeader(&MBB);
for (MachineBasicBlock::iterator MII = MBB.begin(), MIE = MBB.end();
MII != MIE; ) {
MachineInstr *MI = &*MII;
// We may be erasing MI below, increment MII now.
// Skip debug instructions. They should not affect this peephole optimization.
if (MI->isDebugInstr())
if (MI->isPosition())
if (IsLoopHeader && MI->isPHI()) {
if (optimizeRecurrence(*MI)) {
Changed = true;
if (!MI->isCopy()) {
for (const MachineOperand &MO : MI->operands()) {
// Visit all operands: definitions can be implicit or explicit.
if (MO.isReg()) {
unsigned Reg = MO.getReg();
if (MO.isDef() && isNAPhysCopy(Reg)) {
const auto &Def = NAPhysToVirtMIs.find(Reg);
if (Def != NAPhysToVirtMIs.end()) {
// A new definition of the non-allocatable physical register
// invalidates previous copies.
<< "NAPhysCopy: invalidating because of " << *MI);
} else if (MO.isRegMask()) {
const uint32_t *RegMask = MO.getRegMask();
for (auto &RegMI : NAPhysToVirtMIs) {
unsigned Def = RegMI.first;
if (MachineOperand::clobbersPhysReg(RegMask, Def)) {
<< "NAPhysCopy: invalidating because of " << *MI);
if (MI->isImplicitDef() || MI->isKill())
if (MI->isInlineAsm() || MI->hasUnmodeledSideEffects()) {
// Blow away all non-allocatable physical registers knowledge since we
// don't know what's correct anymore.
// FIXME: handle explicit asm clobbers.
LLVM_DEBUG(dbgs() << "NAPhysCopy: blowing away all info due to "
<< *MI);
if ((isUncoalescableCopy(*MI) &&
optimizeUncoalescableCopy(*MI, LocalMIs)) ||
(MI->isCompare() && optimizeCmpInstr(*MI)) ||
(MI->isSelect() && optimizeSelect(*MI, LocalMIs))) {
// MI is deleted.
Changed = true;
if (MI->isConditionalBranch() && optimizeCondBranch(*MI)) {
Changed = true;
if (isCoalescableCopy(*MI) && optimizeCoalescableCopy(*MI)) {
// MI is just rewritten.
Changed = true;
if (MI->isCopy() &&
(foldRedundantCopy(*MI, CopySrcRegs, CopySrcMIs) ||
foldRedundantNAPhysCopy(*MI, NAPhysToVirtMIs))) {
Changed = true;
if (isMoveImmediate(*MI, ImmDefRegs, ImmDefMIs)) {
SeenMoveImm = true;
} else {
Changed |= optimizeExtInstr(*MI, MBB, LocalMIs);
// optimizeExtInstr might have created new instructions after MI
// and before the already incremented MII. Adjust MII so that the
// next iteration sees the new instructions.
if (SeenMoveImm)
Changed |= foldImmediate(*MI, ImmDefRegs, ImmDefMIs);
// Check whether MI is a load candidate for folding into a later
// instruction. If MI is not a candidate, check whether we can fold an
// earlier load into MI.
if (!isLoadFoldable(*MI, FoldAsLoadDefCandidates) &&
!FoldAsLoadDefCandidates.empty()) {
// We visit each operand even after successfully folding a previous
// one. This allows us to fold multiple loads into a single
// instruction. We do assume that optimizeLoadInstr doesn't insert
// foldable uses earlier in the argument list. Since we don't restart
// iteration, we'd miss such cases.
const MCInstrDesc &MIDesc = MI->getDesc();
for (unsigned i = MIDesc.getNumDefs(); i != MI->getNumOperands();
++i) {
const MachineOperand &MOp = MI->getOperand(i);
if (!MOp.isReg())
unsigned FoldAsLoadDefReg = MOp.getReg();
if (FoldAsLoadDefCandidates.count(FoldAsLoadDefReg)) {
// We need to fold load after optimizeCmpInstr, since
// optimizeCmpInstr can enable folding by converting SUB to CMP.
