Revert "[X86] Improvement in CodeGen instruction selection for LEAs."
This reverts r319543, due to ASan bot breakage.
llvm-svn: 319591
diff --git a/llvm/lib/Target/X86/X86OptimizeLEAs.cpp b/llvm/lib/Target/X86/X86OptimizeLEAs.cpp
index 7783c11..cc13686 100644
--- a/llvm/lib/Target/X86/X86OptimizeLEAs.cpp
+++ b/llvm/lib/Target/X86/X86OptimizeLEAs.cpp
@@ -27,7 +27,6 @@
#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"
@@ -59,7 +58,6 @@
cl::init(false));
STATISTIC(NumSubstLEAs, "Number of LEA instruction substitutions");
-STATISTIC(NumFactoredLEAs, "Number of LEAs factorized");
STATISTIC(NumRedundantLEAs, "Number of redundant LEA instructions removed");
/// \brief Returns true if two machine operands are identical and they are not
@@ -67,10 +65,6 @@
static inline bool isIdenticalOp(const MachineOperand &MO1,
const MachineOperand &MO2);
-/// \brief Returns true if two machine instructions have identical operands.
-static bool isIdenticalMI(MachineRegisterInfo *MRI, const MachineOperand &MO1,
- const MachineOperand &MO2);
-
/// \brief Returns true if two address displacement operands are of the same
/// type and use the same symbol/index/address regardless of the offset.
static bool isSimilarDispOp(const MachineOperand &MO1,
@@ -79,9 +73,6 @@
/// \brief Returns true if the instruction is LEA.
static inline bool isLEA(const MachineInstr &MI);
-/// \brief Returns true if Definition of Operand is a copylike instruction.
-static bool isDefCopyLike(MachineRegisterInfo *MRI, const MachineOperand &Opr);
-
namespace {
/// A key based on instruction's memory operands.
@@ -89,35 +80,15 @@
public:
MemOpKey(const MachineOperand *Base, const MachineOperand *Scale,
const MachineOperand *Index, const MachineOperand *Segment,
- const MachineOperand *Disp, bool DispCheck = false)
- : Disp(Disp), DeepCheck(DispCheck) {
+ const MachineOperand *Disp)
+ : Disp(Disp) {
Operands[0] = Base;
Operands[1] = Scale;
Operands[2] = Index;
Operands[3] = Segment;
}
- /// Checks operands of MemOpKey are identical, if Base or Index
- /// operand definitions are of kind SUBREG_TO_REG then compare
- /// operands of defining MI.
- bool performDeepCheck(const MemOpKey &Other) const {
- MachineInstr *MI = const_cast<MachineInstr *>(Operands[0]->getParent());
- MachineRegisterInfo *MRI = MI->getRegInfo();
-
- for (int i = 0; i < 4; i++) {
- bool CopyLike = isDefCopyLike(MRI, *Operands[i]);
- if (CopyLike && !isIdenticalMI(MRI, *Operands[i], *Other.Operands[i]))
- return false;
- else if (!CopyLike && !isIdenticalOp(*Operands[i], *Other.Operands[i]))
- return false;
- }
- return isIdenticalOp(*Disp, *Other.Disp);
- }
-
bool operator==(const MemOpKey &Other) const {
- if (DeepCheck)
- return performDeepCheck(Other);
-
// Addresses' bases, scales, indices and segments must be identical.
for (int i = 0; i < 4; ++i)
if (!isIdenticalOp(*Operands[i], *Other.Operands[i]))
@@ -135,12 +106,6 @@
// Address' displacement operand.
const MachineOperand *Disp;
-
- // If true checks Address' base, index, segment and
- // displacement are identical, in additions if base/index
- // are defined by copylike instruction then futher
- // compare the operands of the defining instruction.
- bool DeepCheck;
};
} // end anonymous namespace
@@ -166,34 +131,12 @@
static unsigned getHashValue(const MemOpKey &Val) {
// Checking any field of MemOpKey is enough to determine if the key is
// empty or tombstone.
