605 lines
20 KiB
C++
605 lines
20 KiB
C++
//===-- MachineCSE.cpp - Machine Common Subexpression Elimination Pass ----===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass performs global common subexpression elimination on machine
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// instructions using a scoped hash table based value numbering scheme. It
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// must be run while the machine function is still in SSA form.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "machine-cse"
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/CodeGen/MachineDominators.h"
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#include "llvm/CodeGen/MachineInstr.h"
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#include "llvm/CodeGen/MachineRegisterInfo.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Target/TargetInstrInfo.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/ScopedHashTable.h"
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#include "llvm/ADT/SmallSet.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/RecyclingAllocator.h"
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using namespace llvm;
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STATISTIC(NumCoalesces, "Number of copies coalesced");
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STATISTIC(NumCSEs, "Number of common subexpression eliminated");
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STATISTIC(NumPhysCSEs,
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"Number of physreg referencing common subexpr eliminated");
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STATISTIC(NumCrossBBCSEs,
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"Number of cross-MBB physreg referencing CS eliminated");
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STATISTIC(NumCommutes, "Number of copies coalesced after commuting");
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namespace {
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class MachineCSE : public MachineFunctionPass {
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const TargetInstrInfo *TII;
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const TargetRegisterInfo *TRI;
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AliasAnalysis *AA;
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MachineDominatorTree *DT;
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MachineRegisterInfo *MRI;
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public:
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static char ID; // Pass identification
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MachineCSE() : MachineFunctionPass(ID), LookAheadLimit(5), CurrVN(0) {
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initializeMachineCSEPass(*PassRegistry::getPassRegistry());
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}
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virtual bool runOnMachineFunction(MachineFunction &MF);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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MachineFunctionPass::getAnalysisUsage(AU);
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AU.addRequired<AliasAnalysis>();
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AU.addPreservedID(MachineLoopInfoID);
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AU.addRequired<MachineDominatorTree>();
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AU.addPreserved<MachineDominatorTree>();
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}
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virtual void releaseMemory() {
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ScopeMap.clear();
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Exps.clear();
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AllocatableRegs.clear();
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ReservedRegs.clear();
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}
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private:
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const unsigned LookAheadLimit;
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typedef RecyclingAllocator<BumpPtrAllocator,
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ScopedHashTableVal<MachineInstr*, unsigned> > AllocatorTy;
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typedef ScopedHashTable<MachineInstr*, unsigned,
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MachineInstrExpressionTrait, AllocatorTy> ScopedHTType;
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typedef ScopedHTType::ScopeTy ScopeType;
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DenseMap<MachineBasicBlock*, ScopeType*> ScopeMap;
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ScopedHTType VNT;
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SmallVector<MachineInstr*, 64> Exps;
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unsigned CurrVN;
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BitVector AllocatableRegs;
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BitVector ReservedRegs;
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bool PerformTrivialCoalescing(MachineInstr *MI, MachineBasicBlock *MBB);
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bool isPhysDefTriviallyDead(unsigned Reg,
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MachineBasicBlock::const_iterator I,
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MachineBasicBlock::const_iterator E) const ;
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bool hasLivePhysRegDefUses(const MachineInstr *MI,
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const MachineBasicBlock *MBB,
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SmallSet<unsigned,8> &PhysRefs,
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SmallVector<unsigned,2> &PhysDefs) const;
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bool PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
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SmallSet<unsigned,8> &PhysRefs,
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SmallVector<unsigned,2> &PhysDefs,
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bool &NonLocal) const;
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bool isCSECandidate(MachineInstr *MI);
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bool isProfitableToCSE(unsigned CSReg, unsigned Reg,
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MachineInstr *CSMI, MachineInstr *MI);
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void EnterScope(MachineBasicBlock *MBB);
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void ExitScope(MachineBasicBlock *MBB);
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bool ProcessBlock(MachineBasicBlock *MBB);
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void ExitScopeIfDone(MachineDomTreeNode *Node,
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DenseMap<MachineDomTreeNode*, unsigned> &OpenChildren,
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DenseMap<MachineDomTreeNode*, MachineDomTreeNode*> &ParentMap);
