LLVM  3.7.0
X86PadShortFunction.cpp
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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"
25 #include "llvm/CodeGen/Passes.h"
26 #include "llvm/IR/Function.h"
27 #include "llvm/Support/Debug.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 
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,
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.
78 
79  // VisitedBBs - Cache of previously visited BBs.
81 
82  const X86Subtarget *STI;
83  const TargetInstrInfo *TII;
84  };
85 
86  char PadShortFunc::ID = 0;
87 }
88 
90  return new PadShortFunc();
91 }
92 
93 /// runOnMachineFunction - Loop over all of the basic blocks, inserting
94 /// NOOP instructions before early exits.
95 bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
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
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.
146 void 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
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.
170 bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
171  unsigned int &Cycles) {
172  // Return cached result if BB was previously visited
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
205 void PadShortFunc::addPadding(MachineBasicBlock *MBB,
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 }
STATISTIC(NumFunctions,"Total number of functions")
A debug info location.
Definition: DebugLoc.h:34
const Function * getFunction() const
getFunction - Return the LLVM function that this machine code represents
const TargetSubtargetInfo & getSubtarget() const
getSubtarget - Return the subtarget for which this machine code is being compiled.
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
const HexagonInstrInfo * TII
FunctionPass * createX86PadShortFunctions()
createX86PadShortFunctions - Return a pass that pads short functions with NOOPs.
std::vector< MachineBasicBlock * >::iterator succ_iterator
#define false
Definition: ConvertUTF.c:65
TargetInstrInfo - Interface to description of machine instruction set.
bundle_iterator< MachineInstr, instr_iterator > iterator
bool isReturn(QueryType Type=AnyInBundle) const
Definition: MachineInstr.h:399
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:294
MachineInstrBuilder BuildMI(MachineFunction &MF, DebugLoc DL, const MCInstrDesc &MCID)
BuildMI - Builder interface.
Representation of each machine instruction.
Definition: MachineInstr.h:51
bool hasFnAttribute(Attribute::AttrKind Kind) const
Return true if the function has the attribute.
Definition: Function.h:217
#define I(x, y, z)
Definition: MD5.cpp:54
bool isCall(QueryType Type=AnyInBundle) const
Definition: MachineInstr.h:403
iterator find(const KeyT &Val)
Definition: DenseMap.h:124
static int const Threshold
TODO: Write a new FunctionPass AliasAnalysis so that it can keep a cache.
Function must be optimized for size first.
Definition: Attributes.h:80