| //===- BranchFolding.cpp - Fold machine code branch instructions ----------===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This pass forwards branches to unconditional branches to make them branch |
| // directly to the target block. This pass often results in dead MBB's, which |
| // it then removes. |
| // |
| // Note that this pass must be run after register allocation, it cannot handle |
| // SSA form. It also must handle virtual registers for targets that emit virtual |
| // ISA (e.g. NVPTX). |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "BranchFolding.h" |
| #include "llvm/ADT/BitVector.h" |
| #include "llvm/ADT/STLExtras.h" |
| #include "llvm/ADT/SmallSet.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/ProfileSummaryInfo.h" |
| #include "llvm/CodeGen/Analysis.h" |
| #include "llvm/CodeGen/MachineBlockFrequencyInfo.h" |
| #include "llvm/CodeGen/MachineBranchProbabilityInfo.h" |
| #include "llvm/CodeGen/MachineFunction.h" |
| #include "llvm/CodeGen/MachineFunctionPass.h" |
| #include "llvm/CodeGen/MachineInstr.h" |
| #include "llvm/CodeGen/MachineInstrBuilder.h" |
| #include "llvm/CodeGen/MachineJumpTableInfo.h" |
| #include "llvm/CodeGen/MachineLoopInfo.h" |
| #include "llvm/CodeGen/MachineModuleInfo.h" |
| #include "llvm/CodeGen/MachineOperand.h" |
| #include "llvm/CodeGen/MachineRegisterInfo.h" |
| #include "llvm/CodeGen/MachineSizeOpts.h" |
| #include "llvm/CodeGen/MBFIWrapper.h" |
| #include "llvm/CodeGen/TargetInstrInfo.h" |
| #include "llvm/CodeGen/TargetOpcodes.h" |
| #include "llvm/CodeGen/TargetPassConfig.h" |
| #include "llvm/CodeGen/TargetRegisterInfo.h" |
| #include "llvm/CodeGen/TargetSubtargetInfo.h" |
| #include "llvm/IR/DebugInfoMetadata.h" |
| #include "llvm/IR/DebugLoc.h" |
| #include "llvm/IR/Function.h" |
| #include "llvm/InitializePasses.h" |
| #include "llvm/MC/LaneBitmask.h" |
| #include "llvm/MC/MCRegisterInfo.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Support/BlockFrequency.h" |
| #include "llvm/Support/BranchProbability.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/ErrorHandling.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include "llvm/Target/TargetMachine.h" |
| #include <cassert> |
| #include <cstddef> |
| #include <iterator> |
| #include <numeric> |
| |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "branch-folder" |
| |
| STATISTIC(NumDeadBlocks, "Number of dead blocks removed"); |
| STATISTIC(NumBranchOpts, "Number of branches optimized"); |
| STATISTIC(NumTailMerge , "Number of block tails merged"); |
| STATISTIC(NumHoist , "Number of times common instructions are hoisted"); |
| STATISTIC(NumTailCalls, "Number of tail calls optimized"); |
| |
| static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", |
| cl::init(cl::BOU_UNSET), cl::Hidden); |
| |
| // Throttle for huge numbers of predecessors (compile speed problems) |
| static cl::opt<unsigned> |
| TailMergeThreshold("tail-merge-threshold", |
| cl::desc("Max number of predecessors to consider tail merging"), |
| cl::init(150), cl::Hidden); |
| |
| // Heuristic for tail merging (and, inversely, tail duplication). |
| // TODO: This should be replaced with a target query. |
| static cl::opt<unsigned> |
| TailMergeSize("tail-merge-size", |
| cl::desc("Min number of instructions to consider tail merging"), |
| cl::init(3), cl::Hidden); |
| |
| namespace { |
| |
| /// BranchFolderPass - Wrap branch folder in a machine function pass. |
| class BranchFolderPass : public MachineFunctionPass { |
| public: |
| static char ID; |
| |
| explicit BranchFolderPass(): MachineFunctionPass(ID) {} |
| |
| bool runOnMachineFunction(MachineFunction &MF) override; |
| |
| void getAnalysisUsage(AnalysisUsage &AU) const override { |
| AU.addRequired<MachineBlockFrequencyInfo>(); |
| AU.addRequired<MachineBranchProbabilityInfo>(); |
| AU.addRequired<ProfileSummaryInfoWrapperPass>(); |
| AU.addRequired<TargetPassConfig>(); |
| MachineFunctionPass::getAnalysisUsage(AU); |
| } |
| }; |
| |
| } // end anonymous namespace |
| |
| char BranchFolderPass::ID = 0; |
| |
| char &llvm::BranchFolderPassID = BranchFolderPass::ID; |
| |
| INITIALIZE_PASS(BranchFolderPass, DEBUG_TYPE, |
| "Control Flow Optimizer", false, false) |
| |
| bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) { |
| if (skipFunction(MF.getFunction())) |
| return false; |
| |
| TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>(); |
| // TailMerge can create jump into if branches that make CFG irreducible for |
| // HW that requires structurized CFG. |
| bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() && |
| PassConfig->getEnableTailMerge(); |
| MBFIWrapper MBBFreqInfo( |
| getAnalysis<MachineBlockFrequencyInfo>()); |
| BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true, MBBFreqInfo, |
| getAnalysis<MachineBranchProbabilityInfo>(), |
| &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI()); |
| return Folder.OptimizeFunction(MF, MF.getSubtarget().getInstrInfo(), |
| MF.getSubtarget().getRegisterInfo()); |
| } |
| |
| BranchFolder::BranchFolder(bool DefaultEnableTailMerge, bool CommonHoist, |
| MBFIWrapper &FreqInfo, |
| const MachineBranchProbabilityInfo &ProbInfo, |
| ProfileSummaryInfo *PSI, unsigned MinTailLength) |
| : EnableHoistCommonCode(CommonHoist), MinCommonTailLength(MinTailLength), |
| MBBFreqInfo(FreqInfo), MBPI(ProbInfo), PSI(PSI) { |
| if (MinCommonTailLength == 0) |
| MinCommonTailLength = TailMergeSize; |
| switch (FlagEnableTailMerge) { |
| case cl::BOU_UNSET: |
| EnableTailMerge = DefaultEnableTailMerge; |
| break; |
| case cl::BOU_TRUE: EnableTailMerge = true; break; |
| case cl::BOU_FALSE: EnableTailMerge = false; break; |
| } |
| } |
| |
| void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { |
| assert(MBB->pred_empty() && "MBB must be dead!"); |
| LLVM_DEBUG(dbgs() << "\nRemoving MBB: " << *MBB); |
| |
| MachineFunction *MF = MBB->getParent(); |
| // drop all successors. |
| while (!MBB->succ_empty()) |
| MBB->removeSuccessor(MBB->succ_end()-1); |
| |
| // Avoid matching if this pointer gets reused. |
| TriedMerging.erase(MBB); |
| |
| // Update call site info. |
| for (const MachineInstr &MI : *MBB) |
| if (MI.shouldUpdateCallSiteInfo()) |
| MF->eraseCallSiteInfo(&MI); |
| |
| // Remove the block. |
| MF->erase(MBB); |
| EHScopeMembership.erase(MBB); |
| if (MLI) |
| MLI->removeBlock(MBB); |
| } |
| |
| bool BranchFolder::OptimizeFunction(MachineFunction &MF, |
| const TargetInstrInfo *tii, |
| const TargetRegisterInfo *tri, |
| MachineLoopInfo *mli, bool AfterPlacement) { |
| if (!tii) return false; |
| |
| TriedMerging.clear(); |
| |
| MachineRegisterInfo &MRI = MF.getRegInfo(); |
| AfterBlockPlacement = AfterPlacement; |
| TII = tii; |
| TRI = tri; |
| MLI = mli; |
| this->MRI = &MRI; |
| |
| UpdateLiveIns = MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF); |
| if (!UpdateLiveIns) |
| MRI.invalidateLiveness(); |
| |
| bool MadeChange = false; |
| |
| // Recalculate EH scope membership. |
| EHScopeMembership = getEHScopeMembership(MF); |
| |
| bool MadeChangeThisIteration = true; |
| while (MadeChangeThisIteration) { |
| MadeChangeThisIteration = TailMergeBlocks(MF); |
| // No need to clean up if tail merging does not change anything after the |
| // block placement. |
| if (!AfterBlockPlacement || MadeChangeThisIteration) |
| MadeChangeThisIteration |= OptimizeBranches(MF); |
| if (EnableHoistCommonCode) |
| MadeChangeThisIteration |= HoistCommonCode(MF); |
| MadeChange |= MadeChangeThisIteration; |
| } |
| |
| // See if any jump tables have become dead as the code generator |
| // did its thing. |
| MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); |
| if (!JTI) |
| return MadeChange; |
| |
| // Walk the function to find jump tables that are live. |
| BitVector JTIsLive(JTI->getJumpTables().size()); |
| for (const MachineBasicBlock &BB : MF) { |
| for (const MachineInstr &I : BB) |
| for (const MachineOperand &Op : I.operands()) { |
| if (!Op.isJTI()) continue; |
| |
| // Remember that this JT is live. |
| JTIsLive.set(Op.getIndex()); |
| } |
| } |
| |
| // Finally, remove dead jump tables. This happens when the |
| // indirect jump was unreachable (and thus deleted). |
| for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) |
| if (!JTIsLive.test(i)) { |
| JTI->RemoveJumpTable(i); |
| MadeChange = true; |
| } |
| |
| return MadeChange; |
| } |
| |
| //===----------------------------------------------------------------------===// |
| // Tail Merging of Blocks |
| //===----------------------------------------------------------------------===// |
| |
| /// HashMachineInstr - Compute a hash value for MI and its operands. |
| static unsigned HashMachineInstr(const MachineInstr &MI) { |
| unsigned Hash = MI.getOpcode(); |
| for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) { |
| const MachineOperand &Op = MI.getOperand(i); |
| |
| // Merge in bits from the operand if easy. We can't use MachineOperand's |
| // hash_code here because it's not deterministic and we sort by hash value |
| // later. |
| unsigned OperandHash = 0; |
| switch (Op.getType()) { |
| case MachineOperand::MO_Register: |
| OperandHash = Op.getReg(); |
| break; |
| case MachineOperand::MO_Immediate: |
| OperandHash = Op.getImm(); |
| break; |
| case MachineOperand::MO_MachineBasicBlock: |
| OperandHash = Op.getMBB()->getNumber(); |
| break; |
| case MachineOperand::MO_FrameIndex: |
| case MachineOperand::MO_ConstantPoolIndex: |
| case MachineOperand::MO_JumpTableIndex: |
| OperandHash = Op.getIndex(); |
| break; |
| case MachineOperand::MO_GlobalAddress: |