// Save FoldAsLoadDefReg because optimizeLoadInstr() resets it and
// we need it for markUsesInDebugValueAsUndef().
unsigned FoldedReg = FoldAsLoadDefReg;
MachineInstr *DefMI = nullptr;
if (MachineInstr *FoldMI =
TII->optimizeLoadInstr(*MI, MRI, FoldAsLoadDefReg, DefMI)) {
// Update LocalMIs since we replaced MI with FoldMI and deleted
// DefMI.
LLVM_DEBUG(dbgs() << "Replacing: " << *MI);
LLVM_DEBUG(dbgs() << " With: " << *FoldMI);
// MI is replaced with FoldMI so we can continue trying to fold
Changed = true;
MI = FoldMI;
// If we run into an instruction we can't fold across, discard
// the load candidates. Note: We might be able to fold *into* this
// instruction, so this needs to be after the folding logic.
if (MI->isLoadFoldBarrier()) {
LLVM_DEBUG(dbgs() << "Encountered load fold barrier on " << *MI);
return Changed;
ValueTrackerResult ValueTracker::getNextSourceFromCopy() {
assert(Def->isCopy() && "Invalid definition");
// Copy instruction are supposed to be: Def = Src.
// If someone breaks this assumption, bad things will happen everywhere.
assert(Def->getNumOperands() == 2 && "Invalid number of operands");
if (Def->getOperand(DefIdx).getSubReg() != DefSubReg)
// If we look for a different subreg, it means we want a subreg of src.
// Bails as we do not support composing subregs yet.
return ValueTrackerResult();
// Otherwise, we want the whole source.
const MachineOperand &Src = Def->getOperand(1);
if (Src.isUndef())
return ValueTrackerResult();
return ValueTrackerResult(Src.getReg(), Src.getSubReg());
ValueTrackerResult ValueTracker::getNextSourceFromBitcast() {
assert(Def->isBitcast() && "Invalid definition");
// Bail if there are effects that a plain copy will not expose.
if (Def->hasUnmodeledSideEffects())
return ValueTrackerResult();
// Bitcasts with more than one def are not supported.
if (Def->getDesc().getNumDefs() != 1)
return ValueTrackerResult();
const MachineOperand DefOp = Def->getOperand(DefIdx);
if (DefOp.getSubReg() != DefSubReg)
// If we look for a different subreg, it means we want a subreg of the src.
// Bails as we do not support composing subregs yet.
return ValueTrackerResult();
unsigned SrcIdx = Def->getNumOperands();
for (unsigned OpIdx = DefIdx + 1, EndOpIdx = SrcIdx; OpIdx != EndOpIdx;
++OpIdx) {
const MachineOperand &MO = Def->getOperand(OpIdx);
if (!MO.isReg() || !MO.getReg())
// Ignore dead implicit defs.
if (MO.isImplicit() && MO.isDead())
assert(!MO.isDef() && "We should have skipped all the definitions by now");
if (SrcIdx != EndOpIdx)
// Multiple sources?
return ValueTrackerResult();
SrcIdx = OpIdx;
// Stop when any user of the bitcast is a SUBREG_TO_REG, replacing with a COPY
// will break the assumed guarantees for the upper bits.
for (const MachineInstr &UseMI : MRI.use_nodbg_instructions(DefOp.getReg())) {
if (UseMI.isSubregToReg())
return ValueTrackerResult();
const MachineOperand &Src = Def->getOperand(SrcIdx);
if (Src.isUndef())
return ValueTrackerResult();
return ValueTrackerResult(Src.getReg(), Src.getSubReg());
ValueTrackerResult ValueTracker::getNextSourceFromRegSequence() {
assert((Def->isRegSequence() || Def->isRegSequenceLike()) &&
"Invalid definition");
if (Def->getOperand(DefIdx).getSubReg())
// If we are composing subregs, bail out.
// The case we are checking is Def.<subreg> = REG_SEQUENCE.
// This should almost never happen as the SSA property is tracked at
// the register level (as opposed to the subreg level).
// I.e.,
// Def.sub0 =
// Def.sub1 =
// is a valid SSA representation for Def.sub0 and Def.sub1, but not for
// Def. Thus, it must not be generated.
// However, some code could theoretically generates a single
// Def.sub0 (i.e, not defining the other subregs) and we would
// have this case.
