llvm-mirror/lib/CodeGen/ImplicitNullChecks.cpp

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//===-- 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<int> 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,
MachineBasicBlock *HandlerMBB);
void rewriteNullChecks(ArrayRef<NullCheck> NullCheckList);
public:
static char ID;
ImplicitNullChecks() : MachineFunctionPass(ID) {
initializeImplicitNullChecksPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &MF) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
MachineFunctionProperties::Property::AllVRegsAllocated);
}
};
/// \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;
int64_t 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 HandlerMBB if the load
/// faults. The FAULTING_LOAD_OP instruction is inserted at the end of MBB.
MachineInstr *
ImplicitNullChecks::insertFaultingLoad(MachineInstr *LoadMI,
MachineBasicBlock *MBB,
MachineBasicBlock *HandlerMBB) {
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)
.addMBB(HandlerMBB)
.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) {
// 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.
MachineInstr *FaultingLoad =
insertFaultingLoad(NC.MemOperation, NC.CheckBlock, NC.NullSucc);
// Now the values defined by MemOperation, if any, are live-in of
// the block of MemOperation.
// The original load operation may define implicit-defs alongside
// the loaded value.
MachineBasicBlock *MBB = NC.MemOperation->getParent();
for (const MachineOperand &MO : FaultingLoad->operands()) {
if (!MO.isReg() || !MO.isDef())
continue;
unsigned Reg = MO.getReg();
if (!Reg || MBB->isLiveIn(Reg))
continue;
MBB->addLiveIn(Reg);
}
NC.MemOperation->eraseFromParent();
NC.CheckOperation->eraseFromParent();
// Insert an *unconditional* branch to not-null successor.
TII->InsertBranch(*NC.CheckBlock, NC.NotNullSucc, nullptr, /*Cond=*/None,
DL);
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)