- hash_code Hash(0);
assert(Val.Disp != PtrInfo::getEmptyKey() && "Cannot hash the empty key");
assert(Val.Disp != PtrInfo::getTombstoneKey() &&
"Cannot hash the tombstone key");
- auto getMIHash = [](MachineInstr *MI) -> hash_code {
- hash_code h(0);
- for (unsigned i = 1, e = MI->getNumOperands(); i < e; i++)
- h = hash_combine(h, MI->getOperand(i));
- return h;
- };
-
- const MachineOperand &Base = *Val.Operands[0];
- const MachineOperand &Index = *Val.Operands[2];
- MachineInstr *MI = const_cast<MachineInstr *>(Base.getParent());
- MachineRegisterInfo *MRI = MI->getRegInfo();
-
- if (isDefCopyLike(MRI, Base))
- Hash = getMIHash(MRI->getVRegDef(Base.getReg()));
- else
- Hash = hash_combine(Hash, Base);
-
- if (isDefCopyLike(MRI, Index))
- Hash = getMIHash(MRI->getVRegDef(Index.getReg()));
- else
- Hash = hash_combine(Hash, Index);
-
- Hash = hash_combine(Hash, *Val.Operands[1], *Val.Operands[3]);
+ hash_code Hash = hash_combine(*Val.Operands[0], *Val.Operands[1],
+ *Val.Operands[2], *Val.Operands[3]);
// If the address displacement is an immediate, it should not affect the
// hash so that memory operands which differ only be immediate displacement
@@ -253,16 +196,6 @@
&MI.getOperand(N + X86::AddrDisp));
}
-static inline MemOpKey getMemOpCSEKey(const MachineInstr &MI, unsigned N) {
- static MachineOperand DummyScale = MachineOperand::CreateImm(1);
- assert((isLEA(MI) || MI.mayLoadOrStore()) &&
- "The instruction must be a LEA, a load or a store");
- return MemOpKey(&MI.getOperand(N + X86::AddrBaseReg), &DummyScale,
- &MI.getOperand(N + X86::AddrIndexReg),
- &MI.getOperand(N + X86::AddrSegmentReg),
- &MI.getOperand(N + X86::AddrDisp), true);
-}
-
static inline bool isIdenticalOp(const MachineOperand &MO1,
const MachineOperand &MO2) {
return MO1.isIdenticalTo(MO2) &&
@@ -270,27 +203,6 @@
!TargetRegisterInfo::isPhysicalRegister(MO1.getReg()));
}
-static bool isIdenticalMI(MachineRegisterInfo *MRI, const MachineOperand &MO1,
- const MachineOperand &MO2) {
- MachineInstr *MI1 = nullptr;
- MachineInstr *MI2 = nullptr;
- if (!MO1.isReg() || !MO2.isReg())
- return false;
-
- MI1 = MRI->getVRegDef(MO1.getReg());
- MI2 = MRI->getVRegDef(MO2.getReg());
- if (!MI1 || !MI2)
- return false;
- if (MI1->getOpcode() != MI2->getOpcode())
- return false;
- if (MI1->getNumOperands() != MI2->getNumOperands())
- return false;
- for (unsigned i = 1, e = MI1->getNumOperands(); i < e; ++i)
- if (!isIdenticalOp(MI1->getOperand(i), MI2->getOperand(i)))
- return false;
- return true;
-}
-
#ifndef NDEBUG
static bool isValidDispOp(const MachineOperand &MO) {
return MO.isImm() || MO.isCPI() || MO.isJTI() || MO.isSymbol() ||
@@ -322,150 +234,8 @@
Opcode == X86::LEA64r || Opcode == X86::LEA64_32r;
}
-static bool isDefCopyLike(MachineRegisterInfo *MRI, const MachineOperand &Opr) {
- bool isInstrErased = !(Opr.isReg() && Opr.getParent()->getParent());
- if (!Opr.isReg() || isInstrErased ||
- TargetRegisterInfo::isPhysicalRegister(Opr.getReg()))
- return false;
- MachineInstr *MI = MRI->getVRegDef(Opr.getReg());
- return MI && MI->isCopyLike();
-}
-
namespace {
-/// This class captures the functions and attributes
-/// needed to factorize LEA within and across basic
-/// blocks.LEA instruction with same BASE,OFFSET and
-/// INDEX are the candidates for factorization.