| case MachineOperand::MO_ExternalSymbol: |
| // Global address / external symbol are too hard, don't bother, but do |
| // pull in the offset. |
| OperandHash = Op.getOffset(); |
| break; |
| default: |
| break; |
| } |
| |
| Hash += ((OperandHash << 3) | Op.getType()) << (i & 31); |
| } |
| return Hash; |
| } |
| |
| /// HashEndOfMBB - Hash the last instruction in the MBB. |
| static unsigned HashEndOfMBB(const MachineBasicBlock &MBB) { |
| MachineBasicBlock::const_iterator I = MBB.getLastNonDebugInstr(false); |
| if (I == MBB.end()) |
| return 0; |
| |
| return HashMachineInstr(*I); |
| } |
| |
| /// Whether MI should be counted as an instruction when calculating common tail. |
| static bool countsAsInstruction(const MachineInstr &MI) { |
| return !(MI.isDebugInstr() || MI.isCFIInstruction()); |
| } |
| |
| /// Iterate backwards from the given iterator \p I, towards the beginning of the |
| /// block. If a MI satisfying 'countsAsInstruction' is found, return an iterator |
| /// pointing to that MI. If no such MI is found, return the end iterator. |
| static MachineBasicBlock::iterator |
| skipBackwardPastNonInstructions(MachineBasicBlock::iterator I, |
| MachineBasicBlock *MBB) { |
| while (I != MBB->begin()) { |
| --I; |
| if (countsAsInstruction(*I)) |
| return I; |
| } |
| return MBB->end(); |
| } |
| |
| /// Given two machine basic blocks, return the number of instructions they |
| /// actually have in common together at their end. If a common tail is found (at |
| /// least by one instruction), then iterators for the first shared instruction |
| /// in each block are returned as well. |
| /// |
| /// Non-instructions according to countsAsInstruction are ignored. |
| static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, |
| MachineBasicBlock *MBB2, |
| MachineBasicBlock::iterator &I1, |
| MachineBasicBlock::iterator &I2) { |
| MachineBasicBlock::iterator MBBI1 = MBB1->end(); |
| MachineBasicBlock::iterator MBBI2 = MBB2->end(); |
| |
| unsigned TailLen = 0; |
| while (true) { |
| MBBI1 = skipBackwardPastNonInstructions(MBBI1, MBB1); |
| MBBI2 = skipBackwardPastNonInstructions(MBBI2, MBB2); |
| if (MBBI1 == MBB1->end() || MBBI2 == MBB2->end()) |
| break; |
| if (!MBBI1->isIdenticalTo(*MBBI2) || |
| // FIXME: This check is dubious. It's used to get around a problem where |
| // people incorrectly expect inline asm directives to remain in the same |
| // relative order. This is untenable because normal compiler |
| // optimizations (like this one) may reorder and/or merge these |
| // directives. |
| MBBI1->isInlineAsm()) { |
| break; |
| } |
| if (MBBI1->getFlag(MachineInstr::NoMerge) || |
| MBBI2->getFlag(MachineInstr::NoMerge)) |
| break; |
| ++TailLen; |
| I1 = MBBI1; |
| I2 = MBBI2; |
| } |
| |
| return TailLen; |
| } |
| |
| void BranchFolder::replaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, |
| MachineBasicBlock &NewDest) { |
| if (UpdateLiveIns) { |
| // OldInst should always point to an instruction. |
| MachineBasicBlock &OldMBB = *OldInst->getParent(); |
| LiveRegs.clear(); |
| LiveRegs.addLiveOuts(OldMBB); |
| // Move backward to the place where will insert the jump. |
| MachineBasicBlock::iterator I = OldMBB.end(); |
| do { |
| --I; |
| LiveRegs.stepBackward(*I); |
| } while (I != OldInst); |
| |
| // Merging the tails may have switched some undef operand to non-undef ones. |
| // Add IMPLICIT_DEFS into OldMBB as necessary to have a definition of the |
| // register. |
| for (MachineBasicBlock::RegisterMaskPair P : NewDest.liveins()) { |
| // We computed the liveins with computeLiveIn earlier and should only see |
| // full registers: |
| assert(P.LaneMask == LaneBitmask::getAll() && |
| "Can only handle full register."); |
| MCPhysReg Reg = P.PhysReg; |
| if (!LiveRegs.available(*MRI, Reg)) |
| continue; |
| DebugLoc DL; |
| BuildMI(OldMBB, OldInst, DL, TII->get(TargetOpcode::IMPLICIT_DEF), Reg); |
| } |
| } |
| |
| TII->ReplaceTailWithBranchTo(OldInst, &NewDest); |
| ++NumTailMerge; |
| } |
| |
| MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, |
| MachineBasicBlock::iterator BBI1, |
| const BasicBlock *BB) { |
| if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1)) |
| return nullptr; |
| |
| MachineFunction &MF = *CurMBB.getParent(); |
| |
| // Create the fall-through block. |
| MachineFunction::iterator MBBI = CurMBB.getIterator(); |
| MachineBasicBlock *NewMBB = MF.CreateMachineBasicBlock(BB); |
| CurMBB.getParent()->insert(++MBBI, NewMBB); |
| |
| // Move all the successors of this block to the specified block. |
| NewMBB->transferSuccessors(&CurMBB); |
| |
| // Add an edge from CurMBB to NewMBB for the fall-through. |
| CurMBB.addSuccessor(NewMBB); |
| |
| // Splice the code over. |
| NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); |
| |
| // NewMBB belongs to the same loop as CurMBB. |
| if (MLI) |
| if (MachineLoop *ML = MLI->getLoopFor(&CurMBB)) |
| ML->addBasicBlockToLoop(NewMBB, MLI->getBase()); |
| |
| // NewMBB inherits CurMBB's block frequency. |
| MBBFreqInfo.setBlockFreq(NewMBB, MBBFreqInfo.getBlockFreq(&CurMBB)); |
| |
| if (UpdateLiveIns) |
| computeAndAddLiveIns(LiveRegs, *NewMBB); |
| |
| // Add the new block to the EH scope. |
| const auto &EHScopeI = EHScopeMembership.find(&CurMBB); |
| if (EHScopeI != EHScopeMembership.end()) { |
| auto n = EHScopeI->second; |
| EHScopeMembership[NewMBB] = n; |
| } |
| |
| return NewMBB; |
| } |
| |
| /// EstimateRuntime - Make a rough estimate for how long it will take to run |
| /// the specified code. |
| static unsigned EstimateRuntime(MachineBasicBlock::iterator I, |
| MachineBasicBlock::iterator E) { |
| unsigned Time = 0; |
| for (; I != E; ++I) { |
| if (!countsAsInstruction(*I)) |
| continue; |
| if (I->isCall()) |
| Time += 10; |
| else if (I->mayLoadOrStore()) |
| Time += 2; |
| else |
| ++Time; |
| } |
| return Time; |
| } |
| |
| // CurMBB needs to add an unconditional branch to SuccMBB (we removed these |
| // branches temporarily for tail merging). In the case where CurMBB ends |
| // with a conditional branch to the next block, optimize by reversing the |
| // test and conditionally branching to SuccMBB instead. |
| static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB, |
| const TargetInstrInfo *TII) { |
| MachineFunction *MF = CurMBB->getParent(); |
| MachineFunction::iterator I = std::next(MachineFunction::iterator(CurMBB)); |
| MachineBasicBlock *TBB = nullptr, *FBB = nullptr; |
| SmallVector<MachineOperand, 4> Cond; |
| DebugLoc dl = CurMBB->findBranchDebugLoc(); |
| if (I != MF->end() && !TII->analyzeBranch(*CurMBB, TBB, FBB, Cond, true)) { |
| MachineBasicBlock *NextBB = &*I; |
| if (TBB == NextBB && !Cond.empty() && !FBB) { |
| if (!TII->reverseBranchCondition(Cond)) { |
| TII->removeBranch(*CurMBB); |
| TII->insertBranch(*CurMBB, SuccBB, nullptr, Cond, dl); |
| return; |
| } |
| } |
| } |
| TII->insertBranch(*CurMBB, SuccBB, nullptr, |
| SmallVector<MachineOperand, 0>(), dl); |
| } |
| |
| bool |
| BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const { |
| if (getHash() < o.getHash()) |
| return true; |
| if (getHash() > o.getHash()) |
| return false; |
| if (getBlock()->getNumber() < o.getBlock()->getNumber()) |
| return true; |
| if (getBlock()->getNumber() > o.getBlock()->getNumber()) |
| return false; |
| // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing |
| // an object with itself. |
| #ifndef _GLIBCXX_DEBUG |
| llvm_unreachable("Predecessor appears twice"); |
| #else |
| return false; |
| #endif |
| } |
| |
| /// CountTerminators - Count the number of terminators in the given |
| /// block and set I to the position of the first non-terminator, if there |
| /// is one, or MBB->end() otherwise. |
| static unsigned CountTerminators(MachineBasicBlock *MBB, |
| MachineBasicBlock::iterator &I) { |
| I = MBB->end(); |
| unsigned NumTerms = 0; |
| while (true) { |
| if (I == MBB->begin()) { |
| I = MBB->end(); |
| break; |
| } |
| --I; |
| if (!I->isTerminator()) break; |
| ++NumTerms; |
| } |
| return NumTerms; |
| } |
| |
| /// A no successor, non-return block probably ends in unreachable and is cold. |
| /// Also consider a block that ends in an indirect branch to be a return block, |
| /// since many targets use plain indirect branches to return. |
| static bool blockEndsInUnreachable(const MachineBasicBlock *MBB) { |
| if (!MBB->succ_empty()) |
| return false; |
| if (MBB->empty()) |
| return true; |
| return !(MBB->back().isReturn() || MBB->back().isIndirectBranch()); |
| } |
| |
| /// ProfitableToMerge - Check if two machine basic blocks have a common tail |
| /// and decide if it would be profitable to merge those tails. Return the |
| /// length of the common tail and iterators to the first common instruction |
| /// in each block. |
| /// MBB1, MBB2 The blocks to check |
| /// MinCommonTailLength Minimum size of tail block to be merged. |
| /// CommonTailLen Out parameter to record the size of the shared tail between |
| /// MBB1 and MBB2 |
| /// I1, I2 Iterator references that will be changed to point to the first |
| /// instruction in the common tail shared by MBB1,MBB2 |
| /// SuccBB A common successor of MBB1, MBB2 which are in a canonical form |
| /// relative to SuccBB |
| /// PredBB The layout predecessor of SuccBB, if any. |
| /// EHScopeMembership map from block to EH scope #. |
| /// AfterPlacement True if we are merging blocks after layout. Stricter |
| /// thresholds apply to prevent undoing tail-duplication. |
| static bool |
| ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2, |
