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//===- MachineCopyPropagation.cpp - Machine Copy Propagation Pass ---------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This is an extremely simple MachineInstr-level copy propagation pass.
// This pass forwards the source of COPYs to the users of their destinations
// when doing so is legal. For example:
// %reg1 = COPY %reg0
// ...
// ... = OP %reg1
// If
// - %reg0 has not been clobbered by the time of the use of %reg1
// - the register class constraints are satisfied
// - the COPY def is the only value that reaches OP
// then this pass replaces the above with:
// %reg1 = COPY %reg0
// ...
// ... = OP %reg0
// This pass also removes some redundant COPYs. For example:
// %R1 = COPY %R0
// ... // No clobber of %R1
// %R0 = COPY %R1 <<< Removed
// or
// %R1 = COPY %R0
// ... // No clobber of %R0
// %R1 = COPY %R0 <<< Removed
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/DebugCounter.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <iterator>
using namespace llvm;
#define DEBUG_TYPE "machine-cp"
STATISTIC(NumDeletes, "Number of dead copies deleted");
STATISTIC(NumCopyForwards, "Number of copy uses forwarded");
DEBUG_COUNTER(FwdCounter, "machine-cp-fwd",
"Controls which register COPYs are forwarded");
namespace {
using RegList = SmallVector<unsigned, 4>;
using SourceMap = DenseMap<unsigned, RegList>;
using Reg2MIMap = DenseMap<unsigned, MachineInstr *>;
class MachineCopyPropagation : public MachineFunctionPass {
const TargetRegisterInfo *TRI;
const TargetInstrInfo *TII;
const MachineRegisterInfo *MRI;
static char ID; // Pass identification, replacement for typeid
MachineCopyPropagation() : MachineFunctionPass(ID) {
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnMachineFunction(MachineFunction &MF) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
void ClobberRegister(unsigned Reg);
void ReadRegister(unsigned Reg);
void CopyPropagateBlock(MachineBasicBlock &MBB);
bool eraseIfRedundant(MachineInstr &Copy, unsigned Src, unsigned Def);
void forwardUses(MachineInstr &MI);
bool isForwardableRegClassCopy(const MachineInstr &Copy,
const MachineInstr &UseI, unsigned UseIdx);
bool hasImplicitOverlap(const MachineInstr &MI, const MachineOperand &Use);
/// Candidates for deletion.
SmallSetVector<MachineInstr*, 8> MaybeDeadCopies;
/// Def -> available copies map.
Reg2MIMap AvailCopyMap;
/// Def -> copies map.
Reg2MIMap CopyMap;
/// Src -> Def map
SourceMap SrcMap;
bool Changed;
} // end anonymous namespace
char MachineCopyPropagation::ID = 0;
char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID;
"Machine Copy Propagation Pass", false, false)
/// Remove any entry in \p Map where the register is a subregister or equal to
/// a register contained in \p Regs.
static void removeRegsFromMap(Reg2MIMap &Map, const RegList &Regs,
const TargetRegisterInfo &TRI) {
for (unsigned Reg : Regs) {
// Source of copy is no longer available for propagation.
for (MCSubRegIterator SR(Reg, &TRI, true); SR.isValid(); ++SR)
/// Remove any entry in \p Map that is marked clobbered in \p RegMask.
/// The map will typically have a lot fewer entries than the regmask clobbers,
/// so this is more efficient than iterating the clobbered registers and calling
/// ClobberRegister() on them.
static void removeClobberedRegsFromMap(Reg2MIMap &Map,
const MachineOperand &RegMask) {
for (Reg2MIMap::iterator I = Map.begin(), E = Map.end(), Next; I != E;
I = Next) {
Next = std::next(I);
unsigned Reg = I->first;
if (RegMask.clobbersPhysReg(Reg))
void MachineCopyPropagation::ClobberRegister(unsigned Reg) {
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
SourceMap::iterator SI = SrcMap.find(*AI);
if (SI != SrcMap.end()) {
removeRegsFromMap(AvailCopyMap, SI->second, *TRI);
void MachineCopyPropagation::ReadRegister(unsigned Reg) {
// If 'Reg' is defined by a copy, the copy is no longer a candidate
// for elimination.
