| //===-- ImplicitNullChecks.cpp - Fold null checks into memory accesses ----===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This pass turns explicit null checks of the form |
| // |
| // test %r10, %r10 |
| // je throw_npe |
| // movl (%r10), %esi |
| // ... |
| // |
| // to |
| // |
| // faulting_load_op("movl (%r10), %esi", throw_npe) |
| // ... |
| // |
| // With the help of a runtime that understands the .fault_maps section, |
| // faulting_load_op branches to throw_npe if executing movl (%r10), %esi incurs |
| // a page fault. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/ADT/DenseSet.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/CodeGen/Passes.h" |
| #include "llvm/CodeGen/MachineFunction.h" |
| #include "llvm/CodeGen/MachineMemOperand.h" |
| #include "llvm/CodeGen/MachineOperand.h" |
| #include "llvm/CodeGen/MachineFunctionPass.h" |
| #include "llvm/CodeGen/MachineInstrBuilder.h" |
| #include "llvm/CodeGen/MachineRegisterInfo.h" |
| #include "llvm/CodeGen/MachineModuleInfo.h" |
| #include "llvm/IR/BasicBlock.h" |
| #include "llvm/IR/Instruction.h" |
| #include "llvm/IR/LLVMContext.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Target/TargetSubtargetInfo.h" |
| #include "llvm/Target/TargetInstrInfo.h" |
| |
| using namespace llvm; |
| |
| static cl::opt<unsigned> PageSize("imp-null-check-page-size", |
| cl::desc("The page size of the target in " |
| "bytes"), |
| cl::init(4096)); |
| |
| #define DEBUG_TYPE "implicit-null-checks" |
| |
| STATISTIC(NumImplicitNullChecks, |
| "Number of explicit null checks made implicit"); |
| |
| namespace { |
| |
| class ImplicitNullChecks : public MachineFunctionPass { |
| /// Represents one null check that can be made implicit. |
| struct NullCheck { |
| // The memory operation the null check can be folded into. |
| MachineInstr *MemOperation; |
| |
| // The instruction actually doing the null check (Ptr != 0). |
| MachineInstr *CheckOperation; |
| |
| // The block the check resides in. |
| MachineBasicBlock *CheckBlock; |
| |
| // The block branched to if the pointer is non-null. |
| MachineBasicBlock *NotNullSucc; |
| |
| // The block branched to if the pointer is null. |
| MachineBasicBlock *NullSucc; |
| |
| NullCheck() |
| : MemOperation(), CheckOperation(), CheckBlock(), NotNullSucc(), |
| NullSucc() {} |
| |
| explicit NullCheck(MachineInstr *memOperation, MachineInstr *checkOperation, |
| MachineBasicBlock *checkBlock, |
| MachineBasicBlock *notNullSucc, |
| MachineBasicBlock *nullSucc) |
| : MemOperation(memOperation), CheckOperation(checkOperation), |
| CheckBlock(checkBlock), NotNullSucc(notNullSucc), NullSucc(nullSucc) { |
| } |
| }; |
| |
| const TargetInstrInfo *TII = nullptr; |
| const TargetRegisterInfo *TRI = nullptr; |
| MachineModuleInfo *MMI = nullptr; |
| |
| bool analyzeBlockForNullChecks(MachineBasicBlock &MBB, |
| SmallVectorImpl<NullCheck> &NullCheckList); |
| MachineInstr *insertFaultingLoad(MachineInstr *LoadMI, MachineBasicBlock *MBB, |
| MCSymbol *HandlerLabel); |
| void rewriteNullChecks(ArrayRef<NullCheck> NullCheckList); |
| |
| public: |
| static char ID; |
| |
| ImplicitNullChecks() : MachineFunctionPass(ID) { |
| initializeImplicitNullChecksPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool runOnMachineFunction(MachineFunction &MF) override; |
| }; |
| |
| /// \brief Detect re-ordering hazards and dependencies. |
| /// |