-class FactorizeLEAOpt {
-public:
- using LEAListT = std::list<MachineInstr *>;
- using LEAMapT = DenseMap<MemOpKey, LEAListT>;
- using ValueT = DenseMap<MemOpKey, unsigned>;
- using ScopeEntryT = std::pair<MachineBasicBlock *, ValueT>;
- using ScopeStackT = std::vector<ScopeEntryT>;
-
- FactorizeLEAOpt() = default;
- FactorizeLEAOpt(const FactorizeLEAOpt &) = delete;
- FactorizeLEAOpt &operator=(const FactorizeLEAOpt &) = delete;
-
- void performCleanup() {
- for (auto LEA : removedLEAs)
- LEA->eraseFromParent();
- LEAs.clear();
- Stack.clear();
- removedLEAs.clear();
- }
-
- LEAMapT &getLEAMap() { return LEAs; }
- ScopeEntryT *getTopScope() { return &Stack.back(); }
-
- void addForLazyRemoval(MachineInstr *Instr) { removedLEAs.insert(Instr); }
-
- bool checkIfScheduledForRemoval(MachineInstr *Instr) {
- return removedLEAs.find(Instr) != removedLEAs.end();
- }
-
- /// Push the ScopeEntry for the BasicBlock over Stack.
- /// Also traverses over list of instruction and update
- /// LEAs Map and ScopeEntry for each LEA instruction
- /// found using insertLEA().
- void collectDataForBasicBlock(MachineBasicBlock *MBB);
-
- /// Stores the size of MachineInstr list corrosponding
- /// to key K from LEAs MAP into the ScopeEntry of
- /// the basic block, then insert the LEA at the beginning
- /// of the list.
- void insertLEA(MachineInstr *MI);
-
- /// Pops out ScopeEntry of top most BasicBlock from the stack
- /// and remove the LEA instructions contained in the scope
- /// from the LEAs Map.
- void removeDataForBasicBlock();
-
- /// If LEA contains Physical Registers then its not a candidate
- /// for factorizations since physical registers may violate SSA
- /// semantics of MI.
- bool containsPhyReg(MachineInstr *MI, unsigned RecLevel);
-
-private:
- ScopeStackT Stack;
- LEAMapT LEAs;
- std::set<MachineInstr *> removedLEAs;
-};
-
-void FactorizeLEAOpt::collectDataForBasicBlock(MachineBasicBlock *MBB) {
- ValueT EmptyMap;
- ScopeEntryT SE = std::make_pair(MBB, EmptyMap);
- Stack.push_back(SE);
- for (auto &MI : *MBB) {
- if (isLEA(MI))
- insertLEA(&MI);
- }
-}
-
-void FactorizeLEAOpt::removeDataForBasicBlock() {
- ScopeEntryT &SE = Stack.back();
- for (auto MapEntry : SE.second) {
- LEAMapT::iterator Itr = LEAs.find(MapEntry.first);
- assert((Itr != LEAs.end()) &&
- "LEAs map must have a node corresponding to ScopeEntry's Key.");
-
- while (((*Itr).second.size() > MapEntry.second))
- (*Itr).second.pop_front();
- // If list goes empty remove entry from LEAs Map.
- if ((*Itr).second.empty())
- LEAs.erase(Itr);
- }
- Stack.pop_back();
-}
-
-bool FactorizeLEAOpt::containsPhyReg(MachineInstr *MI, unsigned RecLevel) {
- if (!MI || !RecLevel)
- return false;
-
- MachineRegisterInfo *MRI = MI->getRegInfo();
- for (auto Operand : MI->operands()) {
- if (!Operand.isReg())
- continue;
- if (TargetRegisterInfo::isPhysicalRegister(Operand.getReg()))
- return true;
- MachineInstr *OperDefMI = MRI->getVRegDef(Operand.getReg());
- if (OperDefMI && (MI != OperDefMI) && OperDefMI->isCopyLike() &&
- containsPhyReg(OperDefMI, RecLevel - 1))
- return true;
- }
- return false;
-}
-
-void FactorizeLEAOpt::insertLEA(MachineInstr *MI) {
- unsigned lsize;
- if (containsPhyReg(MI, 2))
- return;
-
- // Factorization is beneficial only for complex LEAs.
