lib/Target/X86/X86VZeroUpper.cpp

//===-- X86VZeroUpper.cpp - AVX vzeroupper instruction inserter -----------===//
//
//                     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 inserts x86 AVX vzeroupper instructions
// before calls to SSE encoded functions. This avoids transition latency
// penalty when transferring control between AVX encoded instructions and old
// SSE encoding mode.
//
//===----------------------------------------------------------------------===//

#include "X86.h"
#include "X86InstrInfo.h"
#include "X86Subtarget.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/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
using namespace llvm;

#define DEBUG_TYPE "x86-vzeroupper"

STATISTIC(NumVZU, "Number of vzeroupper instructions inserted");

namespace {

  class VZeroUpperInserter : public MachineFunctionPass {
  public:

    VZeroUpperInserter() : MachineFunctionPass(ID) {}
    bool runOnMachineFunction(MachineFunction &MF) override;
    const char *getPassName() const override {return "X86 vzeroupper inserter";}

  private:

    void processBasicBlock(MachineBasicBlock &MBB);
    void insertVZeroUpper(MachineBasicBlock::iterator I,
                          MachineBasicBlock &MBB);
    void addDirtySuccessor(MachineBasicBlock &MBB);

    typedef enum { PASS_THROUGH, EXITS_CLEAN, EXITS_DIRTY } BlockExitState;
    static const char* getBlockExitStateName(BlockExitState ST);

    // Core algorithm state:
    // BlockState - Each block is either:
    //   - PASS_THROUGH: There are neither YMM dirtying instructions nor
    //                   vzeroupper instructions in this block.
    //   - EXITS_CLEAN: There is (or will be) a vzeroupper instruction in this
    //                  block that will ensure that YMM is clean on exit.
    //   - EXITS_DIRTY: An instruction in the block dirties YMM and no
    //                  subsequent vzeroupper in the block clears it.
    //
    // AddedToDirtySuccessors - This flag is raised when a block is added to the
    //                          DirtySuccessors list to ensure that it's not
    //                          added multiple times.
    //
    // FirstUnguardedCall - Records the location of the first unguarded call in
    //                      each basic block that may need to be guarded by a
    //                      vzeroupper. We won't know whether it actually needs
    //                      to be guarded until we discover a predecessor that
    //                      is DIRTY_OUT.
    struct BlockState {
      BlockState() : ExitState(PASS_THROUGH), AddedToDirtySuccessors(false) {}
      BlockExitState ExitState;
      bool AddedToDirtySuccessors;
      MachineBasicBlock::iterator FirstUnguardedCall;
    };
    typedef SmallVector<BlockState, 8> BlockStateMap;
    typedef SmallVector<MachineBasicBlock*, 8> DirtySuccessorsWorkList;

    BlockStateMap BlockStates;
    DirtySuccessorsWorkList DirtySuccessors;
    bool EverMadeChange;
    const TargetInstrInfo *TII;

    static char ID;
  };

  char VZeroUpperInserter::ID = 0;
}

FunctionPass *llvm::createX86IssueVZeroUpperPass() {
  return new VZeroUpperInserter();
}

const char* VZeroUpperInserter::getBlockExitStateName(BlockExitState ST) {
  switch (ST) {
    case PASS_THROUGH: return "Pass-through";
    case EXITS_DIRTY: return "Exits-dirty";
    case EXITS_CLEAN: return "Exits-clean";
  }
  llvm_unreachable("Invalid block exit state.");
}

static bool isYmmReg(unsigned Reg) {
  return (Reg >= X86::YMM0 && Reg <= X86::YMM15);
}

static bool checkFnHasLiveInYmm(MachineRegisterInfo &MRI) {
  for (MachineRegisterInfo::livein_iterator I = MRI.livein_begin(),
       E = MRI.livein_end(); I != E; ++I)
    if (isYmmReg(I->first))
      return true;

  return false;
}

static bool clobbersAllYmmRegs(const MachineOperand &MO) {
  for (unsigned reg = X86::YMM0; reg <= X86::YMM15; ++reg) {
    if (!MO.clobbersPhysReg(reg))
      return false;
  }
  return true;
}

static bool hasYmmReg(MachineInstr *MI) {
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    const MachineOperand &MO = MI->getOperand(i);
    if (MI->isCall() && MO.isRegMask() && !clobbersAllYmmRegs(MO))
      return true;
    if (!MO.isReg())
      continue;
    if (MO.isDebug())
      continue;
    if (isYmmReg(MO.getReg()))
      return true;
  }
  return false;
}

