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llvm-mirror/lib/CodeGen/MachineCSE.cpp
Jeremy Morse 7698ed73e1 [DebugInfo] LiveDebugValues: correctly discriminate kinds of variable locations 
The missing line added by this patch ensures that only spilt variable
locations are candidates for being restored from the stack. Otherwise,
register or constant-value information can be interpreted as a spill
location, through a union.

The added regression test replicates a scenario where this occurs: the
stack load from [rsp] causes the register-location DBG_VALUE to be
"restored" to rsi, when it should be left alone. See PR43058 for details.

Un x-fail a test that was suffering from this from a previous patch.

Differential Revision: https://reviews.llvm.org/D66895

llvm-svn: 370648
2019-09-02 12:28:36 +00:00

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32 KiB
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//===- MachineCSE.cpp - Machine Common Subexpression Elimination Pass -----===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This pass performs global common subexpression elimination on machine
// instructions using a scoped hash table based value numbering scheme. It
// must be run while the machine function is still in SSA form.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/ScopedHashTable.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineDominators.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/Passes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/RecyclingAllocator.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <iterator>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "machine-cse"
STATISTIC(NumCoalesces, "Number of copies coalesced");
STATISTIC(NumCSEs, "Number of common subexpression eliminated");
STATISTIC(NumPREs, "Number of partial redundant expression"
" transformed to fully redundant");
STATISTIC(NumPhysCSEs,
"Number of physreg referencing common subexpr eliminated");
STATISTIC(NumCrossBBCSEs,
"Number of cross-MBB physreg referencing CS eliminated");
STATISTIC(NumCommutes, "Number of copies coalesced after commuting");
namespace {
class MachineCSE : public MachineFunctionPass {
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
AliasAnalysis *AA;
MachineDominatorTree *DT;
MachineRegisterInfo *MRI;
MachineBlockFrequencyInfo *MBFI;
public:
static char ID; // Pass identification
MachineCSE() : MachineFunctionPass(ID) {
initializeMachineCSEPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
AU.addRequired<AAResultsWrapperPass>();
AU.addPreservedID(MachineLoopInfoID);
AU.addRequired<MachineDominatorTree>();
AU.addPreserved<MachineDominatorTree>();
AU.addRequired<MachineBlockFrequencyInfo>();
AU.addPreserved<MachineBlockFrequencyInfo>();
}
void releaseMemory() override {
ScopeMap.clear();
PREMap.clear();
Exps.clear();
}
private:
using AllocatorTy = RecyclingAllocator<BumpPtrAllocator,
ScopedHashTableVal<MachineInstr *, unsigned>>;
using ScopedHTType =
ScopedHashTable<MachineInstr *, unsigned, MachineInstrExpressionTrait,
AllocatorTy>;
using ScopeType = ScopedHTType::ScopeTy;
using PhysDefVector = SmallVector<std::pair<unsigned, unsigned>, 2>;
unsigned LookAheadLimit = 0;
DenseMap<MachineBasicBlock *, ScopeType *> ScopeMap;
DenseMap<MachineInstr *, MachineBasicBlock *, MachineInstrExpressionTrait>
PREMap;
ScopedHTType VNT;
SmallVector<MachineInstr *, 64> Exps;
unsigned CurrVN = 0;
bool PerformTrivialCopyPropagation(MachineInstr *MI,
MachineBasicBlock *MBB);
bool isPhysDefTriviallyDead(unsigned Reg,
MachineBasicBlock::const_iterator I,
MachineBasicBlock::const_iterator E) const;
bool hasLivePhysRegDefUses(const MachineInstr *MI,
const MachineBasicBlock *MBB,
SmallSet<unsigned, 8> &PhysRefs,
PhysDefVector &PhysDefs, bool &PhysUseDef) const;
bool PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
SmallSet<unsigned, 8> &PhysRefs,
PhysDefVector &PhysDefs, bool &NonLocal) const;
bool isCSECandidate(MachineInstr *MI);
bool isProfitableToCSE(unsigned CSReg, unsigned Reg,
MachineBasicBlock *CSBB, MachineInstr *MI);
void EnterScope(MachineBasicBlock *MBB);
void ExitScope(MachineBasicBlock *MBB);
bool ProcessBlockCSE(MachineBasicBlock *MBB);
void ExitScopeIfDone(MachineDomTreeNode *Node,
DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren);
bool PerformCSE(MachineDomTreeNode *Node);
bool isPRECandidate(MachineInstr *MI);
bool ProcessBlockPRE(MachineDominatorTree *MDT, MachineBasicBlock *MBB);
bool PerformSimplePRE(MachineDominatorTree *DT);
/// Heuristics to see if it's profitable to move common computations of MBB
/// and MBB1 to CandidateBB.
