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 
46     VisitedBBInfo() : HasReturn(false), Cycles(0) {}
47     VisitedBBInfo(bool HasReturn, unsigned int Cycles)
48       : HasReturn(HasReturn), Cycles(Cycles) {}
49   };
50 
51   struct PadShortFunc : public MachineFunctionPass {
52     static char ID;
53     PadShortFunc() : MachineFunctionPass(ID)
54                    , Threshold(4) {}
55 
56     bool runOnMachineFunction(MachineFunction &MF) override;
57 
58     void getAnalysisUsage(AnalysisUsage &AU) const override {
59       AU.addRequired<ProfileSummaryInfoWrapperPass>();
60       AU.addRequired<LazyMachineBlockFrequencyInfoPass>();
61       MachineFunctionPass::getAnalysisUsage(AU);
62     }
63 
64     MachineFunctionProperties getRequiredProperties() const override {
65       return MachineFunctionProperties().set(
66           MachineFunctionProperties::Property::NoVRegs);
67     }
68 
69     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;
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 
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.
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 (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
132        I != ReturnBBs.end(); ++I) {
133     MachineBasicBlock *MBB = I->first;
134     unsigned Cycles = I->second;
135 
136     // Function::hasOptSize is already checked above.
137     bool OptForSize = llvm::shouldOptimizeForSize(MBB, PSI, MBFI);
138     if (OptForSize)
139       continue;
140 
141     if (Cycles < Threshold) {
142       // BB ends in a return. Skip over any DBG_VALUE instructions
143       // trailing the terminator.
144       assert(MBB->size() > 0 &&
145              "Basic block should contain at least a RET but is empty");
146       MachineBasicBlock::iterator ReturnLoc = --MBB->end();
147 
148       while (ReturnLoc->isDebugInstr())
149         --ReturnLoc;
150       assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
151              "Basic block does not end with RET");
152 
153       addPadding(MBB, ReturnLoc, Threshold - Cycles);
154       NumBBsPadded++;
155       MadeChange = true;
156     }
157   }
158 
159   return MadeChange;
160 }
161 
162 /// findReturn - Starting at MBB, follow control flow and add all
163 /// basic blocks that contain a return to ReturnBBs.
164 void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
165   // If this BB has a return, note how many cycles it takes to get there.
166   bool hasReturn = cyclesUntilReturn(MBB, Cycles);
167   if (Cycles >= Threshold)
168     return;
169 
170   if (hasReturn) {
171     ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
172     return;
173   }
174 
175   // Follow branches in BB and look for returns
176   for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
177        I != MBB->succ_end(); ++I) {
178     if (*I == MBB)
179       continue;
180     findReturns(*I, Cycles);
181   }
182 }
183 
184 /// cyclesUntilReturn - return true if the MBB has a return instruction,
185 /// and return false otherwise.
186 /// Cycles will be incremented by the number of cycles taken to reach the
187 /// return or the end of the BB, whichever occurs first.
188 bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
189                                      unsigned int &Cycles) {
190   // Return cached result if BB was previously visited
191   DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
192     = VisitedBBs.find(MBB);
193   if (it != VisitedBBs.end()) {
194     VisitedBBInfo BBInfo = it->second;
195     Cycles += BBInfo.Cycles;
196     return BBInfo.HasReturn;
197   }
198 
199   unsigned int CyclesToEnd = 0;
200 
201   for (MachineInstr &MI : *MBB) {
202     // Mark basic blocks with a return instruction. Calls to other
203     // functions do not count because the called function will be padded,
204     // if necessary.
205     if (MI.isReturn() && !MI.isCall()) {
206       VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
207       Cycles += CyclesToEnd;
208       return true;
209     }
210 
211     CyclesToEnd += TSM.computeInstrLatency(&MI);
212   }
213 
214   VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
215   Cycles += CyclesToEnd;
216   return false;
217 }
218 
219 /// addPadding - Add the given number of NOOP instructions to the function
220 /// just prior to the return at MBBI
221 void PadShortFunc::addPadding(MachineBasicBlock *MBB,
222                               MachineBasicBlock::iterator &MBBI,
223                               unsigned int NOOPsToAdd) {
224   DebugLoc DL = MBBI->getDebugLoc();
225   unsigned IssueWidth = TSM.getIssueWidth();
226 
227   for (unsigned i = 0, e = IssueWidth * NOOPsToAdd; i != e; ++i)
228     BuildMI(*MBB, MBBI, DL, TSM.getInstrInfo()->get(X86::NOOP));
229 }
230