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bool PerformCSE(MachineDomTreeNode *Node);
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};
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} // end anonymous namespace
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char MachineCSE::ID = 0;
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char &llvm::MachineCSEID = MachineCSE::ID;
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INITIALIZE_PASS_BEGIN(MachineCSE, "machine-cse",
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"Machine Common Subexpression Elimination", false, false)
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INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
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INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
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INITIALIZE_PASS_END(MachineCSE, "machine-cse",
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"Machine Common Subexpression Elimination", false, false)
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bool MachineCSE::PerformTrivialCoalescing(MachineInstr *MI,
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MachineBasicBlock *MBB) {
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bool Changed = false;
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for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
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MachineOperand &MO = MI->getOperand(i);
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if (!MO.isReg() || !MO.isUse())
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continue;
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unsigned Reg = MO.getReg();
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if (!TargetRegisterInfo::isVirtualRegister(Reg))
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continue;
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if (!MRI->hasOneNonDBGUse(Reg))
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// Only coalesce single use copies. This ensure the copy will be
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// deleted.
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continue;
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MachineInstr *DefMI = MRI->getVRegDef(Reg);
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if (DefMI->getParent() != MBB)
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continue;
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if (!DefMI->isCopy())
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continue;
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unsigned SrcReg = DefMI->getOperand(1).getReg();
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if (!TargetRegisterInfo::isVirtualRegister(SrcReg))
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continue;
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if (DefMI->getOperand(0).getSubReg() || DefMI->getOperand(1).getSubReg())
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continue;
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if (!MRI->constrainRegClass(SrcReg, MRI->getRegClass(Reg)))
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continue;
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DEBUG(dbgs() << "Coalescing: " << *DefMI);
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DEBUG(dbgs() << "*** to: " << *MI);
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MO.setReg(SrcReg);
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MRI->clearKillFlags(SrcReg);
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DefMI->eraseFromParent();
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++NumCoalesces;
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Changed = true;
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}
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return Changed;
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}
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bool
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MachineCSE::isPhysDefTriviallyDead(unsigned Reg,
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MachineBasicBlock::const_iterator I,
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MachineBasicBlock::const_iterator E) const {
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unsigned LookAheadLeft = LookAheadLimit;
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while (LookAheadLeft) {
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// Skip over dbg_value's.
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while (I != E && I->isDebugValue())
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++I;
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if (I == E)
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// Reached end of block, register is obviously dead.
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return true;
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bool SeenDef = false;
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
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const MachineOperand &MO = I->getOperand(i);
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if (MO.isRegMask() && MO.clobbersPhysReg(Reg))
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SeenDef = true;
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if (!MO.isReg() || !MO.getReg())
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continue;
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if (!TRI->regsOverlap(MO.getReg(), Reg))
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continue;
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if (MO.isUse())
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// Found a use!
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return false;
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SeenDef = true;
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}
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if (SeenDef)
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// See a def of Reg (or an alias) before encountering any use, it's
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// trivially dead.
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return true;
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--LookAheadLeft;
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++I;
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}
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return false;
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}
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/// hasLivePhysRegDefUses - Return true if the specified instruction read/write
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/// physical registers (except for dead defs of physical registers). It also
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/// returns the physical register def by reference if it's the only one and the
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/// instruction does not uses a physical register.