bool isProfitableToHoistInto(MachineBasicBlock *CandidateBB,
MachineBasicBlock *MBB,
MachineBasicBlock *MBB1);
};
} // end anonymous namespace
char MachineCSE::ID = 0;
char &llvm::MachineCSEID = MachineCSE::ID;
INITIALIZE_PASS_BEGIN(MachineCSE, DEBUG_TYPE,
"Machine Common Subexpression Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(MachineCSE, DEBUG_TYPE,
"Machine Common Subexpression Elimination", false, false)
/// The source register of a COPY machine instruction can be propagated to all
/// its users, and this propagation could increase the probability of finding
/// common subexpressions. If the COPY has only one user, the COPY itself can
/// be removed.
bool MachineCSE::PerformTrivialCopyPropagation(MachineInstr *MI,
MachineBasicBlock *MBB) {
bool Changed = false;
for (MachineOperand &MO : MI->operands()) {
if (!MO.isReg() || !MO.isUse())
continue;
Register Reg = MO.getReg();
if (!Register::isVirtualRegister(Reg))
continue;
bool OnlyOneUse = MRI->hasOneNonDBGUse(Reg);
MachineInstr *DefMI = MRI->getVRegDef(Reg);
if (!DefMI->isCopy())
continue;
Register SrcReg = DefMI->getOperand(1).getReg();
if (!Register::isVirtualRegister(SrcReg))
continue;
if (DefMI->getOperand(0).getSubReg())
continue;
// FIXME: We should trivially coalesce subregister copies to expose CSE
// opportunities on instructions with truncated operands (see
// cse-add-with-overflow.ll). This can be done here as follows:
// if (SrcSubReg)
// RC = TRI->getMatchingSuperRegClass(MRI->getRegClass(SrcReg), RC,
// SrcSubReg);
// MO.substVirtReg(SrcReg, SrcSubReg, *TRI);
//
// The 2-addr pass has been updated to handle coalesced subregs. However,
// some machine-specific code still can't handle it.
// To handle it properly we also need a way find a constrained subregister
// class given a super-reg class and subreg index.
if (DefMI->getOperand(1).getSubReg())
continue;
if (!MRI->constrainRegAttrs(SrcReg, Reg))
continue;
LLVM_DEBUG(dbgs() << "Coalescing: " << *DefMI);
LLVM_DEBUG(dbgs() << "*** to: " << *MI);
// Propagate SrcReg of copies to MI.
MO.setReg(SrcReg);
MRI->clearKillFlags(SrcReg);
// Coalesce single use copies.
if (OnlyOneUse) {
// If (and only if) we've eliminated all uses of the copy, also
// copy-propagate to any debug-users of MI, or they'll be left using
// an undefined value.
DefMI->changeDebugValuesDefReg(SrcReg);
DefMI->eraseFromParent();
++NumCoalesces;
}
Changed = true;
}
return Changed;
}
bool
MachineCSE::isPhysDefTriviallyDead(unsigned Reg,
MachineBasicBlock::const_iterator I,
MachineBasicBlock::const_iterator E) const {
unsigned LookAheadLeft = LookAheadLimit;
while (LookAheadLeft) {
// Skip over dbg_value's.
