#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");
-
-namespace llvm {
- template<> struct DenseMapInfo<MachineInstr*> {
- static inline MachineInstr *getEmptyKey() {
- return 0;
- }
-
- static inline MachineInstr *getTombstoneKey() {
- return reinterpret_cast<MachineInstr*>(-1);
- }
-
- static unsigned getHashValue(const MachineInstr* const &MI) {
- unsigned Hash = MI->getOpcode() * 37;
- for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
- const MachineOperand &MO = MI->getOperand(i);
- uint64_t Key = (uint64_t)MO.getType() << 32;
- switch (MO.getType()) {
- default: break;
- case MachineOperand::MO_Register:
- if (MO.isDef() && TargetRegisterInfo::isVirtualRegister(MO.getReg()))
- continue; // Skip virtual register defs.
- Key |= MO.getReg();
- break;
- case MachineOperand::MO_Immediate:
- Key |= MO.getImm();
- break;
- case MachineOperand::MO_FrameIndex:
- case MachineOperand::MO_ConstantPoolIndex:
- case MachineOperand::MO_JumpTableIndex:
- Key |= MO.getIndex();
- break;
- case MachineOperand::MO_MachineBasicBlock:
- Key |= DenseMapInfo<void*>::getHashValue(MO.getMBB());
- break;
- case MachineOperand::MO_GlobalAddress:
- Key |= DenseMapInfo<void*>::getHashValue(MO.getGlobal());
- break;
- case MachineOperand::MO_BlockAddress:
- Key |= DenseMapInfo<void*>::getHashValue(MO.getBlockAddress());
- break;
- }
- Key += ~(Key << 32);
- Key ^= (Key >> 22);
- Key += ~(Key << 13);
- Key ^= (Key >> 8);
- Key += (Key << 3);
- Key ^= (Key >> 15);
- Key += ~(Key << 27);
- Key ^= (Key >> 31);
- Hash = (unsigned)Key + Hash * 37;
- }
- return Hash;
- }
-
- static bool isEqual(const MachineInstr* const &LHS,
- const MachineInstr* const &RHS) {
- if (RHS == getEmptyKey() || RHS == getTombstoneKey() ||
- LHS == getEmptyKey() || LHS == getTombstoneKey())
- return LHS == RHS;
- return LHS->isIdenticalTo(RHS, MachineInstr::IgnoreVRegDefs);
- }
- };
-} // end llvm namespace
+STATISTIC(NumPhysCSEs, "Number of phyreg defining common subexpr eliminated");
namespace {
class MachineCSE : public MachineFunctionPass {
const TargetInstrInfo *TII;
- MachineRegisterInfo *MRI;
+ const TargetRegisterInfo *TRI;
+ AliasAnalysis *AA;
MachineDominatorTree *DT;
+ MachineRegisterInfo *MRI;
public:
static char ID; // Pass identification
- MachineCSE() : MachineFunctionPass(&ID), CurrVN(0) {}
+ MachineCSE() : MachineFunctionPass(ID), LookAheadLimit(5), CurrVN(0) {
+ initializeMachineCSEPass(*PassRegistry::getPassRegistry());
+ }
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:
- unsigned CurrVN;
- ScopedHashTable<MachineInstr*, unsigned> VNT;
+ 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 ProcessBlock(MachineDomTreeNode *Node);
+ 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;
-static RegisterPass<MachineCSE>
-X("machine-cse", "Machine Common Subexpression Elimination");
+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(); }
unsigned Reg = MO.getReg();
if (!Reg || TargetRegisterInfo::isPhysicalRegister(Reg))
continue;
- if (!MRI->hasOneUse(Reg))
+ 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;
- unsigned SrcReg, DstReg, SrcSubIdx, DstSubIdx;
- if (TII->isMoveInstr(*DefMI, SrcReg, DstReg, SrcSubIdx, DstSubIdx) &&
- TargetRegisterInfo::isVirtualRegister(SrcReg) &&
- !SrcSubIdx && !DstSubIdx) {
- MO.setReg(SrcReg);
- DefMI->eraseFromParent();
- ++NumCoalesces;
- Changed = true;
- }
+ 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;
}
-static bool hasLivePhysRegDefUse(MachineInstr *MI) {
+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) {
- MachineOperand &MO = MI->getOperand(i);
+ const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (!Reg)
continue;
- // FIXME: This is obviously overly conservative. On x86 lots of instructions
- // will def EFLAGS and they are not marked dead at this point.
- if (TargetRegisterInfo::isPhysicalRegister(Reg) &&
- !(MO.isDef() && MO.isDead()))
+ 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::ProcessBlock(MachineDomTreeNode *Node) {
+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;
- ScopedHashTableScope<MachineInstr*, unsigned> VNTS(VNT);
- MachineBasicBlock *MBB = Node->getBlock();
+ SmallVector<std::pair<unsigned, unsigned>, 8> CSEPairs;
for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E; ) {
MachineInstr *MI = &*I;
++I;
- bool SawStore = false;
- if (!MI->isSafeToMove(TII, 0, SawStore))
- continue;
- // Ignore copies or instructions that read / write physical registers
- // (except for dead defs of physical registers).
- unsigned SrcReg, DstReg, SrcSubIdx, DstSubIdx;
- if (TII->isMoveInstr(*MI, SrcReg, DstReg, SrcSubIdx, DstSubIdx) ||
- MI->isExtractSubreg() || MI->isInsertSubreg() || MI->isSubregToReg())
- continue;
- if (hasLivePhysRegDefUse(MI))
+
+ if (!isCSECandidate(MI))
continue;
+ bool DefPhys = false;
bool FoundCSE = VNT.count(MI);
if (!FoundCSE) {
// Look for trivial copy coalescing opportunities.
- if (PerformTrivialCoalescing(MI, MBB))
+ 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) {
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);
continue;
unsigned OldReg = MO.getReg();
unsigned NewReg = CSMI->getOperand(i).getReg();
- assert(OldReg != NewReg &&
- TargetRegisterInfo::isVirtualRegister(OldReg) &&
+ if (OldReg == NewReg)
+ continue;
+ assert(TargetRegisterInfo::isVirtualRegister(OldReg) &&
TargetRegisterInfo::isVirtualRegister(NewReg) &&
"Do not CSE physical register defs!");
- MRI->replaceRegWith(OldReg, NewReg);
+ if (!isProfitableToCSE(NewReg, OldReg, CSMI, MI)) {
+ DoCSE = false;
+ break;
+ }
+ CSEPairs.push_back(std::make_pair(OldReg, NewReg));
--NumDefs;
}
- MI->eraseFromParent();
- ++NumCSEs;
+
+ // 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;
}
+}
- // Recursively call ProcessBlock with childred.
- const std::vector<MachineDomTreeNode*> &Children = Node->getChildren();
- for (unsigned i = 0, e = Children.size(); i != e; ++i)
- Changed |= ProcessBlock(Children[i]);
+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 ProcessBlock(DT->getRootNode());
+ return PerformCSE(DT->getRootNode());
}
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