| unsigned MinCommonTailLength, unsigned &CommonTailLen, |
| MachineBasicBlock::iterator &I1, |
| MachineBasicBlock::iterator &I2, MachineBasicBlock *SuccBB, |
| MachineBasicBlock *PredBB, |
| DenseMap<const MachineBasicBlock *, int> &EHScopeMembership, |
| bool AfterPlacement, |
| MBFIWrapper &MBBFreqInfo, |
| ProfileSummaryInfo *PSI) { |
| // It is never profitable to tail-merge blocks from two different EH scopes. |
| if (!EHScopeMembership.empty()) { |
| auto EHScope1 = EHScopeMembership.find(MBB1); |
| assert(EHScope1 != EHScopeMembership.end()); |
| auto EHScope2 = EHScopeMembership.find(MBB2); |
| assert(EHScope2 != EHScopeMembership.end()); |
| if (EHScope1->second != EHScope2->second) |
| return false; |
| } |
| |
| CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2); |
| if (CommonTailLen == 0) |
| return false; |
| LLVM_DEBUG(dbgs() << "Common tail length of " << printMBBReference(*MBB1) |
| << " and " << printMBBReference(*MBB2) << " is " |
| << CommonTailLen << '\n'); |
| |
| // Move the iterators to the beginning of the MBB if we only got debug |
| // instructions before the tail. This is to avoid splitting a block when we |
| // only got debug instructions before the tail (to be invariant on -g). |
| if (skipDebugInstructionsForward(MBB1->begin(), MBB1->end(), false) == I1) |
| I1 = MBB1->begin(); |
| if (skipDebugInstructionsForward(MBB2->begin(), MBB2->end(), false) == I2) |
| I2 = MBB2->begin(); |
| |
| bool FullBlockTail1 = I1 == MBB1->begin(); |
| bool FullBlockTail2 = I2 == MBB2->begin(); |
| |
| // It's almost always profitable to merge any number of non-terminator |
| // instructions with the block that falls through into the common successor. |
| // This is true only for a single successor. For multiple successors, we are |
| // trading a conditional branch for an unconditional one. |
| // TODO: Re-visit successor size for non-layout tail merging. |
| if ((MBB1 == PredBB || MBB2 == PredBB) && |
| (!AfterPlacement || MBB1->succ_size() == 1)) { |
| MachineBasicBlock::iterator I; |
| unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I); |
| if (CommonTailLen > NumTerms) |
| return true; |
| } |
| |
| // If these are identical non-return blocks with no successors, merge them. |
| // Such blocks are typically cold calls to noreturn functions like abort, and |
| // are unlikely to become a fallthrough target after machine block placement. |
| // Tail merging these blocks is unlikely to create additional unconditional |
| // branches, and will reduce the size of this cold code. |
| if (FullBlockTail1 && FullBlockTail2 && |
| blockEndsInUnreachable(MBB1) && blockEndsInUnreachable(MBB2)) |
| return true; |
| |
| // If one of the blocks can be completely merged and happens to be in |
| // a position where the other could fall through into it, merge any number |
| // of instructions, because it can be done without a branch. |
| // TODO: If the blocks are not adjacent, move one of them so that they are? |
| if (MBB1->isLayoutSuccessor(MBB2) && FullBlockTail2) |
| return true; |
| if (MBB2->isLayoutSuccessor(MBB1) && FullBlockTail1) |
| return true; |
| |
| // If both blocks are identical and end in a branch, merge them unless they |
| // both have a fallthrough predecessor and successor. |
| // We can only do this after block placement because it depends on whether |
| // there are fallthroughs, and we don't know until after layout. |
| if (AfterPlacement && FullBlockTail1 && FullBlockTail2) { |
| auto BothFallThrough = [](MachineBasicBlock *MBB) { |
| if (!MBB->succ_empty() && !MBB->canFallThrough()) |
| return false; |
| MachineFunction::iterator I(MBB); |
| MachineFunction *MF = MBB->getParent(); |
| return (MBB != &*MF->begin()) && std::prev(I)->canFallThrough(); |
| }; |
| if (!BothFallThrough(MBB1) || !BothFallThrough(MBB2)) |
| return true; |
| } |
| |
| // If both blocks have an unconditional branch temporarily stripped out, |
| // count that as an additional common instruction for the following |
| // heuristics. This heuristic is only accurate for single-succ blocks, so to |
| // make sure that during layout merging and duplicating don't crash, we check |
| // for that when merging during layout. |
| unsigned EffectiveTailLen = CommonTailLen; |
| if (SuccBB && MBB1 != PredBB && MBB2 != PredBB && |
| (MBB1->succ_size() == 1 || !AfterPlacement) && |
| !MBB1->back().isBarrier() && |
| !MBB2->back().isBarrier()) |
| ++EffectiveTailLen; |
| |
| // Check if the common tail is long enough to be worthwhile. |
| if (EffectiveTailLen >= MinCommonTailLength) |
| return true; |
| |
| // If we are optimizing for code size, 2 instructions in common is enough if |
| // we don't have to split a block. At worst we will be introducing 1 new |
| // branch instruction, which is likely to be smaller than the 2 |
| // instructions that would be deleted in the merge. |
| MachineFunction *MF = MBB1->getParent(); |
| bool OptForSize = |
| MF->getFunction().hasOptSize() || |
| (llvm::shouldOptimizeForSize(MBB1, PSI, &MBBFreqInfo) && |
| llvm::shouldOptimizeForSize(MBB2, PSI, &MBBFreqInfo)); |
| return EffectiveTailLen >= 2 && OptForSize && |
| (FullBlockTail1 || FullBlockTail2); |
| } |
| |
| unsigned BranchFolder::ComputeSameTails(unsigned CurHash, |
| unsigned MinCommonTailLength, |
| MachineBasicBlock *SuccBB, |
| MachineBasicBlock *PredBB) { |
| unsigned maxCommonTailLength = 0U; |
| SameTails.clear(); |
| MachineBasicBlock::iterator TrialBBI1, TrialBBI2; |
| MPIterator HighestMPIter = std::prev(MergePotentials.end()); |
| for (MPIterator CurMPIter = std::prev(MergePotentials.end()), |
| B = MergePotentials.begin(); |
| CurMPIter != B && CurMPIter->getHash() == CurHash; --CurMPIter) { |
| for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) { |
| unsigned CommonTailLen; |
| if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(), |
| MinCommonTailLength, |
| CommonTailLen, TrialBBI1, TrialBBI2, |
| SuccBB, PredBB, |
| EHScopeMembership, |
| AfterBlockPlacement, MBBFreqInfo, PSI)) { |
| if (CommonTailLen > maxCommonTailLength) { |
| SameTails.clear(); |
| maxCommonTailLength = CommonTailLen; |
| HighestMPIter = CurMPIter; |
| SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1)); |
| } |
| if (HighestMPIter == CurMPIter && |
| CommonTailLen == maxCommonTailLength) |
| SameTails.push_back(SameTailElt(I, TrialBBI2)); |
| } |
| if (I == B) |
| break; |
| } |
| } |
| return maxCommonTailLength; |
| } |
| |
| void BranchFolder::RemoveBlocksWithHash(unsigned CurHash, |
| MachineBasicBlock *SuccBB, |
| MachineBasicBlock *PredBB) { |
| MPIterator CurMPIter, B; |
| for (CurMPIter = std::prev(MergePotentials.end()), |
| B = MergePotentials.begin(); |
| CurMPIter->getHash() == CurHash; --CurMPIter) { |
| // Put the unconditional branch back, if we need one. |
| MachineBasicBlock *CurMBB = CurMPIter->getBlock(); |
| if (SuccBB && CurMBB != PredBB) |
| FixTail(CurMBB, SuccBB, TII); |
| if (CurMPIter == B) |
| break; |
| } |
| if (CurMPIter->getHash() != CurHash) |
| CurMPIter++; |
| MergePotentials.erase(CurMPIter, MergePotentials.end()); |
| } |
| |
| bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, |
| MachineBasicBlock *SuccBB, |
| unsigned maxCommonTailLength, |
| unsigned &commonTailIndex) { |
| commonTailIndex = 0; |
| unsigned TimeEstimate = ~0U; |
| for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { |
| // Use PredBB if possible; that doesn't require a new branch. |
| if (SameTails[i].getBlock() == PredBB) { |
| commonTailIndex = i; |
| break; |
| } |
| // Otherwise, make a (fairly bogus) choice based on estimate of |
| // how long it will take the various blocks to execute. |
| unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(), |
| SameTails[i].getTailStartPos()); |
| if (t <= TimeEstimate) { |
| TimeEstimate = t; |
| commonTailIndex = i; |
| } |
| } |
| |
| MachineBasicBlock::iterator BBI = |
| SameTails[commonTailIndex].getTailStartPos(); |
| MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); |
| |
| LLVM_DEBUG(dbgs() << "\nSplitting " << printMBBReference(*MBB) << ", size " |
| << maxCommonTailLength); |
| |
| // If the split block unconditionally falls-thru to SuccBB, it will be |
| // merged. In control flow terms it should then take SuccBB's name. e.g. If |
| // SuccBB is an inner loop, the common tail is still part of the inner loop. |
| const BasicBlock *BB = (SuccBB && MBB->succ_size() == 1) ? |
| SuccBB->getBasicBlock() : MBB->getBasicBlock(); |
| MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI, BB); |
| if (!newMBB) { |
| LLVM_DEBUG(dbgs() << "... failed!"); |
| return false; |
| } |
| |
| SameTails[commonTailIndex].setBlock(newMBB); |
| SameTails[commonTailIndex].setTailStartPos(newMBB->begin()); |
| |
| // If we split PredBB, newMBB is the new predecessor. |
| if (PredBB == MBB) |
| PredBB = newMBB; |
| |
| return true; |
| } |
| |
| static void |
| mergeOperations(MachineBasicBlock::iterator MBBIStartPos, |
| MachineBasicBlock &MBBCommon) { |
| MachineBasicBlock *MBB = MBBIStartPos->getParent(); |
| // Note CommonTailLen does not necessarily matches the size of |
| // the common BB nor all its instructions because of debug |
| // instructions differences. |
| unsigned CommonTailLen = 0; |
| for (auto E = MBB->end(); MBBIStartPos != E; ++MBBIStartPos) |
| ++CommonTailLen; |
| |
| MachineBasicBlock::reverse_iterator MBBI = MBB->rbegin(); |
| MachineBasicBlock::reverse_iterator MBBIE = MBB->rend(); |
| MachineBasicBlock::reverse_iterator MBBICommon = MBBCommon.rbegin(); |
