1 //===--------------- PPCVSXFMAMutate.cpp - VSX FMA Mutation ---------------===// 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 pass mutates the form of VSX FMA instructions to avoid unnecessary 10 // copies. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "MCTargetDesc/PPCPredicates.h" 15 #include "PPC.h" 16 #include "PPCInstrBuilder.h" 17 #include "PPCInstrInfo.h" 18 #include "PPCMachineFunctionInfo.h" 19 #include "PPCTargetMachine.h" 20 #include "llvm/ADT/STLExtras.h" 21 #include "llvm/ADT/Statistic.h" 22 #include "llvm/CodeGen/LiveIntervals.h" 23 #include "llvm/CodeGen/MachineDominators.h" 24 #include "llvm/CodeGen/MachineFrameInfo.h" 25 #include "llvm/CodeGen/MachineFunctionPass.h" 26 #include "llvm/CodeGen/MachineInstrBuilder.h" 27 #include "llvm/CodeGen/MachineMemOperand.h" 28 #include "llvm/CodeGen/MachineRegisterInfo.h" 29 #include "llvm/CodeGen/PseudoSourceValue.h" 30 #include "llvm/CodeGen/ScheduleDAG.h" 31 #include "llvm/CodeGen/SlotIndexes.h" 32 #include "llvm/InitializePasses.h" 33 #include "llvm/MC/MCAsmInfo.h" 34 #include "llvm/MC/TargetRegistry.h" 35 #include "llvm/Support/CommandLine.h" 36 #include "llvm/Support/Debug.h" 37 #include "llvm/Support/ErrorHandling.h" 38 #include "llvm/Support/raw_ostream.h" 39 40 using namespace llvm; 41 42 // Temporarily disable FMA mutation by default, since it doesn't handle 43 // cross-basic-block intervals well. 44 // See: http://lists.llvm.org/pipermail/llvm-dev/2016-February/095669.html 45 // http://reviews.llvm.org/D17087 46 static cl::opt<bool> DisableVSXFMAMutate( 47 "disable-ppc-vsx-fma-mutation", 48 cl::desc("Disable VSX FMA instruction mutation"), cl::init(true), 49 cl::Hidden); 50 51 #define DEBUG_TYPE "ppc-vsx-fma-mutate" 52 53 namespace llvm { namespace PPC { 54 int getAltVSXFMAOpcode(uint16_t Opcode); 55 } } 56 57 namespace { 58 // PPCVSXFMAMutate pass - For copies between VSX registers and non-VSX registers 59 // (Altivec and scalar floating-point registers), we need to transform the 60 // copies into subregister copies with other restrictions. 61 struct PPCVSXFMAMutate : public MachineFunctionPass { 62 static char ID; 63 PPCVSXFMAMutate() : MachineFunctionPass(ID) { 64 initializePPCVSXFMAMutatePass(*PassRegistry::getPassRegistry()); 65 } 66 67 LiveIntervals *LIS; 68 const PPCInstrInfo *TII; 69 70 protected: 71 bool processBlock(MachineBasicBlock &MBB) { 72 bool Changed = false; 73 74 MachineRegisterInfo &MRI = MBB.getParent()->getRegInfo(); 75 const TargetRegisterInfo *TRI = &TII->getRegisterInfo(); 76 for (MachineBasicBlock::iterator I = MBB.begin(), IE = MBB.end(); 77 I != IE; ++I) { 78 MachineInstr &MI = *I; 79 80 // The default (A-type) VSX FMA form kills the addend (it is taken from 81 // the target register, which is then updated to reflect the result of 82 // the FMA). If the instruction, however, kills one of the registers 83 // used for the product, then we can use the M-form instruction (which 84 // will take that value from the to-be-defined register). 85 86 int AltOpc = PPC::getAltVSXFMAOpcode(MI.getOpcode()); 87 if (AltOpc == -1) 88 continue; 89 90 // This pass is run after register coalescing, and so we're looking for 91 // a situation like this: 92 // ... 