lib/Target/BPF/BPFMIChecking.cpp

   1 //===-------------- BPFMIChecking.cpp - MI Checking Legality -------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This pass performs checking to signal errors for certain illegal usages at
  10 // MachineInstruction layer. Specially, the result of XADD{32,64} insn should
  11 // not be used. The pass is done at the PreEmit pass right before the
  12 // machine code is emitted at which point the register liveness information
  13 // is still available.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #include "BPF.h"
  18 #include "BPFInstrInfo.h"
  19 #include "BPFTargetMachine.h"
  20 #include "llvm/CodeGen/MachineInstrBuilder.h"
  21 #include "llvm/CodeGen/MachineRegisterInfo.h"
  22 #include "llvm/Support/Debug.h"
  23
  24 using namespace llvm;
  25
  26 #define DEBUG_TYPE "bpf-mi-checking"
  27
  28 namespace {
  29
  30 struct BPFMIPreEmitChecking : public MachineFunctionPass {
  31
  32   static char ID;
  33   MachineFunction *MF;
  34   const TargetRegisterInfo *TRI;
  35
  36   BPFMIPreEmitChecking() : MachineFunctionPass(ID) {
  37     initializeBPFMIPreEmitCheckingPass(*PassRegistry::getPassRegistry());
  38   }
  39
  40 private:
  41   // Initialize class variables.
  42   void initialize(MachineFunction &MFParm);
  43
  44   void checkingIllegalXADD(void);
  45
  46 public:
  47
  48   // Main entry point for this pass.
  49   bool runOnMachineFunction(MachineFunction &MF) override {
  50     if (!skipFunction(MF.getFunction())) {
  51       initialize(MF);
  52       checkingIllegalXADD();
  53     }
  54     return false;
  55   }
  56 };
  57
  58 // Initialize class variables.
  59 void BPFMIPreEmitChecking::initialize(MachineFunction &MFParm) {
  60   MF = &MFParm;
  61   TRI = MF->getSubtarget<BPFSubtarget>().getRegisterInfo();
  62   LLVM_DEBUG(dbgs() << "*** BPF PreEmit checking pass ***\n\n");
  63 }
  64
  65 // Make sure all Defs of XADD are dead, meaning any result of XADD insn is not
  66 // used.
  67 //
  68 // NOTE: BPF backend hasn't enabled sub-register liveness track, so when the
  69 // source and destination operands of XADD are GPR32, there is no sub-register
  70 // dead info. If we rely on the generic MachineInstr::allDefsAreDead, then we
  71 // will raise false alarm on GPR32 Def.
  72 //
  73 // To support GPR32 Def, ideally we could just enable sub-registr liveness track
  74 // on BPF backend, then allDefsAreDead could work on GPR32 Def. This requires
  75 // implementing TargetSubtargetInfo::enableSubRegLiveness on BPF.
  76 //
  77 // However, sub-register liveness tracking module inside LLVM is actually
  78 // designed for the situation where one register could be split into more than
  79 // one sub-registers for which case each sub-register could have their own
  80 // liveness and kill one of them doesn't kill others. So, tracking liveness for
  81 // each make sense.
  82 //
  83 // For BPF, each 64-bit register could only have one 32-bit sub-register. This
  84 // is exactly the case which LLVM think brings no benefits for doing
  85 // sub-register tracking, because the live range of sub-register must always
  86 // equal to its parent register, therefore liveness tracking is disabled even
  87 // the back-end has implemented enableSubRegLiveness. The detailed information
  88 // is at r232695:
  89 //
  90 //   Author: Matthias Braun <matze@braunis.de>
  91 //   Date:   Thu Mar 19 00:21:58 2015 +0000
  92 //   Do not track subregister liveness when it brings no benefits
  93 //
  94 // Hence, for BPF, we enhance MachineInstr::allDefsAreDead. Given the solo
  95 // sub-register always has the same liveness as its parent register, LLVM is
  96 // already attaching a implicit 64-bit register Def whenever the there is
  97 // a sub-register Def. The liveness of the implicit 64-bit Def is available.