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
Reg2MIMap::iterator CI = CopyMap.find(*AI);
if (CI != CopyMap.end()) {
LLVM_DEBUG(dbgs() << "MCP: Copy is used - not dead: ";
/// Return true if \p PreviousCopy did copy register \p Src to register \p Def.
/// This fact may have been obscured by sub register usage or may not be true at
/// all even though Src and Def are subregisters of the registers used in
/// PreviousCopy. e.g.
/// isNopCopy("ecx = COPY eax", AX, CX) == true
/// isNopCopy("ecx = COPY eax", AH, CL) == false
static bool isNopCopy(const MachineInstr &PreviousCopy, unsigned Src,
unsigned Def, const TargetRegisterInfo *TRI) {
unsigned PreviousSrc = PreviousCopy.getOperand(1).getReg();
unsigned PreviousDef = PreviousCopy.getOperand(0).getReg();
if (Src == PreviousSrc) {
assert(Def == PreviousDef);
return true;
if (!TRI->isSubRegister(PreviousSrc, Src))
return false;
unsigned SubIdx = TRI->getSubRegIndex(PreviousSrc, Src);
return SubIdx == TRI->getSubRegIndex(PreviousDef, Def);
/// Remove instruction \p Copy if there exists a previous copy that copies the
/// register \p Src to the register \p Def; This may happen indirectly by
/// copying the super registers.
bool MachineCopyPropagation::eraseIfRedundant(MachineInstr &Copy, unsigned Src,
unsigned Def) {
// Avoid eliminating a copy from/to a reserved registers as we cannot predict
// the value (Example: The sparc zero register is writable but stays zero).
if (MRI->isReserved(Src) || MRI->isReserved(Def))
return false;
// Search for an existing copy.
Reg2MIMap::iterator CI = AvailCopyMap.find(Def);
if (CI == AvailCopyMap.end())
return false;
// Check that the existing copy uses the correct sub registers.
MachineInstr &PrevCopy = *CI->second;
if (PrevCopy.getOperand(0).isDead())
return false;
if (!isNopCopy(PrevCopy, Src, Def, TRI))
return false;
LLVM_DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; Copy.dump());
// Copy was redundantly redefining either Src or Def. Remove earlier kill
// flags between Copy and PrevCopy because the value will be reused now.
unsigned CopyDef = Copy.getOperand(0).getReg();
assert(CopyDef == Src || CopyDef == Def);
for (MachineInstr &MI :
make_range(PrevCopy.getIterator(), Copy.getIterator()))
MI.clearRegisterKills(CopyDef, TRI);
Changed = true;
return true;
/// Decide whether we should forward the source of \param Copy to its use in
/// \param UseI based on the physical register class constraints of the opcode
/// and avoiding introducing more cross-class COPYs.
bool MachineCopyPropagation::isForwardableRegClassCopy(const MachineInstr &Copy,
const MachineInstr &UseI,
unsigned UseIdx) {
unsigned CopySrcReg = Copy.getOperand(1).getReg();
// If the new register meets the opcode register constraints, then allow
// forwarding.
if (const TargetRegisterClass *URC =
UseI.getRegClassConstraint(UseIdx, TII, TRI))
return URC->contains(CopySrcReg);
if (!UseI.isCopy())
return false;
/// COPYs don't have register class constraints, so if the user instruction
/// is a COPY, we just try to avoid introducing additional cross-class
/// COPYs. For example:
/// RegClassA = COPY RegClassB // Copy parameter
/// ...
/// RegClassB = COPY RegClassA // UseI parameter
/// which after forwarding becomes
/// RegClassA = COPY RegClassB
/// ...
/// RegClassB = COPY RegClassB
/// so we have reduced the number of cross-class COPYs and potentially
/// introduced a nop COPY that can be removed.
const TargetRegisterClass *UseDstRC =
const TargetRegisterClass *SuperRC = UseDstRC;
for (TargetRegisterClass::sc_iterator SuperRCI = UseDstRC->getSuperClasses();
SuperRC; SuperRC = *SuperRCI++)
if (SuperRC->contains(CopySrcReg))
return true;
return false;
/// Check that \p MI does not have implicit uses that overlap with it's \p Use
/// operand (the register being replaced), since these can sometimes be
/// implicitly tied to other operands. For example, on AMDGPU:
/// V_MOVRELS_B32_e32 %VGPR2, %M0<imp-use>, %EXEC<imp-use>, %VGPR2_VGPR3_VGPR4_VGPR5<imp-use>
/// the %VGPR2 is implicitly tied to the larger reg operand, but we have no
/// way of knowing we need to update the latter when updating the former.