93 // %5 = COPY %9; VSLRC:%5,%9 94 // %5<def,tied1> = XSMADDADP %5<tied0>, %17, %16, 95 // implicit %rm; VSLRC:%5,%17,%16 96 // ... 97 // %9<def,tied1> = XSMADDADP %9<tied0>, %17, %19, 98 // implicit %rm; VSLRC:%9,%17,%19 99 // ... 100 // Where we can eliminate the copy by changing from the A-type to the 101 // M-type instruction. Specifically, for this example, this means: 102 // %5<def,tied1> = XSMADDADP %5<tied0>, %17, %16, 103 // implicit %rm; VSLRC:%5,%17,%16 104 // is replaced by: 105 // %16<def,tied1> = XSMADDMDP %16<tied0>, %18, %9, 106 // implicit %rm; VSLRC:%16,%18,%9 107 // and we remove: %5 = COPY %9; VSLRC:%5,%9 108 109 SlotIndex FMAIdx = LIS->getInstructionIndex(MI); 110 111 VNInfo *AddendValNo = 112 LIS->getInterval(MI.getOperand(1).getReg()).Query(FMAIdx).valueIn(); 113 114 // This can be null if the register is undef. 115 if (!AddendValNo) 116 continue; 117 118 MachineInstr *AddendMI = LIS->getInstructionFromIndex(AddendValNo->def); 119 120 // The addend and this instruction must be in the same block. 121 122 if (!AddendMI || AddendMI->getParent() != MI.getParent()) 123 continue; 124 125 // The addend must be a full copy within the same register class. 126 127 if (!AddendMI->isFullCopy()) 128 continue; 129 130 Register AddendSrcReg = AddendMI->getOperand(1).getReg(); 131 if (AddendSrcReg.isVirtual()) { 132 if (MRI.getRegClass(AddendMI->getOperand(0).getReg()) != 133 MRI.getRegClass(AddendSrcReg)) 134 continue; 135 } else { 136 // If AddendSrcReg is a physical register, make sure the destination 137 // register class contains it. 138 if (!MRI.getRegClass(AddendMI->getOperand(0).getReg()) 139 ->contains(AddendSrcReg)) 140 continue; 141 } 142 143 // In theory, there could be other uses of the addend copy before this 144 // fma. We could deal with this, but that would require additional 145 // logic below and I suspect it will not occur in any relevant 146 // situations. Additionally, check whether the copy source is killed 147 // prior to the fma. In order to replace the addend here with the 148 // source of the copy, it must still be live here. We can't use 149 // interval testing for a physical register, so as long as we're 150 // walking the MIs we may as well test liveness here. 151 // 152 // FIXME: There is a case that occurs in practice, like this: 153 // %9 = COPY %f1; VSSRC:%9 154 // ... 155 // %6 = COPY %9; VSSRC:%6,%9 156 // %7 = COPY %9; VSSRC:%7,%9 157 // %9<def,tied1> = XSMADDASP %9<tied0>, %1, %4; VSSRC: 158 // %6<def,tied1> = XSMADDASP %6<tied0>, %1, %2; VSSRC: 159 // %7<def,tied1> = XSMADDASP %7<tied0>, %1, %3; VSSRC: 160 // which prevents an otherwise-profitable transformation. 161 bool OtherUsers = false, KillsAddendSrc = false; 162 for (auto J = std::prev(I), JE = MachineBasicBlock::iterator(AddendMI); 163 J != JE; --J) { 164 if (J->readsVirtualRegister(AddendMI->getOperand(0).getReg())) { 165 OtherUsers = true; 166 break; 167 } 168 if (J->modifiesRegister(AddendSrcReg, TRI) || 169 J->killsRegister(AddendSrcReg, TRI)) { 170 KillsAddendSrc = true; 171 break; 172 } 173 } 174 175 if (OtherUsers || KillsAddendSrc) 176 continue; 177 178 179 // The transformation doesn't work well with things like: 180 // %5 = A-form-op %5, %11, %5; 181 // unless %11 is also a kill, so skip when it is not, 182 // and check operand 3 to see it is also a kill to handle the case: 183 // %5 = A-form-op %5, %5, %11; 184 // where %5 and %11 are both kills. This case would be skipped 185 // otherwise. 