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//===-------------- BPFMIChecking.cpp - MI Checking Legality -------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This pass performs checking to signal errors for certain illegal usages at
// MachineInstruction layer. Specially, the result of XADD{32,64} insn should
// not be used. The pass is done at the PreEmit pass right before the
// machine code is emitted at which point the register liveness information
// is still available.
#include "BPF.h"
#include "BPFInstrInfo.h"
#include "BPFTargetMachine.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
#define DEBUG_TYPE "bpf-mi-checking"
namespace {
struct BPFMIPreEmitChecking : public MachineFunctionPass {
static char ID;
MachineFunction *MF;
const TargetRegisterInfo *TRI;
BPFMIPreEmitChecking() : MachineFunctionPass(ID) {
// Initialize class variables.
void initialize(MachineFunction &MFParm);
bool processAtomicInsts(void);
// Main entry point for this pass.
bool runOnMachineFunction(MachineFunction &MF) override {
if (!skipFunction(MF.getFunction())) {
return processAtomicInsts();
return false;
// Initialize class variables.
void BPFMIPreEmitChecking::initialize(MachineFunction &MFParm) {
MF = &MFParm;
TRI = MF->getSubtarget<BPFSubtarget>().getRegisterInfo();
LLVM_DEBUG(dbgs() << "*** BPF PreEmit checking pass ***\n\n");
// Make sure all Defs of XADD are dead, meaning any result of XADD insn is not
// used.
// NOTE: BPF backend hasn't enabled sub-register liveness track, so when the
// source and destination operands of XADD are GPR32, there is no sub-register
// dead info. If we rely on the generic MachineInstr::allDefsAreDead, then we
// will raise false alarm on GPR32 Def.
// To support GPR32 Def, ideally we could just enable sub-registr liveness track
// on BPF backend, then allDefsAreDead could work on GPR32 Def. This requires
// implementing TargetSubtargetInfo::enableSubRegLiveness on BPF.
// However, sub-register liveness tracking module inside LLVM is actually
// designed for the situation where one register could be split into more than
// one sub-registers for which case each sub-register could have their own
// liveness and kill one of them doesn't kill others. So, tracking liveness for
// each make sense.
// For BPF, each 64-bit register could only have one 32-bit sub-register. This
// is exactly the case which LLVM think brings no benefits for doing
// sub-register tracking, because the live range of sub-register must always
// equal to its parent register, therefore liveness tracking is disabled even
// the back-end has implemented enableSubRegLiveness. The detailed information
// is at r232695:
// Author: Matthias Braun <>
// Date: Thu Mar 19 00:21:58 2015 +0000
// Do not track subregister liveness when it brings no benefits
// Hence, for BPF, we enhance MachineInstr::allDefsAreDead. Given the solo
// sub-register always has the same liveness as its parent register, LLVM is
// already attaching a implicit 64-bit register Def whenever the there is
// a sub-register Def. The liveness of the implicit 64-bit Def is available.
// For example, for "lock *(u32 *)(r0 + 4) += w9", the MachineOperand info could
// be:
// $w9 = XADDW32 killed $r0, 4, $w9(tied-def 0),
// implicit killed $r9, implicit-def dead $r9
// Even though w9 is not marked as Dead, the parent register r9 is marked as
// Dead correctly, and it is safe to use such information or our purpose.
static bool hasLiveDefs(const MachineInstr &MI, const TargetRegisterInfo *TRI) {
const MCRegisterClass *GPR64RegClass =
std::vector<unsigned> GPR32LiveDefs;
std::vector<unsigned> GPR64DeadDefs;
for (const MachineOperand &MO : MI.operands()) {
bool RegIsGPR64;
if (!MO.isReg() || MO.isUse())
RegIsGPR64 = GPR64RegClass->contains(MO.getReg());
if (!MO.isDead()) {
// It is a GPR64 live Def, we are sure it is live. */
if (RegIsGPR64)
return true;
// It is a GPR32 live Def, we are unsure whether it is really dead due to
// no sub-register liveness tracking. Push it to vector for deferred
// check.
// Record any GPR64 dead Def as some unmarked GPR32 could be alias of its
// low 32-bit.
if (RegIsGPR64)
// No GPR32 live Def, safe to return false.
if (GPR32LiveDefs.empty())
return false;
// No GPR64 dead Def, so all those GPR32 live Def can't have alias, therefore
// must be truely live, safe to return true.
if (GPR64DeadDefs.empty())
return true;
// Otherwise, return true if any aliased SuperReg of GPR32 is not dead.
for (auto I : GPR32LiveDefs)
for (MCSuperRegIterator SR(I, TRI); SR.isValid(); ++SR)
if (!llvm::is_contained(GPR64DeadDefs, *SR))
return true;
return false;
bool BPFMIPreEmitChecking::processAtomicInsts(void) {
for (MachineBasicBlock &MBB : *MF) {
for (MachineInstr &MI : MBB) {
if (MI.getOpcode() != BPF::XADDW &&
MI.getOpcode() != BPF::XADDD &&
MI.getOpcode() != BPF::XADDW32)
if (hasLiveDefs(MI, TRI)) {
DebugLoc Empty;
const DebugLoc &DL = MI.getDebugLoc();
if (DL != Empty)
report_fatal_error(Twine("line ") + std::to_string(DL.getLine()) +
": Invalid usage of the XADD return value", false);
report_fatal_error("Invalid usage of the XADD return value", false);
// Check return values of atomic_fetch_and_{add,and,or,xor}.
// If the return is not used, the atomic_fetch_and_<op> instruction
// is replaced with atomic_<op> instruction.
MachineInstr *ToErase = nullptr;
bool Changed = false;
const BPFInstrInfo *TII = MF->getSubtarget<BPFSubtarget>().getInstrInfo();
for (MachineBasicBlock &MBB : *MF) {
for (MachineInstr &MI : MBB) {
if (ToErase) {
ToErase = nullptr;
if (MI.getOpcode() != BPF::XFADDW32 && MI.getOpcode() != BPF::XFADDD &&
MI.getOpcode() != BPF::XFANDW32 && MI.getOpcode() != BPF::XFANDD &&
MI.getOpcode() != BPF::XFXORW32 && MI.getOpcode() != BPF::XFXORD &&
MI.getOpcode() != BPF::XFORW32 && MI.getOpcode() != BPF::XFORD)
if (hasLiveDefs(MI, TRI))
LLVM_DEBUG(dbgs() << "Transforming "; MI.dump());
unsigned newOpcode;
switch (MI.getOpcode()) {
case BPF::XFADDW32:
newOpcode = BPF::XADDW32;
newOpcode = BPF::XADDD;
case BPF::XFANDW32:
newOpcode = BPF::XANDW32;
newOpcode = BPF::XANDD;
case BPF::XFXORW32:
newOpcode = BPF::XXORW32;
newOpcode = BPF::XXORD;
case BPF::XFORW32:
newOpcode = BPF::XORW32;
case BPF::XFORD:
newOpcode = BPF::XORD;
llvm_unreachable("Incorrect Atomic Instruction Opcode");
BuildMI(MBB, MI, MI.getDebugLoc(), TII->get(newOpcode))
ToErase = &MI;
Changed = true;
return Changed;
} // end default namespace
INITIALIZE_PASS(BPFMIPreEmitChecking, "bpf-mi-pemit-checking",
"BPF PreEmit Checking", false, false)
char BPFMIPreEmitChecking::ID = 0;
FunctionPass* llvm::createBPFMIPreEmitCheckingPass()
return new BPFMIPreEmitChecking();