//
// The LLVM Compiler Infrastructure
//
-// This file was developed by the LLVM research group and is distributed under
-// the University of Illinois Open Source License. See LICENSE.TXT for details.
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// to:
//
// A = B
-// A = A op C
+// A op= C
+//
+// Note that if a register allocator chooses to use this pass, that it
+// has to be capable of handling the non-SSA nature of these rewritten
+// virtual registers.
+//
+// It is also worth noting that the duplicate operand of the two
+// address instruction is removed.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "twoaddrinstr"
+#include "llvm/CodeGen/Passes.h"
#include "llvm/Function.h"
#include "llvm/CodeGen/LiveVariables.h"
-#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
-#include "llvm/CodeGen/Passes.h"
-#include "llvm/CodeGen/SSARegMap.h"
-#include "llvm/Target/MRegisterInfo.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
-#include "llvm/Target/TargetRegInfo.h"
-#include "Support/Debug.h"
-#include "Support/Statistic.h"
-#include "Support/STLExtras.h"
-#include <iostream>
-
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/ADT/BitVector.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/STLExtras.h"
using namespace llvm;
+STATISTIC(NumTwoAddressInstrs, "Number of two-address instructions");
+STATISTIC(NumCommuted , "Number of instructions commuted to coalesce");
+STATISTIC(NumConvertedTo3Addr, "Number of instructions promoted to 3-address");
+STATISTIC(Num3AddrSunk, "Number of 3-address instructions sunk");
+STATISTIC(NumReMats, "Number of instructions re-materialized");
+
namespace {
- class TwoAddressInstructionPass : public MachineFunctionPass
- {
- private:
- MachineFunction* mf_;
- const TargetMachine* tm_;
- const MRegisterInfo* mri_;
- LiveVariables* lv_;
-
- public:
- virtual void getAnalysisUsage(AnalysisUsage &AU) const;
-
- private:
- /// runOnMachineFunction - pass entry point
- bool runOnMachineFunction(MachineFunction&);
- };
-
- RegisterPass<TwoAddressInstructionPass> X(
- "twoaddressinstruction", "Two-Address instruction pass");
-
- Statistic<> numTwoAddressInstrs("twoaddressinstruction",
- "Number of two-address instructions");
- Statistic<> numInstrsAdded("twoaddressinstruction",
- "Number of instructions added");
-};
-
-const PassInfo *llvm::TwoAddressInstructionPassID = X.getPassInfo();
-
-void TwoAddressInstructionPass::getAnalysisUsage(AnalysisUsage &AU) const
-{
- AU.addPreserved<LiveVariables>();
- AU.addRequired<LiveVariables>();
- AU.addPreservedID(PHIEliminationID);
- AU.addRequiredID(PHIEliminationID);
- MachineFunctionPass::getAnalysisUsage(AU);
+ class VISIBILITY_HIDDEN TwoAddressInstructionPass
+ : public MachineFunctionPass {
+ const TargetInstrInfo *TII;
+ const TargetRegisterInfo *TRI;
+ MachineRegisterInfo *MRI;
+ LiveVariables *LV;
+
+ bool Sink3AddrInstruction(MachineBasicBlock *MBB, MachineInstr *MI,
+ unsigned Reg,
+ MachineBasicBlock::iterator OldPos);
+
+ bool isSafeToReMat(unsigned DstReg, MachineInstr *MI);
+ bool isProfitableToReMat(unsigned Reg, const TargetRegisterClass *RC,
+ MachineInstr *MI, MachineInstr *DefMI,
+ MachineBasicBlock *MBB, unsigned Loc,
+ DenseMap<MachineInstr*, unsigned> &DistanceMap);
+ public:
+ static char ID; // Pass identification, replacement for typeid
+ TwoAddressInstructionPass() : MachineFunctionPass((intptr_t)&ID) {}
+
+ virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.addPreserved<LiveVariables>();
+ AU.addPreservedID(MachineLoopInfoID);
+ AU.addPreservedID(MachineDominatorsID);
+ AU.addPreservedID(PHIEliminationID);
+ MachineFunctionPass::getAnalysisUsage(AU);
+ }
+
+ /// runOnMachineFunction - Pass entry point.
