llvm/lib/Target/X86/X86PadShortFunction.cpp

//===-------- X86PadShortFunction.cpp - pad short functions -----------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the pass which will pad short functions to prevent
// a stall if a function returns before the return address is ready. This
// is needed for some Intel Atom processors.
//
//===----------------------------------------------------------------------===//

#include <algorithm>

#define DEBUG_TYPE "x86-pad-short-functions"
#include "X86.h"
#include "X86InstrInfo.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"

using namespace llvm;

STATISTIC(NumBBsPadded, "Number of basic blocks padded");

namespace {
  struct PadShortFunc : public MachineFunctionPass {
    static char ID;
    PadShortFunc() : MachineFunctionPass(ID)
                   , Threshold(4), TM(0), TII(0) {}

    virtual bool runOnMachineFunction(MachineFunction &MF);

    virtual const char *getPassName() const {
      return "X86 Atom pad short functions";
    }

  private:
    void findReturns(MachineBasicBlock *MBB,
                     unsigned int Cycles = 0);

    bool cyclesUntilReturn(MachineBasicBlock *MBB,
                           unsigned int &Cycles,
                           MachineBasicBlock::iterator *Location = 0);

    void addPadding(MachineBasicBlock *MBB,
                    MachineBasicBlock::iterator &MBBI,
                    unsigned int NOOPsToAdd);

    const unsigned int Threshold;
    DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;

    const TargetMachine *TM;
    const TargetInstrInfo *TII;
  };

  char PadShortFunc::ID = 0;
}

FunctionPass *llvm::createX86PadShortFunctions() {
  return new PadShortFunc();
}

/// runOnMachineFunction - Loop over all of the basic blocks, inserting
/// NOOP instructions before early exits.
bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
  bool OptForSize = MF.getFunction()->getAttributes().
    hasAttribute(AttributeSet::FunctionIndex, Attribute::OptimizeForSize);

  if (OptForSize)
    return false;

  TM = &MF.getTarget();
  TII = TM->getInstrInfo();

  // Search through basic blocks and mark the ones that have early returns
  ReturnBBs.clear();
  findReturns(MF.begin());

  bool MadeChange = false;

  MachineBasicBlock::iterator ReturnLoc;
  MachineBasicBlock *MBB;
  unsigned int Cycles = 0;
  unsigned int BBCycles;

  // Pad the identified basic blocks with NOOPs
  for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
       I != ReturnBBs.end(); ++I) {
    MBB = I->first;
    Cycles = I->second;

    if (Cycles < Threshold) {
      if (!cyclesUntilReturn(MBB, BBCycles, &ReturnLoc))
        continue;

      addPadding(MBB, ReturnLoc, Threshold - Cycles);
      NumBBsPadded++;
      MadeChange = true;
    }
  }

  return MadeChange;
}

/// findReturn - Starting at MBB, follow control flow and add all
/// basic blocks that contain a return to ReturnBBs.
void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
  // If this BB has a return, note how many cycles it takes to get there.
  bool hasReturn = cyclesUntilReturn(MBB, Cycles);
  if (Cycles >= Threshold)
    return;

  if (hasReturn) {
    ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
    return;
  }

  // Follow branches in BB and look for returns
  for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
      I != MBB->succ_end(); ++I) {
    findReturns(*I, Cycles);
  }
}

/// cyclesUntilReturn - if the MBB has a return instruction, set Location
/// to the instruction and return true. Return false otherwise.
/// Cycles will be incremented by the number of cycles taken to reach the
/// return or the end of the BB, whichever occurs first.
bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
                                     unsigned int &Cycles,
                                     MachineBasicBlock::iterator *Location) {
  for (MachineBasicBlock::iterator MBBI = MBB->begin();
        MBBI != MBB->end(); ++MBBI) {
    MachineInstr *MI = MBBI;
    // Mark basic blocks with a return instruction. Calls to other
    // functions do not count because the called function will be padded,
    // if necessary.
    if (MI->isReturn() && !MI->isCall()) {
      if (Location)
        *Location = MBBI;
      return true;
    }

    Cycles += TII->getInstrLatency(TM->getInstrItineraryData(), MI);
  }

  return false;
}

/// addPadding - Add the given number of NOOP instructions to the function
/// just prior to the return at MBBI
void PadShortFunc::addPadding(MachineBasicBlock *MBB,
                              MachineBasicBlock::iterator &MBBI,
                              unsigned int NOOPsToAdd) {
  DebugLoc DL = MBBI->getDebugLoc();

  while (NOOPsToAdd-- > 0) {
    BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
    BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
  }
}