X86PadShortFunction.cpp revision 36b56886974eae4f9c5ebc96befd3e7bfe5de338 
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#define DEBUG_TYPE "x86-pad-short-functions"
19#include "X86.h"
20#include "X86InstrInfo.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
33STATISTIC(NumBBsPadded, "Number of basic blocks padded");
34
35namespace {
36  struct VisitedBBInfo {
37    // HasReturn - Whether the BB contains a return instruction
38    bool HasReturn;
39
40    // Cycles - Number of cycles until return if HasReturn is true, otherwise
41    // number of cycles until end of the BB
42    unsigned int Cycles;
43
44    VisitedBBInfo() : HasReturn(false), Cycles(0) {}
45    VisitedBBInfo(bool HasReturn, unsigned int Cycles)
46      : HasReturn(HasReturn), Cycles(Cycles) {}
47  };
48
49  struct PadShortFunc : public MachineFunctionPass {
50    static char ID;
51    PadShortFunc() : MachineFunctionPass(ID)
52                   , Threshold(4), TM(0), TII(0) {}
53
54    bool runOnMachineFunction(MachineFunction &MF) override;
55
56    const char *getPassName() const override {
57      return "X86 Atom pad short functions";
58    }
59
60  private:
61    void findReturns(MachineBasicBlock *MBB,
62                     unsigned int Cycles = 0);
63
64    bool cyclesUntilReturn(MachineBasicBlock *MBB,
65                           unsigned int &Cycles);
66
67    void addPadding(MachineBasicBlock *MBB,
68                    MachineBasicBlock::iterator &MBBI,
69                    unsigned int NOOPsToAdd);
70
71    const unsigned int Threshold;
72
73    // ReturnBBs - Maps basic blocks that return to the minimum number of
74    // cycles until the return, starting from the entry block.
75    DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
76
77    // VisitedBBs - Cache of previously visited BBs.
78    DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs;
79
80    const TargetMachine *TM;
81    const TargetInstrInfo *TII;
82  };
83
84  char PadShortFunc::ID = 0;
85}
86
87FunctionPass *llvm::createX86PadShortFunctions() {
88  return new PadShortFunc();
89}
90
91/// runOnMachineFunction - Loop over all of the basic blocks, inserting
92/// NOOP instructions before early exits.
93bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
94  const AttributeSet &FnAttrs = MF.getFunction()->getAttributes();
95  if (FnAttrs.hasAttribute(AttributeSet::FunctionIndex,
96                           Attribute::OptimizeForSize) ||
97      FnAttrs.hasAttribute(AttributeSet::FunctionIndex,
98                           Attribute::MinSize)) {
99    return false;
100  }
101
102  TM = &MF.getTarget();
103  TII = TM->getInstrInfo();
104
105  // Search through basic blocks and mark the ones that have early returns
106  ReturnBBs.clear();
107  VisitedBBs.clear();
108  findReturns(MF.begin());
109
110  bool MadeChange = false;
111
112  MachineBasicBlock *MBB;
113  unsigned int Cycles = 0;
114
115  // Pad the identified basic blocks with NOOPs
116  for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
117       I != ReturnBBs.end(); ++I) {
118    MBB = I->first;
119    Cycles = I->second;
120
121    if (Cycles < Threshold) {
122      // BB ends in a return. Skip over any DBG_VALUE instructions
123      // trailing the terminator.
124      assert(MBB->size() > 0 &&
125             "Basic block should contain at least a RET but is empty");
126      MachineBasicBlock::iterator ReturnLoc = --MBB->end();
127
128      while (ReturnLoc->isDebugValue())
129        --ReturnLoc;
130      assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
131             "Basic block does not end with RET");
132
133      addPadding(MBB, ReturnLoc, Threshold - Cycles);
134      NumBBsPadded++;
135      MadeChange = true;
136    }
137  }
138
139  return MadeChange;
140}
141
142/// findReturn - Starting at MBB, follow control flow and add all
143/// basic blocks that contain a return to ReturnBBs.
144void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
145  // If this BB has a return, note how many cycles it takes to get there.
146  bool hasReturn = cyclesUntilReturn(MBB, Cycles);
147  if (Cycles >= Threshold)
148    return;
149
150  if (hasReturn) {
151    ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
152    return;
153  }
154
155  // Follow branches in BB and look for returns
156  for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
157       I != MBB->succ_end(); ++I) {
158    if (*I == MBB)
159      continue;
160    findReturns(*I, Cycles);
161  }
162}
163
164/// cyclesUntilReturn - return true if the MBB has a return instruction,
165/// and return false otherwise.
166/// Cycles will be incremented by the number of cycles taken to reach the
167/// return or the end of the BB, whichever occurs first.
168bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
169                                     unsigned int &Cycles) {
170  // Return cached result if BB was previously visited
171  DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
172    = VisitedBBs.find(MBB);
173  if (it != VisitedBBs.end()) {
174    VisitedBBInfo BBInfo = it->second;
175    Cycles += BBInfo.Cycles;
176    return BBInfo.HasReturn;
177  }
178
179  unsigned int CyclesToEnd = 0;
180
181  for (MachineBasicBlock::iterator MBBI = MBB->begin();
182        MBBI != MBB->end(); ++MBBI) {
183    MachineInstr *MI = MBBI;
184    // Mark basic blocks with a return instruction. Calls to other
185    // functions do not count because the called function will be padded,
186    // if necessary.
187    if (MI->isReturn() && !MI->isCall()) {
188      VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
189      Cycles += CyclesToEnd;
190      return true;
191    }
192
193    CyclesToEnd += TII->getInstrLatency(TM->getInstrItineraryData(), MI);
194  }
195
196  VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
197  Cycles += CyclesToEnd;
198  return false;
199}
200
201/// addPadding - Add the given number of NOOP instructions to the function
202/// just prior to the return at MBBI
203void PadShortFunc::addPadding(MachineBasicBlock *MBB,
204                              MachineBasicBlock::iterator &MBBI,
205                              unsigned int NOOPsToAdd) {
206  DebugLoc DL = MBBI->getDebugLoc();
207
208  while (NOOPsToAdd-- > 0) {
209    BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
210    BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
211  }
212}
213