+ bool runOnMachineFunction(MachineFunction&);
+ };
+}
+
+char TwoAddressInstructionPass::ID = 0;
+static RegisterPass<TwoAddressInstructionPass>
+X("twoaddressinstruction", "Two-Address instruction pass");
+
+const PassInfo *const llvm::TwoAddressInstructionPassID = &X;
+
+/// Sink3AddrInstruction - A two-address instruction has been converted to a
+/// three-address instruction to avoid clobbering a register. Try to sink it
+/// past the instruction that would kill the above mentioned register to reduce
+/// register pressure.
+bool TwoAddressInstructionPass::Sink3AddrInstruction(MachineBasicBlock *MBB,
+ MachineInstr *MI, unsigned SavedReg,
+ MachineBasicBlock::iterator OldPos) {
+ // Check if it's safe to move this instruction.
+ bool SeenStore = true; // Be conservative.
+ if (!MI->isSafeToMove(TII, SeenStore))
+ return false;
+
+ unsigned DefReg = 0;
+ SmallSet<unsigned, 4> UseRegs;
+
+ for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+ const MachineOperand &MO = MI->getOperand(i);
+ if (!MO.isRegister())
+ continue;
+ unsigned MOReg = MO.getReg();
+ if (!MOReg)
+ continue;
+ if (MO.isUse() && MOReg != SavedReg)
+ UseRegs.insert(MO.getReg());
+ if (!MO.isDef())
+ continue;
+ if (MO.isImplicit())
+ // Don't try to move it if it implicitly defines a register.
+ return false;
+ if (DefReg)
+ // For now, don't move any instructions that define multiple registers.
+ return false;
+ DefReg = MO.getReg();
+ }
+
+ // Find the instruction that kills SavedReg.
+ MachineInstr *KillMI = NULL;
+ for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(SavedReg),
+ UE = MRI->use_end(); UI != UE; ++UI) {
+ MachineOperand &UseMO = UI.getOperand();
+ if (!UseMO.isKill())
+ continue;
+ KillMI = UseMO.getParent();
+ break;
+ }
+
+ if (!KillMI || KillMI->getParent() != MBB)
+ return false;
+
+ // If any of the definitions are used by another instruction between the
+ // position and the kill use, then it's not safe to sink it.
+ //
+ // FIXME: This can be sped up if there is an easy way to query whether an
+ // instruction is before or after another instruction. Then we can use
+ // MachineRegisterInfo def / use instead.
+ MachineOperand *KillMO = NULL;
+ MachineBasicBlock::iterator KillPos = KillMI;
+ ++KillPos;
+
+ unsigned NumVisited = 0;
+ for (MachineBasicBlock::iterator I = next(OldPos); I != KillPos; ++I) {
+ MachineInstr *OtherMI = I;
+ if (NumVisited > 30) // FIXME: Arbitrary limit to reduce compile time cost.
+ return false;
+ ++NumVisited;
+ for (unsigned i = 0, e = OtherMI->getNumOperands(); i != e; ++i) {
+ MachineOperand &MO = OtherMI->getOperand(i);
+ if (!MO.isRegister())
+ continue;
+ unsigned MOReg = MO.getReg();
+ if (!MOReg)
+ continue;
+ if (DefReg == MOReg)
+ return false;
+
+ if (MO.isKill()) {
+ if (OtherMI == KillMI && MOReg == SavedReg)
+ // Save the operand that kills the register. We want to unset the kill
+ // marker if we can sink MI past it.
+ KillMO = &MO;
+ else if (UseRegs.count(MOReg))
+ // One of the uses is killed before the destination.
+ return false;
+ }
+ }
+ }
+
+ // Update kill and LV information.
+ KillMO->setIsKill(false);
+ KillMO = MI->findRegisterUseOperand(SavedReg, false, TRI);
+ KillMO->setIsKill(true);
+
+ if (LV) {
+ LiveVariables::VarInfo& VarInfo = LV->getVarInfo(SavedReg);
+ VarInfo.removeKill(KillMI);
+ VarInfo.Kills.push_back(MI);
+ }
+
+ // Move instruction to its destination.
