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Diffstat (limited to 'contrib/llvm/lib/CodeGen/MachineCSE.cpp')
| -rw-r--r-- | contrib/llvm/lib/CodeGen/MachineCSE.cpp | 508 |
1 files changed, 508 insertions, 0 deletions
diff --git a/contrib/llvm/lib/CodeGen/MachineCSE.cpp b/contrib/llvm/lib/CodeGen/MachineCSE.cpp new file mode 100644 index 000000000000..92e2299ec62f --- /dev/null +++ b/contrib/llvm/lib/CodeGen/MachineCSE.cpp @@ -0,0 +1,508 @@ +//===-- 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>(); + } + + 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(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->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; + 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; + 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); + MRI->clearKillFlags(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) 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; + + // 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()); +} |