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//===-- X86VZeroUpper.cpp - AVX vzeroupper instruction inserter -----------===//
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// The LLVM Compiler Infrastructure
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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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// This file defines the pass which inserts x86 AVX vzeroupper instructions
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// before calls to SSE encoded functions. This avoids transition latency
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// penalty when transferring control between AVX encoded instructions and old
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//===----------------------------------------------------------------------===//
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#include "X86InstrInfo.h"
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#include "X86Subtarget.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/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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#define DEBUG_TYPE "x86-vzeroupper"
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STATISTIC(NumVZU, "Number of vzeroupper instructions inserted");
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class VZeroUpperInserter : public MachineFunctionPass {
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VZeroUpperInserter() : MachineFunctionPass(ID) {}
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bool runOnMachineFunction(MachineFunction &MF) override;
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const char *getPassName() const override {return "X86 vzeroupper inserter";}
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void processBasicBlock(MachineBasicBlock &MBB);
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void insertVZeroUpper(MachineBasicBlock::iterator I,
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MachineBasicBlock &MBB);
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void addDirtySuccessor(MachineBasicBlock &MBB);
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typedef enum { PASS_THROUGH, EXITS_CLEAN, EXITS_DIRTY } BlockExitState;
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static const char* getBlockExitStateName(BlockExitState ST);
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// Core algorithm state:
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// BlockState - Each block is either:
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// - PASS_THROUGH: There are neither YMM dirtying instructions nor
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// vzeroupper instructions in this block.
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// - EXITS_CLEAN: There is (or will be) a vzeroupper instruction in this
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// block that will ensure that YMM is clean on exit.
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// - EXITS_DIRTY: An instruction in the block dirties YMM and no
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// subsequent vzeroupper in the block clears it.
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// AddedToDirtySuccessors - This flag is raised when a block is added to the
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// DirtySuccessors list to ensure that it's not
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// added multiple times.
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// FirstUnguardedCall - Records the location of the first unguarded call in
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// each basic block that may need to be guarded by a
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// vzeroupper. We won't know whether it actually needs
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// to be guarded until we discover a predecessor that
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BlockState() : ExitState(PASS_THROUGH), AddedToDirtySuccessors(false) {}
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BlockExitState ExitState;
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bool AddedToDirtySuccessors;
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MachineBasicBlock::iterator FirstUnguardedCall;
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typedef SmallVector<BlockState, 8> BlockStateMap;
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typedef SmallVector<MachineBasicBlock*, 8> DirtySuccessorsWorkList;
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BlockStateMap BlockStates;
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DirtySuccessorsWorkList DirtySuccessors;
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const TargetInstrInfo *TII;
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char VZeroUpperInserter::ID = 0;
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FunctionPass *llvm::createX86IssueVZeroUpperPass() {
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return new VZeroUpperInserter();
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const char* VZeroUpperInserter::getBlockExitStateName(BlockExitState ST) {
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case PASS_THROUGH: return "Pass-through";
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case EXITS_DIRTY: return "Exits-dirty";
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case EXITS_CLEAN: return "Exits-clean";
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llvm_unreachable("Invalid block exit state.");
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static bool isYmmReg(unsigned Reg) {
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return (Reg >= X86::YMM0 && Reg <= X86::YMM15);
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static bool checkFnHasLiveInYmm(MachineRegisterInfo &MRI) {
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for (MachineRegisterInfo::livein_iterator I = MRI.livein_begin(),
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E = MRI.livein_end(); I != E; ++I)
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if (isYmmReg(I->first))
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static bool clobbersAllYmmRegs(const MachineOperand &MO) {
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for (unsigned reg = X86::YMM0; reg <= X86::YMM15; ++reg) {
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if (!MO.clobbersPhysReg(reg))
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static bool hasYmmReg(MachineInstr *MI) {
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for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
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const MachineOperand &MO = MI->getOperand(i);
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if (MI->isCall() && MO.isRegMask() && !clobbersAllYmmRegs(MO))
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if (isYmmReg(MO.getReg()))
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/// clobbersAnyYmmReg() - Check if any YMM register will be clobbered by this
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static bool callClobbersAnyYmmReg(MachineInstr *MI) {
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assert(MI->isCall() && "Can only be called on call instructions.");
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for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
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const MachineOperand &MO = MI->getOperand(i);
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for (unsigned reg = X86::YMM0; reg <= X86::YMM15; ++reg) {
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if (MO.clobbersPhysReg(reg))
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// Insert a vzeroupper instruction before I.
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void VZeroUpperInserter::insertVZeroUpper(MachineBasicBlock::iterator I,
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MachineBasicBlock &MBB) {
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DebugLoc dl = I->getDebugLoc();
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BuildMI(MBB, I, dl, TII->get(X86::VZEROUPPER));
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EverMadeChange = true;
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// Add MBB to the DirtySuccessors list if it hasn't already been added.
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void VZeroUpperInserter::addDirtySuccessor(MachineBasicBlock &MBB) {
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if (!BlockStates[MBB.getNumber()].AddedToDirtySuccessors) {
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DirtySuccessors.push_back(&MBB);
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BlockStates[MBB.getNumber()].AddedToDirtySuccessors = true;
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/// processBasicBlock - Loop over all of the instructions in the basic block,
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/// inserting vzeroupper instructions before function calls.
