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llvm-mirror/lib/CodeGen/MachineCSE.cpp
Owen Anderson 63f757463c Begin adding static dependence information to passes, which will allow us to 
perform initialization without static constructors AND without explicit initialization
by the client.  For the moment, passes are required to initialize both their
(potential) dependencies and any passes they preserve.  I hope to be able to relax
the latter requirement in the future.

llvm-svn: 116334
2010-10-12 19:48:12 +00:00

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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/DenseMap.h"
#include "llvm/ADT/ScopedHashTable.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
STATISTIC(NumCoalesces, "Number of copies coalesced");
STATISTIC(NumCSEs, "Number of common subexpression eliminated");
STATISTIC(NumPhysCSEs, "Number of phyreg defining common subexpr 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), LookAheadLimit(5), CurrVN(0) {}
virtual bool runOnMachineFunction(MachineFunction &MF);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
AU.addRequired<AliasAnalysis>();
AU.addPreservedID(MachineLoopInfoID);
AU.addRequired<MachineDominatorTree>();
AU.addPreserved<MachineDominatorTree>();
}
virtual void releaseMemory() {
ScopeMap.clear();
Exps.clear();
}
private:
const unsigned LookAheadLimit;
typedef ScopedHashTableScope<MachineInstr*, unsigned,
MachineInstrExpressionTrait> ScopeType;
DenseMap<MachineBasicBlock*, ScopeType*> ScopeMap;
ScopedHashTable<MachineInstr*, unsigned, MachineInstrExpressionTrait> VNT;
SmallVector<MachineInstr*, 64> Exps;
unsigned CurrVN;
bool PerformTrivialCoalescing(MachineInstr *MI, MachineBasicBlock *MBB);
bool isPhysDefTriviallyDead(unsigned Reg,
MachineBasicBlock::const_iterator I,
MachineBasicBlock::const_iterator E) const ;
bool hasLivePhysRegDefUse(const MachineInstr *MI,
const MachineBasicBlock *MBB,
unsigned &PhysDef) const;
bool PhysRegDefReaches(MachineInstr *CSMI, MachineInstr *MI,
unsigned PhysDef) const;
bool isCSECandidate(MachineInstr *MI);
bool isProfitableToCSE(unsigned CSReg, unsigned Reg,
MachineInstr *CSMI, MachineInstr *MI);
void EnterScope(MachineBasicBlock *MBB);
void ExitScope(MachineBasicBlock *MBB);
bool ProcessBlock(MachineBasicBlock *MBB);
void ExitScopeIfDone(MachineDomTreeNode *Node,
DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren,
DenseMap<MachineDomTreeNode*, MachineDomTreeNode*> &ParentMap);
bool PerformCSE(MachineDomTreeNode *Node);
};
} // end anonymous namespace
char MachineCSE::ID = 0;
INITIALIZE_PASS_BEGIN(MachineCSE, "machine-cse",
"Machine Common Subexpression Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(MachineCSE, "machine-cse",
"Machine Common Subexpression Elimination", false, false)
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->hasOneNonDBGUse(Reg))
// Only coalesce single use copies. This ensure the copy will be
// deleted.
continue;
MachineInstr *DefMI = MRI->getVRegDef(Reg);
if (DefMI->getParent() != MBB)
continue;
if (!DefMI->isCopy())
continue;
unsigned SrcReg = DefMI->getOperand(1).getReg();
if (!TargetRegisterInfo::isVirtualRegister(SrcReg))
continue;
if (DefMI->getOperand(0).getSubReg() || DefMI->getOperand(1).getSubReg())
continue;
if (!MRI->constrainRegClass(SrcReg, MRI->getRegClass(Reg)))
continue;
DEBUG(dbgs() << "Coalescing: " << *DefMI);
DEBUG(dbgs() << "*** to: " << *MI);
MO.setReg(SrcReg);
MRI->clearKillFlags(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.
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())
// 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;
}
/// hasLivePhysRegDefUse - 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::hasLivePhysRegDefUse(const MachineInstr *MI,
const MachineBasicBlock *MBB,
unsigned &PhysDef) const {
PhysDef = 0;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (!Reg)
continue;
if (TargetRegisterInfo::isVirtualRegister(Reg))
continue;
if (MO.isUse()) {
// Can't touch anything to read a physical register.
PhysDef = 0;
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.
