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//===-- 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 tranfering control between AVX encoded instructions and old
// SSE encoding mode.
#define DEBUG_TYPE "x86-vzeroupper"
#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/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
using namespace llvm;
STATISTIC(NumVZU, "Number of vzeroupper instructions inserted");
namespace {
struct VZeroUpperInserter : public MachineFunctionPass {
static char ID;
VZeroUpperInserter() : MachineFunctionPass(ID) {}
virtual bool runOnMachineFunction(MachineFunction &MF);
bool processBasicBlock(MachineFunction &MF, MachineBasicBlock &MBB);
virtual const char *getPassName() const { return "X86 vzeroupper inserter";}
const TargetInstrInfo *TII; // Machine instruction info.
MachineBasicBlock *MBB; // Current basic block
// Any YMM register live-in to this function?
bool FnHasLiveInYmm;
// BBState - Contains the state of each MBB: unknown, clean, dirty
SmallVector<uint8_t, 8> BBState;
// BBSolved - Keep track of all MBB which had been already analyzed
// and there is no further processing required.
BitVector BBSolved;
// Machine Basic Blocks are classified according this pass:
// ST_UNKNOWN - The MBB state is unknown, meaning from the entry state
// until the MBB exit there isn't a instruction using YMM to change
// the state to dirty, or one of the incoming predecessors is unknown
// and there's not a dirty predecessor between them.
// ST_CLEAN - No YMM usage in the end of the MBB. A MBB could have
// instructions using YMM and be marked ST_CLEAN, as long as the state
// is cleaned by a vzeroupper before any call.
// ST_DIRTY - Any MBB ending with a YMM usage not cleaned up by a
// vzeroupper instruction.
// ST_INIT - Placeholder for an empty state set
enum {
// computeState - Given two states, compute the resulting state, in
// the following way
// 1) One dirty state yields another dirty state
// 2) All states must be clean for the result to be clean
// 3) If none above and one unknown, the result state is also unknown
unsigned computeState(unsigned PrevState, unsigned CurState) {
if (PrevState == ST_INIT)
return CurState;
if (PrevState == ST_DIRTY || CurState == ST_DIRTY)
return ST_DIRTY;
if (PrevState == ST_CLEAN && CurState == ST_CLEAN)
return ST_CLEAN;
return ST_UNKNOWN;
char VZeroUpperInserter::ID = 0;
FunctionPass *llvm::createX86IssueVZeroUpperPass() {
return new VZeroUpperInserter();
static bool isYmmReg(unsigned Reg) {
if (Reg >= X86::YMM0 && Reg <= X86::YMM15)
return true;
return false;
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 hasYmmReg(MachineInstr *MI) {
for (int i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg())
if (MO.isDebug())
if (isYmmReg(MO.getReg()))
return true;
return false;
/// runOnMachineFunction - Loop over all of the basic blocks, inserting
/// vzero upper instructions before function calls.
bool VZeroUpperInserter::runOnMachineFunction(MachineFunction &MF) {
TII = MF.getTarget().getInstrInfo();
MachineRegisterInfo &MRI = MF.getRegInfo();
bool EverMadeChange = false;
// 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 = false;
const TargetRegisterClass *RC = X86::VR256RegisterClass;
for (TargetRegisterClass::iterator i = RC->begin(), e = RC->end();
i != e; i++) {
if (MRI.isPhysRegUsed(*i)) {
YMMUsed = true;
if (!YMMUsed)
return EverMadeChange;
// Pre-compute the existence of any live-in YMM registers to this function
FnHasLiveInYmm = checkFnHasLiveInYmm(MRI);
BBState.resize(MF.getNumBlockIDs(), 0);
BBSolved.resize(MF.getNumBlockIDs(), 0);
// Each BB state depends on all predecessors, loop over until everything
// converges. (Once we converge, we can implicitly mark everything that is
// still ST_UNKNOWN as ST_CLEAN.)
while (1) {
bool MadeChange = false;
// Process all basic blocks.
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
MadeChange |= processBasicBlock(MF, *I);
// If this iteration over the code changed anything, keep iterating.
if (!MadeChange) break;
EverMadeChange = true;
return EverMadeChange;
/// processBasicBlock - Loop over all of the instructions in the basic block,
/// inserting vzero upper instructions before function calls.
bool VZeroUpperInserter::processBasicBlock(MachineFunction &MF,
MachineBasicBlock &BB) {
bool Changed = false;
unsigned BBNum = BB.getNumber();
MBB = &BB;
// Don't process already solved BBs
if (BBSolved[BBNum])
return false; // No changes
// Check the state of all predecessors
unsigned EntryState = ST_INIT;
for (MachineBasicBlock::const_pred_iterator PI = BB.pred_begin(),
PE = BB.pred_end(); PI != PE; ++PI) {
EntryState = computeState(EntryState, BBState[(*PI)->getNumber()]);
if (EntryState == ST_DIRTY)
// The entry MBB for the function may set the inital state to dirty if
// the function receives any YMM incoming arguments
if (MBB == MF.begin()) {
EntryState = ST_CLEAN;
if (FnHasLiveInYmm)
EntryState = ST_DIRTY;
// The current state is initialized according to the predecessors
unsigned CurState = EntryState;
bool BBHasCall = false;
for (MachineBasicBlock::iterator I = BB.begin(); I != BB.end(); ++I) {
MachineInstr *MI = I;
DebugLoc dl = I->getDebugLoc();
bool isControlFlow = MI->isCall() || MI->isReturn();
// Shortcut: don't need to check regular instructions in dirty state.
if (!isControlFlow && CurState == ST_DIRTY)
if (hasYmmReg(MI)) {
// We found a ymm-using instruction; this could be an AVX instruction,
// or it could be control flow.
CurState = ST_DIRTY;
// Check for control-flow out of the current function (which might
// indirectly execute SSE instructions).
if (!isControlFlow)
BBHasCall = true;
// 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 == ST_DIRTY) {
// Only insert the VZEROUPPER in case the entry state isn't unknown.
// When unknown, only compute the information within the block to have
// it available in the exit if possible, but don't change the block.
if (EntryState != ST_UNKNOWN) {
BuildMI(*MBB, I, dl, TII->get(X86::VZEROUPPER));
// 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.
CurState = ST_CLEAN;
DEBUG(dbgs() << "MBB #" << BBNum
<< ", current state: " << CurState << '\n');
// A BB can only be considered solved when we both have done all the
// necessary transformations, and have computed the exit state. This happens
// in two cases:
// 1) We know the entry state: this immediately implies the exit state and
// all the necessary transformations.
// 2) There are no calls, and and a non-call instruction marks this block:
// no transformations are necessary, and we know the exit state.
if (EntryState != ST_UNKNOWN || (!BBHasCall && CurState != ST_UNKNOWN))
BBSolved[BBNum] = true;
if (CurState != BBState[BBNum])
Changed = true;
BBState[BBNum] = CurState;
return Changed;