1 //===-- AArch64A53Fix835769.cpp -------------------------------------------===//
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 // This pass changes code to work around Cortex-A53 erratum 835769.
9 // It works around it by inserting a nop instruction in code sequences that
10 // in some circumstances may trigger the erratum.
11 // It inserts a nop instruction between a sequence of the following 2 classes
12 // of instructions:
13 // instr 1: mem-instr (including loads, stores and prefetches).
14 // instr 2: non-SIMD integer multiply-accumulate writing 64-bit X registers.
15 //===----------------------------------------------------------------------===//
16
17 #include "AArch64.h"
18 #include "AArch64Subtarget.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/CodeGen/MachineFunction.h"
21 #include "llvm/CodeGen/MachineFunctionPass.h"
22 #include "llvm/CodeGen/MachineInstr.h"
23 #include "llvm/CodeGen/MachineInstrBuilder.h"
24 #include "llvm/CodeGen/MachineRegisterInfo.h"
25 #include "llvm/CodeGen/TargetInstrInfo.h"
26 #include "llvm/Support/Debug.h"
27 #include "llvm/Support/raw_ostream.h"
28
29 using namespace llvm;
30
31 #define DEBUG_TYPE "aarch64-fix-cortex-a53-835769"
32
33 STATISTIC(NumNopsAdded, "Number of Nops added to work around erratum 835769");
34
35 //===----------------------------------------------------------------------===//
36 // Helper functions
37
38 // Is the instruction a match for the instruction that comes first in the
39 // sequence of instructions that can trigger the erratum?
isFirstInstructionInSequence(MachineInstr * MI)40 static bool isFirstInstructionInSequence(MachineInstr *MI) {
41 // Must return true if this instruction is a load, a store or a prefetch.
42 switch (MI->getOpcode()) {
43 case AArch64::PRFMl:
44 case AArch64::PRFMroW:
45 case AArch64::PRFMroX:
46 case AArch64::PRFMui:
47 case AArch64::PRFUMi:
48 return true;
49 default:
50 return MI->mayLoadOrStore();
51 }
52 }
53
54 // Is the instruction a match for the instruction that comes second in the
55 // sequence that can trigger the erratum?
isSecondInstructionInSequence(MachineInstr * MI)56 static bool isSecondInstructionInSequence(MachineInstr *MI) {
57 // Must return true for non-SIMD integer multiply-accumulates, writing
58 // to a 64-bit register.
59 switch (MI->getOpcode()) {
60 // Erratum cannot be triggered when the destination register is 32 bits,
61 // therefore only include the following.
62 case AArch64::MSUBXrrr:
63 case AArch64::MADDXrrr:
64 case AArch64::SMADDLrrr:
65 case AArch64::SMSUBLrrr:
66 case AArch64::UMADDLrrr:
67 case AArch64::UMSUBLrrr:
68 // Erratum can only be triggered by multiply-adds, not by regular
69 // non-accumulating multiplies, i.e. when Ra=XZR='11111'
70 return MI->getOperand(3).getReg() != AArch64::XZR;
71 default:
72 return false;
73 }
74 }
75
76
77 //===----------------------------------------------------------------------===//
78
79 namespace {
80 class AArch64A53Fix835769 : public MachineFunctionPass {
81 const TargetInstrInfo *TII;
82
83 public:
84 static char ID;
AArch64A53Fix835769()85 explicit AArch64A53Fix835769() : MachineFunctionPass(ID) {
86 initializeAArch64A53Fix835769Pass(*PassRegistry::getPassRegistry());
87 }
88
89 bool runOnMachineFunction(MachineFunction &F) override;
90
getRequiredProperties() const91 MachineFunctionProperties getRequiredProperties() const override {
92 return MachineFunctionProperties().set(
93 MachineFunctionProperties::Property::NoVRegs);
94 }
95
getPassName() const96 StringRef getPassName() const override {
97 return "Workaround A53 erratum 835769 pass";
98 }
99
getAnalysisUsage(AnalysisUsage & AU) const100 void getAnalysisUsage(AnalysisUsage &AU) const override {
101 AU.setPreservesCFG();
102 MachineFunctionPass::getAnalysisUsage(AU);
103 }
104
105 private:
106 bool runOnBasicBlock(MachineBasicBlock &MBB);
107 };
108 char AArch64A53Fix835769::ID = 0;
109
110 } // end anonymous namespace
111
112 INITIALIZE_PASS(AArch64A53Fix835769, "aarch64-fix-cortex-a53-835769-pass",
113 "AArch64 fix for A53 erratum 835769", false, false)
114
115 //===----------------------------------------------------------------------===//
116
117 bool
runOnMachineFunction(MachineFunction & F)118 AArch64A53Fix835769::runOnMachineFunction(MachineFunction &F) {
119 LLVM_DEBUG(dbgs() << "***** AArch64A53Fix835769 *****\n");
120 auto &STI = F.getSubtarget<AArch64Subtarget>();
121 // Fix not requested, skip pass.
122 if (!STI.fixCortexA53_835769())
123 return false;
124
125 bool Changed = false;
126 TII = STI.getInstrInfo();
127
128 for (auto &MBB : F) {
129 Changed |= runOnBasicBlock(MBB);
130 }
131 return Changed;
132 }
133
134 // Return the block that was fallen through to get to MBB, if any,
135 // otherwise nullptr.
getBBFallenThrough(MachineBasicBlock * MBB,const TargetInstrInfo * TII)136 static MachineBasicBlock *getBBFallenThrough(MachineBasicBlock *MBB,
137 const TargetInstrInfo *TII) {
138 // Get the previous machine basic block in the function.
