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//===- X86VZeroUpper.cpp - AVX vzeroupper instruction inserter ------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// 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/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/IR/CallingConv.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
using namespace llvm;
#define DEBUG_TYPE "x86-vzeroupper"
static cl::opt<bool>
UseVZeroUpper("x86-use-vzeroupper", cl::Hidden,
cl::desc("Minimize AVX to SSE transition penalty"),
STATISTIC(NumVZU, "Number of vzeroupper instructions inserted");
namespace {
class VZeroUpperInserter : public MachineFunctionPass {
VZeroUpperInserter() : MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &MF) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
StringRef getPassName() const override { return "X86 vzeroupper inserter"; }
void processBasicBlock(MachineBasicBlock &MBB);
void insertVZeroUpper(MachineBasicBlock::iterator I,
MachineBasicBlock &MBB);
void addDirtySuccessor(MachineBasicBlock &MBB);
using BlockExitState = enum { PASS_THROUGH, EXITS_CLEAN, EXITS_DIRTY };
static const char* getBlockExitStateName(BlockExitState ST);
// Core algorithm state:
// BlockState - Each block is either:
// - PASS_THROUGH: There are neither YMM/ZMM 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/ZMM is clean on exit.
// - EXITS_DIRTY: An instruction in the block dirties YMM/ZMM 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 {
BlockExitState ExitState = PASS_THROUGH;
bool AddedToDirtySuccessors = false;
MachineBasicBlock::iterator FirstUnguardedCall;
BlockState() = default;
using BlockStateMap = SmallVector<BlockState, 8>;
using DirtySuccessorsWorkList = SmallVector<MachineBasicBlock *, 8>;
BlockStateMap BlockStates;
DirtySuccessorsWorkList DirtySuccessors;
bool EverMadeChange;
bool IsX86INTR;
const TargetInstrInfo *TII;
static char ID;
} // end anonymous namespace
char VZeroUpperInserter::ID = 0;
FunctionPass *llvm::createX86IssueVZeroUpperPass() {
return new VZeroUpperInserter();
#ifndef NDEBUG
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.");
/// VZEROUPPER cleans state that is related to Y/ZMM0-15 only.
/// Thus, there is no need to check for Y/ZMM16 and above.
static bool isYmmOrZmmReg(unsigned Reg) {
return (Reg >= X86::YMM0 && Reg <= X86::YMM15) ||
(Reg >= X86::ZMM0 && Reg <= X86::ZMM15);
static bool checkFnHasLiveInYmmOrZmm(MachineRegisterInfo &MRI) {
for (std::pair<unsigned, unsigned> LI : MRI.liveins())
if (isYmmOrZmmReg(LI.first))
return true;
return false;
static bool clobbersAllYmmAndZmmRegs(const MachineOperand &MO) {
for (unsigned reg = X86::YMM0; reg <= X86::YMM15; ++reg) {
if (!MO.clobbersPhysReg(reg))
return false;
for (unsigned reg = X86::ZMM0; reg <= X86::ZMM15; ++reg) {
if (!MO.clobbersPhysReg(reg))
return false;
return true;
static bool hasYmmOrZmmReg(MachineInstr &MI) {
for (const MachineOperand &MO : MI.operands()) {
if (MI.isCall() && MO.isRegMask() && !clobbersAllYmmAndZmmRegs(MO))
return true;
if (!MO.isReg())
if (MO.isDebug())
if (isYmmOrZmmReg(MO.getReg()))
return true;
return false;
/// Check if given call instruction has a RegMask operand.
static bool callHasRegMask(MachineInstr &MI) {
assert(MI.isCall() && "Can only be called on call instructions.");
for (const MachineOperand &MO : MI.operands()) {
if (MO.isRegMask())
return true;
return false;
/// Insert a vzeroupper instruction before I.
void VZeroUpperInserter::insertVZeroUpper(MachineBasicBlock::iterator I,
MachineBasicBlock &MBB) {
BuildMI(MBB, I, I->getDebugLoc(), TII->get(X86::VZEROUPPER));
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) {
BlockStates[MBB.getNumber()].AddedToDirtySuccessors = true;
/// 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 is PASS_THROUGH which implies no unguarded
// calls.
