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/MachineFunctionPass.h"
21#include "llvm/CodeGen/MachineInstrBuilder.h"
22#include "llvm/CodeGen/MachineRegisterInfo.h"
23#include "llvm/IR/DiagnosticInfo.h"
24#include "llvm/Support/Debug.h"
25
26using namespace llvm;
27
28#define DEBUG_TYPE "bpf-mi-checking"
29
30namespace {
31
32struct BPFMIPreEmitChecking : public MachineFunctionPass {
33
34 static char ID;
35 MachineFunction *MF;
36 const TargetRegisterInfo *TRI;
37
38 BPFMIPreEmitChecking() : MachineFunctionPass(ID) {
39 initializeBPFMIPreEmitCheckingPass(*PassRegistry::getPassRegistry());
40 }
41
42private:
43 // Initialize class variables.
44 void initialize(MachineFunction &MFParm);
45
46 bool processAtomicInsts();
47
48public:
49
50 // Main entry point for this pass.
51 bool runOnMachineFunction(MachineFunction &MF) override {
52 if (!skipFunction(F: MF.getFunction())) {
53 initialize(MFParm&: MF);
54 return processAtomicInsts();
55 }
56 return false;
57 }
58};
59
60// Initialize class variables.
61void BPFMIPreEmitChecking::initialize(MachineFunction &MFParm) {
62 MF = &MFParm;
63 TRI = MF->getSubtarget<BPFSubtarget>().getRegisterInfo();
64 LLVM_DEBUG(dbgs() << "*** BPF PreEmit checking pass ***\n\n");
65}
66
67// Make sure all Defs of XADD are dead, meaning any result of XADD insn is not
68// used.
69//
70// NOTE: BPF backend hasn't enabled sub-register liveness track, so when the
71// source and destination operands of XADD are GPR32, there is no sub-register
72// dead info. If we rely on the generic MachineInstr::allDefsAreDead, then we
73// will raise false alarm on GPR32 Def.
74//
75// To support GPR32 Def, ideally we could just enable sub-registr liveness track
76// on BPF backend, then allDefsAreDead could work on GPR32 Def. This requires
77// implementing TargetSubtargetInfo::enableSubRegLiveness on BPF.
78//
79// However, sub-register liveness tracking module inside LLVM is actually
80// designed for the situation where one register could be split into more than
81// one sub-registers for which case each sub-register could have their own
82// liveness and kill one of them doesn't kill others. So, tracking liveness for
83// each make sense.
84//
85// For BPF, each 64-bit register could only have one 32-bit sub-register. This
86// is exactly the case which LLVM think brings no benefits for doing
87// sub-register tracking, because the live range of sub-register must always
88// equal to its parent register, therefore liveness tracking is disabled even
89// the back-end has implemented enableSubRegLiveness. The detailed information
90// is at r232695:
91//
92// Author: Matthias Braun <matze@braunis.de>
93// Date: Thu Mar 19 00:21:58 2015 +0000
94// Do not track subregister liveness when it brings no benefits
95//
96// Hence, for BPF, we enhance MachineInstr::allDefsAreDead. Given the solo
97// sub-register always has the same liveness as its parent register, LLVM is
98// already attaching a implicit 64-bit register Def whenever the there is
99// a sub-register Def. The liveness of the implicit 64-bit Def is available.
100// For example, for "lock *(u32 *)(r0 + 4) += w9", the MachineOperand info could
101// be:
102//
103// $w9 = XADDW32 killed $r0, 4, $w9(tied-def 0),
104// implicit killed $r9, implicit-def dead $r9
105//
106// Even though w9 is not marked as Dead, the parent register r9 is marked as
107// Dead correctly, and it is safe to use such information or our purpose.
108static bool hasLiveDefs(const MachineInstr &MI, const TargetRegisterInfo *TRI) {
109 const MCRegisterClass *GPR64RegClass =
110 &BPFMCRegisterClasses[BPF::GPRRegClassID];
111 std::vector<unsigned> GPR32LiveDefs;
112 std::vector<unsigned> GPR64DeadDefs;
113
114 for (const MachineOperand &MO : MI.operands()) {
115 bool RegIsGPR64;
116
117 if (!MO.isReg() || MO.isUse())
118 continue;
119
120 RegIsGPR64 = GPR64RegClass->contains(Reg: MO.getReg());
121 if (!MO.isDead()) {
122 // It is a GPR64 live Def, we are sure it is live. */
123 if (RegIsGPR64)
124 return true;
125 // It is a GPR32 live Def, we are unsure whether it is really dead due to
126 // no sub-register liveness tracking. Push it to vector for deferred
127 // check.