| /// This class keeps track of defs and uses, and can be queried if a given |
| /// machine instruction can be re-ordered from after the machine instructions |
| /// seen so far to before them. |
| class HazardDetector { |
| DenseSet<unsigned> RegDefs; |
| DenseSet<unsigned> RegUses; |
| const TargetRegisterInfo &TRI; |
| bool hasSeenClobber; |
| |
| public: |
| explicit HazardDetector(const TargetRegisterInfo &TRI) : |
| TRI(TRI), hasSeenClobber(false) {} |
| |
| /// \brief Make a note of \p MI for later queries to isSafeToHoist. |
| /// |
| /// May clobber this HazardDetector instance. \see isClobbered. |
| void rememberInstruction(MachineInstr *MI); |
| |
| /// \brief Return true if it is safe to hoist \p MI from after all the |
| /// instructions seen so far (via rememberInstruction) to before it. |
| bool isSafeToHoist(MachineInstr *MI); |
| |
| /// \brief Return true if this instance of HazardDetector has been clobbered |
| /// (i.e. has no more useful information). |
| /// |
| /// A HazardDetecter is clobbered when it sees a construct it cannot |
| /// understand, and it would have to return a conservative answer for all |
| /// future queries. Having a separate clobbered state lets the client code |
| /// bail early, without making queries about all of the future instructions |
| /// (which would have returned the most conservative answer anyway). |
| /// |
| /// Calling rememberInstruction or isSafeToHoist on a clobbered HazardDetector |
| /// is an error. |
| bool isClobbered() { return hasSeenClobber; } |
| }; |
| } |
| |
| |
| void HazardDetector::rememberInstruction(MachineInstr *MI) { |
| assert(!isClobbered() && |
| "Don't add instructions to a clobbered hazard detector"); |
| |
| if (MI->mayStore() || MI->hasUnmodeledSideEffects()) { |
| hasSeenClobber = true; |
| return; |
| } |
| |
| for (auto *MMO : MI->memoperands()) { |
| // Right now we don't want to worry about LLVM's memory model. |
| if (!MMO->isUnordered()) { |
| hasSeenClobber = true; |
| return; |
| } |
| } |
| |
| for (auto &MO : MI->operands()) { |
| if (!MO.isReg() || !MO.getReg()) |
| continue; |
| |
| if (MO.isDef()) |
| RegDefs.insert(MO.getReg()); |
| else |
| RegUses.insert(MO.getReg()); |
| } |
| } |
| |
| bool HazardDetector::isSafeToHoist(MachineInstr *MI) { |
| assert(!isClobbered() && "isSafeToHoist cannot do anything useful!"); |
| |
| // Right now we don't want to worry about LLVM's memory model. This can be |
| // made more precise later. |
| for (auto *MMO : MI->memoperands()) |
| if (!MMO->isUnordered()) |
| return false; |
| |
| for (auto &MO : MI->operands()) { |
| if (MO.isReg() && MO.getReg()) { |
| for (unsigned Reg : RegDefs) |
| if (TRI.regsOverlap(Reg, MO.getReg())) |
| return false; // We found a write-after-write or read-after-write |
| |
| if (MO.isDef()) |
| for (unsigned Reg : RegUses) |
| if (TRI.regsOverlap(Reg, MO.getReg())) |
| return false; // We found a write-after-read |
| } |
| } |
| |
| return true; |
| } |
| |
| bool ImplicitNullChecks::runOnMachineFunction(MachineFunction &MF) { |
| TII = MF.getSubtarget().getInstrInfo(); |
| TRI = MF.getRegInfo().getTargetRegisterInfo(); |
| MMI = &MF.getMMI(); |
| |
| SmallVector<NullCheck, 16> NullCheckList; |
| |
| for (auto &MBB : MF) |
| analyzeBlockForNullChecks(MBB, NullCheckList); |
| |
| if (!NullCheckList.empty()) |
| rewriteNullChecks(NullCheckList); |
| |
| return !NullCheckList.empty(); |
| } |
| |