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void VZeroUpperInserter::processBasicBlock(MachineBasicBlock &MBB) {
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// Start by assuming that the block PASS_THROUGH, which implies no unguarded
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BlockExitState CurState = PASS_THROUGH;
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BlockStates[MBB.getNumber()].FirstUnguardedCall = MBB.end();
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for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); ++I) {
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MachineInstr *MI = I;
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bool isControlFlow = MI->isCall() || MI->isReturn();
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// Shortcut: don't need to check regular instructions in dirty state.
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if (!isControlFlow && CurState == EXITS_DIRTY)
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// We found a ymm-using instruction; this could be an AVX instruction,
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// or it could be control flow.
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CurState = EXITS_DIRTY;
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// Check for control-flow out of the current function (which might
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// indirectly execute SSE instructions).
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// If the call won't clobber any YMM register, skip it as well. It usually
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// happens on helper function calls (such as '_chkstk', '_ftol2') where
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// standard calling convention is not used (RegMask is not used to mark
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// register clobbered and register usage (def/imp-def/use) is well-defined
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// and explicitly specified.
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if (MI->isCall() && !callClobbersAnyYmmReg(MI))
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// The VZEROUPPER instruction resets the upper 128 bits of all Intel AVX
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// registers. This instruction has zero latency. In addition, the processor
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// changes back to Clean state, after which execution of Intel SSE
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// instructions or Intel AVX instructions has no transition penalty. Add
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// the VZEROUPPER instruction before any function call/return that might
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// FIXME: In some cases, we may want to move the VZEROUPPER into a
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// predecessor block.
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if (CurState == EXITS_DIRTY) {
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// After the inserted VZEROUPPER the state becomes clean again, but
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// other YMM may appear before other subsequent calls or even before
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// the end of the BB.
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insertVZeroUpper(I, MBB);
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CurState = EXITS_CLEAN;
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} else if (CurState == PASS_THROUGH) {
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// If this block is currently in pass-through state and we encounter a
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// call then whether we need a vzeroupper or not depends on whether this
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// block has successors that exit dirty. Record the location of the call,
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// and set the state to EXITS_CLEAN, but do not insert the vzeroupper yet.
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// It will be inserted later if necessary.
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BlockStates[MBB.getNumber()].FirstUnguardedCall = I;
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CurState = EXITS_CLEAN;
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DEBUG(dbgs() << "MBB #" << MBB.getNumber() << " exit state: "
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<< getBlockExitStateName(CurState) << '\n');
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if (CurState == EXITS_DIRTY)
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for (MachineBasicBlock::succ_iterator SI = MBB.succ_begin(),
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addDirtySuccessor(**SI);
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BlockStates[MBB.getNumber()].ExitState = CurState;
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/// runOnMachineFunction - Loop over all of the basic blocks, inserting
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/// vzeroupper instructions before function calls.
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bool VZeroUpperInserter::runOnMachineFunction(MachineFunction &MF) {
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const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
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if (!ST.hasAVX() || ST.hasAVX512())
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TII = ST.getInstrInfo();
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MachineRegisterInfo &MRI = MF.getRegInfo();
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EverMadeChange = false;
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bool FnHasLiveInYmm = checkFnHasLiveInYmm(MRI);
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// Fast check: if the function doesn't use any ymm registers, we don't need
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// to insert any VZEROUPPER instructions. This is constant-time, so it is
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// cheap in the common case of no ymm use.
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bool YMMUsed = FnHasLiveInYmm;
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const TargetRegisterClass *RC = &X86::VR256RegClass;
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for (TargetRegisterClass::iterator i = RC->begin(), e = RC->end(); i != e;
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if (!MRI.reg_nodbg_empty(*i)) {
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assert(BlockStates.empty() && DirtySuccessors.empty() &&
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"X86VZeroUpper state should be clear");
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BlockStates.resize(MF.getNumBlockIDs());
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// Process all blocks. This will compute block exit states, record the first
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// unguarded call in each block, and add successors of dirty blocks to the
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// DirtySuccessors list.
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for (MachineBasicBlock &MBB : MF)
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processBasicBlock(MBB);
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// If any YMM regs are live in to this function, add the entry block to the
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// DirtySuccessors list
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addDirtySuccessor(MF.front());
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// Re-visit all blocks that are successors of EXITS_DIRTY bsocks. Add
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// vzeroupper instructions to unguarded calls, and propagate EXITS_DIRTY
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// through PASS_THROUGH blocks.
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while (!DirtySuccessors.empty()) {
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MachineBasicBlock &MBB = *DirtySuccessors.back();
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DirtySuccessors.pop_back();
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BlockState &BBState = BlockStates[MBB.getNumber()];
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// MBB is a successor of a dirty block, so its first call needs to be
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if (BBState.FirstUnguardedCall != MBB.end())
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insertVZeroUpper(BBState.FirstUnguardedCall, MBB);
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// If this successor was a pass-through block then it is now dirty, and its
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// successors need to be added to the worklist (if they haven't been
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if (BBState.ExitState == PASS_THROUGH) {
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DEBUG(dbgs() << "MBB #" << MBB.getNumber()
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<< " was Pass-through, is now Dirty-out.\n");
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for (MachineBasicBlock::succ_iterator SI = MBB.succ_begin(),
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addDirtySuccessor(**SI);
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return EverMadeChange;