lib/Target/X86/X86VZeroUpper.cpp

   1 //===- X86VZeroUpper.cpp - AVX vzeroupper instruction inserter ------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file defines the pass which inserts x86 AVX vzeroupper instructions
  10 // before calls to SSE encoded functions. This avoids transition latency
  11 // penalty when transferring control between AVX encoded instructions and old
  12 // SSE encoding mode.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #include "X86.h"
  17 #include "X86InstrInfo.h"
  18 #include "X86Subtarget.h"
  19 #include "llvm/ADT/SmallVector.h"
  20 #include "llvm/ADT/Statistic.h"
  21 #include "llvm/CodeGen/MachineBasicBlock.h"
  22 #include "llvm/CodeGen/MachineFunction.h"
  23 #include "llvm/CodeGen/MachineFunctionPass.h"
  24 #include "llvm/CodeGen/MachineInstr.h"
  25 #include "llvm/CodeGen/MachineInstrBuilder.h"
  26 #include "llvm/CodeGen/MachineOperand.h"
  27 #include "llvm/CodeGen/MachineRegisterInfo.h"
  28 #include "llvm/CodeGen/TargetInstrInfo.h"
  29 #include "llvm/CodeGen/TargetRegisterInfo.h"
  30 #include "llvm/IR/CallingConv.h"
  31 #include "llvm/IR/DebugLoc.h"
  32 #include "llvm/IR/Function.h"
  33 #include "llvm/Support/Debug.h"
  34 #include "llvm/Support/ErrorHandling.h"
  35 #include "llvm/Support/raw_ostream.h"
  36 #include <cassert>
  37
  38 using namespace llvm;
  39
  40 #define DEBUG_TYPE "x86-vzeroupper"
  41
  42 STATISTIC(NumVZU, "Number of vzeroupper instructions inserted");
  43
  44 namespace {
  45
  46   class VZeroUpperInserter : public MachineFunctionPass {
  47   public:
  48     VZeroUpperInserter() : MachineFunctionPass(ID) {}
  49
  50     bool runOnMachineFunction(MachineFunction &MF) override;
  51
  52     MachineFunctionProperties getRequiredProperties() const override {
  53       return MachineFunctionProperties().set(
  54           MachineFunctionProperties::Property::NoVRegs);
  55     }
  56
  57     StringRef getPassName() const override { return "X86 vzeroupper inserter"; }
  58
  59   private:
  60     void processBasicBlock(MachineBasicBlock &MBB);
  61     void insertVZeroUpper(MachineBasicBlock::iterator I,
  62                           MachineBasicBlock &MBB);
  63     void addDirtySuccessor(MachineBasicBlock &MBB);
  64
  65     using BlockExitState = enum { PASS_THROUGH, EXITS_CLEAN, EXITS_DIRTY };
  66
  67     static const char* getBlockExitStateName(BlockExitState ST);
  68
  69     // Core algorithm state:
  70     // BlockState - Each block is either:
  71     //   - PASS_THROUGH: There are neither YMM/ZMM dirtying instructions nor
  72     //                   vzeroupper instructions in this block.
  73     //   - EXITS_CLEAN: There is (or will be) a vzeroupper instruction in this
  74     //                  block that will ensure that YMM/ZMM is clean on exit.
  75     //   - EXITS_DIRTY: An instruction in the block dirties YMM/ZMM and no
  76     //                  subsequent vzeroupper in the block clears it.
  77     //
  78     // AddedToDirtySuccessors - This flag is raised when a block is added to the
  79     //                          DirtySuccessors list to ensure that it's not
  80     //                          added multiple times.
  81     //
  82     // FirstUnguardedCall - Records the location of the first unguarded call in
  83     //                      each basic block that may need to be guarded by a
  84     //                      vzeroupper. We won't know whether it actually needs
  85     //                      to be guarded until we discover a predecessor that
  86     //                      is DIRTY_OUT.