| /// Analyze MBB to check if its terminating branch can be turned into an |
| /// implicit null check. If yes, append a description of the said null check to |
| /// NullCheckList and return true, else return false. |
| bool ImplicitNullChecks::analyzeBlockForNullChecks( |
| MachineBasicBlock &MBB, SmallVectorImpl<NullCheck> &NullCheckList) { |
| typedef TargetInstrInfo::MachineBranchPredicate MachineBranchPredicate; |
| |
| MDNode *BranchMD = nullptr; |
| if (auto *BB = MBB.getBasicBlock()) |
| BranchMD = BB->getTerminator()->getMetadata(LLVMContext::MD_make_implicit); |
| |
| if (!BranchMD) |
| return false; |
| |
| MachineBranchPredicate MBP; |
| |
| if (TII->AnalyzeBranchPredicate(MBB, MBP, true)) |
| return false; |
| |
| // Is the predicate comparing an integer to zero? |
| if (!(MBP.LHS.isReg() && MBP.RHS.isImm() && MBP.RHS.getImm() == 0 && |
| (MBP.Predicate == MachineBranchPredicate::PRED_NE || |
| MBP.Predicate == MachineBranchPredicate::PRED_EQ))) |
| return false; |
| |
| // If we cannot erase the test instruction itself, then making the null check |
| // implicit does not buy us much. |
| if (!MBP.SingleUseCondition) |
| return false; |
| |
| MachineBasicBlock *NotNullSucc, *NullSucc; |
| |
| if (MBP.Predicate == MachineBranchPredicate::PRED_NE) { |
| NotNullSucc = MBP.TrueDest; |
| NullSucc = MBP.FalseDest; |
| } else { |
| NotNullSucc = MBP.FalseDest; |
| NullSucc = MBP.TrueDest; |
| } |
| |
| // We handle the simplest case for now. We can potentially do better by using |
| // the machine dominator tree. |
| if (NotNullSucc->pred_size() != 1) |
| return false; |
| |
| // Starting with a code fragment like: |
| // |
| // test %RAX, %RAX |
| // jne LblNotNull |
| // |
| // LblNull: |
| // callq throw_NullPointerException |
| // |
| // LblNotNull: |
| // Inst0 |
| // Inst1 |
| // ... |
| // Def = Load (%RAX + <offset>) |
| // ... |
| // |
| // |
| // we want to end up with |
| // |
| // Def = FaultingLoad (%RAX + <offset>), LblNull |
| // jmp LblNotNull ;; explicit or fallthrough |
| // |
| // LblNotNull: |
| // Inst0 |
| // Inst1 |
| // ... |
| // |
| // LblNull: |
| // callq throw_NullPointerException |
| // |
| // |
| // To see why this is legal, consider the two possibilities: |
| // |
| // 1. %RAX is null: since we constrain <offset> to be less than PageSize, the |
| // load instruction dereferences the null page, causing a segmentation |
| // fault. |
| // |
| // 2. %RAX is not null: in this case we know that the load cannot fault, as |
| // otherwise the load would've faulted in the original program too and the |
| // original program would've been undefined. |
| // |
| // This reasoning cannot be extended to justify hoisting through arbitrary |
| // control flow. For instance, in the example below (in pseudo-C) |
| // |
| // if (ptr == null) { throw_npe(); unreachable; } |
| // if (some_cond) { return 42; } |
| // v = ptr->field; // LD |
| // ... |
| // |
| // we cannot (without code duplication) use the load marked "LD" to null check |
| // ptr -- clause (2) above does not apply in this case. In the above program |
| // the safety of ptr->field can be dependent on some_cond; and, for instance, |
| // ptr could be some non-null invalid reference that never gets loaded from |
| // because some_cond is always true. |
| |
| unsigned PointerReg = MBP.LHS.getReg(); |
| |
| HazardDetector HD(*TRI); |
| |
| for (auto MII = NotNullSucc->begin(), MIE = NotNullSucc->end(); MII != MIE; |