| MachineBasicBlock::reverse_iterator MBBIECommon = MBBCommon.rend(); |
| |
| while (CommonTailLen--) { |
| assert(MBBI != MBBIE && "Reached BB end within common tail length!"); |
| (void)MBBIE; |
| |
| if (!countsAsInstruction(*MBBI)) { |
| ++MBBI; |
| continue; |
| } |
| |
| while ((MBBICommon != MBBIECommon) && !countsAsInstruction(*MBBICommon)) |
| ++MBBICommon; |
| |
| assert(MBBICommon != MBBIECommon && |
| "Reached BB end within common tail length!"); |
| assert(MBBICommon->isIdenticalTo(*MBBI) && "Expected matching MIIs!"); |
| |
| // Merge MMOs from memory operations in the common block. |
| if (MBBICommon->mayLoadOrStore()) |
| MBBICommon->cloneMergedMemRefs(*MBB->getParent(), {&*MBBICommon, &*MBBI}); |
| // Drop undef flags if they aren't present in all merged instructions. |
| for (unsigned I = 0, E = MBBICommon->getNumOperands(); I != E; ++I) { |
| MachineOperand &MO = MBBICommon->getOperand(I); |
| if (MO.isReg() && MO.isUndef()) { |
| const MachineOperand &OtherMO = MBBI->getOperand(I); |
| if (!OtherMO.isUndef()) |
| MO.setIsUndef(false); |
| } |
| } |
| |
| ++MBBI; |
| ++MBBICommon; |
| } |
| } |
| |
| void BranchFolder::mergeCommonTails(unsigned commonTailIndex) { |
| MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); |
| |
| std::vector<MachineBasicBlock::iterator> NextCommonInsts(SameTails.size()); |
| for (unsigned int i = 0 ; i != SameTails.size() ; ++i) { |
| if (i != commonTailIndex) { |
| NextCommonInsts[i] = SameTails[i].getTailStartPos(); |
| mergeOperations(SameTails[i].getTailStartPos(), *MBB); |
| } else { |
| assert(SameTails[i].getTailStartPos() == MBB->begin() && |
| "MBB is not a common tail only block"); |
| } |
| } |
| |
| for (auto &MI : *MBB) { |
| if (!countsAsInstruction(MI)) |
| continue; |
| DebugLoc DL = MI.getDebugLoc(); |
| for (unsigned int i = 0 ; i < NextCommonInsts.size() ; i++) { |
| if (i == commonTailIndex) |
| continue; |
| |
| auto &Pos = NextCommonInsts[i]; |
| assert(Pos != SameTails[i].getBlock()->end() && |
| "Reached BB end within common tail"); |
| while (!countsAsInstruction(*Pos)) { |
| ++Pos; |
| assert(Pos != SameTails[i].getBlock()->end() && |
| "Reached BB end within common tail"); |
| } |
| assert(MI.isIdenticalTo(*Pos) && "Expected matching MIIs!"); |
| DL = DILocation::getMergedLocation(DL, Pos->getDebugLoc()); |
| NextCommonInsts[i] = ++Pos; |
| } |
| MI.setDebugLoc(DL); |
| } |
| |
| if (UpdateLiveIns) { |
| LivePhysRegs NewLiveIns(*TRI); |
| computeLiveIns(NewLiveIns, *MBB); |
| LiveRegs.init(*TRI); |
| |
| // The flag merging may lead to some register uses no longer using the |
| // <undef> flag, add IMPLICIT_DEFs in the predecessors as necessary. |
| for (MachineBasicBlock *Pred : MBB->predecessors()) { |
| LiveRegs.clear(); |
| LiveRegs.addLiveOuts(*Pred); |
| MachineBasicBlock::iterator InsertBefore = Pred->getFirstTerminator(); |
| for (Register Reg : NewLiveIns) { |
| if (!LiveRegs.available(*MRI, Reg)) |
| continue; |
| DebugLoc DL; |
| BuildMI(*Pred, InsertBefore, DL, TII->get(TargetOpcode::IMPLICIT_DEF), |
| Reg); |
| } |
| } |
| |
| MBB->clearLiveIns(); |
| addLiveIns(*MBB, NewLiveIns); |
| } |
| } |
| |
| // See if any of the blocks in MergePotentials (which all have SuccBB as a |
| // successor, or all have no successor if it is null) can be tail-merged. |
| // If there is a successor, any blocks in MergePotentials that are not |
| // tail-merged and are not immediately before Succ must have an unconditional |
| // branch to Succ added (but the predecessor/successor lists need no |
| // adjustment). The lone predecessor of Succ that falls through into Succ, |
| // if any, is given in PredBB. |
| // MinCommonTailLength - Except for the special cases below, tail-merge if |
| // there are at least this many instructions in common. |
| bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB, |
| MachineBasicBlock *PredBB, |
| unsigned MinCommonTailLength) { |
| bool MadeChange = false; |
| |
| LLVM_DEBUG( |
| dbgs() << "\nTryTailMergeBlocks: "; |
| for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) dbgs() |
| << printMBBReference(*MergePotentials[i].getBlock()) |
| << (i == e - 1 ? "" : ", "); |
| dbgs() << "\n"; if (SuccBB) { |
| dbgs() << " with successor " << printMBBReference(*SuccBB) << '\n'; |
| if (PredBB) |
| dbgs() << " which has fall-through from " |
| << printMBBReference(*PredBB) << "\n"; |
| } dbgs() << "Looking for common tails of at least " |
| << MinCommonTailLength << " instruction" |
| << (MinCommonTailLength == 1 ? "" : "s") << '\n';); |
| |
| // Sort by hash value so that blocks with identical end sequences sort |
| // together. |
| array_pod_sort(MergePotentials.begin(), MergePotentials.end()); |
| |
| // Walk through equivalence sets looking for actual exact matches. |
| while (MergePotentials.size() > 1) { |
| unsigned CurHash = MergePotentials.back().getHash(); |
| |
| // Build SameTails, identifying the set of blocks with this hash code |
| // and with the maximum number of instructions in common. |
| unsigned maxCommonTailLength = ComputeSameTails(CurHash, |
| MinCommonTailLength, |
| SuccBB, PredBB); |
| |
| // If we didn't find any pair that has at least MinCommonTailLength |
| // instructions in common, remove all blocks with this hash code and retry. |
| if (SameTails.empty()) { |
| RemoveBlocksWithHash(CurHash, SuccBB, PredBB); |
| continue; |
| } |
| |
| // If one of the blocks is the entire common tail (and is not the entry |
| // block/an EH pad, which we can't jump to), we can treat all blocks with |
| // this same tail at once. Use PredBB if that is one of the possibilities, |
| // as that will not introduce any extra branches. |
| MachineBasicBlock *EntryBB = |
| &MergePotentials.front().getBlock()->getParent()->front(); |
| unsigned commonTailIndex = SameTails.size(); |
| // If there are two blocks, check to see if one can be made to fall through |
| // into the other. |
| if (SameTails.size() == 2 && |
| SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) && |
| SameTails[1].tailIsWholeBlock() && !SameTails[1].getBlock()->isEHPad()) |
| commonTailIndex = 1; |
| else if (SameTails.size() == 2 && |
| SameTails[1].getBlock()->isLayoutSuccessor( |
| SameTails[0].getBlock()) && |
| SameTails[0].tailIsWholeBlock() && |
| !SameTails[0].getBlock()->isEHPad()) |
| commonTailIndex = 0; |
| else { |
| // Otherwise just pick one, favoring the fall-through predecessor if |
| // there is one. |
| for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { |
| MachineBasicBlock *MBB = SameTails[i].getBlock(); |
| if ((MBB == EntryBB || MBB->isEHPad()) && |
| SameTails[i].tailIsWholeBlock()) |
| continue; |
| if (MBB == PredBB) { |
| commonTailIndex = i; |
| break; |
| } |
| if (SameTails[i].tailIsWholeBlock()) |
| commonTailIndex = i; |
| } |
| } |
| |
| if (commonTailIndex == SameTails.size() || |
| (SameTails[commonTailIndex].getBlock() == PredBB && |
| !SameTails[commonTailIndex].tailIsWholeBlock())) { |
| // None of the blocks consist entirely of the common tail. |
| // Split a block so that one does. |
| if (!CreateCommonTailOnlyBlock(PredBB, SuccBB, |
| maxCommonTailLength, commonTailIndex)) { |
| RemoveBlocksWithHash(CurHash, SuccBB, PredBB); |
| continue; |
| } |
| } |
| |
| MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); |
| |
| // Recompute common tail MBB's edge weights and block frequency. |
| setCommonTailEdgeWeights(*MBB); |
| |
| // Merge debug locations, MMOs and undef flags across identical instructions |
| // for common tail. |
| mergeCommonTails(commonTailIndex); |
| |
| // MBB is common tail. Adjust all other BB's to jump to this one. |
| // Traversal must be forwards so erases work. |
| LLVM_DEBUG(dbgs() << "\nUsing common tail in " << printMBBReference(*MBB) |
| << " for "); |
| for (unsigned int i=0, e = SameTails.size(); i != e; ++i) { |
| if (commonTailIndex == i) |
| continue; |
| LLVM_DEBUG(dbgs() << printMBBReference(*SameTails[i].getBlock()) |
| << (i == e - 1 ? "" : ", ")); |
| // Hack the end off BB i, making it jump to BB commonTailIndex instead. |
| replaceTailWithBranchTo(SameTails[i].getTailStartPos(), *MBB); |
| // BB i is no longer a predecessor of SuccBB; remove it from the worklist. |
| MergePotentials.erase(SameTails[i].getMPIter()); |
| } |
| LLVM_DEBUG(dbgs() << "\n"); |
| // We leave commonTailIndex in the worklist in case there are other blocks |
| // that match it with a smaller number of instructions. |
| MadeChange = true; |
| } |
| return MadeChange; |
| } |
| |
| bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { |
| bool MadeChange = false; |
| if (!EnableTailMerge) |
| return MadeChange; |
| |
| // First find blocks with no successors. |
| // Block placement may create new tail merging opportunities for these blocks. |
| MergePotentials.clear(); |
| for (MachineBasicBlock &MBB : MF) { |
| if (MergePotentials.size() == TailMergeThreshold) |
| break; |
| if (!TriedMerging.count(&MBB) && MBB.succ_empty()) |
| MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(MBB), &MBB)); |
| } |
| |
| // If this is a large problem, avoid visiting the same basic blocks |
| // multiple times. |
| if (MergePotentials.size() == TailMergeThreshold) |
| for (const MergePotentialsElt &Elt : MergePotentials) |
| TriedMerging.insert(Elt.getBlock()); |
| |
| // See if we can do any tail merging on those. |
| if (MergePotentials.size() >= 2) |
| MadeChange |= TryTailMergeBlocks(nullptr, nullptr, MinCommonTailLength); |
| |
| // Look at blocks (IBB) with multiple predecessors (PBB). |
| // We change each predecessor to a canonical form, by |