- MachineOperand &Base = MI->getOperand(1);
- MachineOperand &Index = MI->getOperand(3);
- MachineOperand &Offset = MI->getOperand(4);
- if ((Offset.isImm() && !Offset.getImm()) ||
- (!Base.isReg() || !Base.getReg()) || (!Index.isReg() || !Index.getReg()))
- return;
-
- MemOpKey Key = getMemOpCSEKey(*MI, 1);
- ScopeEntryT *TopScope = getTopScope();
-
- LEAMapT::iterator Itr = LEAs.find(Key);
- if (Itr == LEAs.end()) {
- lsize = 0;
- LEAs[Key].push_front(MI);
- } else {
- lsize = (*Itr).second.size();
- (*Itr).second.push_front(MI);
- }
- if (TopScope->second.find(Key) == TopScope->second.end())
- TopScope->second[Key] = lsize;
-}
-
class OptimizeLEAPass : public MachineFunctionPass {
public:
OptimizeLEAPass() : MachineFunctionPass(ID) {}
@@ -477,12 +247,6 @@
/// been calculated by LEA. Also, remove redundant LEAs.
bool runOnMachineFunction(MachineFunction &MF) override;
- void getAnalysisUsage(AnalysisUsage &AU) const override {
- AU.setPreservesCFG();
- MachineFunctionPass::getAnalysisUsage(AU);
- AU.addRequired<MachineDominatorTree>();
- }
-
private:
using MemOpMap = DenseMap<MemOpKey, SmallVector<MachineInstr *, 16>>;
@@ -528,24 +292,8 @@
/// \brief Removes LEAs which calculate similar addresses.
bool removeRedundantLEAs(MemOpMap &LEAs);
- /// \brief Visit over basic blocks, collect LEAs in a scoped
- /// hash map (FactorizeLEAOpt::LEAs) and try to factor them out.
- bool FactorizeLEAsAllBasicBlocks(MachineFunction &MF);
-
- bool FactorizeLEAsBasicBlock(MachineDomTreeNode *DN);
-
- /// \brief Factor out LEAs which share Base,Index,Offset and Segment.
- bool processBasicBlock(const MachineBasicBlock &MBB);
-
- /// \brief Try to replace LEA with a lower strength instruction
- /// to improves latency and throughput.
- bool strengthReduceLEAs(MemOpMap &LEAs, const MachineBasicBlock &MBB);
-
DenseMap<const MachineInstr *, unsigned> InstrPos;
- FactorizeLEAOpt FactorOpt;
-
- MachineDominatorTree *DT;
MachineRegisterInfo *MRI;
const X86InstrInfo *TII;
const X86RegisterInfo *TRI;
@@ -741,9 +489,7 @@
bool OptimizeLEAPass::removeRedundantAddrCalc(MemOpMap &LEAs) {
bool Changed = false;
- if (LEAs.empty())
- return Changed;
-
+ assert(!LEAs.empty());
MachineBasicBlock *MBB = (*LEAs.begin()->second.begin())->getParent();
// Process all instructions in basic block.
@@ -913,10 +659,6 @@
// Erase removed LEA from the list.
I2 = List.erase(I2);
- // If List becomes empty remove it from LEAs map.
- if (List.empty())
- LEAs.erase(E.first);
-
Changed = true;
}
++I1;
@@ -926,217 +668,6 @@
return Changed;
}
-static inline int getADDrrFromLEA(int LEAOpcode) {
- switch (LEAOpcode) {
- default:
- llvm_unreachable("Unexpected LEA instruction");
- case X86::LEA16r:
- return X86::ADD16rr;
- case X86::LEA32r:
- return X86::ADD32rr;
- case X86::LEA64_32r:
- case X86::LEA64r:
- return X86::ADD64rr;
- }
-}
-
-bool OptimizeLEAPass::strengthReduceLEAs(MemOpMap &LEAs,
- const MachineBasicBlock &BB) {
- bool Changed = false;
-
- // Loop over all entries in the table.
- for (auto &E : LEAs) {
- auto &List = E.second;
-
- // Loop over all LEA pairs.