I = skipDebugInstructionsForward(I, E);
if (I == E)
// Reached end of block, we don't know if register is dead or not.
return false;
bool SeenDef = false;
for (const MachineOperand &MO : I->operands()) {
if (MO.isRegMask() && MO.clobbersPhysReg(Reg))
SeenDef = true;
if (!MO.isReg() || !MO.getReg())
continue;
if (!TRI->regsOverlap(MO.getReg(), Reg))
continue;
if (MO.isUse())
// Found a use!
return false;
SeenDef = true;
}
if (SeenDef)
// See a def of Reg (or an alias) before encountering any use, it's
// trivially dead.
return true;
--LookAheadLeft;
++I;
}
return false;
}
static bool isCallerPreservedOrConstPhysReg(unsigned Reg,
const MachineFunction &MF,
const TargetRegisterInfo &TRI) {
// MachineRegisterInfo::isConstantPhysReg directly called by
// MachineRegisterInfo::isCallerPreservedOrConstPhysReg expects the
// reserved registers to be frozen. That doesn't cause a problem post-ISel as
// most (if not all) targets freeze reserved registers right after ISel.
//
// It does cause issues mid-GlobalISel, however, hence the additional
// reservedRegsFrozen check.
const MachineRegisterInfo &MRI = MF.getRegInfo();
return TRI.isCallerPreservedPhysReg(Reg, MF) ||
(MRI.reservedRegsFrozen() && MRI.isConstantPhysReg(Reg));
}
/// hasLivePhysRegDefUses - Return true if the specified instruction read/write
/// physical registers (except for dead defs of physical registers). It also
/// returns the physical register def by reference if it's the only one and the
/// instruction does not uses a physical register.
bool MachineCSE::hasLivePhysRegDefUses(const MachineInstr *MI,
const MachineBasicBlock *MBB,
SmallSet<unsigned, 8> &PhysRefs,
PhysDefVector &PhysDefs,
bool &PhysUseDef) const {
// First, add all uses to PhysRefs.
for (const MachineOperand &MO : MI->operands()) {
if (!MO.isReg() || MO.isDef())
continue;
Register Reg = MO.getReg();
if (!Reg)
continue;
if (Register::isVirtualRegister(Reg))
continue;
// Reading either caller preserved or constant physregs is ok.
if (!isCallerPreservedOrConstPhysReg(Reg, *MI->getMF(), *TRI))
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
PhysRefs.insert(*AI);
}
// Next, collect all defs into PhysDefs. If any is already in PhysRefs
// (which currently contains only uses), set the PhysUseDef flag.
PhysUseDef = false;
MachineBasicBlock::const_iterator I = MI; I = std::next(I);
for (const auto &MOP : llvm::enumerate(MI->operands())) {
const MachineOperand &MO = MOP.value();
if (!MO.isReg() || !MO.isDef())
continue;
Register Reg = MO.getReg();
if (!Reg)
continue;
if (Register::isVirtualRegister(Reg))
continue;
// Check against PhysRefs even if the def is "dead".
if (PhysRefs.count(Reg))
PhysUseDef = true;
// If the def is dead, it's ok. But the def may not marked "dead". That's
// common since this pass is run before livevariables. We can scan
// forward a few instructions and check if it is obviously dead.
if (!MO.isDead() && !isPhysDefTriviallyDead(Reg, I, MBB->end()))
PhysDefs.push_back(std::make_pair(MOP.index(), Reg));
}
// Finally, add all defs to PhysRefs as well.
for (unsigned i = 0, e = PhysDefs.size(); i != e; ++i)
for (MCRegAliasIterator AI(PhysDefs[i].second, TRI, true); AI.isValid();
++AI)
PhysRefs.insert(*AI);
return !PhysRefs.empty();
}
bool MachineCSE::PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
SmallSet<unsigned, 8> &PhysRefs,
PhysDefVector &PhysDefs,
bool &NonLocal) const {
// For now conservatively returns false if the common subexpression is
// not in the same basic block as the given instruction. The only exception
// is if the common subexpression is in the sole predecessor block.