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bool MachineCSE::hasLivePhysRegDefUses(const MachineInstr *MI,
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const MachineBasicBlock *MBB,
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SmallSet<unsigned,8> &PhysRefs,
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SmallVector<unsigned,2> &PhysDefs) const{
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MachineBasicBlock::const_iterator I = MI; I = llvm::next(I);
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for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
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const MachineOperand &MO = MI->getOperand(i);
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if (!MO.isReg())
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continue;
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unsigned Reg = MO.getReg();
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if (!Reg)
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continue;
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if (TargetRegisterInfo::isVirtualRegister(Reg))
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continue;
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// If the def is dead, it's ok. But the def may not marked "dead". That's
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// common since this pass is run before livevariables. We can scan
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// forward a few instructions and check if it is obviously dead.
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if (MO.isDef() &&
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(MO.isDead() || isPhysDefTriviallyDead(Reg, I, MBB->end())))
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continue;
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PhysRefs.insert(Reg);
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if (MO.isDef())
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PhysDefs.push_back(Reg);
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for (const uint16_t *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias)
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PhysRefs.insert(*Alias);
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}
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return !PhysRefs.empty();
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}
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bool MachineCSE::PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
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SmallSet<unsigned,8> &PhysRefs,
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SmallVector<unsigned,2> &PhysDefs,
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bool &NonLocal) const {
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// For now conservatively returns false if the common subexpression is
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// not in the same basic block as the given instruction. The only exception
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// is if the common subexpression is in the sole predecessor block.
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const MachineBasicBlock *MBB = MI->getParent();
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const MachineBasicBlock *CSMBB = CSMI->getParent();
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bool CrossMBB = false;
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if (CSMBB != MBB) {
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if (MBB->pred_size() != 1 || *MBB->pred_begin() != CSMBB)
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return false;
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for (unsigned i = 0, e = PhysDefs.size(); i != e; ++i) {
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if (AllocatableRegs.test(PhysDefs[i]) || ReservedRegs.test(PhysDefs[i]))
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// Avoid extending live range of physical registers if they are
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//allocatable or reserved.
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return false;
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}
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CrossMBB = true;
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}
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MachineBasicBlock::const_iterator I = CSMI; I = llvm::next(I);
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MachineBasicBlock::const_iterator E = MI;
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MachineBasicBlock::const_iterator EE = CSMBB->end();
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unsigned LookAheadLeft = LookAheadLimit;
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while (LookAheadLeft) {
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// Skip over dbg_value's.
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while (I != E && I != EE && I->isDebugValue())
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++I;
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if (I == EE) {
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assert(CrossMBB && "Reaching end-of-MBB without finding MI?");
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(void)CrossMBB;
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CrossMBB = false;
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NonLocal = true;
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I = MBB->begin();
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EE = MBB->end();
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continue;
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}
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if (I == E)
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return true;
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
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const MachineOperand &MO = I->getOperand(i);
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// RegMasks go on instructions like calls that clobber lots of physregs.
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// Don't attempt to CSE across such an instruction.
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if (MO.isRegMask())
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return false;
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if (!MO.isReg() || !MO.isDef())
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continue;
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unsigned MOReg = MO.getReg();
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if (TargetRegisterInfo::isVirtualRegister(MOReg))
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continue;
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if (PhysRefs.count(MOReg))
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return false;
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}
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--LookAheadLeft;
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++I;
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}
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return false;
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}
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bool MachineCSE::isCSECandidate(MachineInstr *MI) {
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if (MI->isLabel() || MI->isPHI() || MI->isImplicitDef() ||
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MI->isKill() || MI->isInlineAsm() || MI->isDebugValue())
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return false;
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// Ignore copies.
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if (MI->isCopyLike())
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return false;
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// Ignore stuff that we obviously can't move.
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if (MI->mayStore() || MI->isCall() || MI->isTerminator() ||
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MI->hasUnmodeledSideEffects())
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return false;
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if (MI->mayLoad()) {
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// Okay, this instruction does a load. As a refinement, we allow the target
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// to decide whether the loaded value is actually a constant. If so, we can
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// actually use it as a load.
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if (!MI->isInvariantLoad(AA))
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// FIXME: we should be able to hoist loads with no other side effects if
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// there are no other instructions which can change memory in this loop.
