X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FCodeGen%2FEarlyIfConversion.cpp;h=995606f65d77a0961152f9f59fd00cdc7e52e3ac;hb=71649b0d0dbb7b519142fb1a3317c48b43dd0933;hp=0db442a3bc6d4580780b6504b26a9ab0198bece2;hpb=0fac6aa076450f5474feb2ec697b7d63d33fa567;p=oota-llvm.git diff --git a/lib/CodeGen/EarlyIfConversion.cpp b/lib/CodeGen/EarlyIfConversion.cpp index 0db442a3bc6..995606f65d7 100644 --- a/lib/CodeGen/EarlyIfConversion.cpp +++ b/lib/CodeGen/EarlyIfConversion.cpp @@ -16,30 +16,31 @@ // //===----------------------------------------------------------------------===// -#define DEBUG_TYPE "early-ifcvt" -#include "MachineTraceMetrics.h" -#include "llvm/Function.h" #include "llvm/ADT/BitVector.h" #include "llvm/ADT/PostOrderIterator.h" #include "llvm/ADT/SetVector.h" #include "llvm/ADT/SmallPtrSet.h" #include "llvm/ADT/SparseSet.h" +#include "llvm/ADT/Statistic.h" #include "llvm/CodeGen/MachineBranchProbabilityInfo.h" #include "llvm/CodeGen/MachineDominators.h" #include "llvm/CodeGen/MachineFunction.h" #include "llvm/CodeGen/MachineFunctionPass.h" #include "llvm/CodeGen/MachineLoopInfo.h" #include "llvm/CodeGen/MachineRegisterInfo.h" +#include "llvm/CodeGen/MachineTraceMetrics.h" #include "llvm/CodeGen/Passes.h" -#include "llvm/MC/MCInstrItineraries.h" -#include "llvm/Target/TargetInstrInfo.h" -#include "llvm/Target/TargetRegisterInfo.h" #include "llvm/Support/CommandLine.h" #include "llvm/Support/Debug.h" #include "llvm/Support/raw_ostream.h" +#include "llvm/Target/TargetInstrInfo.h" +#include "llvm/Target/TargetRegisterInfo.h" +#include "llvm/Target/TargetSubtargetInfo.h" using namespace llvm; +#define DEBUG_TYPE "early-ifcvt" + // Absolute maximum number of instructions allowed per speculated block. // This bypasses all other heuristics, so it should be set fairly high. static cl::opt @@ -50,7 +51,10 @@ BlockInstrLimit("early-ifcvt-limit", cl::init(30), cl::Hidden, static cl::opt Stress("stress-early-ifcvt", cl::Hidden, cl::desc("Turn all knobs to 11")); -typedef SmallSetVector BlockSetVector; +STATISTIC(NumDiamondsSeen, "Number of diamonds"); +STATISTIC(NumDiamondsConv, "Number of diamonds converted"); +STATISTIC(NumTrianglesSeen, "Number of triangles"); +STATISTIC(NumTrianglesConv, "Number of triangles converted"); //===----------------------------------------------------------------------===// // SSAIfConv @@ -96,6 +100,12 @@ public: /// equal to Tail. bool isTriangle() const { return TBB == Tail || FBB == Tail; } + /// Returns the Tail predecessor for the True side. + MachineBasicBlock *getTPred() const { return TBB == Tail ? Head : TBB; } + + /// Returns the Tail predecessor for the False side. + MachineBasicBlock *getFPred() const { return FBB == Tail ? Head : FBB; } + /// Information about each phi in the Tail block. struct PHIInfo { MachineInstr *PHI; @@ -134,11 +144,17 @@ private: /// Find a valid insertion point in Head. bool findInsertionPoint(); + /// Replace PHI instructions in Tail with selects. + void replacePHIInstrs(); + + /// Insert selects and rewrite PHI operands to use them. + void rewritePHIOperands(); + public: /// runOnMachineFunction - Initialize per-function data structures. void runOnMachineFunction(MachineFunction &MF) { - TII = MF.getTarget().getInstrInfo(); - TRI = MF.getTarget().getRegisterInfo(); + TII = MF.getSubtarget().getInstrInfo(); + TRI = MF.getSubtarget().getRegisterInfo(); MRI = &MF.getRegInfo(); LiveRegUnits.clear(); LiveRegUnits.setUniverse(TRI->getNumRegUnits()); @@ -204,7 +220,7 @@ bool SSAIfConv::canSpeculateInstrs(MachineBasicBlock *MBB) { // We never speculate stores, so an AA pointer isn't necessary. bool DontMoveAcrossStore = true; - if (!I->isSafeToMove(TII, 0, DontMoveAcrossStore)) { + if (!I->isSafeToMove(TII, nullptr, DontMoveAcrossStore)) { DEBUG(dbgs() << "Can't speculate: " << *I); return false; } @@ -229,7 +245,7 @@ bool SSAIfConv::canSpeculateInstrs(MachineBasicBlock *MBB) { MachineInstr *DefMI = MRI->getVRegDef(Reg); if (!DefMI || DefMI->getParent() != Head) continue; - if (InsertAfter.insert(DefMI)) + if (InsertAfter.insert(DefMI).second) DEBUG(dbgs() << "BB#" << MBB->getNumber() << " depends on " << *DefMI); if (DefMI->isTerminator()) { DEBUG(dbgs() << "Can't insert instructions below terminator.\n"); @@ -323,7 +339,7 @@ bool SSAIfConv::findInsertionPoint() { /// bool SSAIfConv::canConvertIf(MachineBasicBlock *MBB) { Head = MBB; - TBB = FBB = Tail = 0; + TBB = FBB = Tail = nullptr; if (Head->succ_size() != 2) return false; @@ -337,11 +353,7 @@ bool SSAIfConv::canConvertIf(MachineBasicBlock *MBB) { if (Succ0->pred_size() != 1 || Succ0->succ_size() != 1) return false; - // We could support additional Tail predecessors by updating phis instead of - // eliminating them. Let's see an example where it matters first. Tail = Succ0->succ_begin()[0]; - if (Tail->pred_size() != 2) - return false; // This is not a triangle. if (Tail != Succ1) { @@ -391,8 +403,8 @@ bool SSAIfConv::canConvertIf(MachineBasicBlock *MBB) { // Any phis in the tail block must be convertible to selects. PHIs.clear(); - MachineBasicBlock *TPred = TBB == Tail ? Head : TBB; - MachineBasicBlock *FPred = FBB == Tail ? Head : FBB; + MachineBasicBlock *TPred = getTPred(); + MachineBasicBlock *FPred = getFPred(); for (MachineBasicBlock::iterator I = Tail->begin(), E = Tail->end(); I != E && I->isPHI(); ++I) { PHIs.push_back(&*I); @@ -428,9 +440,65 @@ bool SSAIfConv::canConvertIf(MachineBasicBlock *MBB) { if (!findInsertionPoint()) return false; + if (isTriangle()) + ++NumTrianglesSeen; + else + ++NumDiamondsSeen; return true; } +/// replacePHIInstrs - Completely replace PHI instructions with selects. +/// This is possible when the only Tail predecessors are the if-converted +/// blocks. +void SSAIfConv::replacePHIInstrs() { + assert(Tail->pred_size() == 2 && "Cannot replace PHIs"); + MachineBasicBlock::iterator FirstTerm = Head->getFirstTerminator(); + assert(FirstTerm != Head->end() && "No terminators"); + DebugLoc HeadDL = FirstTerm->getDebugLoc(); + + // Convert all PHIs to select instructions inserted before FirstTerm. + for (unsigned i = 0, e = PHIs.size(); i != e; ++i) { + PHIInfo &PI = PHIs[i]; + DEBUG(dbgs() << "If-converting " << *PI.PHI); + unsigned DstReg = PI.PHI->getOperand(0).getReg(); + TII->insertSelect(*Head, FirstTerm, HeadDL, DstReg, Cond, PI.TReg, PI.FReg); + DEBUG(dbgs() << " --> " << *std::prev(FirstTerm)); + PI.PHI->eraseFromParent(); + PI.PHI = nullptr; + } +} + +/// rewritePHIOperands - When there are additional Tail predecessors, insert +/// select instructions in Head and rewrite PHI operands to use the selects. +/// Keep the PHI instructions in Tail to handle the other predecessors. +void SSAIfConv::rewritePHIOperands() { + MachineBasicBlock::iterator FirstTerm = Head->getFirstTerminator(); + assert(FirstTerm != Head->end() && "No terminators"); + DebugLoc HeadDL = FirstTerm->getDebugLoc(); + + // Convert all PHIs to select instructions inserted before FirstTerm. + for (unsigned i = 0, e = PHIs.size(); i != e; ++i) { + PHIInfo &PI = PHIs[i]; + DEBUG(dbgs() << "If-converting " << *PI.PHI); + unsigned PHIDst = PI.PHI->getOperand(0).getReg(); + unsigned DstReg = MRI->createVirtualRegister(MRI->getRegClass(PHIDst)); + TII->insertSelect(*Head, FirstTerm, HeadDL, DstReg, Cond, PI.TReg, PI.FReg); + DEBUG(dbgs() << " --> " << *std::prev(FirstTerm)); + + // Rewrite PHI operands TPred -> (DstReg, Head), remove FPred. + for (unsigned i = PI.PHI->getNumOperands(); i != 1; i -= 2) { + MachineBasicBlock *MBB = PI.PHI->getOperand(i-1).getMBB(); + if (MBB == getTPred()) { + PI.PHI->getOperand(i-1).setMBB(Head); + PI.PHI->getOperand(i-2).setReg(DstReg); + } else if (MBB == getFPred()) { + PI.PHI->RemoveOperand(i-1); + PI.PHI->RemoveOperand(i-2); + } + } + DEBUG(dbgs() << " --> " << *PI.PHI); + } +} /// convertIf - Execute the if conversion after canConvertIf has determined the /// feasibility. @@ -440,27 +508,24 @@ bool SSAIfConv::canConvertIf(MachineBasicBlock *MBB) { void SSAIfConv::convertIf(SmallVectorImpl &RemovedBlocks) { assert(Head && Tail && TBB && FBB && "Call canConvertIf first."); + // Update statistics. + if (isTriangle()) + ++NumTrianglesConv; + else + ++NumDiamondsConv; + // Move all instructions into Head, except for the terminators. if (TBB != Tail) Head->splice(InsertionPoint, TBB, TBB->begin(), TBB->getFirstTerminator()); if (FBB != Tail) Head->splice(InsertionPoint, FBB, FBB->begin(), FBB->getFirstTerminator()); - MachineBasicBlock::iterator FirstTerm = Head->getFirstTerminator(); - assert(FirstTerm != Head->end() && "No terminators"); - DebugLoc HeadDL = FirstTerm->getDebugLoc(); - - // Convert all PHIs to select instructions inserted before FirstTerm. - for (unsigned i = 0, e = PHIs.size(); i != e; ++i) { - PHIInfo &PI = PHIs[i]; - DEBUG(dbgs() << "If-converting " << *PI.PHI); - assert(PI.PHI->getNumOperands() == 5 && "Unexpected PHI operands."); - unsigned DstReg = PI.PHI->getOperand(0).getReg(); - TII->insertSelect(*Head, FirstTerm, HeadDL, DstReg, Cond, PI.TReg, PI.FReg); - DEBUG(dbgs() << " --> " << *llvm::prior(FirstTerm)); - PI.PHI->eraseFromParent(); - PI.PHI = 0; - } + // Are there extra Tail predecessors? + bool ExtraPreds = Tail->pred_size() != 2; + if (ExtraPreds) + rewritePHIOperands(); + else + replacePHIInstrs(); // Fix up the CFG, temporarily leave Head without any successors. Head->removeSuccessor(TBB); @@ -472,6 +537,7 @@ void SSAIfConv::convertIf(SmallVectorImpl &RemovedBlocks) { // Fix up Head's terminators. // It should become a single branch or a fallthrough. + DebugLoc HeadDL = Head->getFirstTerminator()->getDebugLoc(); TII->RemoveBranch(*Head); // Erase the now empty conditional blocks. It is likely that Head can fall @@ -486,7 +552,7 @@ void SSAIfConv::convertIf(SmallVectorImpl &RemovedBlocks) { } assert(Head->succ_empty() && "Additional head successors?"); - if (Head->isLayoutSuccessor(Tail)) { + if (!ExtraPreds && Head->isLayoutSuccessor(Tail)) { // Splice Tail onto the end of Head. DEBUG(dbgs() << "Joining tail BB#" << Tail->getNumber() << " into head BB#" << Head->getNumber() << '\n'); @@ -499,7 +565,7 @@ void SSAIfConv::convertIf(SmallVectorImpl &RemovedBlocks) { // We need a branch to Tail, let code placement work it out later. DEBUG(dbgs() << "Converting to unconditional branch.