- auto I1 = List.begin();
- while (I1 != List.end()) {
- MachineInstrBuilder NewMI;
- MachineInstr &First = **I1;
- MachineOperand &Res = First.getOperand(0);
- MachineOperand &Base = First.getOperand(1);
- MachineOperand &Scale = First.getOperand(2);
- MachineOperand &Index = First.getOperand(3);
- MachineOperand &Offset = First.getOperand(4);
-
- const MCInstrDesc &ADDrr = TII->get(getADDrrFromLEA(First.getOpcode()));
- const DebugLoc DL = First.getDebugLoc();
-
- if (!Base.isReg() || !Index.isReg() || !Index.getReg()) {
- I1++;
- continue;
- }
-
- if (TargetRegisterInfo::isPhysicalRegister(Res.getReg()) ||
- TargetRegisterInfo::isPhysicalRegister(Base.getReg()) ||
- TargetRegisterInfo::isPhysicalRegister(Index.getReg())) {
- I1++;
- continue;
- }
-
- // Check for register class compatibility between Result and
- // Index operands.
- const TargetRegisterClass *ResRC = MRI->getRegClass(Res.getReg());
- const TargetRegisterClass *IndexRC = MRI->getRegClass(Index.getReg());
- if (TRI->getRegSizeInBits(*ResRC) != TRI->getRegSizeInBits(*IndexRC)) {
- I1++;
- continue;
- }
-
- MachineBasicBlock &MBB = *(const_cast<MachineBasicBlock *>(&BB));
- // R = B + I
- if (Scale.isImm() && Scale.getImm() == 1 && Offset.isImm() &&
- !Offset.getImm()) {
- NewMI = BuildMI(MBB, &First, DL, ADDrr)
- .addDef(Res.getReg())
- .addUse(Base.getReg())
- .addUse(Index.getReg());
- Changed = NewMI.getInstr() != nullptr;
- First.eraseFromParent();
- I1 = List.erase(I1);
-
- // If List becomes empty remove it from LEAs map.
- if (List.empty())
- LEAs.erase(E.first);
- } else
- I1++;
- }
- }
- return Changed;
-}
-
-bool OptimizeLEAPass::processBasicBlock(const MachineBasicBlock &MBB) {
- bool cseDone = false;
-
- // Legal scale value (1,2,4 & 8) vector.
- auto LegalScale = [](int scale) {
- return scale == 1 || scale == 2 || scale == 4 || scale == 8;
- };
-
- auto CompareFn = [](const MachineInstr *Arg1,
- const MachineInstr *Arg2) -> bool {
- return Arg1->getOperand(2).getImm() >= Arg2->getOperand(2).getImm();
- };
-
- // Loop over all entries in the table.
- for (auto &E : FactorOpt.getLEAMap()) {
- auto &List = E.second;
- if (List.size() > 1)
- List.sort(CompareFn);
-
- // Loop over all LEA pairs.
- for (auto Iter1 = List.begin(); Iter1 != List.end(); Iter1++) {
- for (auto Iter2 = std::next(Iter1); Iter2 != List.end(); Iter2++) {
- MachineInstr &LI1 = **Iter1;
- MachineInstr &LI2 = **Iter2;
-
- if (!DT->dominates(&LI2, &LI1))
- continue;
-
- int Scale1 = LI1.getOperand(2).getImm();
- int Scale2 = LI2.getOperand(2).getImm();
- assert(LI2.getOperand(0).isReg() && "Result is a VirtualReg");
- DebugLoc DL = LI1.getDebugLoc();
-
- // Continue if instruction has already been factorized.
- if (FactorOpt.checkIfScheduledForRemoval(&LI1))
- continue;
-
- int Factor = Scale1 - Scale2;
- if (Factor > 0 && LegalScale(Factor)) {
- MachineOperand NewBase = LI2.getOperand(0);
- MachineOperand NewIndex = LI1.getOperand(3);
-
- const TargetRegisterClass *LI2ResRC =
- MRI->getRegClass(LI2.getOperand(0).getReg());
- const TargetRegisterClass *LI1BaseRC =
- MRI->getRegClass(LI1.getOperand(1).getReg());
-
- if (TRI->getRegSizeInBits(*LI1BaseRC) >
- TRI->getRegSizeInBits(*LI2ResRC)) {
- MachineInstr *LI1IndexDef =
- MRI->getVRegDef(LI1.getOperand(3).getReg());
- if (LI1IndexDef->getOpcode() != TargetOpcode::SUBREG_TO_REG)
- continue;
- MachineOperand &SubReg = LI1IndexDef->getOperand(2);
- const TargetRegisterClass *SubRegRC =
- MRI->getRegClass(SubReg.getReg());
- if (TRI->getRegSizeInBits(*SubRegRC) !=
- TRI->getRegSizeInBits(*LI2ResRC))
- continue;
- NewIndex = SubReg;
- }
-
- DEBUG(dbgs() << "CSE LEAs: Candidate to replace: "; LI1.dump(););
- MachineInstrBuilder NewMI =
- BuildMI(*(const_cast<MachineBasicBlock *>(&MBB)), &LI1, DL,
- TII->get(LI1.getOpcode()))
- .addDef(LI1.getOperand(0).getReg()) // Dst = Dst of LI1.
