src/llvm/lib/Target/X86/X86PadShortFunction.cpp

   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
  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), TM(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 TargetMachine *TM;
  83     const TargetInstrInfo *TII;
  84   };
  85
  86   char PadShortFunc::ID = 0;
  87 }
  88
  89 FunctionPass *llvm::createX86PadShortFunctions() {
  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) {
  96   const AttributeSet &FnAttrs = MF.getFunction()->getAttributes();
  97   if (FnAttrs.hasAttribute(AttributeSet::FunctionIndex,
  98                            Attribute::OptimizeForSize) ||
  99       FnAttrs.hasAttribute(AttributeSet::FunctionIndex,
 100                            Attribute::MinSize)) {
 101     return false;
 102   }
 103
 104   TM = &MF.getTarget();
 105   if (!TM->getSubtarget<X86Subtarget>().padShortFunctions())
 106     return false;
 107
 108   TII = TM->getSubtargetImpl()->getInstrInfo();
 109
 110   // Search through basic blocks and mark the ones that have early returns
 111   ReturnBBs.clear();
 112   VisitedBBs.clear();
 113   findReturns(MF.begin());
 114
 115   bool MadeChange = false;
 116
 117   MachineBasicBlock *MBB;
 118   unsigned int Cycles = 0;
 119
 120   // Pad the identified basic blocks with NOOPs
 121   for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
 122        I != ReturnBBs.end(); ++I) {
 123     MBB = I->first;
 124     Cycles = I->second;
 125
 126     if (Cycles < Threshold) {
 127       // BB ends in a return. Skip over any DBG_VALUE instructions
 128       // trailing the terminator.
 129       assert(MBB->size() > 0 &&
 130              "Basic block should contain at least a RET but is empty");
 131       MachineBasicBlock::iterator ReturnLoc = --MBB->end();
 132
 133       while (ReturnLoc->isDebugValue())
 134         --ReturnLoc;
 135       assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
 136              "Basic block does not end with RET");
 137
 138       addPadding(MBB, ReturnLoc, Threshold - Cycles);
 139       NumBBsPadded++;
 140       MadeChange = true;
 141     }
 142   }
 143
 144   return MadeChange;
 145 }
 146
 147 /// findReturn - Starting at MBB, follow control flow and add all
 148 /// basic blocks that contain a return to ReturnBBs.
 149 void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
 150   // If this BB has a return, note how many cycles it takes to get there.
 151   bool hasReturn = cyclesUntilReturn(MBB, Cycles);
 152   if (Cycles >= Threshold)
 153     return;
 154
 155   if (hasReturn) {
 156     ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
 157     return;
 158   }
 159
 160   // Follow branches in BB and look for returns
 161   for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
 162        I != MBB->succ_end(); ++I) {
 163     if (*I == MBB)
 164       continue;
 165     findReturns(*I, Cycles);
 166   }
 167 }
 168
 169 /// cyclesUntilReturn - return true if the MBB has a return instruction,
 170 /// and return false otherwise.
 171 /// Cycles will be incremented by the number of cycles taken to reach the
 172 /// return or the end of the BB, whichever occurs first.
 173 bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
 174                                      unsigned int &Cycles) {
 175   // Return cached result if BB was previously visited
 176   DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
 177     = VisitedBBs.find(MBB);
 178   if (it != VisitedBBs.end()) {
 179     VisitedBBInfo BBInfo = it->second;
 180     Cycles += BBInfo.Cycles;
 181     return BBInfo.HasReturn;
 182   }
 183
 184   unsigned int CyclesToEnd = 0;
 185
 186   for (MachineBasicBlock::iterator MBBI = MBB->begin();
 187         MBBI != MBB->end(); ++MBBI) {
 188     MachineInstr *MI = MBBI;
 189     // Mark basic blocks with a return instruction. Calls to other
 190     // functions do not count because the called function will be padded,
 191     // if necessary.
 192     if (MI->isReturn() && !MI->isCall()) {
 193       VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
 194       Cycles += CyclesToEnd;
 195       return true;
 196     }
 197
 198     CyclesToEnd += TII->getInstrLatency(
 199         TM->getSubtargetImpl()->getInstrItineraryData(), MI);
 200   }
 201
 202   VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
 203   Cycles += CyclesToEnd;
 204   return false;
 205 }
 206
 207 /// addPadding - Add the given number of NOOP instructions to the function
 208 /// just prior to the return at MBBI
 209 void PadShortFunc::addPadding(MachineBasicBlock *MBB,
 210                               MachineBasicBlock::iterator &MBBI,
 211                               unsigned int NOOPsToAdd) {
 212   DebugLoc DL = MBBI->getDebugLoc();
 213
 214   while (NOOPsToAdd-- > 0) {
 215     BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
 216     BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
 217   }
 218 }