const MachineBasicBlock *MBB = MI->getParent();
const MachineBasicBlock *CSMBB = CSMI->getParent();
bool CrossMBB = false;
if (CSMBB != MBB) {
if (MBB->pred_size() != 1 || *MBB->pred_begin() != CSMBB)
return false;
for (unsigned i = 0, e = PhysDefs.size(); i != e; ++i) {
if (MRI->isAllocatable(PhysDefs[i].second) ||
MRI->isReserved(PhysDefs[i].second))
// Avoid extending live range of physical registers if they are
//allocatable or reserved.
return false;
}
CrossMBB = true;
}
MachineBasicBlock::const_iterator I = CSMI; I = std::next(I);
MachineBasicBlock::const_iterator E = MI;
MachineBasicBlock::const_iterator EE = CSMBB->end();
unsigned LookAheadLeft = LookAheadLimit;
while (LookAheadLeft) {
// Skip over dbg_value's.
while (I != E && I != EE && I->isDebugInstr())
++I;
if (I == EE) {
assert(CrossMBB && "Reaching end-of-MBB without finding MI?");
(void)CrossMBB;
CrossMBB = false;
NonLocal = true;
I = MBB->begin();
EE = MBB->end();
continue;
}
if (I == E)
return true;
for (const MachineOperand &MO : I->operands()) {
// RegMasks go on instructions like calls that clobber lots of physregs.
// Don't attempt to CSE across such an instruction.
if (MO.isRegMask())
return false;
if (!MO.isReg() || !MO.isDef())
continue;
Register MOReg = MO.getReg();
if (Register::isVirtualRegister(MOReg))
continue;
if (PhysRefs.count(MOReg))
return false;
}
--LookAheadLeft;
++I;
}
return false;
}
bool MachineCSE::isCSECandidate(MachineInstr *MI) {
if (MI->isPosition() || MI->isPHI() || MI->isImplicitDef() || MI->isKill() ||
MI->isInlineAsm() || MI->isDebugInstr())
return false;
// Ignore copies.
if (MI->isCopyLike())
return false;
// Ignore stuff that we obviously can't move.
if (MI->mayStore() || MI->isCall() || MI->isTerminator() ||
MI->mayRaiseFPException() || MI->hasUnmodeledSideEffects())
return false;
if (MI->mayLoad()) {
// Okay, this instruction does a load. As a refinement, we allow the target
// to decide whether the loaded value is actually a constant. If so, we can
// actually use it as a load.
if (!MI->isDereferenceableInvariantLoad(AA))
// FIXME: we should be able to hoist loads with no other side effects if
// there are no other instructions which can change memory in this loop.
// This is a trivial form of alias analysis.
return false;
}
// Ignore stack guard loads, otherwise the register that holds CSEed value may
// be spilled and get loaded back with corrupted data.
if (MI->getOpcode() == TargetOpcode::LOAD_STACK_GUARD)
return false;
return true;
}
/// isProfitableToCSE - Return true if it's profitable to eliminate MI with a
/// common expression that defines Reg. CSBB is basic block where CSReg is
/// defined.
bool MachineCSE::isProfitableToCSE(unsigned CSReg, unsigned Reg,
MachineBasicBlock *CSBB, MachineInstr *MI) {
// FIXME: Heuristics that works around the lack the live range splitting.
// If CSReg is used at all uses of Reg, CSE should not increase register
// pressure of CSReg.
bool MayIncreasePressure = true;
if (Register::isVirtualRegister(CSReg) && Register::isVirtualRegister(Reg)) {
MayIncreasePressure = false;
SmallPtrSet<MachineInstr*, 8> CSUses;
for (MachineInstr &MI : MRI->use_nodbg_instructions(CSReg)) {
CSUses.insert(&MI);
}
for (MachineInstr &MI : MRI->use_nodbg_instructions(Reg)) {
if (!CSUses.count(&MI)) {
MayIncreasePressure = true;
break;
}
}
}
if (!MayIncreasePressure) return true;
// Heuristics #1: Don't CSE "cheap" computation if the def is not local or in
// an immediate predecessor. We don't want to increase register pressure and
// end up causing other computation to be spilled.