| // (1) temporarily removing any unconditional branch from the predecessor |
| // to IBB, and |
| // (2) alter conditional branches so they branch to the other block |
| // not IBB; this may require adding back an unconditional branch to IBB |
| // later, where there wasn't one coming in. E.g. |
| // Bcc IBB |
| // fallthrough to QBB |
| // here becomes |
| // Bncc QBB |
| // with a conceptual B to IBB after that, which never actually exists. |
| // With those changes, we see whether the predecessors' tails match, |
| // and merge them if so. We change things out of canonical form and |
| // back to the way they were later in the process. (OptimizeBranches |
| // would undo some of this, but we can't use it, because we'd get into |
| // a compile-time infinite loop repeatedly doing and undoing the same |
| // transformations.) |
| |
| for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end(); |
| I != E; ++I) { |
| if (I->pred_size() < 2) continue; |
| SmallPtrSet<MachineBasicBlock *, 8> UniquePreds; |
| MachineBasicBlock *IBB = &*I; |
| MachineBasicBlock *PredBB = &*std::prev(I); |
| MergePotentials.clear(); |
| MachineLoop *ML; |
| |
| // Bail if merging after placement and IBB is the loop header because |
| // -- If merging predecessors that belong to the same loop as IBB, the |
| // common tail of merged predecessors may become the loop top if block |
| // placement is called again and the predecessors may branch to this common |
| // tail and require more branches. This can be relaxed if |
| // MachineBlockPlacement::findBestLoopTop is more flexible. |
| // --If merging predecessors that do not belong to the same loop as IBB, the |
| // loop info of IBB's loop and the other loops may be affected. Calling the |
| // block placement again may make big change to the layout and eliminate the |
| // reason to do tail merging here. |
| if (AfterBlockPlacement && MLI) { |
| ML = MLI->getLoopFor(IBB); |
| if (ML && IBB == ML->getHeader()) |
| continue; |
| } |
| |
| for (MachineBasicBlock *PBB : I->predecessors()) { |
| if (MergePotentials.size() == TailMergeThreshold) |
| break; |
| |
| if (TriedMerging.count(PBB)) |
| continue; |
| |
| // Skip blocks that loop to themselves, can't tail merge these. |
| if (PBB == IBB) |
| continue; |
| |
| // Visit each predecessor only once. |
| if (!UniquePreds.insert(PBB).second) |
| continue; |
| |
| // Skip blocks which may jump to a landing pad or jump from an asm blob. |
| // Can't tail merge these. |
| if (PBB->hasEHPadSuccessor() || PBB->mayHaveInlineAsmBr()) |
| continue; |
| |
| // After block placement, only consider predecessors that belong to the |
| // same loop as IBB. The reason is the same as above when skipping loop |
| // header. |
| if (AfterBlockPlacement && MLI) |
| if (ML != MLI->getLoopFor(PBB)) |
| continue; |
| |
| MachineBasicBlock *TBB = nullptr, *FBB = nullptr; |
| SmallVector<MachineOperand, 4> Cond; |
| if (!TII->analyzeBranch(*PBB, TBB, FBB, Cond, true)) { |
| // Failing case: IBB is the target of a cbr, and we cannot reverse the |
| // branch. |
| SmallVector<MachineOperand, 4> NewCond(Cond); |
| if (!Cond.empty() && TBB == IBB) { |
| if (TII->reverseBranchCondition(NewCond)) |
| continue; |
| // This is the QBB case described above |
| if (!FBB) { |
| auto Next = ++PBB->getIterator(); |
| if (Next != MF.end()) |
| FBB = &*Next; |
| } |
| } |
| |
| // Remove the unconditional branch at the end, if any. |
| if (TBB && (Cond.empty() || FBB)) { |
| DebugLoc dl = PBB->findBranchDebugLoc(); |
| TII->removeBranch(*PBB); |
| if (!Cond.empty()) |
| // reinsert conditional branch only, for now |
| TII->insertBranch(*PBB, (TBB == IBB) ? FBB : TBB, nullptr, |
| NewCond, dl); |
| } |
| |
| MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(*PBB), PBB)); |
| } |
| } |
| |
| // If this is a large problem, avoid visiting the same basic blocks multiple |
| // times. |
| if (MergePotentials.size() == TailMergeThreshold) |
| for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) |
| TriedMerging.insert(MergePotentials[i].getBlock()); |
| |
| if (MergePotentials.size() >= 2) |
| MadeChange |= TryTailMergeBlocks(IBB, PredBB, MinCommonTailLength); |
| |
| // Reinsert an unconditional branch if needed. The 1 below can occur as a |
| // result of removing blocks in TryTailMergeBlocks. |
| PredBB = &*std::prev(I); // this may have been changed in TryTailMergeBlocks |
| if (MergePotentials.size() == 1 && |
| MergePotentials.begin()->getBlock() != PredBB) |
| FixTail(MergePotentials.begin()->getBlock(), IBB, TII); |
| } |
| |
| return MadeChange; |
| } |
| |
| void BranchFolder::setCommonTailEdgeWeights(MachineBasicBlock &TailMBB) { |
| SmallVector<BlockFrequency, 2> EdgeFreqLs(TailMBB.succ_size()); |
| BlockFrequency AccumulatedMBBFreq; |
| |
| // Aggregate edge frequency of successor edge j: |
| // edgeFreq(j) = sum (freq(bb) * edgeProb(bb, j)), |
| // where bb is a basic block that is in SameTails. |
| for (const auto &Src : SameTails) { |
| const MachineBasicBlock *SrcMBB = Src.getBlock(); |
| BlockFrequency BlockFreq = MBBFreqInfo.getBlockFreq(SrcMBB); |
| AccumulatedMBBFreq += BlockFreq; |
| |
| // It is not necessary to recompute edge weights if TailBB has less than two |
| // successors. |
| if (TailMBB.succ_size() <= 1) |
| continue; |
| |
| auto EdgeFreq = EdgeFreqLs.begin(); |
| |
| for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end(); |
| SuccI != SuccE; ++SuccI, ++EdgeFreq) |
| *EdgeFreq += BlockFreq * MBPI.getEdgeProbability(SrcMBB, *SuccI); |
| } |
| |
| MBBFreqInfo.setBlockFreq(&TailMBB, AccumulatedMBBFreq); |
| |
| if (TailMBB.succ_size() <= 1) |
| return; |
| |
| auto SumEdgeFreq = |
| std::accumulate(EdgeFreqLs.begin(), EdgeFreqLs.end(), BlockFrequency(0)) |
| .getFrequency(); |
| auto EdgeFreq = EdgeFreqLs.begin(); |
| |
| if (SumEdgeFreq > 0) { |
| for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end(); |
| SuccI != SuccE; ++SuccI, ++EdgeFreq) { |
| auto Prob = BranchProbability::getBranchProbability( |
| EdgeFreq->getFrequency(), SumEdgeFreq); |
| TailMBB.setSuccProbability(SuccI, Prob); |
| } |
| } |
| } |
| |
| //===----------------------------------------------------------------------===// |
| // Branch Optimization |
| //===----------------------------------------------------------------------===// |
| |
| bool BranchFolder::OptimizeBranches(MachineFunction &MF) { |
| bool MadeChange = false; |
| |
| // Make sure blocks are numbered in order |
| MF.RenumberBlocks(); |
| // Renumbering blocks alters EH scope membership, recalculate it. |
| EHScopeMembership = getEHScopeMembership(MF); |
| |
| for (MachineBasicBlock &MBB : |
| llvm::make_early_inc_range(llvm::drop_begin(MF))) { |
| MadeChange |= OptimizeBlock(&MBB); |
| |
| // If it is dead, remove it. |
| if (MBB.pred_empty()) { |
| RemoveDeadBlock(&MBB); |
| MadeChange = true; |
| ++NumDeadBlocks; |
| } |
| } |
| |
| return MadeChange; |
| } |
| |
| // Blocks should be considered empty if they contain only debug info; |
| // else the debug info would affect codegen. |
| static bool IsEmptyBlock(MachineBasicBlock *MBB) { |
| return MBB->getFirstNonDebugInstr(true) == MBB->end(); |
| } |
| |
| // Blocks with only debug info and branches should be considered the same |
| // as blocks with only branches. |
| static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) { |
| MachineBasicBlock::iterator I = MBB->getFirstNonDebugInstr(); |
| assert(I != MBB->end() && "empty block!"); |
| return I->isBranch(); |
| } |
| |
| /// IsBetterFallthrough - Return true if it would be clearly better to |
| /// fall-through to MBB1 than to fall through into MBB2. This has to return |
| /// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will |
| /// result in infinite loops. |
| static bool IsBetterFallthrough(MachineBasicBlock *MBB1, |
| MachineBasicBlock *MBB2) { |
| assert(MBB1 && MBB2 && "Unknown MachineBasicBlock"); |
| |
| // Right now, we use a simple heuristic. If MBB2 ends with a call, and |
| // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to |
| // optimize branches that branch to either a return block or an assert block |
| // into a fallthrough to the return. |
| MachineBasicBlock::iterator MBB1I = MBB1->getLastNonDebugInstr(); |
| MachineBasicBlock::iterator MBB2I = MBB2->getLastNonDebugInstr(); |
| if (MBB1I == MBB1->end() || MBB2I == MBB2->end()) |
| return false; |
| |
| // If there is a clear successor ordering we make sure that one block |
| // will fall through to the next |
| if (MBB1->isSuccessor(MBB2)) return true; |
| if (MBB2->isSuccessor(MBB1)) return false; |
| |
| return MBB2I->isCall() && !MBB1I->isCall(); |
| } |
| |
| /// getBranchDebugLoc - Find and return, if any, the DebugLoc of the branch |
| /// instructions on the block. |
| static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) { |
| MachineBasicBlock::iterator I = MBB.getLastNonDebugInstr(); |
| if (I != MBB.end() && I->isBranch()) |
| return I->getDebugLoc(); |
| return DebugLoc(); |
| } |
| |
| static void copyDebugInfoToPredecessor(const TargetInstrInfo *TII, |
| MachineBasicBlock &MBB, |
| MachineBasicBlock &PredMBB) { |
| auto InsertBefore = PredMBB.getFirstTerminator(); |
| for (MachineInstr &MI : MBB.instrs()) |
| if (MI.isDebugInstr()) { |
| TII->duplicate(PredMBB, InsertBefore, MI); |
| LLVM_DEBUG(dbgs() << "Copied debug entity from empty block to pred: " |
| << MI); |
| } |
| } |
| |
| static void copyDebugInfoToSuccessor(const TargetInstrInfo *TII, |
| MachineBasicBlock &MBB, |
| MachineBasicBlock &SuccMBB) { |
