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llvm-project/llvm/lib/CodeGen/MachineCSE.cpp

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//===-- MachineCSE.cpp - Machine Common Subexpression Elimination Pass ----===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "machine-cse"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/ADT/ScopedHashTable.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
STATISTIC(NumCoalesces, "Number of copies coalesced");
STATISTIC(NumCSEs, "Number of common subexpression eliminated");
namespace {
class MachineCSE : public MachineFunctionPass {
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
AliasAnalysis *AA;
MachineDominatorTree *DT;
MachineRegisterInfo *MRI;
public:
static char ID; // Pass identification
MachineCSE() : MachineFunctionPass(&ID), CurrVN(0) {}
virtual bool runOnMachineFunction(MachineFunction &MF);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
AU.addRequired<AliasAnalysis>();
AU.addRequired<MachineDominatorTree>();
AU.addPreserved<MachineDominatorTree>();
}
private:
unsigned CurrVN;
ScopedHashTable<MachineInstr*, unsigned, MachineInstrExpressionTrait> VNT;
SmallVector<MachineInstr*, 64> Exps;
bool PerformTrivialCoalescing(MachineInstr *MI, MachineBasicBlock *MBB);
bool isPhysDefTriviallyDead(unsigned Reg,
MachineBasicBlock::const_iterator I,
MachineBasicBlock::const_iterator E);
bool hasLivePhysRegDefUse(MachineInstr *MI, MachineBasicBlock *MBB);
bool isCSECandidate(MachineInstr *MI);
bool isProfitableToCSE(unsigned CSReg, unsigned Reg,
MachineInstr *CSMI, MachineInstr *MI);
bool ProcessBlock(MachineDomTreeNode *Node);
};
} // end anonymous namespace
char MachineCSE::ID = 0;
static RegisterPass<MachineCSE>
X("machine-cse", "Machine Common Subexpression Elimination");
FunctionPass *llvm::createMachineCSEPass() { return new MachineCSE(); }
bool MachineCSE::PerformTrivialCoalescing(MachineInstr *MI,
MachineBasicBlock *MBB) {
bool Changed = false;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || !MO.isUse())
continue;
unsigned Reg = MO.getReg();
if (!Reg || TargetRegisterInfo::isPhysicalRegister(Reg))
continue;
if (!MRI->hasOneUse(Reg))
// Only coalesce single use copies. This ensure the copy will be
// deleted.
continue;
MachineInstr *DefMI = MRI->getVRegDef(Reg);
if (DefMI->getParent() != MBB)
continue;
unsigned SrcReg, DstReg, SrcSubIdx, DstSubIdx;
if (TII->isMoveInstr(*DefMI, SrcReg, DstReg, SrcSubIdx, DstSubIdx) &&
TargetRegisterInfo::isVirtualRegister(SrcReg) &&
!SrcSubIdx && !DstSubIdx) {
const TargetRegisterClass *SRC = MRI->getRegClass(SrcReg);
const TargetRegisterClass *RC = MRI->getRegClass(Reg);
const TargetRegisterClass *NewRC = getCommonSubClass(RC, SRC);
if (!NewRC)
continue;
DEBUG(dbgs() << "Coalescing: " << *DefMI);
DEBUG(dbgs() << "*** to: " << *MI);
MO.setReg(SrcReg);
if (NewRC != SRC)
MRI->setRegClass(SrcReg, NewRC);
DefMI->eraseFromParent();
++NumCoalesces;
Changed = true;
}
}
return Changed;
}
bool MachineCSE::isPhysDefTriviallyDead(unsigned Reg,
MachineBasicBlock::const_iterator I,
MachineBasicBlock::const_iterator E) {
unsigned LookAheadLeft = 5;
while (LookAheadLeft) {
// Skip over dbg_value's.
while (I != E && I->isDebugValue())
++I;
if (I == E)
// Reached end of block, register is obviously dead.
return true;
bool SeenDef = false;
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = I->getOperand(i);
if (!MO.isReg() || !MO.getReg())
continue;
if (!TRI->regsOverlap(MO.getReg(), Reg))
continue;
if (MO.isUse())
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;
}
/// hasLivePhysRegDefUse - Return true if the specified instruction read / write
/// physical registers (except for dead defs of physical registers).
bool MachineCSE::hasLivePhysRegDefUse(MachineInstr *MI, MachineBasicBlock *MBB){
unsigned PhysDef = 0;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (!Reg)
continue;
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
if (MO.isUse())
// Can't touch anything to read a physical register.
return true;
if (MO.isDead())
// If the def is dead, it's ok.
continue;
// Ok, this is a physical register def that's 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 (PhysDef)
// Multiple physical register defs. These are rare, forget about it.
return true;
PhysDef = Reg;
}
}
if (PhysDef) {
MachineBasicBlock::iterator I = MI; I = llvm::next(I);
if (!isPhysDefTriviallyDead(PhysDef, I, MBB->end()))
return true;
}
return false;
}
static bool isCopy(const MachineInstr *MI, const TargetInstrInfo *TII) {
unsigned SrcReg, DstReg, SrcSubIdx, DstSubIdx;
return TII->isMoveInstr(*MI, SrcReg, DstReg, SrcSubIdx, DstSubIdx) ||
MI->isExtractSubreg() || MI->isInsertSubreg() || MI->isSubregToReg();
}
bool MachineCSE::isCSECandidate(MachineInstr *MI) {
if (MI->isLabel() || MI->isPHI() || MI->isImplicitDef() ||
MI->isKill() || MI->isInlineAsm() || MI->isDebugValue())
return false;
// Ignore copies.