+ MBB->remove(MI);
+ MBB->insert(KillPos, MI);
+
+ ++Num3AddrSunk;
+ return true;
+}
+
+/// isSafeToReMat - Return true if it's safe to rematerialize the specified
+/// instruction which defined the specified register instead of copying it.
+bool
+TwoAddressInstructionPass::isSafeToReMat(unsigned DstReg, MachineInstr *MI) {
+ const TargetInstrDesc &TID = MI->getDesc();
+ if (!TID.isAsCheapAsAMove())
+ return false;
+ bool SawStore = false;
+ if (!MI->isSafeToMove(TII, SawStore))
+ return false;
+ for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+ MachineOperand &MO = MI->getOperand(i);
+ if (!MO.isRegister())
+ continue;
+ // FIXME: For now, do not remat any instruction with register operands.
+ // Later on, we can loosen the restriction is the register operands have
+ // not been modified between the def and use. Note, this is different from
+ // MachineSink because the code in no longer in two-address form (at least
+ // partially).
+ if (MO.isUse())
+ return false;
+ else if (!MO.isDead() && MO.getReg() != DstReg)
+ return false;
+ }
+ return true;
}
-/// runOnMachineFunction - Reduce two-address instructions to two
-/// operands
+/// isTwoAddrUse - Return true if the specified MI is using the specified
+/// register as a two-address operand.
+static bool isTwoAddrUse(MachineInstr *UseMI, unsigned Reg) {
+ const TargetInstrDesc &TID = UseMI->getDesc();
+ for (unsigned i = 0, e = TID.getNumOperands(); i != e; ++i) {
+ MachineOperand &MO = UseMI->getOperand(i);
+ if (MO.isRegister() && MO.getReg() == Reg &&
+ (MO.isDef() || TID.getOperandConstraint(i, TOI::TIED_TO) != -1))
+ // Earlier use is a two-address one.
+ return true;
+ }
+ return false;
+}
+
+/// isProfitableToReMat - Return true if the heuristics determines it is likely
+/// to be profitable to re-materialize the definition of Reg rather than copy
+/// the register.
+bool
+TwoAddressInstructionPass::isProfitableToReMat(unsigned Reg,
+ const TargetRegisterClass *RC,
+ MachineInstr *MI, MachineInstr *DefMI,
+ MachineBasicBlock *MBB, unsigned Loc,
+ DenseMap<MachineInstr*, unsigned> &DistanceMap){
+ bool OtherUse = false;
+ for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(Reg),
+ UE = MRI->use_end(); UI != UE; ++UI) {
+ MachineOperand &UseMO = UI.getOperand();
+ if (!UseMO.isUse())
+ continue;
+ MachineInstr *UseMI = UseMO.getParent();
+ MachineBasicBlock *UseMBB = UseMI->getParent();
+ if (UseMBB == MBB) {
+ DenseMap<MachineInstr*, unsigned>::iterator DI = DistanceMap.find(UseMI);
+ if (DI != DistanceMap.end() && DI->second == Loc)
+ continue; // Current use.
+ OtherUse = true;
+ // There is at least one other use in the MBB that will clobber the
+ // register.
+ if (isTwoAddrUse(UseMI, Reg))
+ return true;
+ }
+ }
+
+ // If other uses in MBB are not two-address uses, then don't remat.
+ if (OtherUse)
+ return false;
+
+ // No other uses in the same block, remat if it's defined in the same
+ // block so it does not unnecessarily extend the live range.
+ return MBB == DefMI->getParent();
+}
+
+/// runOnMachineFunction - Reduce two-address instructions to two operands.