bool MachineCopyPropagation::hasImplicitOverlap(const MachineInstr &MI,
const MachineOperand &Use) {
for (const MachineOperand &MIUse : MI.uses())
if (&MIUse != &Use && MIUse.isReg() && MIUse.isImplicit() &&
MIUse.isUse() && TRI->regsOverlap(Use.getReg(), MIUse.getReg()))
return true;
return false;
/// Look for available copies whose destination register is used by \p MI and
/// replace the use in \p MI with the copy's source register.
void MachineCopyPropagation::forwardUses(MachineInstr &MI) {
if (AvailCopyMap.empty())
// Look for non-tied explicit vreg uses that have an active COPY
// instruction that defines the physical register allocated to them.
// Replace the vreg with the source of the active COPY.
for (unsigned OpIdx = 0, OpEnd = MI.getNumOperands(); OpIdx < OpEnd;
++OpIdx) {
MachineOperand &MOUse = MI.getOperand(OpIdx);
// Don't forward into undef use operands since doing so can cause problems
// with the machine verifier, since it doesn't treat undef reads as reads,
// so we can end up with a live range that ends on an undef read, leading to
// an error that the live range doesn't end on a read of the live range
// register.
if (!MOUse.isReg() || MOUse.isTied() || MOUse.isUndef() || MOUse.isDef() ||
if (!MOUse.getReg())
// Check that the register is marked 'renamable' so we know it is safe to
// rename it without violating any constraints that aren't expressed in the
// IR (e.g. ABI or opcode requirements).
if (!MOUse.isRenamable())
auto CI = AvailCopyMap.find(MOUse.getReg());
if (CI == AvailCopyMap.end())
MachineInstr &Copy = *CI->second;
unsigned CopyDstReg = Copy.getOperand(0).getReg();
const MachineOperand &CopySrc = Copy.getOperand(1);
unsigned CopySrcReg = CopySrc.getReg();
// FIXME: Don't handle partial uses of wider COPYs yet.
if (MOUse.getReg() != CopyDstReg) {
dbgs() << "MCP: FIXME! Not forwarding COPY to sub-register use:\n "
<< MI);
// Don't forward COPYs of reserved regs unless they are constant.
if (MRI->isReserved(CopySrcReg) && !MRI->isConstantPhysReg(CopySrcReg))
if (!isForwardableRegClassCopy(Copy, MI, OpIdx))
if (hasImplicitOverlap(MI, MOUse))
if (!DebugCounter::shouldExecute(FwdCounter)) {
LLVM_DEBUG(dbgs() << "MCP: Skipping forwarding due to debug counter:\n "
<< MI);
LLVM_DEBUG(dbgs() << "MCP: Replacing " << printReg(MOUse.getReg(), TRI)
<< "\n with " << printReg(CopySrcReg, TRI)
<< "\n in " << MI << " from " << Copy);
if (!CopySrc.isRenamable())
LLVM_DEBUG(dbgs() << "MCP: After replacement: " << MI << "\n");
// Clear kill markers that may have been invalidated.
for (MachineInstr &KMI :
make_range(Copy.getIterator(), std::next(MI.getIterator())))
KMI.clearRegisterKills(CopySrcReg, TRI);
Changed = true;
void MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
LLVM_DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n");
for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) {
MachineInstr *MI = &*I;
// Analyze copies (which don't overlap themselves).
if (MI->isCopy() && !TRI->regsOverlap(MI->getOperand(0).getReg(),
MI->getOperand(1).getReg())) {
unsigned Def = MI->getOperand(0).getReg();
unsigned Src = MI->getOperand(1).getReg();
assert(!TargetRegisterInfo::isVirtualRegister(Def) &&
!TargetRegisterInfo::isVirtualRegister(Src) &&
"MachineCopyPropagation should be run after register allocation!");
// The two copies cancel out and the source of the first copy
// hasn't been overridden, eliminate the second one. e.g.
// %ecx = COPY %eax
// ... nothing clobbered eax.
// %eax = COPY %ecx
// =>
// %ecx = COPY %eax
// or
// %ecx = COPY %eax
// ... nothing clobbered eax.