// If we can ascertain (or force) that this never happens, we could
// turn that into an assertion.
return ValueTrackerResult();
if (!TII)
// We could handle the REG_SEQUENCE here, but we do not want to
// duplicate the code from the generic TII.
return ValueTrackerResult();
SmallVector<RegSubRegPairAndIdx, 8> RegSeqInputRegs;
if (!TII->getRegSequenceInputs(*Def, DefIdx, RegSeqInputRegs))
return ValueTrackerResult();
// We are looking at:
// Def = REG_SEQUENCE v0, sub0, v1, sub1, ...
// Check if one of the operand defines the subreg we are interested in.
for (const RegSubRegPairAndIdx &RegSeqInput : RegSeqInputRegs) {
if (RegSeqInput.SubIdx == DefSubReg) {
if (RegSeqInput.SubReg)
// Bail if we have to compose sub registers.
return ValueTrackerResult();
return ValueTrackerResult(RegSeqInput.Reg, RegSeqInput.SubReg);
// If the subreg we are tracking is super-defined by another subreg,
// we could follow this value. However, this would require to compose
// the subreg and we do not do that for now.
return ValueTrackerResult();
ValueTrackerResult ValueTracker::getNextSourceFromInsertSubreg() {
assert((Def->isInsertSubreg() || Def->isInsertSubregLike()) &&
"Invalid definition");
if (Def->getOperand(DefIdx).getSubReg())
// If we are composing subreg, bail out.
// Same remark as getNextSourceFromRegSequence.
// I.e., this may be turned into an assert.
return ValueTrackerResult();
if (!TII)
// We could handle the REG_SEQUENCE here, but we do not want to
// duplicate the code from the generic TII.
return ValueTrackerResult();
RegSubRegPair BaseReg;
RegSubRegPairAndIdx InsertedReg;
if (!TII->getInsertSubregInputs(*Def, DefIdx, BaseReg, InsertedReg))
return ValueTrackerResult();
// We are looking at:
// Def = INSERT_SUBREG v0, v1, sub1
// There are two cases:
// 1. DefSubReg == sub1, get v1.
// 2. DefSubReg != sub1, the value may be available through v0.
// #1 Check if the inserted register matches the required sub index.
if (InsertedReg.SubIdx == DefSubReg) {
return ValueTrackerResult(InsertedReg.Reg, InsertedReg.SubReg);
// #2 Otherwise, if the sub register we are looking for is not partial
// defined by the inserted element, we can look through the main
// register (v0).
const MachineOperand &MODef = Def->getOperand(DefIdx);
// If the result register (Def) and the base register (v0) do not
// have the same register class or if we have to compose
// subregisters, bail out.
if (MRI.getRegClass(MODef.getReg()) != MRI.getRegClass(BaseReg.Reg) ||
return ValueTrackerResult();
// Get the TRI and check if the inserted sub-register overlaps with the
// sub-register we are tracking.
const TargetRegisterInfo *TRI = MRI.getTargetRegisterInfo();
if (!TRI ||
!(TRI->getSubRegIndexLaneMask(DefSubReg) &
return ValueTrackerResult();
// At this point, the value is available in v0 via the same subreg
// we used for Def.
return ValueTrackerResult(BaseReg.Reg, DefSubReg);
ValueTrackerResult ValueTracker::getNextSourceFromExtractSubreg() {
assert((Def->isExtractSubreg() ||
Def->isExtractSubregLike()) && "Invalid definition");
// We are looking at:
// Def = EXTRACT_SUBREG v0, sub0
// Bail if we have to compose sub registers.
// Indeed, if DefSubReg != 0, we would have to compose it with sub0.
if (DefSubReg)
return ValueTrackerResult();
if (!TII)
// We could handle the EXTRACT_SUBREG here, but we do not want to
// duplicate the code from the generic TII.
return ValueTrackerResult();
RegSubRegPairAndIdx ExtractSubregInputReg;
if (!TII->getExtractSubregInputs(*Def, DefIdx, ExtractSubregInputReg))
return ValueTrackerResult();
// Bail if we have to compose sub registers.
