--- /dev/null
+//===-- 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/SmallVector.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/MachineFunction.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/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));
+
+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 the pointer is non-null.
+ MachineBasicBlock *NotNullSucc;
+
+ // The block branched to if the 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;
+};
+}
+
+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;
+
+ 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:
+ // Def = Load (%RAX + <offset>)
+ // ...
+ //
+ //
+ // we want to end up with
+ //
+ // Def = TrappingLoad (%RAX + <offset>), LblNull
+ // jmp LblNotNull ;; explicit or fallthrough
+ //
+ // LblNotNull:
+ // ...
+ //
+ // LblNull:
+ // callq throw_NullPointerException
+ //
+
+ unsigned PointerReg = MBP.LHS.getReg();
+ MachineInstr *MemOp = &*NotNullSucc->begin();
+ unsigned BaseReg, Offset;
+ if (TII->getMemOpBaseRegImmOfs(MemOp, BaseReg, Offset, TRI))
+ if (MemOp->mayLoad() && !MemOp->isPredicable() && BaseReg == PointerReg &&
+ Offset < PageSize && MemOp->getDesc().getNumDefs() == 1) {
+ NullCheckList.emplace_back(MemOp, MBP.ConditionDef, &MBB, NotNullSucc,
+ NullSucc);
+ return true;
+ }
+
+ 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) {
+ DebugLoc DL;
+ unsigned NumDefs = LoadMI->getDesc().getNumDefs();
+ assert(NumDefs == 1 && "other cases unhandled!");
+ (void)NumDefs;
+
+ unsigned 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->removeFromParent();
+ 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);
+ }
+}
+
+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)
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