forked from OSchip/llvm-project
172 lines
5.2 KiB
C++
172 lines
5.2 KiB
C++
//===-------- X86PadShortFunction.cpp - pad short functions -----------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file defines the pass which will pad short functions to prevent
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// a stall if a function returns before the return address is ready. This
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// is needed for some Intel Atom processors.
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//
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//===----------------------------------------------------------------------===//
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#include <algorithm>
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#define DEBUG_TYPE "x86-pad-short-functions"
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#include "X86.h"
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#include "X86InstrInfo.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/CodeGen/MachineFunctionPass.h"
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#include "llvm/CodeGen/MachineInstrBuilder.h"
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#include "llvm/CodeGen/MachineRegisterInfo.h"
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/IR/Function.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Target/TargetInstrInfo.h"
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using namespace llvm;
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STATISTIC(NumBBsPadded, "Number of basic blocks padded");
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namespace {
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struct PadShortFunc : public MachineFunctionPass {
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static char ID;
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PadShortFunc() : MachineFunctionPass(ID)
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, Threshold(4), TM(0), TII(0) {}
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virtual bool runOnMachineFunction(MachineFunction &MF);
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virtual const char *getPassName() const {
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return "X86 Atom pad short functions";
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}
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private:
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void findReturns(MachineBasicBlock *MBB,
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unsigned int Cycles = 0);
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bool cyclesUntilReturn(MachineBasicBlock *MBB,
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unsigned int &Cycles,
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MachineBasicBlock::iterator *Location = 0);
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void addPadding(MachineBasicBlock *MBB,
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MachineBasicBlock::iterator &MBBI,
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unsigned int NOOPsToAdd);
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const unsigned int Threshold;
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DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
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const TargetMachine *TM;
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const TargetInstrInfo *TII;
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};
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char PadShortFunc::ID = 0;
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}
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FunctionPass *llvm::createX86PadShortFunctions() {
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return new PadShortFunc();
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}
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/// runOnMachineFunction - Loop over all of the basic blocks, inserting
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/// NOOP instructions before early exits.
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bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
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bool OptForSize = MF.getFunction()->getAttributes().
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hasAttribute(AttributeSet::FunctionIndex, Attribute::OptimizeForSize);
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if (OptForSize)
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return false;
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TM = &MF.getTarget();
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TII = TM->getInstrInfo();
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// Search through basic blocks and mark the ones that have early returns
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ReturnBBs.clear();
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findReturns(MF.begin());
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bool MadeChange = false;
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MachineBasicBlock::iterator ReturnLoc;
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MachineBasicBlock *MBB;
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unsigned int Cycles = 0;
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unsigned int BBCycles;
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// Pad the identified basic blocks with NOOPs
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for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
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I != ReturnBBs.end(); ++I) {
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MBB = I->first;
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Cycles = I->second;
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if (Cycles < Threshold) {
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if (!cyclesUntilReturn(MBB, BBCycles, &ReturnLoc))
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continue;
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addPadding(MBB, ReturnLoc, Threshold - Cycles);
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NumBBsPadded++;
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MadeChange = true;
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}
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}
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return MadeChange;
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}
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/// findReturn - Starting at MBB, follow control flow and add all
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/// basic blocks that contain a return to ReturnBBs.
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void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
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// If this BB has a return, note how many cycles it takes to get there.
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bool hasReturn = cyclesUntilReturn(MBB, Cycles);
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if (Cycles >= Threshold)
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return;
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if (hasReturn) {
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ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
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return;
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}
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// Follow branches in BB and look for returns
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for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
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I != MBB->succ_end(); ++I) {
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findReturns(*I, Cycles);
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}
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}
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/// cyclesUntilReturn - if the MBB has a return instruction, set Location
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/// to the instruction and return true. Return false otherwise.
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/// Cycles will be incremented by the number of cycles taken to reach the
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/// return or the end of the BB, whichever occurs first.
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bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
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unsigned int &Cycles,
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MachineBasicBlock::iterator *Location) {
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for (MachineBasicBlock::iterator MBBI = MBB->begin();
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MBBI != MBB->end(); ++MBBI) {
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MachineInstr *MI = MBBI;
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// Mark basic blocks with a return instruction. Calls to other
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// functions do not count because the called function will be padded,
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// if necessary.
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if (MI->isReturn() && !MI->isCall()) {
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if (Location)
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*Location = MBBI;
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return true;
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}
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Cycles += TII->getInstrLatency(TM->getInstrItineraryData(), MI);
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}
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return false;
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}
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/// addPadding - Add the given number of NOOP instructions to the function
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/// just prior to the return at MBBI
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void PadShortFunc::addPadding(MachineBasicBlock *MBB,
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MachineBasicBlock::iterator &MBBI,
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unsigned int NOOPsToAdd) {
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DebugLoc DL = MBBI->getDebugLoc();
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while (NOOPsToAdd-- > 0) {
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BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
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BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
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}
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}
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