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// This is a trivial form of alias analysis.
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return false;
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}
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return true;
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}
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/// isProfitableToCSE - Return true if it's profitable to eliminate MI with a
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/// common expression that defines Reg.
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bool MachineCSE::isProfitableToCSE(unsigned CSReg, unsigned Reg,
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MachineInstr *CSMI, MachineInstr *MI) {
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// FIXME: Heuristics that works around the lack the live range splitting.
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// Heuristics #1: Don't CSE "cheap" computation if the def is not local or in
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// an immediate predecessor. We don't want to increase register pressure and
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// end up causing other computation to be spilled.
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if (MI->isAsCheapAsAMove()) {
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MachineBasicBlock *CSBB = CSMI->getParent();
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MachineBasicBlock *BB = MI->getParent();
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if (CSBB != BB && !CSBB->isSuccessor(BB))
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return false;
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}
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// Heuristics #2: If the expression doesn't not use a vr and the only use
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// of the redundant computation are copies, do not cse.
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bool HasVRegUse = false;
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for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
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const MachineOperand &MO = MI->getOperand(i);
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if (MO.isReg() && MO.isUse() &&
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TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
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HasVRegUse = true;
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break;
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}
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}
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if (!HasVRegUse) {
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bool HasNonCopyUse = false;
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for (MachineRegisterInfo::use_nodbg_iterator I = MRI->use_nodbg_begin(Reg),
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E = MRI->use_nodbg_end(); I != E; ++I) {
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MachineInstr *Use = &*I;
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// Ignore copies.
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if (!Use->isCopyLike()) {
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HasNonCopyUse = true;
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break;
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}
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}
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if (!HasNonCopyUse)
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return false;
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}
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// Heuristics #3: If the common subexpression is used by PHIs, do not reuse
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// it unless the defined value is already used in the BB of the new use.
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bool HasPHI = false;
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SmallPtrSet<MachineBasicBlock*, 4> CSBBs;
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for (MachineRegisterInfo::use_nodbg_iterator I = MRI->use_nodbg_begin(CSReg),
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E = MRI->use_nodbg_end(); I != E; ++I) {
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MachineInstr *Use = &*I;
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HasPHI |= Use->isPHI();
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CSBBs.insert(Use->getParent());
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}
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if (!HasPHI)
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return true;
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return CSBBs.count(MI->getParent());
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}
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void MachineCSE::EnterScope(MachineBasicBlock *MBB) {
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DEBUG(dbgs() << "Entering: " << MBB->getName() << '\n');
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ScopeType *Scope = new ScopeType(VNT);
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ScopeMap[MBB] = Scope;
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}
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void MachineCSE::ExitScope(MachineBasicBlock *MBB) {
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DEBUG(dbgs() << "Exiting: " << MBB->getName() << '\n');
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DenseMap<MachineBasicBlock*, ScopeType*>::iterator SI = ScopeMap.find(MBB);
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assert(SI != ScopeMap.end());
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ScopeMap.erase(SI);
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delete SI->second;
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}
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bool MachineCSE::ProcessBlock(MachineBasicBlock *MBB) {
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bool Changed = false;
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SmallVector<std::pair<unsigned, unsigned>, 8> CSEPairs;
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for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E; ) {
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MachineInstr *MI = &*I;
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++I;
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if (!isCSECandidate(MI))
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continue;
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bool FoundCSE = VNT.count(MI);
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if (!FoundCSE) {
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// Look for trivial copy coalescing opportunities.
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if (PerformTrivialCoalescing(MI, MBB)) {
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Changed = true;
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// After coalescing MI itself may become a copy.
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if (MI->isCopyLike())
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continue;
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FoundCSE = VNT.count(MI);
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}
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}
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// Commute commutable instructions.