- .addUse(NewBase.getReg()) // Base = Dst to LI2.
- .addImm(Factor) // Scale = Diff b/w scales.
- .addUse(NewIndex.getReg()) // Index = Index of LI1.
- .addImm(0) // Disp = 0
- .addUse(
- LI1.getOperand(5).getReg()); // Segment = Segmant of LI1.
-
- cseDone = NewMI.getInstr() != nullptr;
-
- /// To preserve the SSA nature we need to remove Def flag
- /// from old result.
- LI1.getOperand(0).setIsDef(false);
-
- /// Lazy removal shall ensure that replaced LEA remains
- /// till we finish processing all the basic block. This shall
- /// provide opportunity for further factorization based on
- /// the replaced LEA which will be legal since it has same
- /// destination as newly formed LEA.
- FactorOpt.addForLazyRemoval(&LI1);
-
- NumFactoredLEAs++;
- DEBUG(dbgs() << "CSE LEAs: Replaced by: "; NewMI->dump(););
- }
- }
- }
- }
- return cseDone;
-}
-
-bool OptimizeLEAPass::FactorizeLEAsBasicBlock(MachineDomTreeNode *DN) {
- bool Changed = false;
- using StackT = SmallVector<MachineDomTreeNode *, 16>;
- using VisitedNodeMapT = SmallSet<MachineDomTreeNode *, 16>;
-
- StackT WorkList;
- VisitedNodeMapT DomNodeMap;
-
- WorkList.push_back(DN);
- while (!WorkList.empty()) {
- MachineDomTreeNode *MDN = WorkList.back();
- FactorOpt.collectDataForBasicBlock(MDN->getBlock());
- Changed |= processBasicBlock(*MDN->getBlock());
-
- if (DomNodeMap.find(MDN) == DomNodeMap.end()) {
- DomNodeMap.insert(MDN);
- for (auto Child : MDN->getChildren())
- WorkList.push_back(Child);
- }
-
- MachineDomTreeNode *TDM = WorkList.back();
- if (MDN->getLevel() == TDM->getLevel()) {
- FactorOpt.removeDataForBasicBlock();
- DomNodeMap.erase(MDN);
- WorkList.pop_back();
- }
- }
- return Changed;
-}
-
-bool OptimizeLEAPass::FactorizeLEAsAllBasicBlocks(MachineFunction &MF) {
- bool Changed = FactorizeLEAsBasicBlock(DT->getRootNode());
- FactorOpt.performCleanup();
- return Changed;
-}
-
bool OptimizeLEAPass::runOnMachineFunction(MachineFunction &MF) {
bool Changed = false;
@@ -1146,10 +677,6 @@
MRI = &MF.getRegInfo();
TII = MF.getSubtarget<X86Subtarget>().getInstrInfo();
TRI = MF.getSubtarget<X86Subtarget>().getRegisterInfo();
- DT = &getAnalysis<MachineDominatorTree>();
-
- // Attempt factorizing LEAs.
- Changed |= FactorizeLEAsAllBasicBlocks(MF);
// Process all basic blocks.
for (auto &MBB : MF) {
@@ -1166,9 +693,6 @@
// Remove redundant LEA instructions.
Changed |= removeRedundantLEAs(LEAs);
- // Strength reduce LEA instructions.
- Changed |= strengthReduceLEAs(LEAs, MBB);
-
// Remove redundant address calculations. Do it only for -Os/-Oz since only
// a code size gain is expected from this part of the pass.
if (MF.getFunction()->optForSize())