\n"); SmallVector EmptyCond; - TII->InsertBranch(*Head, Tail, 0, EmptyCond, HeadDL); + TII->InsertBranch(*Head, Tail, nullptr, EmptyCond, HeadDL); Head->addSuccessor(Tail); } DEBUG(dbgs() << *Head); @@ -514,7 +580,7 @@ namespace { class EarlyIfConverter : public MachineFunctionPass { const TargetInstrInfo *TII; const TargetRegisterInfo *TRI; - const MCSchedModel *SchedModel; + MCSchedModel SchedModel; MachineRegisterInfo *MRI; MachineDominatorTree *DomTree; MachineLoopInfo *Loops; @@ -525,8 +591,9 @@ class EarlyIfConverter : public MachineFunctionPass { public: static char ID; EarlyIfConverter() : MachineFunctionPass(ID) {} - void getAnalysisUsage(AnalysisUsage &AU) const; - bool runOnMachineFunction(MachineFunction &MF); + void getAnalysisUsage(AnalysisUsage &AU) const override; + bool runOnMachineFunction(MachineFunction &MF) override; + const char *getPassName() const override { return "Early If-Conversion"; } private: bool tryConvertIf(MachineBasicBlock*); @@ -593,32 +660,100 @@ void EarlyIfConverter::invalidateTraces() { Traces->invalidate(IfConv.Tail); Traces->invalidate(IfConv.TBB); Traces->invalidate(IfConv.FBB); - DEBUG(if (MinInstr) MinInstr->print(dbgs())); Traces->verifyAnalysis(); } +// Adjust cycles with downward saturation. +static unsigned adjCycles(unsigned Cyc, int Delta) { + if (Delta < 0 && Cyc + Delta > Cyc) + return 0; + return Cyc + Delta; +} + /// Apply cost model and heuristics to the if-conversion in IfConv. /// Return true if the conversion is a good idea. /// bool EarlyIfConverter::shouldConvertIf() { + // Stress testing mode disables all cost considerations. + if (Stress) + return true; + if (!MinInstr) MinInstr = Traces->getEnsemble(MachineTraceMetrics::TS_MinInstrCount); - // Compare the critical path through TBB and FBB. If the difference is - // greater than the branch misprediction penalty, it would never pay to - // if-convert. The triangle/diamond topology guarantees that these traces - // have the same head and tail, so they can be compared. - MachineTraceMetrics::Trace TBBTrace = MinInstr->getTrace(IfConv.TBB); - MachineTraceMetrics::Trace FBBTrace = MinInstr->getTrace(IfConv.FBB); + MachineTraceMetrics::Trace TBBTrace = MinInstr->getTrace(IfConv.getTPred()); + MachineTraceMetrics::Trace FBBTrace = MinInstr->getTrace(IfConv.getFPred()); DEBUG(dbgs() << "TBB: " << TBBTrace << "FBB: " << FBBTrace); - unsigned TBBCrit = TBBTrace.getCriticalPath(); - unsigned FBBCrit = FBBTrace.getCriticalPath(); - unsigned ExtraCrit = TBBCrit > FBBCrit ? TBBCrit-FBBCrit : FBBCrit-TBBCrit; - if (ExtraCrit >= SchedModel->MispredictPenalty) { - DEBUG(dbgs() << "Critical path difference larger than " - << SchedModel->MispredictPenalty << ".\n"); + unsigned MinCrit = std::min(TBBTrace.getCriticalPath(), + FBBTrace.getCriticalPath()); + + // Set a somewhat arbitrary limit on the critical path extension we accept. + unsigned CritLimit = SchedModel.MispredictPenalty/2; + + // If-conversion only makes sense when there is unexploited ILP. Compute the + // maximum-ILP resource length of the trace after if-conversion. Compare it + // to the shortest critical path. + SmallVector ExtraBlocks; + if (IfConv.TBB != IfConv.Tail) + ExtraBlocks.push_back(IfConv.TBB); + unsigned ResLength = FBBTrace.getResourceLength(ExtraBlocks); + DEBUG(dbgs() << "Resource length " << ResLength + << ", minimal critical path " << MinCrit << '\n'); + if (ResLength > MinCrit + CritLimit) { + DEBUG(dbgs() << "Not enough available ILP.