  87     struct BlockState {
  88       BlockExitState ExitState = PASS_THROUGH;
  89       bool AddedToDirtySuccessors = false;
  90       MachineBasicBlock::iterator FirstUnguardedCall;
  91
  92       BlockState() = default;
  93     };
  94
  95     using BlockStateMap = SmallVector<BlockState, 8>;
  96     using DirtySuccessorsWorkList = SmallVector<MachineBasicBlock *, 8>;
  97
  98     BlockStateMap BlockStates;
  99     DirtySuccessorsWorkList DirtySuccessors;
 100     bool EverMadeChange;
 101     bool IsX86INTR;
 102     const TargetInstrInfo *TII;
 103
 104     static char ID;
 105   };
 106
 107 } // end anonymous namespace
 108
 109 char VZeroUpperInserter::ID = 0;
 110
 111 FunctionPass *llvm::createX86IssueVZeroUpperPass() {
 112   return new VZeroUpperInserter();
 113 }
 114
 115 #ifndef NDEBUG
 116 const char* VZeroUpperInserter::getBlockExitStateName(BlockExitState ST) {
 117   switch (ST) {
 118     case PASS_THROUGH: return "Pass-through";
 119     case EXITS_DIRTY: return "Exits-dirty";
 120     case EXITS_CLEAN: return "Exits-clean";
 121   }
 122   llvm_unreachable("Invalid block exit state.");
 123 }
 124 #endif
 125
 126 /// VZEROUPPER cleans state that is related to Y/ZMM0-15 only.
 127 /// Thus, there is no need to check for Y/ZMM16 and above.
 128 static bool isYmmOrZmmReg(unsigned Reg) {
 129   return (Reg >= X86::YMM0 && Reg <= X86::YMM15) ||
 130          (Reg >= X86::ZMM0 && Reg <= X86::ZMM15);
 131 }
 132
 133 static bool checkFnHasLiveInYmmOrZmm(MachineRegisterInfo &MRI) {
 134   for (std::pair<unsigned, unsigned> LI : MRI.liveins())
 135     if (isYmmOrZmmReg(LI.first))
 136       return true;
 137
 138   return false;
 139 }
 140
 141 static bool clobbersAllYmmAndZmmRegs(const MachineOperand &MO) {
 142   for (unsigned reg = X86::YMM0; reg <= X86::YMM15; ++reg) {
 143     if (!MO.clobbersPhysReg(reg))
 144       return false;
 145   }
 146   for (unsigned reg = X86::ZMM0; reg <= X86::ZMM15; ++reg) {
 147     if (!MO.clobbersPhysReg(reg))
 148       return false;
 149   }
 150   return true;
 151 }
 152
 153 static bool hasYmmOrZmmReg(MachineInstr &MI) {
 154   for (const MachineOperand &MO : MI.operands()) {
 155     if (MI.isCall() && MO.isRegMask() && !clobbersAllYmmAndZmmRegs(MO))
 156       return true;
 157     if (!MO.isReg())
 158       continue;
 159     if (MO.isDebug())
 160       continue;
 161     if (isYmmOrZmmReg(MO.getReg()))
 162       return true;
 163   }
 164   return false;
 165 }
 166
 167 /// Check if given call instruction has a RegMask operand.
 168 static bool callHasRegMask(MachineInstr &MI) {
 169   assert(MI.isCall() && "Can only be called on call instructions.");
 170   for (const MachineOperand &MO : MI.operands()) {
 171     if (MO.isRegMask())
 172       return true;
 173   }
 174   return false;
 175 }
 176
 177 /// Insert a vzeroupper instruction before I.
 178 void VZeroUpperInserter::insertVZeroUpper(MachineBasicBlock::iterator I,
 179                                           MachineBasicBlock &MBB) {
 180   DebugLoc dl = I->getDebugLoc();
 181   BuildMI(MBB, I, dl, TII->get(X86::VZEROUPPER));
 182   ++NumVZU;
 183   EverMadeChange = true;
 184 }
 185
 186 /// Add MBB to the DirtySuccessors list if it hasn't already been added.
