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Diffstat (limited to 'contrib/llvm/lib/CodeGen/BranchFolding.cpp')
| -rw-r--r-- | contrib/llvm/lib/CodeGen/BranchFolding.cpp | 2106 |
1 files changed, 0 insertions, 2106 deletions
diff --git a/contrib/llvm/lib/CodeGen/BranchFolding.cpp b/contrib/llvm/lib/CodeGen/BranchFolding.cpp deleted file mode 100644 index fb54b5d6c8d8..000000000000 --- a/contrib/llvm/lib/CodeGen/BranchFolding.cpp +++ /dev/null @@ -1,2106 +0,0 @@ -//===- 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/DenseMap.h" -#include "llvm/ADT/STLExtras.h" -#include "llvm/ADT/SmallPtrSet.h" -#include "llvm/ADT/SmallSet.h" -#include "llvm/ADT/SmallVector.h" -#include "llvm/ADT/Statistic.h" -#include "llvm/CodeGen/Analysis.h" -#include "llvm/CodeGen/LivePhysRegs.h" -#include "llvm/CodeGen/MachineBasicBlock.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/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/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> -#include <vector> - -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<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(); - BranchFolder::MBFIWrapper MBBFreqInfo( - getAnalysis<MachineBlockFrequencyInfo>()); - BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true, MBBFreqInfo, - getAnalysis<MachineBranchProbabilityInfo>()); - return Folder.OptimizeFunction(MF, MF.getSubtarget().getInstrInfo(), - MF.getSubtarget().getRegisterInfo(), - getAnalysisIfAvailable<MachineModuleInfo>()); -} - -BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist, - MBFIWrapper &FreqInfo, - const MachineBranchProbabilityInfo &ProbInfo, - unsigned MinTailLength) - : EnableHoistCommonCode(CommonHoist), MinCommonTailLength(MinTailLength), - MBBFreqInfo(FreqInfo), MBPI(ProbInfo) { - 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); - - // 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, - MachineModuleInfo *mmi, - MachineLoopInfo *mli, bool AfterPlacement) { - if (!tii) return false; - - TriedMerging.clear(); - - MachineRegisterInfo &MRI = MF.getRegInfo(); - AfterBlockPlacement = AfterPlacement; - TII = tii; - TRI = tri; - MMI = mmi; - MLI = mli; - this->MRI = &MRI; - - UpdateLiveIns = MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF); - if (!UpdateLiveIns) - MRI.invalidateLiveness(); - - // Fix CFG. The later algorithms expect it to be right. - bool MadeChange = false; - for (MachineBasicBlock &MBB : MF) { - MachineBasicBlock *TBB = nullptr, *FBB = nullptr; - SmallVector<MachineOperand, 4> Cond; - if (!TII->analyzeBranch(MBB, TBB, FBB, Cond, true)) - MadeChange |= MBB.CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); - } - - // 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(); - 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()); -} - -/// ComputeCommonTailLength - Given two machine basic blocks, compute the number -/// of instructions they actually have in common together at their end. Return -/// iterators for the first shared instruction in each block. -static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, - MachineBasicBlock *MBB2, - MachineBasicBlock::iterator &I1, - MachineBasicBlock::iterator &I2) { - I1 = MBB1->end(); - I2 = MBB2->end(); - - unsigned TailLen = 0; - while (I1 != MBB1->begin() && I2 != MBB2->begin()) { - --I1; --I2; - // Skip debugging pseudos; necessary to avoid changing the code. - while (!countsAsInstruction(*I1)) { - if (I1==MBB1->begin()) { - while (!countsAsInstruction(*I2)) { - if (I2==MBB2->begin()) { - // I1==DBG at begin; I2==DBG at begin - goto SkipTopCFIAndReturn; - } - --I2; - } - ++I2; - // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin - goto SkipTopCFIAndReturn; - } - --I1; - } - // I1==first (untested) non-DBG preceding known match - while (!countsAsInstruction(*I2)) { - if (I2==MBB2->begin()) { - ++I1; - // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin - goto SkipTopCFIAndReturn; - } - --I2; - } - // I1, I2==first (untested) non-DBGs preceding known match - if (!I1->isIdenticalTo(*I2) || - // 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. - I1->isInlineAsm()) { - ++I1; ++I2; - break; - } - ++TailLen; - } - // Back past possible debugging pseudos at beginning of block. This matters - // when one block differs from the other only by whether debugging pseudos - // are present at the beginning. (This way, the various checks later for - // I1==MBB1->begin() work as expected.) - if (I1 == MBB1->begin() && I2 != MBB2->begin()) { - --I2; - while (I2->isDebugInstr()) { - if (I2 == MBB2->begin()) - return TailLen; - --I2; - } - ++I2; - } - if (I2 == MBB2->begin() && I1 != MBB1->begin()) { - --I1; - while (I1->isDebugInstr()) { - if (I1 == MBB1->begin()) - return TailLen; - --I1; - } - ++I1; - } - -SkipTopCFIAndReturn: - // Ensure that I1 and I2 do not point to a CFI_INSTRUCTION. This can happen if - // I1 and I2 are non-identical when compared and then one or both of them ends - // up pointing to a CFI instruction after being incremented. For example: - /* - BB1: - ... - INSTRUCTION_A - ADD32ri8 <- last common instruction - ... - BB2: - ... - INSTRUCTION_B - CFI_INSTRUCTION - ADD32ri8 <- last common instruction - ... - */ - // When INSTRUCTION_A and INSTRUCTION_B are compared as not equal, after - // incrementing the iterators, I1 will point to ADD, however I2 will point to - // the CFI instruction. Later on, this leads to BB2 being 'hacked off' at the - // wrong place (in ReplaceTailWithBranchTo()) which results in losing this CFI - // instruction. - while (I1 != MBB1->end() && I1->isCFIInstruction()) { - ++I1; - } - - while (I2 != MBB2->end() && I2->isCFIInstruction()) { - ++I2; - } - - 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->mayLoad() || I->mayStore()) - 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 -} - -BlockFrequency -BranchFolder::MBFIWrapper::getBlockFreq(const MachineBasicBlock *MBB) const { - auto I = MergedBBFreq.find(MBB); - - if (I != MergedBBFreq.end()) - return I->second; - - return MBFI.getBlockFreq(MBB); -} - -void BranchFolder::MBFIWrapper::setBlockFreq(const MachineBasicBlock *MBB, - BlockFrequency