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
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) {}
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
99FunctionPass *llvm::createX86PadShortFunctions() {
100  return new PadShortFunc();
101}
102
103/// runOnMachineFunction - Loop over all of the basic blocks, inserting
104/// NOOP instructions before early exits.
105bool 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.
164void 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.
188bool 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
221void 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