1//===-- X86VZeroUpper.cpp - AVX vzeroupper instruction inserter -----------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This file defines the pass which inserts x86 AVX vzeroupper instructions 11// before calls to SSE encoded functions. This avoids transition latency 12// penalty when tranfering control between AVX encoded instructions and old 13// SSE encoding mode. 14// 15//===----------------------------------------------------------------------===// 16 17#define DEBUG_TYPE "x86-vzeroupper" 18#include "X86.h" 19#include "X86InstrInfo.h" 20#include "llvm/ADT/Statistic.h" 21#include "llvm/CodeGen/MachineFunctionPass.h" 22#include "llvm/CodeGen/MachineInstrBuilder.h" 23#include "llvm/CodeGen/MachineRegisterInfo.h" 24#include "llvm/CodeGen/Passes.h" 25#include "llvm/Support/Debug.h" 26#include "llvm/Support/raw_ostream.h" 27#include "llvm/Target/TargetInstrInfo.h" 28using namespace llvm; 29 30STATISTIC(NumVZU, "Number of vzeroupper instructions inserted"); 31 32namespace { 33 struct VZeroUpperInserter : public MachineFunctionPass { 34 static char ID; 35 VZeroUpperInserter() : MachineFunctionPass(ID) {} 36 37 virtual bool runOnMachineFunction(MachineFunction &MF); 38 39 bool processBasicBlock(MachineFunction &MF, MachineBasicBlock &MBB); 40 41 virtual const char *getPassName() const { return "X86 vzeroupper inserter";} 42 43 private: 44 const TargetInstrInfo *TII; // Machine instruction info. 45 46 // Any YMM register live-in to this function? 47 bool FnHasLiveInYmm; 48 49 // BBState - Contains the state of each MBB: unknown, clean, dirty 50 SmallVector<uint8_t, 8> BBState; 51 52 // BBSolved - Keep track of all MBB which had been already analyzed 53 // and there is no further processing required. 54 BitVector BBSolved; 55 56 // Machine Basic Blocks are classified according this pass: 57 // 58 // ST_UNKNOWN - The MBB state is unknown, meaning from the entry state 59 // until the MBB exit there isn't a instruction using YMM to change 60 // the state to dirty, or one of the incoming predecessors is unknown 61 // and there's not a dirty predecessor between them. 62 // 63 // ST_CLEAN - No YMM usage in the end of the MBB. A MBB could have 64 // instructions using YMM and be marked ST_CLEAN, as long as the state 65 // is cleaned by a vzeroupper before any call. 66 // 67 // ST_DIRTY - Any MBB ending with a YMM usage not cleaned up by a 68 // vzeroupper instruction. 69 // 70 // ST_INIT - Placeholder for an empty state set 71 // 72 enum { 73 ST_UNKNOWN = 0, 74 ST_CLEAN = 1, 75 ST_DIRTY = 2, 76 ST_INIT = 3 77 }; 78 79 // computeState - Given two states, compute the resulting state, in 80 // the following way 81 // 82 // 1) One dirty state yields another dirty state 83 // 2) All states must be clean for the result to be clean 84 // 3) If none above and one unknown, the result state is also unknown 85 // 86 static unsigned computeState(unsigned PrevState, unsigned CurState) { 87 if (PrevState == ST_INIT) 88 return CurState; 89 90 if (PrevState == ST_DIRTY || CurState == ST_DIRTY) 91 return ST_DIRTY; 92 93 if (PrevState == ST_CLEAN && CurState == ST_CLEAN) 94 return ST_CLEAN; 95 96 return ST_UNKNOWN; 97 } 98 99 }; 100 char VZeroUpperInserter::ID = 0; 101} 102 103FunctionPass *llvm::createX86IssueVZeroUpperPass() { 104 return new VZeroUpperInserter(); 105} 106 107static bool isYmmReg(unsigned Reg) { 108 if (Reg >= X86::YMM0 && Reg <= X86::YMM15) 109 return true; 110 111 return false; 112} 113 114static bool checkFnHasLiveInYmm(MachineRegisterInfo &MRI) { 115 for (MachineRegisterInfo::livein_iterator I = MRI.livein_begin(), 116 E = MRI.livein_end(); I != E; ++I) 117 if (isYmmReg(I->first)) 118 return true; 119 120 return false; 121} 122 123static bool hasYmmReg(MachineInstr *MI) { 124 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 125 const MachineOperand &MO = MI->getOperand(i); 126 if (!MO.isReg()) 127 continue; 128 if (MO.isDebug()) 129 continue; 130 if (isYmmReg(MO.getReg())) 131 return true; 132 } 133 return false; 134} 135 136/// runOnMachineFunction - Loop over all of the basic blocks, inserting 137/// vzero upper instructions before function calls. 