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1 //===-------- X86PadShortFunction.cpp - pad short functions -----------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file defines the pass which will pad short functions to prevent
11 // a stall if a function returns before the return address is ready. This
12 // is needed for some Intel Atom processors.
13 //
14 //===----------------------------------------------------------------------===//
15 
16 #include <algorithm>
17 
18 #include "X86.h"
19 #include "X86InstrInfo.h"
20 #include "X86Subtarget.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/CodeGen/MachineFunctionPass.h"
23 #include "llvm/CodeGen/MachineInstrBuilder.h"
24 #include "llvm/CodeGen/MachineRegisterInfo.h"
25 #include "llvm/CodeGen/Passes.h"
26 #include "llvm/IR/Function.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Support/raw_ostream.h"
29 #include "llvm/Target/TargetInstrInfo.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__anon535e01920111::VisitedBBInfo46     VisitedBBInfo() : HasReturn(false), Cycles(0) {}
VisitedBBInfo__anon535e01920111::VisitedBBInfo47     VisitedBBInfo(bool HasReturn, unsigned int Cycles)
48       : HasReturn(HasReturn), Cycles(Cycles) {}
49   };
50 
51   struct PadShortFunc : public MachineFunctionPass {
52     static char ID;
PadShortFunc__anon535e01920111::PadShortFunc53     PadShortFunc() : MachineFunctionPass(ID)
54                    , Threshold(4), TM(nullptr), TII(nullptr) {}
55 
56     bool runOnMachineFunction(MachineFunction &MF) override;
57 
getPassName__anon535e01920111::PadShortFunc58     const char *getPassName() const override {
59       return "X86 Atom pad short functions";
60     }
61 
62   private:
63     void findReturns(MachineBasicBlock *MBB,
64                      unsigned int Cycles = 0);
65 
66     bool cyclesUntilReturn(MachineBasicBlock *MBB,
67                            unsigned int &Cycles);
68 
69     void addPadding(MachineBasicBlock *MBB,
70                     MachineBasicBlock::iterator &MBBI,
71                     unsigned int NOOPsToAdd);
72 
73     const unsigned int Threshold;
74 
75     // ReturnBBs - Maps basic blocks that return to the minimum number of
76     // cycles until the return, starting from the entry block.
77     DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
78 
79     // VisitedBBs - Cache of previously visited BBs.
80     DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs;
81 
82     const TargetMachine *TM;
83     const TargetInstrInfo *TII;
84   };
85 
86   char PadShortFunc::ID = 0;
87 }
88 
createX86PadShortFunctions()89 FunctionPass *llvm::createX86PadShortFunctions() {
90   return new PadShortFunc();
91 }
92 
93 /// runOnMachineFunction - Loop over all of the basic blocks, inserting
94 /// NOOP instructions before early exits.
runOnMachineFunction(MachineFunction & MF)95 bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
96   const AttributeSet &FnAttrs = MF.getFunction()->getAttributes();
97   if (FnAttrs.hasAttribute(AttributeSet::FunctionIndex,
98                            Attribute::OptimizeForSize) ||
99       FnAttrs.hasAttribute(AttributeSet::FunctionIndex,
100                            Attribute::MinSize)) {
101     return false;
102   }
103 
104   TM = &MF.getTarget();
105   if (!TM->getSubtarget<X86Subtarget>().padShortFunctions())
106     return false;
107 
108   TII = TM->getInstrInfo();
109 
110   // Search through basic blocks and mark the ones that have early returns
111   ReturnBBs.clear();
112   VisitedBBs.clear();
113   findReturns(MF.begin());
114 
115   bool MadeChange = false;
116 
117   MachineBasicBlock *MBB;
118   unsigned int Cycles = 0;
119 
120   // Pad the identified basic blocks with NOOPs
121   for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
122        I != ReturnBBs.end(); ++I) {
123     MBB = I->first;
124     Cycles = I->second;
125 
126     if (Cycles < Threshold) {
127       // BB ends in a return. Skip over any DBG_VALUE instructions
128       // trailing the terminator.
129       assert(MBB->size() > 0 &&
130              "Basic block should contain at least a RET but is empty");
131       MachineBasicBlock::iterator ReturnLoc = --MBB->end();
132 
133       while (ReturnLoc->isDebugValue())
134         --ReturnLoc;
135       assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
136              "Basic block does not end with RET");
137 
138       addPadding(MBB, ReturnLoc, Threshold - Cycles);
139       NumBBsPadded++;
140       MadeChange = true;
141     }
142   }
143 
144   return MadeChange;
145 }
146 
147 /// findReturn - Starting at MBB, follow control flow and add all
148 /// basic blocks that contain a return to ReturnBBs.
findReturns(MachineBasicBlock * MBB,unsigned int Cycles)149 void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
150   // If this BB has a return, note how many cycles it takes to get there.
151   bool hasReturn = cyclesUntilReturn(MBB, Cycles);
152   if (Cycles >= Threshold)
153     return;
154 
155   if (hasReturn) {
156     ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
157     return;
158   }
159 
160   // Follow branches in BB and look for returns
161   for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
162        I != MBB->succ_end(); ++I) {
163     if (*I == MBB)
164       continue;
165     findReturns(*I, Cycles);
166   }
167 }
168 
169 /// cyclesUntilReturn - return true if the MBB has a return instruction,
170 /// and return false otherwise.
171 /// Cycles will be incremented by the number of cycles taken to reach the
172 /// return or the end of the BB, whichever occurs first.
cyclesUntilReturn(MachineBasicBlock * MBB,unsigned int & Cycles)173 bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
174                                      unsigned int &Cycles) {
175   // Return cached result if BB was previously visited
176   DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
177     = VisitedBBs.find(MBB);
178   if (it != VisitedBBs.end()) {
179     VisitedBBInfo BBInfo = it->second;
180     Cycles += BBInfo.Cycles;
181     return BBInfo.HasReturn;
182   }
183 
184   unsigned int CyclesToEnd = 0;
185 
186   for (MachineBasicBlock::iterator MBBI = MBB->begin();
187         MBBI != MBB->end(); ++MBBI) {
188     MachineInstr *MI = MBBI;
189     // Mark basic blocks with a return instruction. Calls to other
190     // functions do not count because the called function will be padded,
191     // if necessary.
192     if (MI->isReturn() && !MI->isCall()) {
193       VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
194       Cycles += CyclesToEnd;
195       return true;
196     }
197 
198     CyclesToEnd += TII->getInstrLatency(TM->getInstrItineraryData(), MI);
199   }
200 
201   VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
202   Cycles += CyclesToEnd;
203   return false;
204 }
205 
206 /// addPadding - Add the given number of NOOP instructions to the function
207 /// just prior to the return at MBBI
addPadding(MachineBasicBlock * MBB,MachineBasicBlock::iterator & MBBI,unsigned int NOOPsToAdd)208 void PadShortFunc::addPadding(MachineBasicBlock *MBB,
209                               MachineBasicBlock::iterator &MBBI,
210                               unsigned int NOOPsToAdd) {
211   DebugLoc DL = MBBI->getDebugLoc();
212 
213   while (NOOPsToAdd-- > 0) {
214     BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
215     BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
216   }
217 }
218