// %ecx = COPY %eax
// =>
// %ecx = COPY %eax
if (eraseIfRedundant(*MI, Def, Src) || eraseIfRedundant(*MI, Src, Def))
// Src may have been changed by forwardUses()
Src = MI->getOperand(1).getReg();
// If Src is defined by a previous copy, the previous copy cannot be
// eliminated.
for (const MachineOperand &MO : MI->implicit_operands()) {
if (!MO.isReg() || !MO.readsReg())
unsigned Reg = MO.getReg();
if (!Reg)
LLVM_DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump());
// Copy is now a candidate for deletion.
if (!MRI->isReserved(Def))
// If 'Def' is previously source of another copy, then this earlier copy's
// source is no longer available. e.g.
// %xmm9 = copy %xmm2
// ...
// %xmm2 = copy %xmm0
// ...
// %xmm2 = copy %xmm9
for (const MachineOperand &MO : MI->implicit_operands()) {
if (!MO.isReg() || !MO.isDef())
unsigned Reg = MO.getReg();
if (!Reg)
// Remember Def is defined by the copy.
for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid();
++SR) {
CopyMap[*SR] = MI;
AvailCopyMap[*SR] = MI;
// Remember source that's copied to Def. Once it's clobbered, then
// it's no longer available for copy propagation.
RegList &DestList = SrcMap[Src];
if (!is_contained(DestList, Def))
// Clobber any earlyclobber regs first.
for (const MachineOperand &MO : MI->operands())
if (MO.isReg() && MO.isEarlyClobber()) {
unsigned Reg = MO.getReg();
// If we have a tied earlyclobber, that means it is also read by this
// instruction, so we need to make sure we don't remove it as dead
// later.
if (MO.isTied())
// Not a copy.
SmallVector<unsigned, 2> Defs;
const MachineOperand *RegMask = nullptr;
for (const MachineOperand &MO : MI->operands()) {
if (MO.isRegMask())
RegMask = &MO;
if (!MO.isReg())
unsigned Reg = MO.getReg();
if (!Reg)
assert(!TargetRegisterInfo::isVirtualRegister(Reg) &&
"MachineCopyPropagation should be run after register allocation!");
if (MO.isDef() && !MO.isEarlyClobber()) {
} else if (!MO.isDebug() && MO.readsReg())
// The instruction has a register mask operand which means that it clobbers
// a large set of registers. Treat clobbered registers the same way as
// defined registers.
if (RegMask) {
// Erase any MaybeDeadCopies whose destination register is clobbered.
for (SmallSetVector<MachineInstr *, 8>::iterator DI =
DI != MaybeDeadCopies.end();) {
MachineInstr *MaybeDead = *DI;
unsigned Reg = MaybeDead->getOperand(0).getReg();
if (!RegMask->clobbersPhysReg(Reg)) {
LLVM_DEBUG(dbgs() << "MCP: Removing copy due to regmask clobbering: ";
// erase() will return the next valid iterator pointing to the next
// element after the erased one.
DI = MaybeDeadCopies.erase(DI);
Changed = true;
removeClobberedRegsFromMap(AvailCopyMap, *RegMask);
removeClobberedRegsFromMap(CopyMap, *RegMask);
for (SourceMap::iterator I = SrcMap.begin(), E = SrcMap.end(), Next;
I != E; I = Next) {
Next = std::next(I);
if (RegMask->clobbersPhysReg(I->first)) {
removeRegsFromMap(AvailCopyMap, I->second, *TRI);
// Any previous copy definition or reading the Defs is no longer available.
for (unsigned Reg : Defs)
// If MBB doesn't have successors, delete the copies whose defs are not used.
// If MBB does have successors, then conservative assume the defs are live-out
// since we don't want to trust live-in lists.
if (MBB.succ_empty()) {
for (MachineInstr *MaybeDead : MaybeDeadCopies) {
LLVM_DEBUG(dbgs() << "MCP: Removing copy due to no live-out succ: ";
Changed = true;
bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
Changed = false;
TRI = MF.getSubtarget().getRegisterInfo();
TII = MF.getSubtarget().getInstrInfo();
MRI = &MF.getRegInfo();
for (MachineBasicBlock &MBB : MF)
return Changed;