X86PadShortFunction.cpp

Go to the documentation of this file.

//===-------- X86PadShortFunction.cpp - pad short functions -----------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// 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 "X86.h"
#include "X86InstrInfo.h"
#include "X86Subtarget.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/CodeGen/LazyMachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineSizeOpts.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetSchedule.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
 
using namespace llvm;
 
#define DEBUG_TYPE "x86-pad-short-functions"
 
STATISTIC(NumBBsPadded, "Number of basic blocks padded");
 
namespace {
  struct VisitedBBInfo {
    // HasReturn - Whether the BB contains a return instruction
    bool HasReturn = false;
 
    // Cycles - Number of cycles until return if HasReturn is true, otherwise
    // number of cycles until end of the BB
    unsigned int Cycles = 0;
 
    VisitedBBInfo() = default;
    VisitedBBInfo(bool HasReturn, unsigned int Cycles)
      : HasReturn(HasReturn), Cycles(Cycles) {}
  };
 
  struct PadShortFunc : public MachineFunctionPass {
    static char ID;
    PadShortFunc() : MachineFunctionPass(ID) {}
 
    bool runOnMachineFunction(MachineFunction &MF) override;
 
    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.addRequired<ProfileSummaryInfoWrapperPass>();
      AU.addRequired<LazyMachineBlockFrequencyInfoPass>();
      AU.addPreserved<LazyMachineBlockFrequencyInfoPass>();
      MachineFunctionPass::getAnalysisUsage(AU);
    }
 
    MachineFunctionProperties getRequiredProperties() const override {
      return MachineFunctionProperties().set(
          MachineFunctionProperties::Property::NoVRegs);
    }
 
    StringRef getPassName() const override {
      return "X86 Atom pad short functions";
    }
 
  private:
    void findReturns(MachineBasicBlock *MBB,
                     unsigned int Cycles = 0);
 
    bool cyclesUntilReturn(MachineBasicBlock *MBB,
                           unsigned int &Cycles);
 
    void addPadding(MachineBasicBlock *MBB,
                    MachineBasicBlock::iterator &MBBI,
                    unsigned int NOOPsToAdd);
 
    const unsigned int Threshold = 4;
 
    // ReturnBBs - Maps basic blocks that return to the minimum number of
    // cycles until the return, starting from the entry block.
    DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
 
    // VisitedBBs - Cache of previously visited BBs.
    DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs;
 
    TargetSchedModel TSM;
  };
 
  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) {
  if (skipFunction(MF.getFunction()))
    return false;
 
  if (MF.getFunction().hasOptSize())
    return false;
 
  if (!MF.getSubtarget<X86Subtarget>().padShortFunctions())
    return false;
 
  TSM.init(&MF.getSubtarget());
 
  auto *PSI =
      &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
  auto *MBFI = (PSI && PSI->hasProfileSummary()) ?
               &getAnalysis<LazyMachineBlockFrequencyInfoPass>().getBFI() :
               nullptr;
 
  // Search through basic blocks and mark the ones that have early returns
  ReturnBBs.clear();
  VisitedBBs.clear();
  findReturns(&MF.front());
 
  bool MadeChange = false;
 
  // Pad the identified basic blocks with NOOPs
  for (const auto &ReturnBB : ReturnBBs) {
    MachineBasicBlock *MBB = ReturnBB.first;
    unsigned Cycles = ReturnBB.second;
 
    // Function::hasOptSize is already checked above.
    bool OptForSize = llvm::shouldOptimizeForSize(MBB, PSI, MBFI);
    if (OptForSize)
      continue;
 
    if (Cycles < Threshold) {
      // BB ends in a return. Skip over any DBG_VALUE instructions
      // trailing the terminator.
      assert(MBB->size() > 0 &&
             "Basic block should contain at least a RET but is empty");
      MachineBasicBlock::iterator ReturnLoc = --MBB->end();
 
      while (ReturnLoc->isDebugInstr())
        --ReturnLoc;
      assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
             "Basic block does not end with RET");
 
      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 : MBB->successors())
    if (Succ != MBB)
      findReturns(Succ, Cycles);
}
 
/// cyclesUntilReturn - return true if the MBB has a return instruction,
/// and 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) {
  // Return cached result if BB was previously visited
  DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
    = VisitedBBs.find(MBB);
  if (it != VisitedBBs.end()) {
    VisitedBBInfo BBInfo = it->second;
    Cycles += BBInfo.Cycles;
    return BBInfo.HasReturn;
  }
 
  unsigned int CyclesToEnd = 0;
 
  for (MachineInstr &MI : *MBB) {
    // 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()) {
      VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
      Cycles += CyclesToEnd;
      return true;
    }
 
    CyclesToEnd += TSM.computeInstrLatency(&MI);
  }
 
  VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
  Cycles += CyclesToEnd;
  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) {
  const DebugLoc &DL = MBBI->getDebugLoc();
  unsigned IssueWidth = TSM.getIssueWidth();
 
  for (unsigned i = 0, e = IssueWidth * NOOPsToAdd; i != e; ++i)
    BuildMI(*MBB, MBBI, DL, TSM.getInstrInfo()->get(X86::NOOP));
}