/// clobbersAnyYmmReg() - Check if any YMM register will be clobbered by this
/// instruction.
static bool callClobbersAnyYmmReg(MachineInstr *MI) {
  assert(MI->isCall() && "Can only be called on call instructions.");
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    const MachineOperand &MO = MI->getOperand(i);
    if (!MO.isRegMask())
      continue;
    for (unsigned reg = X86::YMM0; reg <= X86::YMM15; ++reg) {
      if (MO.clobbersPhysReg(reg))
        return true;
    }
  }
  return false;
}

// Insert a vzeroupper instruction before I.
void VZeroUpperInserter::insertVZeroUpper(MachineBasicBlock::iterator I,
                                              MachineBasicBlock &MBB) {
  DebugLoc dl = I->getDebugLoc();
  BuildMI(MBB, I, dl, TII->get(X86::VZEROUPPER));
  ++NumVZU;
  EverMadeChange = true;
}

// Add MBB to the DirtySuccessors list if it hasn't already been added.
void VZeroUpperInserter::addDirtySuccessor(MachineBasicBlock &MBB) {
  if (!BlockStates[MBB.getNumber()].AddedToDirtySuccessors) {
    DirtySuccessors.push_back(&MBB);
    BlockStates[MBB.getNumber()].AddedToDirtySuccessors = true;
  }
}

/// processBasicBlock - Loop over all of the instructions in the basic block,
/// inserting vzeroupper instructions before function calls.
void VZeroUpperInserter::processBasicBlock(MachineBasicBlock &MBB) {

  // Start by assuming that the block PASS_THROUGH, which implies no unguarded
  // calls.
  BlockExitState CurState = PASS_THROUGH;
  BlockStates[MBB.getNumber()].FirstUnguardedCall = MBB.end();

  for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); ++I) {
    MachineInstr *MI = I;
    bool isControlFlow = MI->isCall() || MI->isReturn();

    // Shortcut: don't need to check regular instructions in dirty state.
    if (!isControlFlow && CurState == EXITS_DIRTY)
      continue;

    if (hasYmmReg(MI)) {
      // We found a ymm-using instruction; this could be an AVX instruction,
      // or it could be control flow.
      CurState = EXITS_DIRTY;
      continue;
    }

    // Check for control-flow out of the current function (which might
    // indirectly execute SSE instructions).
    if (!isControlFlow)
      continue;

    // If the call won't clobber any YMM register, skip it as well. It usually
    // happens on helper function calls (such as '_chkstk', '_ftol2') where
    // standard calling convention is not used (RegMask is not used to mark
    // register clobbered and register usage (def/imp-def/use) is well-defined
    // and explicitly specified.
    if (MI->isCall() && !callClobbersAnyYmmReg(MI))
      continue;

    // The VZEROUPPER instruction resets the upper 128 bits of all Intel AVX
    // registers. This instruction has zero latency. In addition, the processor
    // changes back to Clean state, after which execution of Intel SSE
    // instructions or Intel AVX instructions has no transition penalty. Add
    // the VZEROUPPER instruction before any function call/return that might
    // execute SSE code.
    // FIXME: In some cases, we may want to move the VZEROUPPER into a
    // predecessor block.
    if (CurState == EXITS_DIRTY) {
      // After the inserted VZEROUPPER the state becomes clean again, but
      // other YMM may appear before other subsequent calls or even before
      // the end of the BB.
      insertVZeroUpper(I, MBB);
      CurState = EXITS_CLEAN;
    } else if (CurState == PASS_THROUGH) {
      // If this block is currently in pass-through state and we encounter a
      // call then whether we need a vzeroupper or not depends on whether this
      // block has successors that exit dirty. Record the location of the call,
      // and set the state to EXITS_CLEAN, but do not insert the vzeroupper yet.
      // It will be inserted later if necessary.
      BlockStates[MBB.getNumber()].FirstUnguardedCall = I;
      CurState = EXITS_CLEAN;
    }
  }