| ++MII) { |
| MachineInstr *MI = &*MII; |
| unsigned BaseReg, Offset; |
| if (TII->getMemOpBaseRegImmOfs(MI, BaseReg, Offset, TRI)) |
| if (MI->mayLoad() && !MI->isPredicable() && BaseReg == PointerReg && |
| Offset < PageSize && MI->getDesc().getNumDefs() <= 1 && |
| HD.isSafeToHoist(MI)) { |
| NullCheckList.emplace_back(MI, MBP.ConditionDef, &MBB, NotNullSucc, |
| NullSucc); |
| return true; |
| } |
| |
| HD.rememberInstruction(MI); |
| if (HD.isClobbered()) |
| return false; |
| } |
| |
| return false; |
| } |
| |
| /// Wrap a machine load instruction, LoadMI, into a FAULTING_LOAD_OP machine |
| /// instruction. The FAULTING_LOAD_OP instruction does the same load as LoadMI |
| /// (defining the same register), and branches to HandlerLabel if the load |
| /// faults. The FAULTING_LOAD_OP instruction is inserted at the end of MBB. |
| MachineInstr *ImplicitNullChecks::insertFaultingLoad(MachineInstr *LoadMI, |
| MachineBasicBlock *MBB, |
| MCSymbol *HandlerLabel) { |
| const unsigned NoRegister = 0; // Guaranteed to be the NoRegister value for |
| // all targets. |
| |
| DebugLoc DL; |
| unsigned NumDefs = LoadMI->getDesc().getNumDefs(); |
| assert(NumDefs <= 1 && "other cases unhandled!"); |
| |
| unsigned DefReg = NoRegister; |
| if (NumDefs != 0) { |
| DefReg = LoadMI->defs().begin()->getReg(); |
| assert(std::distance(LoadMI->defs().begin(), LoadMI->defs().end()) == 1 && |
| "expected exactly one def!"); |
| } |
| |
| auto MIB = BuildMI(MBB, DL, TII->get(TargetOpcode::FAULTING_LOAD_OP), DefReg) |
| .addSym(HandlerLabel) |
| .addImm(LoadMI->getOpcode()); |
| |
| for (auto &MO : LoadMI->uses()) |
| MIB.addOperand(MO); |
| |
| MIB.setMemRefs(LoadMI->memoperands_begin(), LoadMI->memoperands_end()); |
| |
| return MIB; |
| } |
| |
| /// Rewrite the null checks in NullCheckList into implicit null checks. |
| void ImplicitNullChecks::rewriteNullChecks( |
| ArrayRef<ImplicitNullChecks::NullCheck> NullCheckList) { |
| DebugLoc DL; |
| |
| for (auto &NC : NullCheckList) { |
| MCSymbol *HandlerLabel = MMI->getContext().createTempSymbol(); |
| |
| // Remove the conditional branch dependent on the null check. |
| unsigned BranchesRemoved = TII->RemoveBranch(*NC.CheckBlock); |
| (void)BranchesRemoved; |
| assert(BranchesRemoved > 0 && "expected at least one branch!"); |
| |
| // Insert a faulting load where the conditional branch was originally. We |
| // check earlier ensures that this bit of code motion is legal. We do not |
| // touch the successors list for any basic block since we haven't changed |
| // control flow, we've just made it implicit. |
| insertFaultingLoad(NC.MemOperation, NC.CheckBlock, HandlerLabel); |
| NC.MemOperation->eraseFromParent(); |
| NC.CheckOperation->eraseFromParent(); |
| |
| // Insert an *unconditional* branch to not-null successor. |
| TII->InsertBranch(*NC.CheckBlock, NC.NotNullSucc, nullptr, /*Cond=*/None, |
| DL); |
| |
| // Emit the HandlerLabel as an EH_LABEL. |
| BuildMI(*NC.NullSucc, NC.NullSucc->begin(), DL, |
| TII->get(TargetOpcode::EH_LABEL)).addSym(HandlerLabel); |
| |
| NumImplicitNullChecks++; |
| } |
| } |
| |
| char ImplicitNullChecks::ID = 0; |
| char &llvm::ImplicitNullChecksID = ImplicitNullChecks::ID; |
| INITIALIZE_PASS_BEGIN(ImplicitNullChecks, "implicit-null-checks", |
| "Implicit null checks", false, false) |
| INITIALIZE_PASS_END(ImplicitNullChecks, "implicit-null-checks", |
| "Implicit null checks", false, false) |