1 //===-------- X86PadShortFunction.cpp - pad short functions -----------===//
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 file defines the pass which will pad short functions to prevent
10 // a stall if a function returns before the return address is ready. This
11 // is needed for some Intel Atom processors.
12 //
13 //===----------------------------------------------------------------------===//
14
15
16 #include "X86.h"
17 #include "X86InstrInfo.h"
18 #include "X86Subtarget.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/ProfileSummaryInfo.h"
21 #include "llvm/CodeGen/LazyMachineBlockFrequencyInfo.h"
22 #include "llvm/CodeGen/MachineFunctionPass.h"
23 #include "llvm/CodeGen/MachineInstrBuilder.h"
24 #include "llvm/CodeGen/MachineSizeOpts.h"
25 #include "llvm/CodeGen/Passes.h"
26 #include "llvm/CodeGen/TargetSchedule.h"
27 #include "llvm/IR/Function.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Support/raw_ostream.h"
30
31 using namespace llvm;
32
33 #define DEBUG_TYPE "x86-pad-short-functions"
34
35 STATISTIC(NumBBsPadded, "Number of basic blocks padded");
36
37 namespace {
38 struct VisitedBBInfo {
39 // HasReturn - Whether the BB contains a return instruction
40 bool HasReturn;
41
42 // Cycles - Number of cycles until return if HasReturn is true, otherwise
43 // number of cycles until end of the BB
44 unsigned int Cycles;
45
VisitedBBInfo__anon45ff49640111::VisitedBBInfo46 VisitedBBInfo() : HasReturn(false), Cycles(0) {}
VisitedBBInfo__anon45ff49640111::VisitedBBInfo47 VisitedBBInfo(bool HasReturn, unsigned int Cycles)
48 : HasReturn(HasReturn), Cycles(Cycles) {}
49 };
50
51 struct PadShortFunc : public MachineFunctionPass {
52 static char ID;
PadShortFunc__anon45ff49640111::PadShortFunc53 PadShortFunc() : MachineFunctionPass(ID)
54 , Threshold(4) {}
55
56 bool runOnMachineFunction(MachineFunction &MF) override;
57
getAnalysisUsage__anon45ff49640111::PadShortFunc58 void getAnalysisUsage(AnalysisUsage &AU) const override {
59 AU.addRequired<ProfileSummaryInfoWrapperPass>();
60 AU.addRequired<LazyMachineBlockFrequencyInfoPass>();
61 AU.addPreserved<LazyMachineBlockFrequencyInfoPass>();
62 MachineFunctionPass::getAnalysisUsage(AU);
63 }
64
getRequiredProperties__anon45ff49640111::PadShortFunc65 MachineFunctionProperties getRequiredProperties() const override {
66 return MachineFunctionProperties().set(
67 MachineFunctionProperties::Property::NoVRegs);
68 }
69
getPassName__anon45ff49640111::PadShortFunc70 StringRef getPassName() const override {
71 return "X86 Atom pad short functions";
72 }
73
74 private:
75 void findReturns(MachineBasicBlock *MBB,
76 unsigned int Cycles = 0);
77
78 bool cyclesUntilReturn(MachineBasicBlock *MBB,
79 unsigned int &Cycles);
80
81 void addPadding(MachineBasicBlock *MBB,
82 MachineBasicBlock::iterator &MBBI,
83 unsigned int NOOPsToAdd);
84
85 const unsigned int Threshold;
86
87 // ReturnBBs - Maps basic blocks that return to the minimum number of
88 // cycles until the return, starting from the entry block.
89 DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
90
91 // VisitedBBs - Cache of previously visited BBs.
92 DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs;
93
94 TargetSchedModel TSM;
95 };
96
97 char PadShortFunc::ID = 0;
98 }
99
createX86PadShortFunctions()100 FunctionPass *llvm::createX86PadShortFunctions() {
101 return new PadShortFunc();
102 }
103
104 /// runOnMachineFunction - Loop over all of the basic blocks, inserting
105 /// NOOP instructions before early exits.
runOnMachineFunction(MachineFunction & MF)106 bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
107 if (skipFunction(MF.getFunction()))
108 return false;
109
110 if (MF.getFunction().hasOptSize())
111 return false;
112
113 if (!MF.getSubtarget<X86Subtarget>().padShortFunctions())
114 return false;
115
116 TSM.init(&MF.getSubtarget());
117
118 auto *PSI =
119 &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
120 auto *MBFI = (PSI && PSI->hasProfileSummary()) ?
