X86PadShortFunction.cpp revision 288943 
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
31using namespace llvm;
32
33#define DEBUG_TYPE "x86-pad-short-functions"
34
35STATISTIC(NumBBsPadded, "Number of basic blocks padded");
36
37namespace {
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), STI(nullptr), TII(nullptr) {}
55
56    bool runOnMachineFunction(MachineFunction &MF) override;
57
58    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 X86Subtarget *STI;
83    const TargetInstrInfo *TII;
84  };
85
86  char PadShortFunc::ID = 0;
87}
88
89FunctionPass *llvm::createX86PadShortFunctions() {
90  return new PadShortFunc();
91}
92
93/// runOnMachineFunction - Loop over all of the basic blocks, inserting
94/// NOOP instructions before early exits.
95bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
96  if (MF.getFunction()->hasFnAttribute(Attribute::OptimizeForSize) ||
97      MF.getFunction()->hasFnAttribute(Attribute::MinSize)) {
98    return false;
99  }
100
101  STI = &MF.getSubtarget<X86Subtarget>();
102  if (!STI->padShortFunctions())
103    return false;
104
105  TII = STI->getInstrInfo();
106
107  // Search through basic blocks and mark the ones that have early returns
108  ReturnBBs.clear();
109  VisitedBBs.clear();
110  findReturns(MF.begin());
111
112  bool MadeChange = false;
113
114  MachineBasicBlock *MBB;
115  unsigned int Cycles = 0;
116
117  // Pad the identified basic blocks with NOOPs
118  for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
119       I != ReturnBBs.end(); ++I) {
120    MBB = I->first;
121    Cycles = I->second;
122
123    if (Cycles < Threshold) {
124      // BB ends in a return. Skip over any DBG_VALUE instructions
125      // trailing the terminator.
126      assert(MBB->size() > 0 &&
127             "Basic block should contain at least a RET but is empty");
128      MachineBasicBlock::iterator ReturnLoc = --MBB->end();
129
130      while (ReturnLoc->isDebugValue())
131        --ReturnLoc;
132      assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
133             "Basic block does not end with RET");
134
135      addPadding(MBB, ReturnLoc, Threshold - Cycles);
136      NumBBsPadded++;
137      MadeChange = true;
138    }
139  }
140
141  return MadeChange;
142}
143
144/// findReturn - Starting at MBB, follow control flow and add all
145/// basic blocks that contain a return to ReturnBBs.
146void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
147  // If this BB has a return, note how many cycles it takes to get there.
148  bool hasReturn = cyclesUntilReturn(MBB, Cycles);
149  if (Cycles >= Threshold)
150    return;
151
152  if (hasReturn) {
153    ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
154    return;
155  }
156
157  // Follow branches in BB and look for returns
158  for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
159       I != MBB->succ_end(); ++I) {
160    if (*I == MBB)
161      continue;
162    findReturns(*I, Cycles);
163  }
164}
165
166/// cyclesUntilReturn - return true if the MBB has a return instruction,
167/// and return false otherwise.
168/// Cycles will be incremented by the number of cycles taken to reach the
169/// return or the end of the BB, whichever occurs first.
170bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
171                                     unsigned int &Cycles) {
172  // Return cached result if BB was previously visited
173  DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
174    = VisitedBBs.find(MBB);
175  if (it != VisitedBBs.end()) {
176    VisitedBBInfo BBInfo = it->second;
177    Cycles += BBInfo.Cycles;
178    return BBInfo.HasReturn;
179  }
180
181  unsigned int CyclesToEnd = 0;
182
183  for (MachineBasicBlock::iterator MBBI = MBB->begin();
184        MBBI != MBB->end(); ++MBBI) {
185    MachineInstr *MI = MBBI;
186    // Mark basic blocks with a return instruction. Calls to other
187    // functions do not count because the called function will be padded,
188    // if necessary.
189    if (MI->isReturn() && !MI->isCall()) {
190      VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
191      Cycles += CyclesToEnd;
192      return true;
193    }
194
195    CyclesToEnd += TII->getInstrLatency(STI->getInstrItineraryData(), MI);
196  }
197
198  VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
199  Cycles += CyclesToEnd;
200  return false;
201}
202
203/// addPadding - Add the given number of NOOP instructions to the function
204/// just prior to the return at MBBI
205void PadShortFunc::addPadding(MachineBasicBlock *MBB,
206                              MachineBasicBlock::iterator &MBBI,
207                              unsigned int NOOPsToAdd) {
208  DebugLoc DL = MBBI->getDebugLoc();
209
210  while (NOOPsToAdd-- > 0) {
211    BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
212    BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
213  }
214}
215