  98 // For example, for "lock *(u32 *)(r0 + 4) += w9", the MachineOperand info could
  99 // be:
 100 //
 101 //   $w9 = XADDW32 killed $r0, 4, $w9(tied-def 0),
 102 //                        implicit killed $r9, implicit-def dead $r9
 103 //
 104 // Even though w9 is not marked as Dead, the parent register r9 is marked as
 105 // Dead correctly, and it is safe to use such information or our purpose.
 106 static bool hasLiveDefs(const MachineInstr &MI, const TargetRegisterInfo *TRI) {
 107   const MCRegisterClass *GPR64RegClass =
 108     &BPFMCRegisterClasses[BPF::GPRRegClassID];
 109   std::vector<unsigned> GPR32LiveDefs;
 110   std::vector<unsigned> GPR64DeadDefs;
 111
 112   for (const MachineOperand &MO : MI.operands()) {
 113     bool RegIsGPR64;
 114
 115     if (!MO.isReg() || MO.isUse())
 116       continue;
 117
 118     RegIsGPR64 = GPR64RegClass->contains(MO.getReg());
 119     if (!MO.isDead()) {
 120       // It is a GPR64 live Def, we are sure it is live. */
 121       if (RegIsGPR64)
 122         return true;
 123       // It is a GPR32 live Def, we are unsure whether it is really dead due to
 124       // no sub-register liveness tracking. Push it to vector for deferred
 125       // check.
 126       GPR32LiveDefs.push_back(MO.getReg());
 127       continue;
 128     }
 129
 130     // Record any GPR64 dead Def as some unmarked GPR32 could be alias of its
 131     // low 32-bit.
 132     if (RegIsGPR64)
 133       GPR64DeadDefs.push_back(MO.getReg());
 134   }
 135
 136   // No GPR32 live Def, safe to return false.
 137   if (GPR32LiveDefs.empty())
 138     return false;
 139
 140   // No GPR64 dead Def, so all those GPR32 live Def can't have alias, therefore
 141   // must be truely live, safe to return true.
 142   if (GPR64DeadDefs.empty())
 143     return true;
 144
 145   // Otherwise, return true if any aliased SuperReg of GPR32 is not dead.
 146   std::vector<unsigned>::iterator search_begin = GPR64DeadDefs.begin();
 147   std::vector<unsigned>::iterator search_end = GPR64DeadDefs.end();
 148   for (auto I : GPR32LiveDefs)
 149     for (MCSuperRegIterator SR(I, TRI); SR.isValid(); ++SR)
 150        if (std::find(search_begin, search_end, *SR) == search_end)
 151          return true;
 152
 153   return false;
 154 }
 155
 156 void BPFMIPreEmitChecking::checkingIllegalXADD(void) {
 157   for (MachineBasicBlock &MBB : *MF) {
 158     for (MachineInstr &MI : MBB) {
 159       if (MI.getOpcode() != BPF::XADDW &&
 160           MI.getOpcode() != BPF::XADDD &&
 161           MI.getOpcode() != BPF::XADDW32)
 162         continue;
 163
 164       LLVM_DEBUG(MI.dump());
 165       if (hasLiveDefs(MI, TRI)) {
 166         DebugLoc Empty;
 167         const DebugLoc &DL = MI.getDebugLoc();
 168         if (DL != Empty)
 169           report_fatal_error("line " + std::to_string(DL.getLine()) +
 170                              ": Invalid usage of the XADD return value", false);
 171         else
 172           report_fatal_error("Invalid usage of the XADD return value", false);
 173       }
 174     }
 175   }
 176
 177   return;
 178 }
 179
 180 } // end default namespace
 181
 182 INITIALIZE_PASS(BPFMIPreEmitChecking, "bpf-mi-pemit-checking",
 183                 "BPF PreEmit Checking", false, false)
 184
 185 char BPFMIPreEmitChecking::ID = 0;
 186 FunctionPass* llvm::createBPFMIPreEmitCheckingPass()
 187 {
 188   return new BPFMIPreEmitChecking();
 189 }