///
-bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &fn) {
- DEBUG(std::cerr << "Machine Function\n");
- mf_ = &fn;
- tm_ = &fn.getTarget();
- mri_ = tm_->getRegisterInfo();
- lv_ = &getAnalysis<LiveVariables>();
-
- const TargetInstrInfo& tii = tm_->getInstrInfo();
-
- for (MachineFunction::iterator mbbi = mf_->begin(), mbbe = mf_->end();
- mbbi != mbbe; ++mbbi) {
- for (MachineBasicBlock::iterator mii = mbbi->begin();
- mii != mbbi->end(); ++mii) {
- MachineInstr* mi = *mii;
-
- unsigned opcode = mi->getOpcode();
- // ignore if it is not a two-address instruction
- if (!tii.isTwoAddrInstr(opcode))
- continue;
-
- ++numTwoAddressInstrs;
-
- DEBUG(std::cerr << "\tinstruction: "; mi->print(std::cerr, *tm_));
-
- // we have nothing to do if the two operands are the same
- if (mi->getOperand(0).getAllocatedRegNum() ==
- mi->getOperand(1).getAllocatedRegNum())
- continue;
-
- assert(mi->getOperand(1).isRegister() &&
- mi->getOperand(1).getAllocatedRegNum() &&
- mi->getOperand(1).isUse() &&
- "two address instruction invalid");
-
- // rewrite:
- // a = b op c
- // to:
- // a = b
- // a = a op c
- unsigned regA = mi->getOperand(0).getAllocatedRegNum();
- unsigned regB = mi->getOperand(1).getAllocatedRegNum();
-
- assert(regA >= MRegisterInfo::FirstVirtualRegister &&
- regB >= MRegisterInfo::FirstVirtualRegister &&
- "cannot update physical register live information");
-
- // first make sure we do not have a use of a in the
- // instruction (a = b + a for example) because our
- // transofrmation will not work. This should never occur
- // because of SSA.
- for (unsigned i = 1; i < mi->getNumOperands(); ++i) {
- assert(!mi->getOperand(i).isRegister() ||
- mi->getOperand(i).getAllocatedRegNum() != (int)regA);
+bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
+ DOUT << "Machine Function\n";
+ const TargetMachine &TM = MF.getTarget();
+ MRI = &MF.getRegInfo();
+ TII = TM.getInstrInfo();
+ TRI = TM.getRegisterInfo();
+ LV = getAnalysisToUpdate<LiveVariables>();
+
+ bool MadeChange = false;
+
+ DOUT << "********** REWRITING TWO-ADDR INSTRS **********\n";
+ DOUT << "********** Function: " << MF.getFunction()->getName() << '\n';
+
+ // ReMatRegs - Keep track of the registers whose def's are remat'ed.
+ BitVector ReMatRegs;
+ ReMatRegs.resize(MRI->getLastVirtReg()+1);
+
+ // DistanceMap - Keep track the distance of a MI from the start of the
+ // current basic block.
+ DenseMap<MachineInstr*, unsigned> DistanceMap;
+
+ for (MachineFunction::iterator mbbi = MF.begin(), mbbe = MF.end();
+ mbbi != mbbe; ++mbbi) {
+ unsigned Dist = 0;
+ DistanceMap.clear();
+ for (MachineBasicBlock::iterator mi = mbbi->begin(), me = mbbi->end();
+ mi != me; ) {
+ MachineBasicBlock::iterator nmi = next(mi);
+ const TargetInstrDesc &TID = mi->getDesc();
+ bool FirstTied = true;
+
+ DistanceMap.insert(std::make_pair(mi, ++Dist));
+ for (unsigned si = 1, e = TID.getNumOperands(); si < e; ++si) {
+ int ti = TID.getOperandConstraint(si, TOI::TIED_TO);
+ if (ti == -1)
+ continue;
+
+ if (FirstTied) {
+ ++NumTwoAddressInstrs;
+ DOUT << '\t'; DEBUG(mi->print(*cerr.stream(), &TM));
+ }
+
+ FirstTied = false;
+
+ assert(mi->getOperand(si).isRegister() && mi->getOperand(si).getReg() &&
+ mi->getOperand(si).isUse() && "two address instruction invalid");
+
+ // If the two operands are the same we just remove the use
+ // and mark the def as def&use, otherwise we have to insert a copy.
