ImplicitNullChecks.cpp | Explore in Territory

//===- ImplicitNullChecks.cpp - Fold null checks into memory accesses -----===//
//
// 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 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.
// Store and LoadStore are also supported.
//
//===----------------------------------------------------------------------===//

#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/CodeGen/FaultMaps.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineMemOperand.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/PseudoSourceValue.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include <cassert>
#include <cstdint>
#include <iterator>

usingnamespacellvm;

static cl::opt<int> PageSize("imp-null-check-page-size",
                             cl::desc("The page size of the target in bytes"),
                             cl::init(4096), cl::Hidden);

static cl::opt<unsigned> MaxInstsToConsider(
    "imp-null-max-insts-to-consider",
    cl::desc("The max number of instructions to consider hoisting loads over "
             "(the algorithm is quadratic over this number)"),
    cl::Hidden, cl::init(8));

#define DEBUG_TYPE …

STATISTIC(NumImplicitNullChecks,
          "Number of explicit null checks made implicit");

namespace {

class ImplicitNullChecks : public MachineFunctionPass { … };

} // end anonymous namespace

bool ImplicitNullChecks::canHandle(const MachineInstr *MI) { … }

ImplicitNullChecks::DependenceResult
ImplicitNullChecks::computeDependence(const MachineInstr *MI,
                                      ArrayRef<MachineInstr *> Block) { … }

bool ImplicitNullChecks::canReorder(const MachineInstr *A,
                                    const MachineInstr *B) { … }

bool ImplicitNullChecks::runOnMachineFunction(MachineFunction &MF) { … }

// Return true if any register aliasing \p Reg is live-in into \p MBB.
static bool AnyAliasLiveIn(const TargetRegisterInfo *TRI,
                           MachineBasicBlock *MBB, unsigned Reg) { … }

ImplicitNullChecks::AliasResult
ImplicitNullChecks::areMemoryOpsAliased(const MachineInstr &MI,
                                        const MachineInstr *PrevMI) const { … }

ImplicitNullChecks::SuitabilityResult
ImplicitNullChecks::isSuitableMemoryOp(const MachineInstr &MI,
                                       unsigned PointerReg,
                                       ArrayRef<MachineInstr *> PrevInsts) { … }

bool ImplicitNullChecks::canDependenceHoistingClobberLiveIns(
    MachineInstr *DependenceMI, MachineBasicBlock *NullSucc) { … }

bool ImplicitNullChecks::canHoistInst(MachineInstr *FaultingMI,
                                      ArrayRef<MachineInstr *> InstsSeenSoFar,
                                      MachineBasicBlock *NullSucc,
                                      MachineInstr *&Dependence) { … }

/// 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) { … }

/// Wrap a machine instruction, MI, into a FAULTING machine instruction.
/// The FAULTING instruction does the same load/store as MI
/// (defining the same register), and branches to HandlerMBB if the mem access
/// faults.  The FAULTING instruction is inserted at the end of MBB.
MachineInstr *ImplicitNullChecks::insertFaultingInstr(
    MachineInstr *MI, MachineBasicBlock *MBB, MachineBasicBlock *HandlerMBB) { … }

/// Rewrite the null checks in NullCheckList into implicit null checks.
void ImplicitNullChecks::rewriteNullChecks(
    ArrayRef<ImplicitNullChecks::NullCheck> NullCheckList) { … }

char ImplicitNullChecks::ID = …;

char &llvm::ImplicitNullChecksID = …;

INITIALIZE_PASS_BEGIN(ImplicitNullChecks, DEBUG_TYPE,
                      "Implicit null checks", false, false)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(ImplicitNullChecks, DEBUG_TYPE,
                    "Implicit null checks", false, false)
llvm/llvm/lib/CodeGen/ImplicitNullChecks.cpp