F) { - MergedBBFreq[MBB] = F; -} - -raw_ostream & -BranchFolder::MBFIWrapper::printBlockFreq(raw_ostream &OS, - const MachineBasicBlock *MBB) const { - return MBFI.printBlockFreq(OS, getBlockFreq(MBB)); -} - -raw_ostream & -BranchFolder::MBFIWrapper::printBlockFreq(raw_ostream &OS, - const BlockFrequency Freq) const { - return MBFI.printBlockFreq(OS, Freq); -} - -void BranchFolder::MBFIWrapper::view(const Twine &Name, bool isSimple) { - MBFI.view(Name, isSimple); -} - -uint64_t -BranchFolder::MBFIWrapper::getEntryFreq() const { - return MBFI.getEntryFreq(); -} - -/// 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) { - // 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'); - - // 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 (I1 == MBB1->begin() && I2 == MBB2->begin() && - 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) && I2 == MBB2->begin()) - return true; - if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin()) - 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 && I1 == MBB1->begin() && I2 == MBB2->begin()) { - auto BothFallThrough = [](MachineBasicBlock *MBB) { - if (MBB->succ_size() != 0 && !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(); - return EffectiveTailLen >= 2 && MF->getFunction().hasOptSize() && - (I1 == MBB1->begin() || I2 == MBB2->begin()); -} - -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)) { - 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->mayLoad() || MBBICommon->mayStore()) - 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 (unsigned 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 not the entry - // block, 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()) - commonTailIndex = 1; - else if (SameTails.size() == 2 && - SameTails[1].getBlock()->isLayoutSuccessor( - SameTails[0].getBlock()) && - SameTails[0].tailIsWholeBlock()) - 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 && 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 (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) - TriedMerging.insert(MergePotentials[i].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. Can't tail merge these. - if (PBB->hasEHPadSuccessor()) - 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 (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end(); - I != E; ) { - MachineBasicBlock *MBB = &*I++; - 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() == 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) { - // 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; - } - - // 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 CFG for the prior block has extra edges, remove them. - MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, - !PriorCond.empty()); - - // 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.begin() != PrevBB.end()) { - 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; - } - } - } - } - - if (!IsEmptyBlock(MBB) && MBB->pred_size() == 1 && - MF.getFunction().hasOptSize()) { - // 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. - } - } - - // Analyze the branch in the current block. - MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr; - SmallVector<MachineOperand, 4> CurCond; - bool CurUnAnalyzable = - TII->analyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); - if (!CurUnAnalyzable) { - // If the CFG for the prior block has extra edges, remove them. - MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); - - // 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 (and fix the CFG). - 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; - PMBB->CorrectExtraCFGEdges(NewCurTBB, nullptr, false); - } - } - } - - // 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, 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) && - (!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 all successors to see if we can move this block before it. - for (MachineBasicBlock *SuccBB : MBB->successors()) { - // 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, - // and if the successor isn't an EH destination, we can arrange for the - // fallthrough to happen. - if (SuccBB != MBB && &*SuccPrev != MBB && - !SuccPrev->canFallThrough() && !CurUnAnalyzable && - !SuccBB->isEHPad()) { - 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 (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) { - MachineBasicBlock *MBB = &*I++; - 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(unsigned Reg, const TargetRegisterInfo *TRI, - Container &Set) { - if (TargetRegisterInfo::isPhysicalRegister(Reg)) { - 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<unsigned,4> &Uses, - SmallSet<unsigned,4> &Defs) { - MachineBasicBlock::iterator Loc = MBB->getFirstTerminator(); - if (!TII->isUnpredicatedTerminator(*Loc)) - return MBB->end(); - - for (const MachineOperand &MO : Loc->operands()) { - if (!MO.isReg()) - continue; - unsigned 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 = - skipDebugInstructionsBackward(std::prev(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; - unsigned 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; - unsigned Reg = MO.getReg(); - if (!Reg) - continue; - if (MO.isUse()) { - addRegAndItsAliases(Reg, TRI, Uses); - } else { - if (Uses.erase(Reg)) { - if (TargetRegisterInfo::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<unsigned, 4> Uses, Defs; - MachineBasicBlock::iterator Loc = - findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs); - if (Loc == MBB->end()) - return false; - - bool HasDups = false; - SmallSet<unsigned, 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); - FIB = skipDebugInstructionsForward(FIB, FIE); - 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; - unsigned 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; - unsigned Reg = MO.getReg(); - if (!Reg) - continue; - if (!AllDefsSet.count(Reg)) { - continue; - } - if (TargetRegisterInfo::isPhysicalRegister(Reg)) { - 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; - unsigned Reg = MO.getReg(); - if (!Reg || TargetRegisterInfo::isVirtualRegister(Reg)) - 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; -} |