PhysDef = 0;
return true;
}
PhysDef = Reg;
}
if (PhysDef) {
MachineBasicBlock::const_iterator I = MI; I = llvm::next(I);
if (!isPhysDefTriviallyDead(PhysDef, I, MBB->end()))
return true;
}
return false;
}
bool MachineCSE::PhysRegDefReaches(MachineInstr *CSMI, MachineInstr *MI,
unsigned PhysDef) const {
// For now conservatively returns false if the common subexpression is
// not in the same basic block as the given instruction.
MachineBasicBlock *MBB = MI->getParent();
if (CSMI->getParent() != MBB)
return false;
MachineBasicBlock::const_iterator I = CSMI; I = llvm::next(I);
MachineBasicBlock::const_iterator E = MI;
unsigned LookAheadLeft = LookAheadLimit;
while (LookAheadLeft) {
// Skip over dbg_value's.
while (I != E && I->isDebugValue())
++I;
if (I == E)
return true;
if (I->modifiesRegister(PhysDef, TRI))
return false;
--LookAheadLeft;
++I;
}
return false;
}
bool MachineCSE::isCSECandidate(MachineInstr *MI) {
if (MI->isLabel() || MI->isPHI() || MI->isImplicitDef() ||
MI->isKill() || MI->isInlineAsm() || MI->isDebugValue())
return false;
// Ignore copies.
if (MI->isCopyLike())
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 (!Use->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;
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());
}
void MachineCSE::EnterScope(MachineBasicBlock *MBB) {
DEBUG(dbgs() << "Entering: " << MBB->getName() << '\n');
ScopeType *Scope = new ScopeType(VNT);
ScopeMap[MBB] = Scope;
}
void MachineCSE::ExitScope(MachineBasicBlock *MBB) {
DEBUG(dbgs() << "Exiting: " << MBB->getName() << '\n');
DenseMap<MachineBasicBlock*, ScopeType*>::iterator SI = ScopeMap.find(MBB);
assert(SI != ScopeMap.end());
ScopeMap.erase(SI);
delete SI->second;
}
bool MachineCSE::ProcessBlock(MachineBasicBlock *MBB) {
bool Changed = false;
SmallVector<std::pair<unsigned, unsigned>, 8> CSEPairs;
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E; ) {
MachineInstr *MI = &*I;
++I;
if (!isCSECandidate(MI))
continue;
bool DefPhys = false;
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 (MI->isCopyLike())
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.
unsigned PhysDef = 0;
if (FoundCSE && hasLivePhysRegDefUse(MI, MBB, PhysDef)) {
FoundCSE = false;
// ... Unless the CS is local and it also defines the physical register
// which is not clobbered in between.
if (PhysDef) {
unsigned CSVN = VNT.lookup(MI);
MachineInstr *CSMI = Exps[CSVN];
if (PhysRegDefReaches(CSMI, MI, PhysDef)) {
FoundCSE = true;
DefPhys = 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];
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);
MRI->clearKillFlags(CSEPairs[i].second);
}
MI->eraseFromParent();
++NumCSEs;
if (DefPhys)
++NumPhysCSEs;
} else {
DEBUG(dbgs() << "*** Not profitable, avoid CSE!\n");
VNT.insert(MI, CurrVN++);
Exps.push_back(MI);
}
CSEPairs.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,
DenseMap<MachineDomTreeNode*, MachineDomTreeNode*> &ParentMap) {
if (OpenChildren[Node])
return;
// Pop scope.
ExitScope(Node->getBlock());
// Now traverse upwards to pop ancestors whose offsprings are all done.
while (MachineDomTreeNode *Parent = ParentMap[Node]) {
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*, MachineDomTreeNode*> ParentMap;
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();
unsigned NumChildren = Children.size();
OpenChildren[Node] = NumChildren;
for (unsigned i = 0; i != NumChildren; ++i) {
MachineDomTreeNode *Child = Children[i];
ParentMap[Child] = Node;
WorkList.push_back(Child);
}
} while (!WorkList.empty());
// Now perform CSE.
bool Changed = false;
for (unsigned i = 0, e = Scopes.size(); i != e; ++i) {
MachineDomTreeNode *Node = Scopes[i];
MachineBasicBlock *MBB = Node->getBlock();
EnterScope(MBB);
Changed |= ProcessBlock(MBB);
// If it's a leaf node, it's done. Traverse upwards to pop ancestors.
ExitScopeIfDone(Node, OpenChildren, ParentMap);
}
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 PerformCSE(DT->getRootNode());
}
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