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bool Commuted = false;
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if (!FoundCSE && MI->isCommutable()) {
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MachineInstr *NewMI = TII->commuteInstruction(MI);
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if (NewMI) {
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Commuted = true;
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FoundCSE = VNT.count(NewMI);
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if (NewMI != MI) {
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// New instruction. It doesn't need to be kept.
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NewMI->eraseFromParent();
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Changed = true;
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} else if (!FoundCSE)
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// MI was changed but it didn't help, commute it back!
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(void)TII->commuteInstruction(MI);
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}
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}
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// If the instruction defines physical registers and the values *may* be
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// used, then it's not safe to replace it with a common subexpression.
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// It's also not safe if the instruction uses physical registers.
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bool CrossMBBPhysDef = false;
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SmallSet<unsigned,8> PhysRefs;
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SmallVector<unsigned, 2> PhysDefs;
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if (FoundCSE && hasLivePhysRegDefUses(MI, MBB, PhysRefs, PhysDefs)) {
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FoundCSE = false;
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// ... Unless the CS is local or is in the sole predecessor block
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// and it also defines the physical register which is not clobbered
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// in between and the physical register uses were not clobbered.
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unsigned CSVN = VNT.lookup(MI);
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MachineInstr *CSMI = Exps[CSVN];
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if (PhysRegDefsReach(CSMI, MI, PhysRefs, PhysDefs, CrossMBBPhysDef))
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FoundCSE = true;
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}
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if (!FoundCSE) {
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VNT.insert(MI, CurrVN++);
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Exps.push_back(MI);
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continue;
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}
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// Found a common subexpression, eliminate it.
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unsigned CSVN = VNT.lookup(MI);
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MachineInstr *CSMI = Exps[CSVN];
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DEBUG(dbgs() << "Examining: " << *MI);
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DEBUG(dbgs() << "*** Found a common subexpression: " << *CSMI);
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// Check if it's profitable to perform this CSE.
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bool DoCSE = true;
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unsigned NumDefs = MI->getDesc().getNumDefs();
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for (unsigned i = 0, e = MI->getNumOperands(); NumDefs && i != e; ++i) {
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MachineOperand &MO = MI->getOperand(i);
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if (!MO.isReg() || !MO.isDef())
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continue;
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unsigned OldReg = MO.getReg();
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unsigned NewReg = CSMI->getOperand(i).getReg();
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if (OldReg == NewReg)
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continue;
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assert(TargetRegisterInfo::isVirtualRegister(OldReg) &&
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TargetRegisterInfo::isVirtualRegister(NewReg) &&
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"Do not CSE physical register defs!");
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if (!isProfitableToCSE(NewReg, OldReg, CSMI, MI)) {
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DoCSE = false;
|
|
break;
|
|
}
|
|
|
|
// Don't perform CSE if the result of the old instruction cannot exist
|
|
// within the register class of the new instruction.
|
|
const TargetRegisterClass *OldRC = MRI->getRegClass(OldReg);
|
|
if (!MRI->constrainRegClass(NewReg, OldRC)) {
|
|
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);
|
|
}
|
|
|
|
if (CrossMBBPhysDef) {
|
|
// Add physical register defs now coming in from a predecessor to MBB
|
|
// livein list.
|
|
while (!PhysDefs.empty()) {
|
|
unsigned LiveIn = PhysDefs.pop_back_val();
|
|
if (!MBB->isLiveIn(LiveIn))
|
|
MBB->addLiveIn(LiveIn);
|
|
}
|
|
++NumCrossBBCSEs;
|
|
}
|
|
|
|
MI->eraseFromParent();
|
|
++NumCSEs;
|
|
if (!PhysRefs.empty())
|
|
++NumPhysCSEs;
|
|
if (Commuted)
|
|
++NumCommutes;
|
|
Changed = true;
|
|
} 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>();
|
|
AllocatableRegs = TRI->getAllocatableSet(MF);
|
|
ReservedRegs = TRI->getReservedRegs(MF);
|
|
return PerformCSE(DT->getRootNode());
|
|
}
|