\n"); return false; } + + // Assume that the depth of the first head terminator will also be the depth + // of the select instruction inserted, as determined by the flag dependency. + // TBB / FBB data dependencies may delay the select even more. + MachineTraceMetrics::Trace HeadTrace = MinInstr->getTrace(IfConv.Head); + unsigned BranchDepth = + HeadTrace.getInstrCycles(IfConv.Head->getFirstTerminator()).Depth; + DEBUG(dbgs() << "Branch depth: " << BranchDepth << '\n'); + + // Look at all the tail phis, and compute the critical path extension caused + // by inserting select instructions. + MachineTraceMetrics::Trace TailTrace = MinInstr->getTrace(IfConv.Tail); + for (unsigned i = 0, e = IfConv.PHIs.size(); i != e; ++i) { + SSAIfConv::PHIInfo &PI = IfConv.PHIs[i]; + unsigned Slack = TailTrace.getInstrSlack(PI.PHI); + unsigned MaxDepth = Slack + TailTrace.getInstrCycles(PI.PHI).Depth; + DEBUG(dbgs() << "Slack " << Slack << ":\t" << *PI.PHI); + + // The condition is pulled into the critical path. + unsigned CondDepth = adjCycles(BranchDepth, PI.CondCycles); + if (CondDepth > MaxDepth) { + unsigned Extra = CondDepth - MaxDepth; + DEBUG(dbgs() << "Condition adds " << Extra << " cycles.\n"); + if (Extra > CritLimit) { + DEBUG(dbgs() << "Exceeds limit of " << CritLimit << '\n'); + return false; + } + } + + // The TBB value is pulled into the critical path. + unsigned TDepth = adjCycles(TBBTrace.getPHIDepth(PI.PHI), PI.TCycles); + if (TDepth > MaxDepth) { + unsigned Extra = TDepth - MaxDepth; + DEBUG(dbgs() << "TBB data adds " << Extra << " cycles.\n"); + if (Extra > CritLimit) { + DEBUG(dbgs() << "Exceeds limit of " << CritLimit << '\n'); + return false; + } + } + + // The FBB value is pulled into the critical path. + unsigned FDepth = adjCycles(FBBTrace.getPHIDepth(PI.PHI), PI.FCycles); + if (FDepth > MaxDepth) { + unsigned Extra = FDepth - MaxDepth; + DEBUG(dbgs() << "FBB data adds " << Extra << " cycles.\n"); + if (Extra > CritLimit) { + DEBUG(dbgs() << "Exceeds limit of " << CritLimit << '\n'); + return false; + } + } + } return true; } @@ -640,16 +775,22 @@ bool EarlyIfConverter::tryConvertIf(MachineBasicBlock *MBB) { bool EarlyIfConverter::runOnMachineFunction(MachineFunction &MF) { DEBUG(dbgs() << "********** EARLY IF-CONVERSION **********\n" - << "********** Function: " - << ((Value*)MF.getFunction())->getName() << '\n'); - TII = MF.getTarget().getInstrInfo(); - TRI = MF.getTarget().getRegisterInfo(); - SchedModel = MF.getTarget().getInstrItineraryData()->SchedModel; + << "********** Function: " << MF.getName() << '\n'); + // Only run if conversion if the target wants it. + if (!MF.getTarget() + .getSubtarget() + .enableEarlyIfConversion()) + return false; + + TII = MF.getSubtarget().getInstrInfo(); + TRI = MF.getSubtarget().getRegisterInfo(); + SchedModel = + MF.getTarget().getSubtarget().getSchedModel(); MRI = &MF.getRegInfo(); DomTree = &getAnalysis(); Loops = getAnalysisIfAvailable(); Traces = &getAnalysis(); - MinInstr = 0; + MinInstr = nullptr; bool Changed = false; IfConv.runOnMachineFunction(MF); @@ -663,6 +804,5 @@ bool EarlyIfConverter::runOnMachineFunction(MachineFunction &MF) { if (tryConvertIf(I->getBlock())) Changed = true; - MF.verify(this, "After early if-conversion"); return Changed; }