lib/Target/X86/X86PadShortFunction.cpp

   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/CodeGen/MachineFunctionPass.h"
  21 #include "llvm/CodeGen/MachineInstrBuilder.h"
  22 #include "llvm/CodeGen/Passes.h"
  23 #include "llvm/CodeGen/TargetSchedule.h"
  24 #include "llvm/IR/Function.h"
  25 #include "llvm/Support/Debug.h"
  26 #include "llvm/Support/raw_ostream.h"
  27
  28 using namespace llvm;
  29
  30 #define DEBUG_TYPE "x86-pad-short-functions"
  31
  32 STATISTIC(NumBBsPadded, "Number of basic blocks padded");
  33
  34 namespace {
  35   struct VisitedBBInfo {
  36     // HasReturn - Whether the BB contains a return instruction
  37     bool HasReturn;
  38
  39     // Cycles - Number of cycles until return if HasReturn is true, otherwise
  40     // number of cycles until end of the BB
  41     unsigned int Cycles;
  42
  43     VisitedBBInfo() : HasReturn(false), Cycles(0) {}
  44     VisitedBBInfo(bool HasReturn, unsigned int Cycles)
  45       : HasReturn(HasReturn), Cycles(Cycles) {}
  46   };
  47
  48   struct PadShortFunc : public MachineFunctionPass {
  49     static char ID;
  50     PadShortFunc() : MachineFunctionPass(ID)
  51                    , Threshold(4) {}
  52
  53     bool runOnMachineFunction(MachineFunction &MF) override;
  54
  55     MachineFunctionProperties getRequiredProperties() const override {
  56       return MachineFunctionProperties().set(
  57           MachineFunctionProperties::Property::NoVRegs);
  58     }
  59
  60     StringRef getPassName() const override {
  61       return "X86 Atom pad short functions";
  62     }
  63
  64   private:
  65     void findReturns(MachineBasicBlock *MBB,
  66                      unsigned int Cycles = 0);
  67
  68     bool cyclesUntilReturn(MachineBasicBlock *MBB,
  69                            unsigned int &Cycles);
  70
  71     void addPadding(MachineBasicBlock *MBB,
  72                     MachineBasicBlock::iterator &MBBI,
  73                     unsigned int NOOPsToAdd);
  74
  75     const unsigned int Threshold;
  76
  77     // ReturnBBs - Maps basic blocks that return to the minimum number of
  78     // cycles until the return, starting from the entry block.
  79     DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
  80
  81     // VisitedBBs - Cache of previously visited BBs.
  82     DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs;
  83
  84     TargetSchedModel TSM;
  85   };
  86
  87   char PadShortFunc::ID = 0;
  88 }
  89
  90 FunctionPass *llvm::createX86PadShortFunctions() {
  91   return new PadShortFunc();
  92 }
  93
  94 /// runOnMachineFunction - Loop over all of the basic blocks, inserting
  95 /// NOOP instructions before early exits.
  96 bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
  97   if (skipFunction(MF.getFunction()))
  98     return false;
  99
 100   if (MF.getFunction().hasOptSize())
 101     return false;
 102
 103   if (!MF.getSubtarget<X86Subtarget>().padShortFunctions())
 104     return false;
 105
 106   TSM.init(&MF.getSubtarget());
 107
 108   // Search through basic blocks and mark the ones that have early returns
 109   ReturnBBs.clear();
 110   VisitedBBs.clear();
 111   findReturns(&MF.front());
 112
 113   bool MadeChange = false;
 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     MachineBasicBlock *MBB = I->first;
 119     unsigned 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->isDebugInstr())
 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.
 144 void 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.
 168 bool 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 (MachineInstr &MI : *MBB) {
 182     // Mark basic blocks with a return instruction. Calls to other
 183     // functions do not count because the called function will be padded,
 184     // if necessary.
 185     if (MI.isReturn() && !MI.isCall()) {
 186       VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
 187       Cycles += CyclesToEnd;
 188       return true;
 189     }
 190
 191     CyclesToEnd += TSM.computeInstrLatency(&MI);
 192   }
 193
 194   VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
 195   Cycles += CyclesToEnd;
 196   return false;
 197 }
 198
 199 /// addPadding - Add the given number of NOOP instructions to the function
 200 /// just prior to the return at MBBI
 201 void PadShortFunc::addPadding(MachineBasicBlock *MBB,
 202                               MachineBasicBlock::iterator &MBBI,
 203                               unsigned int NOOPsToAdd) {
 204   DebugLoc DL = MBBI->getDebugLoc();
 205   unsigned IssueWidth = TSM.getIssueWidth();
 206
 207   for (unsigned i = 0, e = IssueWidth * NOOPsToAdd; i != e; ++i)
 208     BuildMI(*MBB, MBBI, DL, TSM.getInstrInfo()->get(X86::NOOP));
 209 }