+ if (mi->getOperand(ti).getReg() != mi->getOperand(si).getReg()) {
+ // Rewrite:
+ // a = b op c
+ // to:
+ // a = b
+ // a = a op c
+ unsigned regA = mi->getOperand(ti).getReg();
+ unsigned regB = mi->getOperand(si).getReg();
+
+ assert(TargetRegisterInfo::isVirtualRegister(regA) &&
+ TargetRegisterInfo::isVirtualRegister(regB) &&
+ "cannot update physical register live information");
+
+#ifndef NDEBUG
+ // First, verify that we don't have a use of a in the instruction (a =
+ // b + a for example) because our transformation will not work. This
+ // should never occur because we are in SSA form.
+ for (unsigned i = 0; i != mi->getNumOperands(); ++i)
+ assert((int)i == ti ||
+ !mi->getOperand(i).isRegister() ||
+ mi->getOperand(i).getReg() != regA);
+#endif
+
+ // If this instruction is not the killing user of B, see if we can
+ // rearrange the code to make it so. Making it the killing user will
+ // allow us to coalesce A and B together, eliminating the copy we are
+ // about to insert.
+ if (!mi->killsRegister(regB)) {
+ // If this instruction is commutative, check to see if C dies. If
+ // so, swap the B and C operands. This makes the live ranges of A
+ // and C joinable.
+ // FIXME: This code also works for A := B op C instructions.
+ if (TID.isCommutable() && mi->getNumOperands() >= 3) {
+ assert(mi->getOperand(3-si).isRegister() &&
+ "Not a proper commutative instruction!");
+ unsigned regC = mi->getOperand(3-si).getReg();
+
+ if (mi->killsRegister(regC)) {
+ DOUT << "2addr: COMMUTING : " << *mi;
+ MachineInstr *NewMI = TII->commuteInstruction(mi);
+
+ if (NewMI == 0) {
+ DOUT << "2addr: COMMUTING FAILED!\n";
+ } else {
+ DOUT << "2addr: COMMUTED TO: " << *NewMI;
+ // If the instruction changed to commute it, update livevar.
+ if (NewMI != mi) {
+ if (LV)
+ // Update live variables
+ LV->replaceKillInstruction(regC, mi, NewMI);
+
+ mbbi->insert(mi, NewMI); // Insert the new inst
+ mbbi->erase(mi); // Nuke the old inst.
+ mi = NewMI;
+ DistanceMap.insert(std::make_pair(NewMI, Dist));
+ }
+
+ ++NumCommuted;
+ regB = regC;
+ goto InstructionRearranged;
+ }
+ }
}
- const TargetRegisterClass* rc =
- mf_->getSSARegMap()->getRegClass(regA);
- numInstrsAdded += mri_->copyRegToReg(*mbbi, mii, regA, regB, rc);
+ // If this instruction is potentially convertible to a true
+ // three-address instruction,
+ if (TID.isConvertibleTo3Addr()) {
+ // FIXME: This assumes there are no more operands which are tied
+ // to another register.
+#ifndef NDEBUG
+ for (unsigned i = si + 1, e = TID.getNumOperands(); i < e; ++i)
+ assert(TID.getOperandConstraint(i, TOI::TIED_TO) == -1);
+#endif
- MachineInstr* prevMi = *(mii - 1);
- DEBUG(std::cerr << "\t\tadded instruction: ";
- prevMi->print(std::cerr, *tm_));
+ MachineInstr *NewMI = TII->convertToThreeAddress(mbbi, mi, LV);
+ if (NewMI) {
+ DOUT << "2addr: CONVERTING 2-ADDR: " << *mi;
+ DOUT << "2addr: TO 3-ADDR: " << *NewMI;
+ bool Sunk = false;
- // update live variables for regA
- LiveVariables::VarInfo& varInfo = lv_->getVarInfo(regA);
- varInfo.DefInst = prevMi;
+ if (NewMI->findRegisterUseOperand(regB, false, TRI))
+ // FIXME: Temporary workaround. If the new instruction doesn't
+ // uses regB, convertToThreeAddress must have created more
+ // then one instruction.