| auto InsertBefore = SuccMBB.SkipPHIsAndLabels(SuccMBB.begin()); |
| for (MachineInstr &MI : MBB.instrs()) |
| if (MI.isDebugInstr()) { |
| TII->duplicate(SuccMBB, InsertBefore, MI); |
| LLVM_DEBUG(dbgs() << "Copied debug entity from empty block to succ: " |
| << MI); |
| } |
| } |
| |
| // Try to salvage DBG_VALUE instructions from an otherwise empty block. If such |
| // a basic block is removed we would lose the debug information unless we have |
| // copied the information to a predecessor/successor. |
| // |
| // TODO: This function only handles some simple cases. An alternative would be |
| // to run a heavier analysis, such as the LiveDebugValues pass, before we do |
| // branch folding. |
| static void salvageDebugInfoFromEmptyBlock(const TargetInstrInfo *TII, |
| MachineBasicBlock &MBB) { |
| assert(IsEmptyBlock(&MBB) && "Expected an empty block (except debug info)."); |
| // If this MBB is the only predecessor of a successor it is legal to copy |
| // DBG_VALUE instructions to the beginning of the successor. |
| for (MachineBasicBlock *SuccBB : MBB.successors()) |
| if (SuccBB->pred_size() == 1) |
| copyDebugInfoToSuccessor(TII, MBB, *SuccBB); |
| // If this MBB is the only successor of a predecessor it is legal to copy the |
| // DBG_VALUE instructions to the end of the predecessor (just before the |
| // terminators, assuming that the terminator isn't affecting the DBG_VALUE). |
| for (MachineBasicBlock *PredBB : MBB.predecessors()) |
| if (PredBB->succ_size() == 1) |
| copyDebugInfoToPredecessor(TII, MBB, *PredBB); |
| } |
| |
| bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { |
| bool MadeChange = false; |
| MachineFunction &MF = *MBB->getParent(); |
| ReoptimizeBlock: |
| |
| MachineFunction::iterator FallThrough = MBB->getIterator(); |
| ++FallThrough; |
| |
| // Make sure MBB and FallThrough belong to the same EH scope. |
| bool SameEHScope = true; |
| if (!EHScopeMembership.empty() && FallThrough != MF.end()) { |
| auto MBBEHScope = EHScopeMembership.find(MBB); |
| assert(MBBEHScope != EHScopeMembership.end()); |
| auto FallThroughEHScope = EHScopeMembership.find(&*FallThrough); |
| assert(FallThroughEHScope != EHScopeMembership.end()); |
| SameEHScope = MBBEHScope->second == FallThroughEHScope->second; |
| } |
| |
| // Analyze the branch in the current block. As a side-effect, this may cause |
| // the block to become empty. |
| MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr; |
| SmallVector<MachineOperand, 4> CurCond; |
| bool CurUnAnalyzable = |
| TII->analyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); |
| |
| // If this block is empty, make everyone use its fall-through, not the block |
| // explicitly. Landing pads should not do this since the landing-pad table |
| // points to this block. Blocks with their addresses taken shouldn't be |
| // optimized away. |
| if (IsEmptyBlock(MBB) && !MBB->isEHPad() && !MBB->hasAddressTaken() && |
| SameEHScope) { |
| salvageDebugInfoFromEmptyBlock(TII, *MBB); |
| // Dead block? Leave for cleanup later. |
| if (MBB->pred_empty()) return MadeChange; |
| |
| if (FallThrough == MF.end()) { |
| // TODO: Simplify preds to not branch here if possible! |
| } else if (FallThrough->isEHPad()) { |
| // Don't rewrite to a landing pad fallthough. That could lead to the case |
| // where a BB jumps to more than one landing pad. |
| // TODO: Is it ever worth rewriting predecessors which don't already |
| // jump to a landing pad, and so can safely jump to the fallthrough? |
| } else if (MBB->isSuccessor(&*FallThrough)) { |
| // Rewrite all predecessors of the old block to go to the fallthrough |
| // instead. |
| while (!MBB->pred_empty()) { |
| MachineBasicBlock *Pred = *(MBB->pred_end()-1); |
| Pred->ReplaceUsesOfBlockWith(MBB, &*FallThrough); |
| } |
| // If MBB was the target of a jump table, update jump tables to go to the |
| // fallthrough instead. |
| if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) |
| MJTI->ReplaceMBBInJumpTables(MBB, &*FallThrough); |
| MadeChange = true; |
| } |
| return MadeChange; |
| } |
| |
| // Check to see if we can simplify the terminator of the block before this |
| // one. |
| MachineBasicBlock &PrevBB = *std::prev(MachineFunction::iterator(MBB)); |
| |
| MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr; |
| SmallVector<MachineOperand, 4> PriorCond; |
| bool PriorUnAnalyzable = |
| TII->analyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); |
| if (!PriorUnAnalyzable) { |
| // If the previous branch is conditional and both conditions go to the same |
| // destination, remove the branch, replacing it with an unconditional one or |
| // a fall-through. |
| if (PriorTBB && PriorTBB == PriorFBB) { |
| DebugLoc dl = getBranchDebugLoc(PrevBB); |
| TII->removeBranch(PrevBB); |
| PriorCond.clear(); |
| if (PriorTBB != MBB) |
| TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl); |
| MadeChange = true; |
| ++NumBranchOpts; |
| goto ReoptimizeBlock; |
| } |
| |
| // If the previous block unconditionally falls through to this block and |
| // this block has no other predecessors, move the contents of this block |
| // into the prior block. This doesn't usually happen when SimplifyCFG |
| // has been used, but it can happen if tail merging splits a fall-through |
| // predecessor of a block. |
| // This has to check PrevBB->succ_size() because EH edges are ignored by |
| // analyzeBranch. |
| if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 && |
| PrevBB.succ_size() == 1 && |
| !MBB->hasAddressTaken() && !MBB->isEHPad()) { |
| LLVM_DEBUG(dbgs() << "\nMerging into block: " << PrevBB |
| << "From MBB: " << *MBB); |
| // Remove redundant DBG_VALUEs first. |
| if (!PrevBB.empty()) { |
| MachineBasicBlock::iterator PrevBBIter = PrevBB.end(); |
| --PrevBBIter; |
| MachineBasicBlock::iterator MBBIter = MBB->begin(); |
| // Check if DBG_VALUE at the end of PrevBB is identical to the |
| // DBG_VALUE at the beginning of MBB. |
| while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end() |
| && PrevBBIter->isDebugInstr() && MBBIter->isDebugInstr()) { |
| if (!MBBIter->isIdenticalTo(*PrevBBIter)) |
| break; |
| MachineInstr &DuplicateDbg = *MBBIter; |
| ++MBBIter; -- PrevBBIter; |
| DuplicateDbg.eraseFromParent(); |
| } |
| } |
| PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end()); |
| PrevBB.removeSuccessor(PrevBB.succ_begin()); |
| assert(PrevBB.succ_empty()); |
| PrevBB.transferSuccessors(MBB); |
| MadeChange = true; |
| return MadeChange; |
| } |
| |
| // If the previous branch *only* branches to *this* block (conditional or |
| // not) remove the branch. |
| if (PriorTBB == MBB && !PriorFBB) { |
| TII->removeBranch(PrevBB); |
| MadeChange = true; |
| ++NumBranchOpts; |
| goto ReoptimizeBlock; |
| } |
| |
| // If the prior block branches somewhere else on the condition and here if |
| // the condition is false, remove the uncond second branch. |
| if (PriorFBB == MBB) { |
| DebugLoc dl = getBranchDebugLoc(PrevBB); |
| TII->removeBranch(PrevBB); |
| TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl); |
| MadeChange = true; |
| ++NumBranchOpts; |
| goto ReoptimizeBlock; |
| } |
| |
| // If the prior block branches here on true and somewhere else on false, and |
| // if the branch condition is reversible, reverse the branch to create a |
| // fall-through. |
| if (PriorTBB == MBB) { |
| SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); |
| if (!TII->reverseBranchCondition(NewPriorCond)) { |
| DebugLoc dl = getBranchDebugLoc(PrevBB); |
| TII->removeBranch(PrevBB); |
| TII->insertBranch(PrevBB, PriorFBB, nullptr, NewPriorCond, dl); |
| MadeChange = true; |
| ++NumBranchOpts; |
| goto ReoptimizeBlock; |
| } |
| } |
| |
| // If this block has no successors (e.g. it is a return block or ends with |
| // a call to a no-return function like abort or __cxa_throw) and if the pred |
| // falls through into this block, and if it would otherwise fall through |
| // into the block after this, move this block to the end of the function. |
| // |
| // We consider it more likely that execution will stay in the function (e.g. |
| // due to loops) than it is to exit it. This asserts in loops etc, moving |
| // the assert condition out of the loop body. |
| if (MBB->succ_empty() && !PriorCond.empty() && !PriorFBB && |
| MachineFunction::iterator(PriorTBB) == FallThrough && |
| !MBB->canFallThrough()) { |
| bool DoTransform = true; |
| |
| // We have to be careful that the succs of PredBB aren't both no-successor |
| // blocks. If neither have successors and if PredBB is the second from |
| // last block in the function, we'd just keep swapping the two blocks for |
| // last. Only do the swap if one is clearly better to fall through than |
| // the other. |
| if (FallThrough == --MF.end() && |
| !IsBetterFallthrough(PriorTBB, MBB)) |
| DoTransform = false; |
| |
| if (DoTransform) { |
| // Reverse the branch so we will fall through on the previous true cond. |
| SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); |
| if (!TII->reverseBranchCondition(NewPriorCond)) { |
| LLVM_DEBUG(dbgs() << "\nMoving MBB: " << *MBB |
| << "To make fallthrough to: " << *PriorTBB << "\n"); |
| |
| DebugLoc dl = getBranchDebugLoc(PrevBB); |
| TII->removeBranch(PrevBB); |
| TII->insertBranch(PrevBB, MBB, nullptr, NewPriorCond, dl); |
| |
| // Move this block to the end of the function. |
| MBB->moveAfter(&MF.back()); |
| MadeChange = true; |
| ++NumBranchOpts; |
| return MadeChange; |
| } |
| } |
| } |
| } |
| |
| bool OptForSize = |
| MF.getFunction().hasOptSize() || |
