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 = false;
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 = 0;
45
46 VisitedBBInfo() = default;
VisitedBBInfo__anonb137836c0111::VisitedBBInfo47 VisitedBBInfo(bool HasReturn, unsigned int Cycles)
48 : HasReturn(HasReturn), Cycles(Cycles) {}
49 };
50
51 struct PadShortFunc : public MachineFunctionPass {
52 static char ID;
PadShortFunc__anonb137836c0111::PadShortFunc53 PadShortFunc() : MachineFunctionPass(ID) {}
54
55 bool runOnMachineFunction(MachineFunction &MF) override;
56
getAnalysisUsage__anonb137836c0111::PadShortFunc57 void getAnalysisUsage(AnalysisUsage &AU) const override {
58 AU.addRequired<ProfileSummaryInfoWrapperPass>();
59 AU.addRequired<LazyMachineBlockFrequencyInfoPass>();
60 AU.addPreserved<LazyMachineBlockFrequencyInfoPass>();
61 MachineFunctionPass::getAnalysisUsage(AU);
62 }
63
getRequiredProperties__anonb137836c0111::PadShortFunc64 MachineFunctionProperties getRequiredProperties() const override {
65 return MachineFunctionProperties().set(
66 MachineFunctionProperties::Property::NoVRegs);
67 }
68
getPassName__anonb137836c0111::PadShortFunc69 StringRef getPassName() const override {
70 return "X86 Atom pad short functions";
71 }
72
73 private:
74 void findReturns(MachineBasicBlock *MBB,
75 unsigned int Cycles = 0);
76
77 bool cyclesUntilReturn(MachineBasicBlock *MBB,
78 unsigned int &Cycles);
79
80 void addPadding(MachineBasicBlock *MBB,
81 MachineBasicBlock::iterator &MBBI,
82 unsigned int NOOPsToAdd);
83
84 const unsigned int Threshold = 4;
85
86 // ReturnBBs - Maps basic blocks that return to the minimum number of
87 // cycles until the return, starting from the entry block.
88 DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
89
90 // VisitedBBs - Cache of previously visited BBs.
91 DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs;
92
93 TargetSchedModel TSM;
94 };
95
96 char PadShortFunc::ID = 0;
97 }
98
createX86PadShortFunctions()99 FunctionPass *llvm::createX86PadShortFunctions() {
100 return new PadShortFunc();
101 }
102
103 /// runOnMachineFunction - Loop over all of the basic blocks, inserting
104 /// NOOP instructions before early exits.
runOnMachineFunction(MachineFunction & MF)105 bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
106 if (skipFunction(MF.getFunction()))
107 return false;
108
109 if (MF.getFunction().hasOptSize())
110 return false;
111
112 if (!MF.getSubtarget<X86Subtarget>().padShortFunctions())
113 return false;
114
115 TSM.init(&MF.getSubtarget());
116
117 auto *PSI =
118 &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
119 auto *MBFI = (PSI && PSI->hasProfileSummary()) ?
120 &getAnalysis<LazyMachineBlockFrequencyInfoPass>().getBFI() :
121 nullptr;
122
123 // Search through basic blocks and mark the ones that have early returns
124 ReturnBBs.clear();
125 VisitedBBs.clear();
126 findReturns(&MF.front());
127
128 bool MadeChange = false;
129
130 // Pad the identified basic blocks with NOOPs
131 for (const auto &ReturnBB : ReturnBBs) {
132 MachineBasicBlock *MBB = ReturnBB.first;
133 unsigned Cycles = ReturnBB.second;
134
135 // Function::hasOptSize is already checked above.
136 bool OptForSize = llvm::shouldOptimizeForSize(MBB, PSI, MBFI);
137 if (OptForSize)
138 continue;
139
140 if (Cycles < Threshold) {
141 // BB ends in a return. Skip over any DBG_VALUE instructions
142 // trailing the terminator.
143 assert(MBB->size() > 0 &&
144 "Basic block should contain at least a RET but is empty");
145 MachineBasicBlock::iterator ReturnLoc = --MBB->end();
146
147 while (ReturnLoc->isDebugInstr())
148 --ReturnLoc;
149 assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
150 "Basic block does not end with RET");
151
152 addPadding(MBB, ReturnLoc, Threshold - Cycles);
153 NumBBsPadded++;
154 MadeChange = true;
155 }
156 }
157
158 return MadeChange;
159 }
160
161 /// findReturn - Starting at MBB, follow control flow and add all
162 /// basic blocks that contain a return to ReturnBBs.
findReturns(MachineBasicBlock * MBB,unsigned int Cycles)163 void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
164 // If this BB has a return, note how many cycles it takes to get there.
165 bool hasReturn = cyclesUntilReturn(MBB, Cycles);
166 if (Cycles >= Threshold)
167 return;
168
169 if (hasReturn) {
170 ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
171 return;
172 }
173
174 // Follow branches in BB and look for returns
175 for (MachineBasicBlock *Succ : MBB->successors())
176 if (Succ != MBB)
177 findReturns(Succ, Cycles);
178 }
179
180 /// cyclesUntilReturn - return true if the MBB has a return instruction,
181 /// and return false otherwise.
182 /// Cycles will be incremented by the number of cycles taken to reach the
183 /// return or the end of the BB, whichever occurs first.
cyclesUntilReturn(MachineBasicBlock * MBB,unsigned int & Cycles)184 bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
185 unsigned int &Cycles) {
186 // Return cached result if BB was previously visited
187 DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
188 = VisitedBBs.find(MBB);
189 if (it != VisitedBBs.end()) {
190 VisitedBBInfo BBInfo = it->second;
191 Cycles += BBInfo.Cycles;
192 return BBInfo.HasReturn;
193 }
194
195 unsigned int CyclesToEnd = 0;
196
197 for (MachineInstr &MI : *MBB) {
198 // Mark basic blocks with a return instruction. Calls to other
199 // functions do not count because the called function will be padded,
200 // if necessary.
201 if (MI.isReturn() && !MI.isCall()) {
202 VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
203 Cycles += CyclesToEnd;
204 return true;
205 }
206
207 CyclesToEnd += TSM.computeInstrLatency(&MI);
208 }
209
210 VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
211 Cycles += CyclesToEnd;
212 return false;
213 }
214
215 /// addPadding - Add the given number of NOOP instructions to the function
216 /// just prior to the return at MBBI
addPadding(MachineBasicBlock * MBB,MachineBasicBlock::iterator & MBBI,unsigned int NOOPsToAdd)217 void PadShortFunc::addPadding(MachineBasicBlock *MBB,
218 MachineBasicBlock::iterator &MBBI,
219 unsigned int NOOPsToAdd) {
220 const DebugLoc &DL = MBBI->getDebugLoc();
221 unsigned IssueWidth = TSM.getIssueWidth();
222
223 for (unsigned i = 0, e = IssueWidth * NOOPsToAdd; i != e; ++i)
224 BuildMI(*MBB, MBBI, DL, TSM.getInstrInfo()->get(X86::NOOP));
225 }
226