BlockExitState CurState = PASS_THROUGH;
BlockStates[MBB.getNumber()].FirstUnguardedCall = MBB.end();
for (MachineInstr &MI : MBB) {
bool IsCall = MI.isCall();
bool IsReturn = MI.isReturn();
bool IsControlFlow = IsCall || IsReturn;
// No need for vzeroupper before iret in interrupt handler function,
// epilogue will restore YMM/ZMM registers if needed.
if (IsX86INTR && IsReturn)
// An existing VZERO* instruction resets the state.
if (MI.getOpcode() == X86::VZEROALL || MI.getOpcode() == X86::VZEROUPPER) {
// Shortcut: don't need to check regular instructions in dirty state.
if (!IsControlFlow && CurState == EXITS_DIRTY)
if (hasYmmOrZmmReg(MI)) {
// We found a ymm/zmm-using instruction; this could be an AVX/AVX512
// instruction, or it could be control flow.
// Check for control-flow out of the current function (which might
// indirectly execute SSE instructions).
if (!IsControlFlow)
// If the call has no RegMask, 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/implicit-def/use) is well-defined and
// explicitly specified.
if (IsCall && !callHasRegMask(MI))
// The VZEROUPPER instruction resets the upper 128 bits of YMM0-YMM15
// registers. In addition, the processor changes back to Clean state, after
// which execution of SSE instructions or 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/ZMM may appear before other subsequent calls or even before
// the end of the BB.
insertVZeroUpper(MI, MBB);
} 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 = MI;
LLVM_DEBUG(dbgs() << "MBB #" << MBB.getNumber() << " exit state: "
<< getBlockExitStateName(CurState) << '\n');
if (CurState == EXITS_DIRTY)
for (MachineBasicBlock *Succ : MBB.successors())
BlockStates[MBB.getNumber()].ExitState = CurState;
/// Loop over all of the basic blocks, inserting vzeroupper instructions before
/// function calls.
bool VZeroUpperInserter::runOnMachineFunction(MachineFunction &MF) {
if (!UseVZeroUpper)
return false;
const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
if (!ST.hasAVX() || !ST.insertVZEROUPPER())
return false;
TII = ST.getInstrInfo();
MachineRegisterInfo &MRI = MF.getRegInfo();
EverMadeChange = false;
IsX86INTR = MF.getFunction().getCallingConv() == CallingConv::X86_INTR;
bool FnHasLiveInYmmOrZmm = checkFnHasLiveInYmmOrZmm(MRI);
// Fast check: if the function doesn't use any ymm/zmm 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/zmm use.
bool YmmOrZmmUsed = FnHasLiveInYmmOrZmm;
for (auto *RC : {&X86::VR256RegClass, &X86::VR512_0_15RegClass}) {
if (!YmmOrZmmUsed) {
for (TargetRegisterClass::iterator i = RC->begin(), e = RC->end(); i != e;
i++) {
if (!MRI.reg_nodbg_empty(*i)) {
YmmOrZmmUsed = true;
if (!YmmOrZmmUsed)
return false;
assert(BlockStates.empty() && DirtySuccessors.empty() &&
"X86VZeroUpper state should be clear");
// 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)
// If any YMM/ZMM regs are live-in to this function, add the entry block to
// the DirtySuccessors list
if (FnHasLiveInYmmOrZmm)
// Re-visit all blocks that are successors of EXITS_DIRTY blocks. Add
// vzeroupper instructions to unguarded calls, and propagate EXITS_DIRTY
// through PASS_THROUGH blocks.
while (!DirtySuccessors.empty()) {
MachineBasicBlock &MBB = *DirtySuccessors.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. Its
// successors need to be added to the worklist (if they haven't been
// already).
if (BBState.ExitState == PASS_THROUGH) {
LLVM_DEBUG(dbgs() << "MBB #" << MBB.getNumber()
<< " was Pass-through, is now Dirty-out.\n");
for (MachineBasicBlock *Succ : MBB.successors())
return EverMadeChange;