if (TII->isAsCheapAsAMove(*MI)) {
MachineBasicBlock *BB = MI->getParent();
if (CSBB != BB && !CSBB->isSuccessor(BB))
return false;
}
// Heuristics #2: If the expression doesn't not use a vr and the only use
// of the redundant computation are copies, do not cse.
bool HasVRegUse = false;
for (const MachineOperand &MO : MI->operands()) {
if (MO.isReg() && MO.isUse() && Register::isVirtualRegister(MO.getReg())) {
HasVRegUse = true;
break;
}
}
if (!HasVRegUse) {
bool HasNonCopyUse = false;
for (MachineInstr &MI : MRI->use_nodbg_instructions(Reg)) {
// Ignore copies.
if (!MI.isCopyLike()) {
HasNonCopyUse = true;
break;
}
}
if (!HasNonCopyUse)
return false;
}
// Heuristics #3: If the common subexpression is used by PHIs, do not reuse
// it unless the defined value is already used in the BB of the new use.
bool HasPHI = false;
for (MachineInstr &UseMI : MRI->use_nodbg_instructions(CSReg)) {
HasPHI |= UseMI.isPHI();
if (UseMI.getParent() == MI->getParent())
return true;
}
return !HasPHI;
}
void MachineCSE::EnterScope(MachineBasicBlock *MBB) {
LLVM_DEBUG(dbgs() << "Entering: " << MBB->getName() << '\n');
ScopeType *Scope = new ScopeType(VNT);
ScopeMap[MBB] = Scope;
}
void MachineCSE::ExitScope(MachineBasicBlock *MBB) {
LLVM_DEBUG(dbgs() << "Exiting: " << MBB->getName() << '\n');
DenseMap<MachineBasicBlock*, ScopeType*>::iterator SI = ScopeMap.find(MBB);
assert(SI != ScopeMap.end());
delete SI->second;
ScopeMap.erase(SI);
}
bool MachineCSE::ProcessBlockCSE(MachineBasicBlock *MBB) {
bool Changed = false;
SmallVector<std::pair<unsigned, unsigned>, 8> CSEPairs;
SmallVector<unsigned, 2> ImplicitDefsToUpdate;
SmallVector<unsigned, 2> ImplicitDefs;
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E; ) {
MachineInstr *MI = &*I;
++I;
if (!isCSECandidate(MI))
continue;
bool FoundCSE = VNT.count(MI);
if (!FoundCSE) {
// Using trivial copy propagation to find more CSE opportunities.
if (PerformTrivialCopyPropagation(MI, MBB)) {
Changed = true;
// After coalescing MI itself may become a copy.
if (MI->isCopyLike())
continue;
// Try again to see if CSE is possible.
FoundCSE = VNT.count(MI);
}
}
// Commute commutable instructions.
bool Commuted = false;
if (!FoundCSE && MI->isCommutable()) {
if (MachineInstr *NewMI = TII->commuteInstruction(*MI)) {
Commuted = true;
FoundCSE = VNT.count(NewMI);
if (NewMI != MI) {
// New instruction. It doesn't need to be kept.
NewMI->eraseFromParent();
Changed = true;
} else if (!FoundCSE)
// MI was changed but it didn't help, commute it back!
(void)TII->commuteInstruction(*MI);
}
}
// If the instruction defines physical registers and the values *may* be
// used, then it's not safe to replace it with a common subexpression.
// It's also not safe if the instruction uses physical registers.
bool CrossMBBPhysDef = false;
SmallSet<unsigned, 8> PhysRefs;
PhysDefVector PhysDefs;
bool PhysUseDef = false;
if (FoundCSE && hasLivePhysRegDefUses(MI, MBB, PhysRefs,
PhysDefs, PhysUseDef)) {
FoundCSE = false;
// ... Unless the CS is local or is in the sole predecessor block
// and it also defines the physical register which is not clobbered
// in between and the physical register uses were not clobbered.