138bool VZeroUpperInserter::runOnMachineFunction(MachineFunction &MF) { 139 TII = MF.getTarget().getInstrInfo(); 140 MachineRegisterInfo &MRI = MF.getRegInfo(); 141 bool EverMadeChange = false; 142 143 // Fast check: if the function doesn't use any ymm registers, we don't need 144 // to insert any VZEROUPPER instructions. This is constant-time, so it is 145 // cheap in the common case of no ymm use. 146 bool YMMUsed = false; 147 const TargetRegisterClass *RC = &X86::VR256RegClass; 148 for (TargetRegisterClass::iterator i = RC->begin(), e = RC->end(); 149 i != e; i++) { 150 if (MRI.isPhysRegUsed(*i)) { 151 YMMUsed = true; 152 break; 153 } 154 } 155 if (!YMMUsed) 156 return EverMadeChange; 157 158 // Pre-compute the existence of any live-in YMM registers to this function 159 FnHasLiveInYmm = checkFnHasLiveInYmm(MRI); 160 161 assert(BBState.empty()); 162 BBState.resize(MF.getNumBlockIDs(), 0); 163 BBSolved.resize(MF.getNumBlockIDs(), 0); 164 165 // Each BB state depends on all predecessors, loop over until everything 166 // converges. (Once we converge, we can implicitly mark everything that is 167 // still ST_UNKNOWN as ST_CLEAN.) 168 while (1) { 169 bool MadeChange = false; 170 171 // Process all basic blocks. 172 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) 173 MadeChange |= processBasicBlock(MF, *I); 174 175 // If this iteration over the code changed anything, keep iterating. 176 if (!MadeChange) break; 177 EverMadeChange = true; 178 } 179 180 BBState.clear(); 181 BBSolved.clear(); 182 return EverMadeChange; 183} 184 185/// processBasicBlock - Loop over all of the instructions in the basic block, 186/// inserting vzero upper instructions before function calls. 187bool VZeroUpperInserter::processBasicBlock(MachineFunction &MF, 188 MachineBasicBlock &BB) { 189 bool Changed = false; 190 unsigned BBNum = BB.getNumber(); 191 192 // Don't process already solved BBs 193 if (BBSolved[BBNum]) 194 return false; // No changes 195 196 // Check the state of all predecessors 197 unsigned EntryState = ST_INIT; 198 for (MachineBasicBlock::const_pred_iterator PI = BB.pred_begin(), 199 PE = BB.pred_end(); PI != PE; ++PI) { 200 EntryState = computeState(EntryState, BBState[(*PI)->getNumber()]); 201 if (EntryState == ST_DIRTY) 202 break; 203 } 204 205 206 // The entry MBB for the function may set the initial state to dirty if 207 // the function receives any YMM incoming arguments 208 if (&BB == MF.begin()) { 209 EntryState = ST_CLEAN; 210 if (FnHasLiveInYmm) 211 EntryState = ST_DIRTY; 212 } 213 214 // The current state is initialized according to the predecessors 215 unsigned CurState = EntryState; 216 bool BBHasCall = false; 217 218 for (MachineBasicBlock::iterator I = BB.begin(); I != BB.end(); ++I) { 219 MachineInstr *MI = I; 220 DebugLoc dl = I->getDebugLoc(); 221 bool isControlFlow = MI->isCall() || MI->isReturn(); 222 223 // Shortcut: don't need to check regular instructions in dirty state. 224 if (!isControlFlow && CurState == ST_DIRTY) 225 continue; 226 227 if (hasYmmReg(MI)) { 228 // We found a ymm-using instruction; this could be an AVX instruction, 229 // or it could be control flow. 230 CurState = ST_DIRTY; 231 continue; 232 } 233 234 // Check for control-flow out of the current function (which might 235 // indirectly execute SSE instructions). 236 if (!isControlFlow) 237 continue; 238 239 BBHasCall = true; 240 241 // The VZEROUPPER instruction resets the upper 128 bits of all Intel AVX 242 // registers. This instruction has zero latency. In addition, the processor 243 // changes back to Clean state, after which execution of Intel SSE 244 // instructions or Intel AVX instructions has no transition penalty. Add 245 // the VZEROUPPER instruction before any function call/return that might 246 // execute SSE code. 247 // FIXME: In some cases, we may want to move the VZEROUPPER into a 248 // predecessor block. 249 if (CurState == ST_DIRTY) { 250 // Only insert the VZEROUPPER in case the entry state isn't unknown. 251 // When unknown, only compute the information within the block to have 252 // it available in the exit if possible, but don't change the block. 253 if (EntryState != ST_UNKNOWN) { 254 BuildMI(BB, I, dl, TII->get(X86::VZEROUPPER)); 255 ++NumVZU; 256 } 257 258 // After the inserted VZEROUPPER the state becomes clean again, but 259 // other YMM may appear before other subsequent calls or even before 260 // the end of the BB. 261 CurState = ST_CLEAN; 262 } 263 } 264 265 DEBUG(dbgs() << "MBB #" << BBNum 266 << ", current state: " << CurState << '\n'); 267 268 // A BB can only be considered solved when we both have done all the 269 // necessary transformations, and have computed the exit state. This happens 270 // in two cases: 271 // 1) We know the entry state: this immediately implies the exit state and 272 // all the necessary transformations. 273 // 2) There are no calls, and and a non-call instruction marks this block: 274 // no transformations are necessary, and we know the exit state. 275 if (EntryState != ST_UNKNOWN || (!BBHasCall && CurState != ST_UNKNOWN)) 276 BBSolved[BBNum] = true; 277 278 if (CurState != BBState[BBNum]) 279 Changed = true; 280 281 BBState[BBNum] = CurState; 282 return Changed; 283} 284