| llvm::shouldOptimizeForSize(MBB, PSI, &MBBFreqInfo); |
| if (!IsEmptyBlock(MBB) && MBB->pred_size() == 1 && OptForSize) { |
| // Changing "Jcc foo; foo: jmp bar;" into "Jcc bar;" might change the branch |
| // direction, thereby defeating careful block placement and regressing |
| // performance. Therefore, only consider this for optsize functions. |
| MachineInstr &TailCall = *MBB->getFirstNonDebugInstr(); |
| if (TII->isUnconditionalTailCall(TailCall)) { |
| MachineBasicBlock *Pred = *MBB->pred_begin(); |
| MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr; |
| SmallVector<MachineOperand, 4> PredCond; |
| bool PredAnalyzable = |
| !TII->analyzeBranch(*Pred, PredTBB, PredFBB, PredCond, true); |
| |
| if (PredAnalyzable && !PredCond.empty() && PredTBB == MBB && |
| PredTBB != PredFBB) { |
| // The predecessor has a conditional branch to this block which consists |
| // of only a tail call. Try to fold the tail call into the conditional |
| // branch. |
| if (TII->canMakeTailCallConditional(PredCond, TailCall)) { |
| // TODO: It would be nice if analyzeBranch() could provide a pointer |
| // to the branch instruction so replaceBranchWithTailCall() doesn't |
| // have to search for it. |
| TII->replaceBranchWithTailCall(*Pred, PredCond, TailCall); |
| ++NumTailCalls; |
| Pred->removeSuccessor(MBB); |
| MadeChange = true; |
| return MadeChange; |
| } |
| } |
| // If the predecessor is falling through to this block, we could reverse |
| // the branch condition and fold the tail call into that. However, after |
| // that we might have to re-arrange the CFG to fall through to the other |
| // block and there is a high risk of regressing code size rather than |
| // improving it. |
| } |
| } |
| |
| if (!CurUnAnalyzable) { |
| // If this is a two-way branch, and the FBB branches to this block, reverse |
| // the condition so the single-basic-block loop is faster. Instead of: |
| // Loop: xxx; jcc Out; jmp Loop |
| // we want: |
| // Loop: xxx; jncc Loop; jmp Out |
| if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { |
| SmallVector<MachineOperand, 4> NewCond(CurCond); |
| if (!TII->reverseBranchCondition(NewCond)) { |
| DebugLoc dl = getBranchDebugLoc(*MBB); |
| TII->removeBranch(*MBB); |
| TII->insertBranch(*MBB, CurFBB, CurTBB, NewCond, dl); |
| MadeChange = true; |
| ++NumBranchOpts; |
| goto ReoptimizeBlock; |
| } |
| } |
| |
| // If this branch is the only thing in its block, see if we can forward |
| // other blocks across it. |
| if (CurTBB && CurCond.empty() && !CurFBB && |
| IsBranchOnlyBlock(MBB) && CurTBB != MBB && |
| !MBB->hasAddressTaken() && !MBB->isEHPad()) { |
| DebugLoc dl = getBranchDebugLoc(*MBB); |
| // This block may contain just an unconditional branch. Because there can |
| // be 'non-branch terminators' in the block, try removing the branch and |
| // then seeing if the block is empty. |
| TII->removeBranch(*MBB); |
| // If the only things remaining in the block are debug info, remove these |
| // as well, so this will behave the same as an empty block in non-debug |
| // mode. |
| if (IsEmptyBlock(MBB)) { |
| // Make the block empty, losing the debug info (we could probably |
| // improve this in some cases.) |
| MBB->erase(MBB->begin(), MBB->end()); |
| } |
| // If this block is just an unconditional branch to CurTBB, we can |
| // usually completely eliminate the block. The only case we cannot |
| // completely eliminate the block is when the block before this one |
| // falls through into MBB and we can't understand the prior block's branch |
| // condition. |
| if (MBB->empty()) { |
| bool PredHasNoFallThrough = !PrevBB.canFallThrough(); |
| if (PredHasNoFallThrough || !PriorUnAnalyzable || |
| !PrevBB.isSuccessor(MBB)) { |
| // If the prior block falls through into us, turn it into an |
| // explicit branch to us to make updates simpler. |
| if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && |
| PriorTBB != MBB && PriorFBB != MBB) { |
| if (!PriorTBB) { |
| assert(PriorCond.empty() && !PriorFBB && |
| "Bad branch analysis"); |
| PriorTBB = MBB; |
| } else { |
| assert(!PriorFBB && "Machine CFG out of date!"); |
| PriorFBB = MBB; |
| } |
| DebugLoc pdl = getBranchDebugLoc(PrevBB); |
| TII->removeBranch(PrevBB); |
| TII->insertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, pdl); |
| } |
| |
| // Iterate through all the predecessors, revectoring each in-turn. |
| size_t PI = 0; |
| bool DidChange = false; |
| bool HasBranchToSelf = false; |
| while(PI != MBB->pred_size()) { |
| MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); |
| if (PMBB == MBB) { |
| // If this block has an uncond branch to itself, leave it. |
| ++PI; |
| HasBranchToSelf = true; |
| } else { |
| DidChange = true; |
| PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); |
| // If this change resulted in PMBB ending in a conditional |
| // branch where both conditions go to the same destination, |
| // change this to an unconditional branch. |
| MachineBasicBlock *NewCurTBB = nullptr, *NewCurFBB = nullptr; |
| SmallVector<MachineOperand, 4> NewCurCond; |
| bool NewCurUnAnalyzable = TII->analyzeBranch( |
| *PMBB, NewCurTBB, NewCurFBB, NewCurCond, true); |
| if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) { |
| DebugLoc pdl = getBranchDebugLoc(*PMBB); |
| TII->removeBranch(*PMBB); |
| NewCurCond.clear(); |
| TII->insertBranch(*PMBB, NewCurTBB, nullptr, NewCurCond, pdl); |
| MadeChange = true; |
| ++NumBranchOpts; |
| } |
| } |
| } |
| |
| // Change any jumptables to go to the new MBB. |
| if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) |
| MJTI->ReplaceMBBInJumpTables(MBB, CurTBB); |
| if (DidChange) { |
| ++NumBranchOpts; |
| MadeChange = true; |
| if (!HasBranchToSelf) return MadeChange; |
| } |
| } |
| } |
| |
| // Add the branch back if the block is more than just an uncond branch. |
| TII->insertBranch(*MBB, CurTBB, nullptr, CurCond, dl); |
| } |
| } |
| |
| // If the prior block doesn't fall through into this block, and if this |
| // block doesn't fall through into some other block, see if we can find a |
| // place to move this block where a fall-through will happen. |
| if (!PrevBB.canFallThrough()) { |
| // Now we know that there was no fall-through into this block, check to |
| // see if it has a fall-through into its successor. |
| bool CurFallsThru = MBB->canFallThrough(); |
| |
| if (!MBB->isEHPad()) { |
| // Check all the predecessors of this block. If one of them has no fall |
| // throughs, and analyzeBranch thinks it _could_ fallthrough to this |
| // block, move this block right after it. |
| for (MachineBasicBlock *PredBB : MBB->predecessors()) { |
| // Analyze the branch at the end of the pred. |
| MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr; |
| SmallVector<MachineOperand, 4> PredCond; |
| if (PredBB != MBB && !PredBB->canFallThrough() && |
| !TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) && |
| (PredTBB == MBB || PredFBB == MBB) && |
| (!CurFallsThru || !CurTBB || !CurFBB) && |
| (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { |
| // If the current block doesn't fall through, just move it. |
| // If the current block can fall through and does not end with a |
| // conditional branch, we need to append an unconditional jump to |
| // the (current) next block. To avoid a possible compile-time |
| // infinite loop, move blocks only backward in this case. |
| // Also, if there are already 2 branches here, we cannot add a third; |
| // this means we have the case |
| // Bcc next |
| // B elsewhere |
| // next: |
| if (CurFallsThru) { |
| MachineBasicBlock *NextBB = &*std::next(MBB->getIterator()); |
| CurCond.clear(); |
| TII->insertBranch(*MBB, NextBB, nullptr, CurCond, DebugLoc()); |
| } |
| MBB->moveAfter(PredBB); |
| MadeChange = true; |
| goto ReoptimizeBlock; |
| } |
| } |
| } |
| |
| if (!CurFallsThru) { |
| // Check analyzable branch-successors to see if we can move this block |
| // before one. |
| if (!CurUnAnalyzable) { |
| for (MachineBasicBlock *SuccBB : {CurFBB, CurTBB}) { |
| if (!SuccBB) |
| continue; |
| // Analyze the branch at the end of the block before the succ. |
| MachineFunction::iterator SuccPrev = --SuccBB->getIterator(); |
| |
| // If this block doesn't already fall-through to that successor, and |
| // if the succ doesn't already have a block that can fall through into |
| // it, we can arrange for the fallthrough to happen. |
| if (SuccBB != MBB && &*SuccPrev != MBB && |
| !SuccPrev->canFallThrough()) { |
| MBB->moveBefore(SuccBB); |
| MadeChange = true; |
| goto ReoptimizeBlock; |
| } |
| } |
| } |
| |
| // Okay, there is no really great place to put this block. If, however, |
| // the block before this one would be a fall-through if this block were |
| // removed, move this block to the end of the function. There is no real |
| // advantage in "falling through" to an EH block, so we don't want to |
| // perform this transformation for that case. |
| // |
| // Also, Windows EH introduced the possibility of an arbitrary number of |
| // successors to a given block. The analyzeBranch call does not consider |
| // exception handling and so we can get in a state where a block |
| // containing a call is followed by multiple EH blocks that would be |
| // rotated infinitely at the end of the function if the transformation |
| // below were performed for EH "FallThrough" blocks. Therefore, even if |
| // that appears not to be happening anymore, we should assume that it is |
| // possible and not remove the "!FallThrough()->isEHPad" condition below. |
| MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr; |
| SmallVector<MachineOperand, 4> PrevCond; |
| if (FallThrough != MF.end() && |
| !FallThrough->isEHPad() && |
| !TII->analyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) && |
| PrevBB.isSuccessor(&*FallThrough)) { |
| MBB->moveAfter(&MF.back()); |
| MadeChange = true; |
| return MadeChange; |
| } |
| } |
| } |
| |
| return MadeChange; |
| } |
| |
| //===----------------------------------------------------------------------===// |
| // Hoist Common Code |
| //===----------------------------------------------------------------------===// |
| |
| bool BranchFolder::HoistCommonCode(MachineFunction &MF) { |
| bool MadeChange = false; |
| for (MachineBasicBlock &MBB : llvm::make_early_inc_range(MF)) |
| MadeChange |= HoistCommonCodeInSuccs(&MBB); |
| |
| return MadeChange; |
| } |
| |
| /// findFalseBlock - BB has a fallthrough. Find its 'false' successor given |
| /// its 'true' successor. |
| static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB, |
| MachineBasicBlock *TrueBB) { |
| for (MachineBasicBlock *SuccBB : BB->successors()) |
| if (SuccBB != TrueBB) |
| return SuccBB; |
| return nullptr; |
| } |
| |
| template <class Container> |
| static void addRegAndItsAliases(Register Reg, const TargetRegisterInfo *TRI, |
| Container &Set) { |
| if (Reg.isPhysical()) { |
| for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) |
| Set.insert(*AI); |
| } else { |
| Set.insert(Reg); |
| } |
| } |
| |
| /// findHoistingInsertPosAndDeps - Find the location to move common instructions |
| /// in successors to. The location is usually just before the terminator, |
| /// however if the terminator is a conditional branch and its previous |
| /// instruction is the flag setting instruction, the previous instruction is |
| /// the preferred location. This function also gathers uses and defs of the |
| /// instructions from the insertion point to the end of the block. The data is |
| /// used by HoistCommonCodeInSuccs to ensure safety. |
| static |
| MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB, |
| const TargetInstrInfo *TII, |
| const TargetRegisterInfo *TRI, |
| SmallSet<Register, 4> &Uses, |
| SmallSet<Register, 4> &Defs) { |
| MachineBasicBlock::iterator Loc = MBB->getFirstTerminator(); |
| if (!TII->isUnpredicatedTerminator(*Loc)) |
| return MBB->end(); |
| |
| for (const MachineOperand &MO : Loc->operands()) { |
| if (!MO.isReg()) |
| continue; |
| Register Reg = MO.getReg(); |
| if (!Reg) |
| continue; |
| if (MO.isUse()) { |
| addRegAndItsAliases(Reg, TRI, Uses); |
| } else { |
| if (!MO.isDead()) |
| // Don't try to hoist code in the rare case the terminator defines a |
| // register that is later used. |
| return MBB->end(); |
| |
| // If the terminator defines a register, make sure we don't hoist |
| // the instruction whose def might be clobbered by the terminator. |
| addRegAndItsAliases(Reg, TRI, Defs); |
| } |
| } |
| |
| if (Uses.empty()) |
| return Loc; |
| // If the terminator is the only instruction in the block and Uses is not |
| // empty (or we would have returned above), we can still safely hoist |
| // instructions just before the terminator as long as the Defs/Uses are not |
| // violated (which is checked in HoistCommonCodeInSuccs). |
| if (Loc == MBB->begin()) |
| return Loc; |
| |
| // The terminator is probably a conditional branch, try not to separate the |
| // branch from condition setting instruction. |
| MachineBasicBlock::iterator PI = prev_nodbg(Loc, MBB->begin()); |
| |
| bool IsDef = false; |
| for (const MachineOperand &MO : PI->operands()) { |
| // If PI has a regmask operand, it is probably a call. Separate away. |
| if (MO.isRegMask()) |
| return Loc; |
| if (!MO.isReg() || MO.isUse()) |
| continue; |
| Register Reg = MO.getReg(); |
| if (!Reg) |
| continue; |
| if (Uses.count(Reg)) { |
| IsDef = true; |
| break; |
| } |
| } |
| if (!IsDef) |
| // The condition setting instruction is not just before the conditional |
| // branch. |
| return Loc; |
| |
| // Be conservative, don't insert instruction above something that may have |
| // side-effects. And since it's potentially bad to separate flag setting |
| // instruction from the conditional branch, just abort the optimization |
| // completely. |
| // Also avoid moving code above predicated instruction since it's hard to |
| // reason about register liveness with predicated instruction. |
| bool DontMoveAcrossStore = true; |
| if (!PI->isSafeToMove(nullptr, DontMoveAcrossStore) || TII->isPredicated(*PI)) |
| return MBB->end(); |
| |
| // Find out what registers are live. Note this routine is ignoring other live |
| // registers which are only used by instructions in successor blocks. |
| for (const MachineOperand &MO : PI->operands()) { |
| if (!MO.isReg()) |
| continue; |
| Register Reg = MO.getReg(); |
| if (!Reg) |
| continue; |
| if (MO.isUse()) { |
| addRegAndItsAliases(Reg, TRI, Uses); |
| } else { |
| if (Uses.erase(Reg)) { |
| if (Register::isPhysicalRegister(Reg)) { |
| for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs) |
| Uses.erase(*SubRegs); // Use sub-registers to be conservative |
| } |
| } |
| addRegAndItsAliases(Reg, TRI, Defs); |
| } |
| } |
| |
| return PI; |
| } |
| |
| bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) { |
| MachineBasicBlock *TBB = nullptr, *FBB = nullptr; |
| SmallVector<MachineOperand, 4> Cond; |
| if (TII->analyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty()) |
| return false; |
| |
| if (!FBB) FBB = findFalseBlock(MBB, TBB); |
| if (!FBB) |
| // Malformed bcc? True and false blocks are the same? |
| return false; |
| |
| // Restrict the optimization to cases where MBB is the only predecessor, |
| // it is an obvious win. |
| if (TBB->pred_size() > 1 || FBB->pred_size() > 1) |
| return false; |
| |
| // Find a suitable position to hoist the common instructions to. Also figure |
| // out which registers are used or defined by instructions from the insertion |
| // point to the end of the block. |
| SmallSet<Register, 4> Uses, Defs; |
| MachineBasicBlock::iterator Loc = |
| findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs); |
| if (Loc == MBB->end()) |
| return false; |
| |
| bool HasDups = false; |
| SmallSet<Register, 4> ActiveDefsSet, AllDefsSet; |
| MachineBasicBlock::iterator TIB = TBB->begin(); |
| MachineBasicBlock::iterator FIB = FBB->begin(); |
| MachineBasicBlock::iterator TIE = TBB->end(); |
| MachineBasicBlock::iterator FIE = FBB->end(); |
| while (TIB != TIE && FIB != FIE) { |
| // Skip dbg_value instructions. These do not count. |
| TIB = skipDebugInstructionsForward(TIB, TIE, false); |
| FIB = skipDebugInstructionsForward(FIB, FIE, false); |
| if (TIB == TIE || FIB == FIE) |
| break; |
| |
| if (!TIB->isIdenticalTo(*FIB, MachineInstr::CheckKillDead)) |
| break; |
| |
| if (TII->isPredicated(*TIB)) |
| // Hard to reason about register liveness with predicated instruction. |
| break; |
| |
| bool IsSafe = true; |
| for (MachineOperand &MO : TIB->operands()) { |
| // Don't attempt to hoist instructions with register masks. |
| if (MO.isRegMask()) { |
| IsSafe = false; |
| break; |
| } |
| if (!MO.isReg()) |
| continue; |
| Register Reg = MO.getReg(); |
| if (!Reg) |
| continue; |
| if (MO.isDef()) { |
| if (Uses.count(Reg)) { |
| // Avoid clobbering a register that's used by the instruction at |
| // the point of insertion. |
| IsSafe = false; |
| break; |
| } |
| |
| if (Defs.count(Reg) && !MO.isDead()) { |
| // Don't hoist the instruction if the def would be clobber by the |
| // instruction at the point insertion. FIXME: This is overly |
| // conservative. It should be possible to hoist the instructions |
| // in BB2 in the following example: |
| // BB1: |
| // r1, eflag = op1 r2, r3 |
| // brcc eflag |
| // |
| // BB2: |
| // r1 = op2, ... |
| // = op3, killed r1 |
| IsSafe = false; |
| break; |
| } |
| } else if (!ActiveDefsSet.count(Reg)) { |
| if (Defs.count(Reg)) { |
| // Use is defined by the instruction at the point of insertion. |
| IsSafe = false; |
| break; |
| } |
| |
| if (MO.isKill() && Uses.count(Reg)) |
| // Kills a register that's read by the instruction at the point of |
| // insertion. Remove the kill marker. |
| MO.setIsKill(false); |
| } |
| } |
| if (!IsSafe) |
| break; |
| |
| bool DontMoveAcrossStore = true; |
| if (!TIB->isSafeToMove(nullptr, DontMoveAcrossStore)) |
| break; |
| |
| // Remove kills from ActiveDefsSet, these registers had short live ranges. |
| for (const MachineOperand &MO : TIB->operands()) { |
| if (!MO.isReg() || !MO.isUse() || !MO.isKill()) |
| continue; |
| Register Reg = MO.getReg(); |
| if (!Reg) |
| continue; |
| if (!AllDefsSet.count(Reg)) { |
| continue; |
| } |
| if (Reg.isPhysical()) { |
| for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) |
| ActiveDefsSet.erase(*AI); |
| } else { |
| ActiveDefsSet.erase(Reg); |
| } |
| } |
| |
| // Track local defs so we can update liveins. |
| for (const MachineOperand &MO : TIB->operands()) { |
| if (!MO.isReg() || !MO.isDef() || MO.isDead()) |
| continue; |
| Register Reg = MO.getReg(); |
| if (!Reg || Reg.isVirtual()) |
| continue; |
| addRegAndItsAliases(Reg, TRI, ActiveDefsSet); |
| addRegAndItsAliases(Reg, TRI, AllDefsSet); |
| } |
| |
| HasDups = true; |
| ++TIB; |
| ++FIB; |
| } |
| |
| if (!HasDups) |
| return false; |
| |
| MBB->splice(Loc, TBB, TBB->begin(), TIB); |
| FBB->erase(FBB->begin(), FIB); |
| |
| if (UpdateLiveIns) { |
| recomputeLiveIns(*TBB); |
| recomputeLiveIns(*FBB); |
| } |
| |
| ++NumHoist; |
| return true; |
| } |