 187 void VZeroUpperInserter::addDirtySuccessor(MachineBasicBlock &MBB) {
 188   if (!BlockStates[MBB.getNumber()].AddedToDirtySuccessors) {
 189     DirtySuccessors.push_back(&MBB);
 190     BlockStates[MBB.getNumber()].AddedToDirtySuccessors = true;
 191   }
 192 }
 193
 194 /// Loop over all of the instructions in the basic block, inserting vzeroupper
 195 /// instructions before function calls.
 196 void VZeroUpperInserter::processBasicBlock(MachineBasicBlock &MBB) {
 197   // Start by assuming that the block is PASS_THROUGH which implies no unguarded
 198   // calls.
 199   BlockExitState CurState = PASS_THROUGH;
 200   BlockStates[MBB.getNumber()].FirstUnguardedCall = MBB.end();
 201
 202   for (MachineInstr &MI : MBB) {
 203     bool IsCall = MI.isCall();
 204     bool IsReturn = MI.isReturn();
 205     bool IsControlFlow = IsCall || IsReturn;
 206
 207     // No need for vzeroupper before iret in interrupt handler function,
 208     // epilogue will restore YMM/ZMM registers if needed.
 209     if (IsX86INTR && IsReturn)
 210       continue;
 211
 212     // An existing VZERO* instruction resets the state.
 213     if (MI.getOpcode() == X86::VZEROALL || MI.getOpcode() == X86::VZEROUPPER) {
 214       CurState = EXITS_CLEAN;
 215       continue;
 216     }
 217
 218     // Shortcut: don't need to check regular instructions in dirty state.
 219     if (!IsControlFlow && CurState == EXITS_DIRTY)
 220       continue;
 221
 222     if (hasYmmOrZmmReg(MI)) {
 223       // We found a ymm/zmm-using instruction; this could be an AVX/AVX512
 224       // instruction, or it could be control flow.
 225       CurState = EXITS_DIRTY;
 226       continue;
 227     }
 228
 229     // Check for control-flow out of the current function (which might
 230     // indirectly execute SSE instructions).
 231     if (!IsControlFlow)
 232       continue;
 233
 234     // If the call has no RegMask, skip it as well. It usually happens on
 235     // helper function calls (such as '_chkstk', '_ftol2') where standard
 236     // calling convention is not used (RegMask is not used to mark register
 237     // clobbered and register usage (def/implicit-def/use) is well-defined and
 238     // explicitly specified.
 239     if (IsCall && !callHasRegMask(MI))
 240       continue;
 241
 242     // The VZEROUPPER instruction resets the upper 128 bits of YMM0-YMM15
 243     // registers. In addition, the processor changes back to Clean state, after
 244     // which execution of SSE instructions or AVX instructions has no transition
 245     // penalty. Add the VZEROUPPER instruction before any function call/return
 246     // that might execute SSE code.
 247     // FIXME: In some cases, we may want to move the VZEROUPPER into a
 248     // predecessor block.
 249     if (CurState == EXITS_DIRTY) {
 250       // After the inserted VZEROUPPER the state becomes clean again, but
 251       // other YMM/ZMM may appear before other subsequent calls or even before
 252       // the end of the BB.
 253       insertVZeroUpper(MI, MBB);
 254       CurState = EXITS_CLEAN;
 255     } else if (CurState == PASS_THROUGH) {
 256       // If this block is currently in pass-through state and we encounter a
 257       // call then whether we need a vzeroupper or not depends on whether this
 258       // block has successors that exit dirty. Record the location of the call,
 259       // and set the state to EXITS_CLEAN, but do not insert the vzeroupper yet.
 260       // It will be inserted later if necessary.