128 GPR32LiveDefs.push_back(x: MO.getReg());
129 continue;
130 }
131
132 // Record any GPR64 dead Def as some unmarked GPR32 could be alias of its
133 // low 32-bit.
134 if (RegIsGPR64)
135 GPR64DeadDefs.push_back(x: MO.getReg());
136 }
137
138 // No GPR32 live Def, safe to return false.
139 if (GPR32LiveDefs.empty())
140 return false;
141
142 // No GPR64 dead Def, so all those GPR32 live Def can't have alias, therefore
143 // must be truely live, safe to return true.
144 if (GPR64DeadDefs.empty())
145 return true;
146
147 // Otherwise, return true if any aliased SuperReg of GPR32 is not dead.
148 for (auto I : GPR32LiveDefs)
149 for (MCPhysReg SR : TRI->superregs(Reg: I))
150 if (!llvm::is_contained(Range&: GPR64DeadDefs, Element: SR))
151 return true;
152
153 return false;
154}
155
156bool BPFMIPreEmitChecking::processAtomicInsts() {
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 const Function &F = MF->getFunction();
169 F.getContext().diagnose(DI: DiagnosticInfoUnsupported{
170 F, "Invalid usage of the XADD return value", DL});
171 }
172 }
173 }
174
175 // Check return values of atomic_fetch_and_{add,and,or,xor}.
176 // If the return is not used, the atomic_fetch_and_<op> instruction
177 // is replaced with atomic_<op> instruction.
178 MachineInstr *ToErase = nullptr;
179 bool Changed = false;
180 const BPFInstrInfo *TII = MF->getSubtarget<BPFSubtarget>().getInstrInfo();
181 for (MachineBasicBlock &MBB : *MF) {
182 for (MachineInstr &MI : MBB) {
183 if (ToErase) {
184 ToErase->eraseFromParent();
185 ToErase = nullptr;
186 }
187
188 if (MI.getOpcode() != BPF::XFADDW32 && MI.getOpcode() != BPF::XFADDD &&
189 MI.getOpcode() != BPF::XFANDW32 && MI.getOpcode() != BPF::XFANDD &&
190 MI.getOpcode() != BPF::XFXORW32 && MI.getOpcode() != BPF::XFXORD &&
191 MI.getOpcode() != BPF::XFORW32 && MI.getOpcode() != BPF::XFORD)
192 continue;
193
194 if (hasLiveDefs(MI, TRI))
195 continue;
196
197 LLVM_DEBUG(dbgs() << "Transforming "; MI.dump());
198 unsigned newOpcode;
199 switch (MI.getOpcode()) {
200 case BPF::XFADDW32:
201 newOpcode = BPF::XADDW32;
202 break;
203 case BPF::XFADDD:
204 newOpcode = BPF::XADDD;
205 break;
206 case BPF::XFANDW32:
207 newOpcode = BPF::XANDW32;
208 break;
209 case BPF::XFANDD:
210 newOpcode = BPF::XANDD;
211 break;
212 case BPF::XFXORW32:
213 newOpcode = BPF::XXORW32;
214 break;
215 case BPF::XFXORD:
216 newOpcode = BPF::XXORD;
217 break;
218 case BPF::XFORW32:
219 newOpcode = BPF::XORW32;
220 break;
221 case BPF::XFORD:
222 newOpcode = BPF::XORD;
223 break;
224 default:
225 llvm_unreachable("Incorrect Atomic Instruction Opcode");
226 }
227
228 BuildMI(BB&: MBB, I&: MI, MIMD: MI.getDebugLoc(), MCID: TII->get(Opcode: newOpcode))
229 .add(MO: MI.getOperand(i: 0))
230 .add(MO: MI.getOperand(i: 1))
231 .add(MO: MI.getOperand(i: 2))
232 .add(MO: MI.getOperand(i: 3));
233
234 ToErase = &MI;
235 Changed = true;
236 }
237 }
238
239 return Changed;
240}
241
242} // end default namespace
243
244INITIALIZE_PASS(BPFMIPreEmitChecking, "bpf-mi-pemit-checking",
245 "BPF PreEmit Checking", false, false)
246
247char BPFMIPreEmitChecking::ID = 0;
248FunctionPass* llvm::createBPFMIPreEmitCheckingPass()
249{
250 return new BPFMIPreEmitChecking();
251}
252