// This can never be the case if the instruction both uses and
// defines the same physical register, which was detected above.
if (!PhysUseDef) {
unsigned CSVN = VNT.lookup(MI);
MachineInstr *CSMI = Exps[CSVN];
if (PhysRegDefsReach(CSMI, MI, PhysRefs, PhysDefs, CrossMBBPhysDef))
FoundCSE = true;
}
}
if (!FoundCSE) {
VNT.insert(MI, CurrVN++);
Exps.push_back(MI);
continue;
}
// Found a common subexpression, eliminate it.
unsigned CSVN = VNT.lookup(MI);
MachineInstr *CSMI = Exps[CSVN];
LLVM_DEBUG(dbgs() << "Examining: " << *MI);
LLVM_DEBUG(dbgs() << "*** Found a common subexpression: " << *CSMI);
// Check if it's profitable to perform this CSE.
bool DoCSE = true;
unsigned NumDefs = MI->getNumDefs();
for (unsigned i = 0, e = MI->getNumOperands(); NumDefs && i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || !MO.isDef())
continue;
Register OldReg = MO.getReg();
Register NewReg = CSMI->getOperand(i).getReg();
// Go through implicit defs of CSMI and MI, if a def is not dead at MI,
// we should make sure it is not dead at CSMI.
if (MO.isImplicit() && !MO.isDead() && CSMI->getOperand(i).isDead())
ImplicitDefsToUpdate.push_back(i);
// Keep track of implicit defs of CSMI and MI, to clear possibly
// made-redundant kill flags.
if (MO.isImplicit() && !MO.isDead() && OldReg == NewReg)
ImplicitDefs.push_back(OldReg);
if (OldReg == NewReg) {
--NumDefs;
continue;
}
assert(Register::isVirtualRegister(OldReg) &&
Register::isVirtualRegister(NewReg) &&
"Do not CSE physical register defs!");
if (!isProfitableToCSE(NewReg, OldReg, CSMI->getParent(), MI)) {
LLVM_DEBUG(dbgs() << "*** Not profitable, avoid CSE!\n");
DoCSE = false;
break;
}
// Don't perform CSE if the result of the new instruction cannot exist
// within the constraints (register class, bank, or low-level type) of
// the old instruction.
if (!MRI->constrainRegAttrs(NewReg, OldReg)) {
LLVM_DEBUG(
dbgs() << "*** Not the same register constraints, avoid CSE!\n");
DoCSE = false;
break;
}
CSEPairs.push_back(std::make_pair(OldReg, NewReg));
--NumDefs;
}
// Actually perform the elimination.
if (DoCSE) {
for (std::pair<unsigned, unsigned> &CSEPair : CSEPairs) {
unsigned OldReg = CSEPair.first;
unsigned NewReg = CSEPair.second;
// OldReg may have been unused but is used now, clear the Dead flag
MachineInstr *Def = MRI->getUniqueVRegDef(NewReg);
assert(Def != nullptr && "CSEd register has no unique definition?");
Def->clearRegisterDeads(NewReg);
// Replace with NewReg and clear kill flags which may be wrong now.
MRI->replaceRegWith(OldReg, NewReg);
MRI->clearKillFlags(NewReg);
}
// Go through implicit defs of CSMI and MI, if a def is not dead at MI,
// we should make sure it is not dead at CSMI.
for (unsigned ImplicitDefToUpdate : ImplicitDefsToUpdate)
CSMI->getOperand(ImplicitDefToUpdate).setIsDead(false);
for (auto PhysDef : PhysDefs)
if (!MI->getOperand(PhysDef.first).isDead())
CSMI->getOperand(PhysDef.first).setIsDead(false);
// Go through implicit defs of CSMI and MI, and clear the kill flags on
// their uses in all the instructions between CSMI and MI.
