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.insertVZEROUPPER()) 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 for (auto *RC : {&X86::VR256RegClass, &X86::VR512_0_15RegClass}) { 296 if (!YmmOrZmmUsed) { 297 for (TargetRegisterClass::iterator i = RC->begin(), e = RC->end(); i != e; 298 i++) { 299 if (!MRI.reg_nodbg_empty(*i)) { 300 YmmOrZmmUsed = true; 301 break; 302 } 303 } 304 } 305 } 306 if (!YmmOrZmmUsed) 307 return false; 308 309 assert(BlockStates.empty() && DirtySuccessors.empty() && 310 "X86VZeroUpper state should be clear"); 311 BlockStates.resize(MF.getNumBlockIDs()); 312 313 // Process all blocks. This will compute block exit states, record the first 314 // unguarded call in each block, and add successors of dirty blocks to the 315 // DirtySuccessors list. 316 for (MachineBasicBlock &MBB : MF) 317 processBasicBlock(MBB); 318 319 // If any YMM/ZMM regs are live-in to this function, add the entry block to 320 // the DirtySuccessors list 321 if (FnHasLiveInYmmOrZmm) 322 addDirtySuccessor(MF.front()); 323 324 // Re-visit all blocks that are successors of EXITS_DIRTY blocks. Add 325 // vzeroupper instructions to unguarded calls, and propagate EXITS_DIRTY 326 // through PASS_THROUGH blocks. 327 while (!DirtySuccessors.empty()) { 328 MachineBasicBlock &MBB = *DirtySuccessors.back(); 329 DirtySuccessors.pop_back(); 330 BlockState &BBState = BlockStates[MBB.getNumber()]; 331 332 // MBB is a successor of a dirty block, so its first call needs to be 333 // guarded. 334 if (BBState.FirstUnguardedCall != MBB.end()) 335 insertVZeroUpper(BBState.FirstUnguardedCall, MBB); 336 337 // If this successor was a pass-through block, then it is now dirty. Its 338 // successors need to be added to the worklist (if they haven't been 339 // already). 340 if (BBState.ExitState == PASS_THROUGH) { 341 LLVM_DEBUG(dbgs() << "MBB #" << MBB.getNumber() 342 << " was Pass-through, is now Dirty-out.\n"); 343 for (MachineBasicBlock *Succ : MBB.successors()) 344 addDirtySuccessor(*Succ); 345 } 346 } 347 348 BlockStates.clear(); 349 return EverMadeChange; 350 } 351