+ Sunk = Sink3AddrInstruction(mbbi, NewMI, regB, mi);
- // update live variables for regB
- if (lv_->removeVirtualRegisterKilled(regB, &*mbbi, mi))
- lv_->addVirtualRegisterKilled(regB, &*mbbi, prevMi);
+ mbbi->erase(mi); // Nuke the old inst.
- if (lv_->removeVirtualRegisterDead(regB, &*mbbi, mi))
- lv_->addVirtualRegisterDead(regB, &*mbbi, prevMi);
+ if (!Sunk) {
+ DistanceMap.insert(std::make_pair(NewMI, Dist));
+ mi = NewMI;
+ nmi = next(mi);
+ }
- // replace all occurences of regB with regA
- for (unsigned i = 1; i < mi->getNumOperands(); ++i) {
- if (mi->getOperand(i).isRegister() &&
- mi->getOperand(i).getReg() == regB)
- mi->SetMachineOperandReg(i, regA);
+ ++NumConvertedTo3Addr;
+ break; // Done with this instruction.
+ }
}
- DEBUG(std::cerr << "\t\tmodified original to: ";
- mi->print(std::cerr, *tm_));
- assert(mi->getOperand(0).getAllocatedRegNum() ==
- mi->getOperand(1).getAllocatedRegNum());
+ }
+
+ InstructionRearranged:
+ const TargetRegisterClass* rc = MRI->getRegClass(regA);
+ MachineInstr *DefMI = MRI->getVRegDef(regB);
+ // If it's safe and profitable, remat the definition instead of
+ // copying it.
+ if (DefMI &&
+ isSafeToReMat(regB, DefMI) &&
+ isProfitableToReMat(regB, rc, mi, DefMI, mbbi, Dist,DistanceMap)){
+ DEBUG(cerr << "2addr: REMATTING : " << *DefMI << "\n");
+ TII->reMaterialize(*mbbi, mi, regA, DefMI);
+ ReMatRegs.set(regB);
+ ++NumReMats;
+ } else {
+ TII->copyRegToReg(*mbbi, mi, regA, regB, rc, rc);
+ }
+
+ MachineBasicBlock::iterator prevMi = prior(mi);
+ DOUT << "\t\tprepend:\t"; DEBUG(prevMi->print(*cerr.stream(), &TM));
+
+ // Update live variables for regB.
+ if (LV) {
+ LiveVariables::VarInfo& varInfoB = LV->getVarInfo(regB);
+
+ // regB is used in this BB.
+ varInfoB.UsedBlocks[mbbi->getNumber()] = true;
+
+ if (LV->removeVirtualRegisterKilled(regB, mbbi, mi))
+ LV->addVirtualRegisterKilled(regB, prevMi);
+
+ if (LV->removeVirtualRegisterDead(regB, mbbi, mi))
+ LV->addVirtualRegisterDead(regB, prevMi);
+ }
+
+ // Replace all occurences of regB with regA.
+ for (unsigned i = 0, e = mi->getNumOperands(); i != e; ++i) {
+ if (mi->getOperand(i).isRegister() &&
+ mi->getOperand(i).getReg() == regB)
+ mi->getOperand(i).setReg(regA);
+ }
}
+
+ assert(mi->getOperand(ti).isDef() && mi->getOperand(si).isUse());
+ mi->getOperand(ti).setReg(mi->getOperand(si).getReg());
+ MadeChange = true;
+
+ DOUT << "\t\trewrite to:\t"; DEBUG(mi->print(*cerr.stream(), &TM));
+ }
+
+ mi = nmi;
+ }
+ }
+
+ // Some remat'ed instructions are dead.
+ int VReg = ReMatRegs.find_first();
+ while (VReg != -1) {
+ if (MRI->use_empty(VReg)) {
+ MachineInstr *DefMI = MRI->getVRegDef(VReg);
+ DefMI->eraseFromParent();
}
+ VReg = ReMatRegs.find_next(VReg);
+ }
- return numInstrsAdded != 0;
+ return MadeChange;
}
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