// We might have made some of the kill flags redundant, consider:
// subs ... implicit-def %nzcv <- CSMI
// csinc ... implicit killed %nzcv <- this kill flag isn't valid anymore
// subs ... implicit-def %nzcv <- MI, to be eliminated
// csinc ... implicit killed %nzcv
// Since we eliminated MI, and reused a register imp-def'd by CSMI
// (here %nzcv), that register, if it was killed before MI, should have
// that kill flag removed, because it's lifetime was extended.
if (CSMI->getParent() == MI->getParent()) {
for (MachineBasicBlock::iterator II = CSMI, IE = MI; II != IE; ++II)
for (auto ImplicitDef : ImplicitDefs)
if (MachineOperand *MO = II->findRegisterUseOperand(
ImplicitDef, /*isKill=*/true, TRI))
MO->setIsKill(false);
} else {
// If the instructions aren't in the same BB, bail out and clear the
// kill flag on all uses of the imp-def'd register.
for (auto ImplicitDef : ImplicitDefs)
MRI->clearKillFlags(ImplicitDef);
}
if (CrossMBBPhysDef) {
// Add physical register defs now coming in from a predecessor to MBB
// livein list.
while (!PhysDefs.empty()) {
auto LiveIn = PhysDefs.pop_back_val();
if (!MBB->isLiveIn(LiveIn.second))
MBB->addLiveIn(LiveIn.second);
}
++NumCrossBBCSEs;
}
MI->eraseFromParent();
++NumCSEs;
if (!PhysRefs.empty())
++NumPhysCSEs;
if (Commuted)
++NumCommutes;
Changed = true;
} else {
VNT.insert(MI, CurrVN++);
Exps.push_back(MI);
}
CSEPairs.clear();
ImplicitDefsToUpdate.clear();
ImplicitDefs.clear();
}
return Changed;
}
/// ExitScopeIfDone - Destroy scope for the MBB that corresponds to the given
/// dominator tree node if its a leaf or all of its children are done. Walk
/// up the dominator tree to destroy ancestors which are now done.
void
MachineCSE::ExitScopeIfDone(MachineDomTreeNode *Node,
DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren) {
if (OpenChildren[Node])
return;
// Pop scope.
ExitScope(Node->getBlock());
// Now traverse upwards to pop ancestors whose offsprings are all done.
while (MachineDomTreeNode *Parent = Node->getIDom()) {
unsigned Left = --OpenChildren[Parent];
if (Left != 0)
break;
ExitScope(Parent->getBlock());
Node = Parent;
}
}
bool MachineCSE::PerformCSE(MachineDomTreeNode *Node) {
SmallVector<MachineDomTreeNode*, 32> Scopes;
SmallVector<MachineDomTreeNode*, 8> WorkList;
DenseMap<MachineDomTreeNode*, unsigned> OpenChildren;
CurrVN = 0;
// Perform a DFS walk to determine the order of visit.
WorkList.push_back(Node);
do {
Node = WorkList.pop_back_val();
Scopes.push_back(Node);
const std::vector<MachineDomTreeNode*> &Children = Node->getChildren();
OpenChildren[Node] = Children.size();
for (MachineDomTreeNode *Child : Children)
WorkList.push_back(Child);
} while (!WorkList.empty());
// Now perform CSE.
bool Changed = false;
for (MachineDomTreeNode *Node : Scopes) {
MachineBasicBlock *MBB = Node->getBlock();
EnterScope(MBB);
Changed |= ProcessBlockCSE(MBB);
// If it's a leaf node, it's done. Traverse upwards to pop ancestors.
ExitScopeIfDone(Node, OpenChildren);
}
return Changed;
}
// We use stronger checks for PRE candidate rather than for CSE ones to embrace
// checks inside ProcessBlockCSE(), not only inside isCSECandidate(). This helps
// to exclude instrs created by PRE that won't be CSEed later.