 261       BlockStates[MBB.getNumber()].FirstUnguardedCall = MI;
 262       CurState = EXITS_CLEAN;
 263     }
 264   }
 265
 266   LLVM_DEBUG(dbgs() << "MBB #" << MBB.getNumber() << " exit state: "
 267                     << getBlockExitStateName(CurState) << '\n');
 268
 269   if (CurState == EXITS_DIRTY)
 270     for (MachineBasicBlock::succ_iterator SI = MBB.succ_begin(),
 271                                           SE = MBB.succ_end();
 272          SI != SE; ++SI)
 273       addDirtySuccessor(**SI);
 274
 275   BlockStates[MBB.getNumber()].ExitState = CurState;
 276 }
 277
 278 /// Loop over all of the basic blocks, inserting vzeroupper instructions before
 279 /// function calls.
 280 bool VZeroUpperInserter::runOnMachineFunction(MachineFunction &MF) {
 281   const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
 282   if (!ST.hasAVX() || ST.hasFastPartialYMMorZMMWrite())
 283     return false;
 284   TII = ST.getInstrInfo();
 285   MachineRegisterInfo &MRI = MF.getRegInfo();
 286   EverMadeChange = false;
 287   IsX86INTR = MF.getFunction().getCallingConv() == CallingConv::X86_INTR;
 288
 289   bool FnHasLiveInYmmOrZmm = checkFnHasLiveInYmmOrZmm(MRI);
 290
 291   // Fast check: if the function doesn't use any ymm/zmm registers, we don't
 292   // need to insert any VZEROUPPER instructions.  This is constant-time, so it
 293   // is cheap in the common case of no ymm/zmm use.
 294   bool YmmOrZmmUsed = FnHasLiveInYmmOrZmm;
 295   const TargetRegisterClass *RCs[2] = {&X86::VR256RegClass, &X86::VR512RegClass};
 296   for (auto *RC : RCs) {
 297     if (!YmmOrZmmUsed) {
 298       for (TargetRegisterClass::iterator i = RC->begin(), e = RC->end(); i != e;
 299            i++) {
 300         if (!MRI.reg_nodbg_empty(*i)) {
 301           YmmOrZmmUsed = true;
 302           break;
 303         }
 304       }
 305     }
 306   }
 307   if (!YmmOrZmmUsed) {
 308     return false;
 309   }
 310
 311   assert(BlockStates.empty() && DirtySuccessors.empty() &&
 312          "X86VZeroUpper state should be clear");
 313   BlockStates.resize(MF.getNumBlockIDs());
 314
 315   // Process all blocks. This will compute block exit states, record the first
 316   // unguarded call in each block, and add successors of dirty blocks to the
 317   // DirtySuccessors list.
 318   for (MachineBasicBlock &MBB : MF)
 319     processBasicBlock(MBB);
 320
 321   // If any YMM/ZMM regs are live-in to this function, add the entry block to
 322   // the DirtySuccessors list
 323   if (FnHasLiveInYmmOrZmm)
 324     addDirtySuccessor(MF.front());
 325
 326   // Re-visit all blocks that are successors of EXITS_DIRTY blocks. Add
 327   // vzeroupper instructions to unguarded calls, and propagate EXITS_DIRTY
 328   // through PASS_THROUGH blocks.
 329   while (!DirtySuccessors.empty()) {
 330     MachineBasicBlock &MBB = *DirtySuccessors.back();
 331     DirtySuccessors.pop_back();
 332     BlockState &BBState = BlockStates[MBB.getNumber()];
 333
 334     // MBB is a successor of a dirty block, so its first call needs to be
 335     // guarded.
 336     if (BBState.FirstUnguardedCall != MBB.end())
 337       insertVZeroUpper(BBState.FirstUnguardedCall, MBB);
 338
 339     // If this successor was a pass-through block, then it is now dirty. Its
 340     // successors need to be added to the worklist (if they haven't been
 341     // already).
 342     if (BBState.ExitState == PASS_THROUGH) {
 343       LLVM_DEBUG(dbgs() << "MBB #" << MBB.getNumber()
 344                         << " was Pass-through, is now Dirty-out.\n");
 345       for (MachineBasicBlock *Succ : MBB.successors())
 346         addDirtySuccessor(*Succ);
 347     }
 348   }
 349
 350   BlockStates.clear();
 351   return EverMadeChange;
 352 }