// Likewise, if v0.subreg != 0, we would have to compose v0.subreg with sub0.
if (ExtractSubregInputReg.SubReg)
return ValueTrackerResult();
// Otherwise, the value is available in the v0.sub0.
return ValueTrackerResult(ExtractSubregInputReg.Reg,
ValueTrackerResult ValueTracker::getNextSourceFromSubregToReg() {
assert(Def->isSubregToReg() && "Invalid definition");
// We are looking at:
// Def = SUBREG_TO_REG Imm, v0, sub0
// Bail if we have to compose sub registers.
// If DefSubReg != sub0, we would have to check that all the bits
// we track are included in sub0 and if yes, we would have to
// determine the right subreg in v0.
if (DefSubReg != Def->getOperand(3).getImm())
return ValueTrackerResult();
// Bail if we have to compose sub registers.
// Likewise, if v0.subreg != 0, we would have to compose it with sub0.
if (Def->getOperand(2).getSubReg())
return ValueTrackerResult();
return ValueTrackerResult(Def->getOperand(2).getReg(),
/// Explore each PHI incoming operand and return its sources.
ValueTrackerResult ValueTracker::getNextSourceFromPHI() {
assert(Def->isPHI() && "Invalid definition");
ValueTrackerResult Res;
// If we look for a different subreg, bail as we do not support composing
// subregs yet.
if (Def->getOperand(0).getSubReg() != DefSubReg)
return ValueTrackerResult();
// Return all register sources for PHI instructions.
for (unsigned i = 1, e = Def->getNumOperands(); i < e; i += 2) {
const MachineOperand &MO = Def->getOperand(i);
assert(MO.isReg() && "Invalid PHI instruction");
// We have no code to deal with undef operands. They shouldn't happen in
// normal programs anyway.
if (MO.isUndef())
return ValueTrackerResult();
Res.addSource(MO.getReg(), MO.getSubReg());
return Res;
ValueTrackerResult ValueTracker::getNextSourceImpl() {
assert(Def && "This method needs a valid definition");
assert(((Def->getOperand(DefIdx).isDef() &&
(DefIdx < Def->getDesc().getNumDefs() ||
Def->getDesc().isVariadic())) ||
Def->getOperand(DefIdx).isImplicit()) &&
"Invalid DefIdx");
if (Def->isCopy())
return getNextSourceFromCopy();
if (Def->isBitcast())
return getNextSourceFromBitcast();
// All the remaining cases involve "complex" instructions.
// Bail if we did not ask for the advanced tracking.
if (DisableAdvCopyOpt)
return ValueTrackerResult();
if (Def->isRegSequence() || Def->isRegSequenceLike())
return getNextSourceFromRegSequence();
if (Def->isInsertSubreg() || Def->isInsertSubregLike())
return getNextSourceFromInsertSubreg();
if (Def->isExtractSubreg() || Def->isExtractSubregLike())
return getNextSourceFromExtractSubreg();
if (Def->isSubregToReg())
return getNextSourceFromSubregToReg();
if (Def->isPHI())
return getNextSourceFromPHI();
return ValueTrackerResult();
ValueTrackerResult ValueTracker::getNextSource() {
// If we reach a point where we cannot move up in the use-def chain,
// there is nothing we can get.
if (!Def)
return ValueTrackerResult();
ValueTrackerResult Res = getNextSourceImpl();
if (Res.isValid()) {
// Update definition, definition index, and subregister for the
// next call of getNextSource.
// Update the current register.
bool OneRegSrc = Res.getNumSources() == 1;
if (OneRegSrc)
Reg = Res.getSrcReg(0);
// Update the result before moving up in the use-def chain
// with the instruction containing the last found sources.
// If we can still move up in the use-def chain, move to the next
// definition.
if (!TargetRegisterInfo::isPhysicalRegister(Reg) && OneRegSrc) {
MachineRegisterInfo::def_iterator DI = MRI.def_begin(Reg);
if (DI != MRI.def_end()) {
Def = DI->getParent();
DefIdx = DI.getOperandNo();
DefSubReg = Res.getSrcSubReg(0);
} else {
Def = nullptr;
return Res;
// If we end up here, this means we will not be able to find another source
// for the next iteration. Make sure any new call to getNextSource bails out
// early by cutting the use-def chain.
Def = nullptr;
return Res;