bool MachineCSE::isPRECandidate(MachineInstr *MI) {
if (!isCSECandidate(MI) ||
MI->isNotDuplicable() ||
MI->mayLoad() ||
MI->isAsCheapAsAMove() ||
MI->getNumDefs() != 1 ||
MI->getNumExplicitDefs() != 1)
return false;
for (auto def : MI->defs())
if (!Register::isVirtualRegister(def.getReg()))
return false;
for (auto use : MI->uses())
if (use.isReg() && !Register::isVirtualRegister(use.getReg()))
return false;
return true;
}
bool MachineCSE::ProcessBlockPRE(MachineDominatorTree *DT,
MachineBasicBlock *MBB) {
bool Changed = false;
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E;) {
MachineInstr *MI = &*I;
++I;
if (!isPRECandidate(MI))
continue;
if (!PREMap.count(MI)) {
PREMap[MI] = MBB;
continue;
}
auto MBB1 = PREMap[MI];
assert(
!DT->properlyDominates(MBB, MBB1) &&
"MBB cannot properly dominate MBB1 while DFS through dominators tree!");
auto CMBB = DT->findNearestCommonDominator(MBB, MBB1);
if (!CMBB->isLegalToHoistInto())
continue;
if (!isProfitableToHoistInto(CMBB, MBB, MBB1))
continue;
// Two instrs are partial redundant if their basic blocks are reachable
// from one to another but one doesn't dominate another.
if (CMBB != MBB1) {
auto BB = MBB->getBasicBlock(), BB1 = MBB1->getBasicBlock();
if (BB != nullptr && BB1 != nullptr &&
(isPotentiallyReachable(BB1, BB) ||
isPotentiallyReachable(BB, BB1))) {
assert(MI->getOperand(0).isDef() &&
"First operand of instr with one explicit def must be this def");
Register VReg = MI->getOperand(0).getReg();
Register NewReg = MRI->cloneVirtualRegister(VReg);
if (!isProfitableToCSE(NewReg, VReg, CMBB, MI))
continue;
MachineInstr &NewMI =
TII->duplicate(*CMBB, CMBB->getFirstTerminator(), *MI);
NewMI.getOperand(0).setReg(NewReg);
PREMap[MI] = CMBB;
++NumPREs;
Changed = true;
}
}
}
return Changed;
}
// This simple PRE (partial redundancy elimination) pass doesn't actually
// eliminate partial redundancy but transforms it to full redundancy,
// anticipating that the next CSE step will eliminate this created redundancy.
// If CSE doesn't eliminate this, than created instruction will remain dead
// and eliminated later by Remove Dead Machine Instructions pass.
bool MachineCSE::PerformSimplePRE(MachineDominatorTree *DT) {
SmallVector<MachineDomTreeNode *, 32> BBs;
PREMap.clear();
bool Changed = false;
BBs.push_back(DT->getRootNode());
do {
auto Node = BBs.pop_back_val();
const std::vector<MachineDomTreeNode *> &Children = Node->getChildren();
for (MachineDomTreeNode *Child : Children)
BBs.push_back(Child);
MachineBasicBlock *MBB = Node->getBlock();
Changed |= ProcessBlockPRE(DT, MBB);
} while (!BBs.empty());
return Changed;
}
bool MachineCSE::isProfitableToHoistInto(MachineBasicBlock *CandidateBB,
MachineBasicBlock *MBB,
MachineBasicBlock *MBB1) {
if (CandidateBB->getParent()->getFunction().hasMinSize())
return true;
assert(DT->dominates(CandidateBB, MBB) && "CandidateBB should dominate MBB");
assert(DT->dominates(CandidateBB, MBB1) &&
"CandidateBB should dominate MBB1");
return MBFI->getBlockFreq(CandidateBB) <=
MBFI->getBlockFreq(MBB) + MBFI->getBlockFreq(MBB1);
}
bool MachineCSE::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
TII = MF.getSubtarget().getInstrInfo();
TRI = MF.getSubtarget().getRegisterInfo();
MRI = &MF.getRegInfo();
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
DT = &getAnalysis<MachineDominatorTree>();
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
LookAheadLimit = TII->getMachineCSELookAheadLimit();
bool ChangedPRE, ChangedCSE;
ChangedPRE = PerformSimplePRE(DT);
ChangedCSE = PerformCSE(DT->getRootNode());
return ChangedPRE || ChangedCSE;
}
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