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      AU.addPreserved<LazyMachineBlockFrequencyInfoPass>();
62      MachineFunctionPass::getAnalysisUsage(AU);
63    }
64
65    MachineFunctionProperties getRequiredProperties() const override {
66      return MachineFunctionProperties().set(
67          MachineFunctionProperties::Property::NoVRegs);
68    }
69
70    StringRef getPassName() const override {
71      return "X86 Atom pad short functions";
72    }
73
74  private:
75    void findReturns(MachineBasicBlock *MBB,
76                     unsigned int Cycles = 0);
77
78    bool cyclesUntilReturn(MachineBasicBlock *MBB,
79                           unsigned int &Cycles);
80
81    void addPadding(MachineBasicBlock *MBB,
82                    MachineBasicBlock::iterator &MBBI,
83                    unsigned int NOOPsToAdd);
84
85    const unsigned int Threshold;
86
87    // ReturnBBs - Maps basic blocks that return to the minimum number of
88    // cycles until the return, starting from the entry block.
89    DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
90
91    // VisitedBBs - Cache of previously visited BBs.
92    DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs;
93
94    TargetSchedModel TSM;
95  };
96
97  char PadShortFunc::ID = 0;
98}
99
100FunctionPass *llvm::createX86PadShortFunctions() {
101  return new PadShortFunc();
102}
103
104/// runOnMachineFunction - Loop over all of the basic blocks, inserting
105/// NOOP instructions before early exits.
106bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
107  if (skipFunction(MF.getFunction()))
108    return false;
109
110  if (MF.getFunction().hasOptSize())
111    return false;
112
113  if (!MF.getSubtarget<X86Subtarget>().padShortFunctions())
114    return false;
115
116  TSM.init(&MF.getSubtarget());
117
118  auto *PSI =
119      &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
120  auto *MBFI = (PSI && PSI->hasProfileSummary()) ?
121               &getAnalysis<LazyMachineBlockFrequencyInfoPass>().getBFI() :
122               nullptr;
123
124  // Search through basic blocks and mark the ones that have early returns
125  ReturnBBs.clear();
126  VisitedBBs.clear();
127  findReturns(&MF.front());
128
129  bool MadeChange = false;
130
131  // Pad the identified basic blocks with NOOPs
132  for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
133       I != ReturnBBs.end(); ++I) {
134    MachineBasicBlock *MBB = I->first;
135    unsigned Cycles = I->second;
136
137    // Function::hasOptSize is already checked above.
138    bool OptForSize = llvm::shouldOptimizeForSize(MBB, PSI, MBFI);
139    if (OptForSize)
140      continue;
141
142    if (Cycles < Threshold) {
143      // BB ends in a return. Skip over any DBG_VALUE instructions
144      // trailing the terminator.
145      assert(MBB->size() > 0 &&
146             "Basic block should contain at least a RET but is empty");
147      MachineBasicBlock::iterator ReturnLoc = --MBB->end();
148
149      while (ReturnLoc->isDebugInstr())
150        --ReturnLoc;
151      assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
152             "Basic block does not end with RET");
153
154      addPadding(MBB, ReturnLoc, Threshold - Cycles);
155      NumBBsPadded++;
156      MadeChange = true;
157    }
158  }
159
160  return MadeChange;
161}
162
163/// findReturn - Starting at MBB, follow control flow and add all
164/// basic blocks that contain a return to ReturnBBs.
165void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
166  // If this BB has a return, note how many cycles it takes to get there.
167  bool hasReturn = cyclesUntilReturn(MBB, Cycles);
168  if (Cycles >= Threshold)
169    return;
170
171  if (hasReturn) {
172    ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
173    return;
174  }
175
176  // Follow branches in BB and look for returns
177  for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
178       I != MBB->succ_end(); ++I) {
179    if (*I == MBB)
180      continue;
181    findReturns(*I, Cycles);
182  }
183}
184
185/// cyclesUntilReturn - return true if the MBB has a return instruction,
186/// and return false otherwise.
187/// Cycles will be incremented by the number of cycles taken to reach the
188/// return or the end of the BB, whichever occurs first.
189bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
190                                     unsigned int &Cycles) {
191  // Return cached result if BB was previously visited
192  DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
193    = VisitedBBs.find(MBB);
194  if (it != VisitedBBs.end()) {
195    VisitedBBInfo BBInfo = it->second;
196    Cycles += BBInfo.Cycles;
197    return BBInfo.HasReturn;
198  }
199
200  unsigned int CyclesToEnd = 0;
201
202  for (MachineInstr &MI : *MBB) {
203    // Mark basic blocks with a return instruction. Calls to other
204    // functions do not count because the called function will be padded,
205    // if necessary.
206    if (MI.isReturn() && !MI.isCall()) {
207      VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
208      Cycles += CyclesToEnd;
209      return true;
210    }
211
212    CyclesToEnd += TSM.computeInstrLatency(&MI);
213  }
214
215  VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
216  Cycles += CyclesToEnd;
217  return false;
218}
219
220/// addPadding - Add the given number of NOOP instructions to the function
221/// just prior to the return at MBBI
222void PadShortFunc::addPadding(MachineBasicBlock *MBB,
223                              MachineBasicBlock::iterator &MBBI,
224                              unsigned int NOOPsToAdd) {
225  DebugLoc DL = MBBI->getDebugLoc();
226  unsigned IssueWidth = TSM.getIssueWidth();
227
228  for (unsigned i = 0, e = IssueWidth * NOOPsToAdd; i != e; ++i)
229    BuildMI(*MBB, MBBI, DL, TSM.getInstrInfo()->get(X86::NOOP));
230}
231