121 &getAnalysis<LazyMachineBlockFrequencyInfoPass>().getBFI() :
122 nullptr;
123
124 // Search through basic blocks and mark the ones that have early returns
125 ReturnBBs.clear();
126 VisitedBBs.clear();
127 findReturns(&MF.front());
128
129 bool MadeChange = false;
130
131 // Pad the identified basic blocks with NOOPs
132 for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
133 I != ReturnBBs.end(); ++I) {
134 MachineBasicBlock *MBB = I->first;
135 unsigned Cycles = I->second;
136
137 // Function::hasOptSize is already checked above.
138 bool OptForSize = llvm::shouldOptimizeForSize(MBB, PSI, MBFI);
139 if (OptForSize)
140 continue;
141
142 if (Cycles < Threshold) {
143 // BB ends in a return. Skip over any DBG_VALUE instructions
144 // trailing the terminator.
145 assert(MBB->size() > 0 &&
146 "Basic block should contain at least a RET but is empty");
147 MachineBasicBlock::iterator ReturnLoc = --MBB->end();
148
149 while (ReturnLoc->isDebugInstr())
150 --ReturnLoc;
151 assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
152 "Basic block does not end with RET");
153
154 addPadding(MBB, ReturnLoc, Threshold - Cycles);
155 NumBBsPadded++;
156 MadeChange = true;
157 }
158 }
159
160 return MadeChange;
161 }
162
163 /// findReturn - Starting at MBB, follow control flow and add all
164 /// basic blocks that contain a return to ReturnBBs.
findReturns(MachineBasicBlock * MBB,unsigned int Cycles)165 void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
166 // If this BB has a return, note how many cycles it takes to get there.
167 bool hasReturn = cyclesUntilReturn(MBB, Cycles);
168 if (Cycles >= Threshold)
169 return;
170
171 if (hasReturn) {
172 ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
173 return;
174 }
175
176 // Follow branches in BB and look for returns
177 for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
178 I != MBB->succ_end(); ++I) {
179 if (*I == MBB)
180 continue;
181 findReturns(*I, Cycles);
182 }
183 }
184
185 /// cyclesUntilReturn - return true if the MBB has a return instruction,
186 /// and return false otherwise.
187 /// Cycles will be incremented by the number of cycles taken to reach the
188 /// return or the end of the BB, whichever occurs first.
cyclesUntilReturn(MachineBasicBlock * MBB,unsigned int & Cycles)189 bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
190 unsigned int &Cycles) {
191 // Return cached result if BB was previously visited
192 DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
193 = VisitedBBs.find(MBB);
194 if (it != VisitedBBs.end()) {
195 VisitedBBInfo BBInfo = it->second;
196 Cycles += BBInfo.Cycles;
197 return BBInfo.HasReturn;
198 }
199
200 unsigned int CyclesToEnd = 0;
201
202 for (MachineInstr &MI : *MBB) {
203 // Mark basic blocks with a return instruction. Calls to other
204 // functions do not count because the called function will be padded,
205 // if necessary.
206 if (MI.isReturn() && !MI.isCall()) {
207 VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
208 Cycles += CyclesToEnd;
209 return true;
210 }
211
212 CyclesToEnd += TSM.computeInstrLatency(&MI);
213 }
214
215 VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
216 Cycles += CyclesToEnd;
217 return false;
218 }
219
220 /// addPadding - Add the given number of NOOP instructions to the function
221 /// just prior to the return at MBBI
addPadding(MachineBasicBlock * MBB,MachineBasicBlock::iterator & MBBI,unsigned int NOOPsToAdd)222 void PadShortFunc::addPadding(MachineBasicBlock *MBB,
223 MachineBasicBlock::iterator &MBBI,
224 unsigned int NOOPsToAdd) {
225 DebugLoc DL = MBBI->getDebugLoc();
226 unsigned IssueWidth = TSM.getIssueWidth();
227
228 for (unsigned i = 0, e = IssueWidth * NOOPsToAdd; i != e; ++i)
229 BuildMI(*MBB, MBBI, DL, TSM.getInstrInfo()->get(X86::NOOP));
230 }
231