  DEBUG(dbgs() << "MBB #" << MBB.getNumber() << " exit state: "
               << getBlockExitStateName(CurState) << '\n');

  if (CurState == EXITS_DIRTY)
    for (MachineBasicBlock::succ_iterator SI = MBB.succ_begin(),
                                          SE = MBB.succ_end();
         SI != SE; ++SI)
      addDirtySuccessor(**SI);

  BlockStates[MBB.getNumber()].ExitState = CurState;
}

/// runOnMachineFunction - Loop over all of the basic blocks, inserting
/// vzeroupper instructions before function calls.
bool VZeroUpperInserter::runOnMachineFunction(MachineFunction &MF) {
  const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
  if (!ST.hasAVX() || ST.hasAVX512())
    return false;
  TII = ST.getInstrInfo();
  MachineRegisterInfo &MRI = MF.getRegInfo();
  EverMadeChange = false;

  bool FnHasLiveInYmm = checkFnHasLiveInYmm(MRI);

  // Fast check: if the function doesn't use any ymm registers, we don't need
  // to insert any VZEROUPPER instructions.  This is constant-time, so it is
  // cheap in the common case of no ymm use.
  bool YMMUsed = FnHasLiveInYmm;
  if (!YMMUsed) {
    const TargetRegisterClass *RC = &X86::VR256RegClass;
    for (TargetRegisterClass::iterator i = RC->begin(), e = RC->end(); i != e;
         i++) {
      if (!MRI.reg_nodbg_empty(*i)) {
        YMMUsed = true;
        break;
      }
    }
  }
  if (!YMMUsed) {
    return false;
  }

  assert(BlockStates.empty() && DirtySuccessors.empty() &&
         "X86VZeroUpper state should be clear");
  BlockStates.resize(MF.getNumBlockIDs());

  // Process all blocks. This will compute block exit states, record the first
  // unguarded call in each block, and add successors of dirty blocks to the
  // DirtySuccessors list.
  for (MachineBasicBlock &MBB : MF)
    processBasicBlock(MBB);

  // If any YMM regs are live in to this function, add the entry block to the
  // DirtySuccessors list
  if (FnHasLiveInYmm)
    addDirtySuccessor(MF.front());

  // Re-visit all blocks that are successors of EXITS_DIRTY bsocks. Add
  // vzeroupper instructions to unguarded calls, and propagate EXITS_DIRTY
  // through PASS_THROUGH blocks.
  while (!DirtySuccessors.empty()) {
    MachineBasicBlock &MBB = *DirtySuccessors.back();
    DirtySuccessors.pop_back();
    BlockState &BBState = BlockStates[MBB.getNumber()];

    // MBB is a successor of a dirty block, so its first call needs to be
    // guarded.
    if (BBState.FirstUnguardedCall != MBB.end())
      insertVZeroUpper(BBState.FirstUnguardedCall, MBB);

    // If this successor was a pass-through block then it is now dirty, and its
    // successors need to be added to the worklist (if they haven't been
    // already).
    if (BBState.ExitState == PASS_THROUGH) {
      DEBUG(dbgs() << "MBB #" << MBB.getNumber()
                   << " was Pass-through, is now Dirty-out.\n");
      for (MachineBasicBlock::succ_iterator SI = MBB.succ_begin(),
                                            SE = MBB.succ_end();
           SI != SE; ++SI)
        addDirtySuccessor(**SI);
    }
  }

  BlockStates.clear();
  return EverMadeChange;
}