if (isCopy(MI, TII))
return false;
// Ignore stuff that we obviously can't move.
const TargetInstrDesc &TID = MI->getDesc();
if (TID.mayStore() || TID.isCall() || TID.isTerminator() ||
TID.hasUnmodeledSideEffects())
return false;
if (TID.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->isInvariantLoad(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;
}
return true;
}
/// isProfitableToCSE - Return true if it's profitable to eliminate MI with a
/// common expression that defines Reg.
bool MachineCSE::isProfitableToCSE(unsigned CSReg, unsigned Reg,
MachineInstr *CSMI, MachineInstr *MI) {
// FIXME: Heuristics that works around the lack the live range splitting.
// Heuristics #1: Don't cse "cheap" computating 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 (MI->getDesc().isAsCheapAsAMove()) {
MachineBasicBlock *CSBB = CSMI->getParent();
MachineBasicBlock *BB = MI->getParent();
if (CSBB != BB &&
find(CSBB->succ_begin(), CSBB->succ_end(), BB) == CSBB->succ_end())
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 (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (MO.isReg() && MO.isUse() && MO.getReg() &&
TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
HasVRegUse = true;
break;
}
}
if (!HasVRegUse) {
bool HasNonCopyUse = false;
for (MachineRegisterInfo::use_nodbg_iterator I = MRI->use_nodbg_begin(Reg),
E = MRI->use_nodbg_end(); I != E; ++I) {
MachineInstr *Use = &*I;
// Ignore copies.
if (!isCopy(Use, TII)) {
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;
SmallPtrSet<MachineBasicBlock*, 4> CSBBs;
for (MachineRegisterInfo::use_nodbg_iterator I = MRI->use_nodbg_begin(CSReg),
E = MRI->use_nodbg_end(); I != E; ++I) {
MachineInstr *Use = &*I;
HasPHI |= Use->isPHI();
CSBBs.insert(Use->getParent());
}
if (!HasPHI)
return true;
return CSBBs.count(MI->getParent());
}
bool MachineCSE::ProcessBlock(MachineDomTreeNode *Node) {
bool Changed = false;
SmallVector<std::pair<unsigned, unsigned>, 8> CSEPairs;
ScopedHashTableScope<MachineInstr*, unsigned,
MachineInstrExpressionTrait> VNTS(VNT);
MachineBasicBlock *MBB = Node->getBlock();
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) {
// Look for trivial copy coalescing opportunities.
if (PerformTrivialCoalescing(MI, MBB)) {
// After coalescing MI itself may become a copy.
if (isCopy(MI, TII))
continue;
FoundCSE = VNT.count(MI);
}
}
// FIXME: commute commutable instructions?
// If the instruction defines a physical register and the value *may* be
// used, then it's not safe to replace it with a common subexpression.
if (FoundCSE && hasLivePhysRegDefUse(MI, MBB))
FoundCSE = false;
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];
DEBUG(dbgs() << "Examining: " << *MI);
DEBUG(dbgs() << "*** Found a common subexpression: " << *CSMI);
// Check if it's profitable to perform this CSE.
bool DoCSE = true;
unsigned NumDefs = MI->getDesc().getNumDefs();
for (unsigned i = 0, e = MI->getNumOperands(); NumDefs && i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg() || !MO.isDef())
continue;
unsigned OldReg = MO.getReg();
unsigned NewReg = CSMI->getOperand(i).getReg();
if (OldReg == NewReg)
continue;
assert(TargetRegisterInfo::isVirtualRegister(OldReg) &&
TargetRegisterInfo::isVirtualRegister(NewReg) &&
"Do not CSE physical register defs!");
if (!isProfitableToCSE(NewReg, OldReg, CSMI, MI)) {
DoCSE = false;
break;
}
CSEPairs.push_back(std::make_pair(OldReg, NewReg));
--NumDefs;
}
// Actually perform the elimination.
if (DoCSE) {
for (unsigned i = 0, e = CSEPairs.size(); i != e; ++i)
MRI->replaceRegWith(CSEPairs[i].first, CSEPairs[i].second);
MI->eraseFromParent();
++NumCSEs;
} else {
DEBUG(dbgs() << "*** Not profitable, avoid CSE!\n");
VNT.insert(MI, CurrVN++);
Exps.push_back(MI);
}
CSEPairs.clear();
}
// Recursively call ProcessBlock with children.
const std::vector<MachineDomTreeNode*> &Children = Node->getChildren();
for (unsigned i = 0, e = Children.size(); i != e; ++i)
Changed |= ProcessBlock(Children[i]);
return Changed;
}
bool MachineCSE::runOnMachineFunction(MachineFunction &MF) {
TII = MF.getTarget().getInstrInfo();
TRI = MF.getTarget().getRegisterInfo();
MRI = &MF.getRegInfo();
AA = &getAnalysis<AliasAnalysis>();
DT = &getAnalysis<MachineDominatorTree>();
return ProcessBlock(DT->getRootNode());
}
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