139 MachineFunction::iterator MBBI(MBB);
140
141 // Can't go off top of function.
142 if (MBBI == MBB->getParent()->begin())
143 return nullptr;
144
145 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
146 SmallVector<MachineOperand, 2> Cond;
147
148 MachineBasicBlock *PrevBB = &*std::prev(MBBI);
149 for (MachineBasicBlock *S : MBB->predecessors())
150 if (S == PrevBB && !TII->analyzeBranch(*PrevBB, TBB, FBB, Cond) && !TBB &&
151 !FBB)
152 return S;
153
154 return nullptr;
155 }
156
157 // Iterate through fallen through blocks trying to find a previous non-pseudo if
158 // there is one, otherwise return nullptr. Only look for instructions in
159 // previous blocks, not the current block, since we only use this to look at
160 // previous blocks.
getLastNonPseudo(MachineBasicBlock & MBB,const TargetInstrInfo * TII)161 static MachineInstr *getLastNonPseudo(MachineBasicBlock &MBB,
162 const TargetInstrInfo *TII) {
163 MachineBasicBlock *FMBB = &MBB;
164
165 // If there is no non-pseudo in the current block, loop back around and try
166 // the previous block (if there is one).
167 while ((FMBB = getBBFallenThrough(FMBB, TII))) {
168 for (MachineInstr &I : llvm::reverse(*FMBB))
169 if (!I.isPseudo())
170 return &I;
171 }
172
173 // There was no previous non-pseudo in the fallen through blocks
174 return nullptr;
175 }
176
insertNopBeforeInstruction(MachineBasicBlock & MBB,MachineInstr * MI,const TargetInstrInfo * TII)177 static void insertNopBeforeInstruction(MachineBasicBlock &MBB, MachineInstr* MI,
178 const TargetInstrInfo *TII) {
179 // If we are the first instruction of the block, put the NOP at the end of
180 // the previous fallthrough block
181 if (MI == &MBB.front()) {
182 MachineInstr *I = getLastNonPseudo(MBB, TII);
183 assert(I && "Expected instruction");
184 DebugLoc DL = I->getDebugLoc();
185 BuildMI(I->getParent(), DL, TII->get(AArch64::HINT)).addImm(0);
186 }
187 else {
188 DebugLoc DL = MI->getDebugLoc();
189 BuildMI(MBB, MI, DL, TII->get(AArch64::HINT)).addImm(0);
190 }
191
192 ++NumNopsAdded;
193 }
194
195 bool
runOnBasicBlock(MachineBasicBlock & MBB)196 AArch64A53Fix835769::runOnBasicBlock(MachineBasicBlock &MBB) {
197 bool Changed = false;
198 LLVM_DEBUG(dbgs() << "Running on MBB: " << MBB
199 << " - scanning instructions...\n");
200
201 // First, scan the basic block, looking for a sequence of 2 instructions
202 // that match the conditions under which the erratum may trigger.
203
204 // List of terminating instructions in matching sequences
205 std::vector<MachineInstr*> Sequences;
206 unsigned Idx = 0;
207 MachineInstr *PrevInstr = nullptr;
208
209 // Try and find the last non-pseudo instruction in any fallen through blocks,
210 // if there isn't one, then we use nullptr to represent that.
211 PrevInstr = getLastNonPseudo(MBB, TII);
212
213 for (auto &MI : MBB) {
214 MachineInstr *CurrInstr = &MI;
215 LLVM_DEBUG(dbgs() << " Examining: " << MI);
216 if (PrevInstr) {
217 LLVM_DEBUG(dbgs() << " PrevInstr: " << *PrevInstr
218 << " CurrInstr: " << *CurrInstr
219 << " isFirstInstructionInSequence(PrevInstr): "
220 << isFirstInstructionInSequence(PrevInstr) << "\n"
221 << " isSecondInstructionInSequence(CurrInstr): "
222 << isSecondInstructionInSequence(CurrInstr) << "\n");
223 if (isFirstInstructionInSequence(PrevInstr) &&
224 isSecondInstructionInSequence(CurrInstr)) {
225 LLVM_DEBUG(dbgs() << " ** pattern found at Idx " << Idx << "!\n");
226 (void) Idx;
227 Sequences.push_back(CurrInstr);
228 }
229 }
230 if (!CurrInstr->isPseudo())
231 PrevInstr = CurrInstr;
232 ++Idx;
233 }
234
235 LLVM_DEBUG(dbgs() << "Scan complete, " << Sequences.size()
236 << " occurrences of pattern found.\n");
237
238 // Then update the basic block, inserting nops between the detected sequences.
239 for (auto &MI : Sequences) {
240 Changed = true;
241 insertNopBeforeInstruction(MBB, MI, TII);
242 }
243
244 return Changed;
245 }
246
247 // Factory function used by AArch64TargetMachine to add the pass to
248 // the passmanager.
createAArch64A53Fix835769()249 FunctionPass *llvm::createAArch64A53Fix835769() {
250 return new AArch64A53Fix835769();
251 }
252