186 Register OldFMAReg = MI.getOperand(0).getReg(); 187 188 // Find one of the product operands that is killed by this instruction. 189 unsigned KilledProdOp = 0, OtherProdOp = 0; 190 Register Reg2 = MI.getOperand(2).getReg(); 191 Register Reg3 = MI.getOperand(3).getReg(); 192 if (LIS->getInterval(Reg2).Query(FMAIdx).isKill() 193 && Reg2 != OldFMAReg) { 194 KilledProdOp = 2; 195 OtherProdOp = 3; 196 } else if (LIS->getInterval(Reg3).Query(FMAIdx).isKill() 197 && Reg3 != OldFMAReg) { 198 KilledProdOp = 3; 199 OtherProdOp = 2; 200 } 201 202 // If there are no usable killed product operands, then this 203 // transformation is likely not profitable. 204 if (!KilledProdOp) 205 continue; 206 207 // If the addend copy is used only by this MI, then the addend source 208 // register is likely not live here. This could be fixed (based on the 209 // legality checks above, the live range for the addend source register 210 // could be extended), but it seems likely that such a trivial copy can 211 // be coalesced away later, and thus is not worth the effort. 212 if (AddendSrcReg.isVirtual() && 213 !LIS->getInterval(AddendSrcReg).liveAt(FMAIdx)) 214 continue; 215 216 // Transform: (O2 * O3) + O1 -> (O2 * O1) + O3. 217 218 Register KilledProdReg = MI.getOperand(KilledProdOp).getReg(); 219 Register OtherProdReg = MI.getOperand(OtherProdOp).getReg(); 220 221 unsigned AddSubReg = AddendMI->getOperand(1).getSubReg(); 222 unsigned KilledProdSubReg = MI.getOperand(KilledProdOp).getSubReg(); 223 unsigned OtherProdSubReg = MI.getOperand(OtherProdOp).getSubReg(); 224 225 bool AddRegKill = AddendMI->getOperand(1).isKill(); 226 bool KilledProdRegKill = MI.getOperand(KilledProdOp).isKill(); 227 bool OtherProdRegKill = MI.getOperand(OtherProdOp).isKill(); 228 229 bool AddRegUndef = AddendMI->getOperand(1).isUndef(); 230 bool KilledProdRegUndef = MI.getOperand(KilledProdOp).isUndef(); 231 bool OtherProdRegUndef = MI.getOperand(OtherProdOp).isUndef(); 232 233 // If there isn't a class that fits, we can't perform the transform. 234 // This is needed for correctness with a mixture of VSX and Altivec 235 // instructions to make sure that a low VSX register is not assigned to 236 // the Altivec instruction. 237 if (!MRI.constrainRegClass(KilledProdReg, 238 MRI.getRegClass(OldFMAReg))) 239 continue; 240 241 assert(OldFMAReg == AddendMI->getOperand(0).getReg() && 242 "Addend copy not tied to old FMA output!"); 243 244 LLVM_DEBUG(dbgs() << "VSX FMA Mutation:\n " << MI); 245 246 MI.getOperand(0).setReg(KilledProdReg); 247 MI.getOperand(1).setReg(KilledProdReg); 248 MI.getOperand(3).setReg(AddendSrcReg); 249 250 MI.getOperand(0).setSubReg(KilledProdSubReg); 251 MI.getOperand(1).setSubReg(KilledProdSubReg); 252 MI.getOperand(3).setSubReg(AddSubReg); 253 254 MI.getOperand(1).setIsKill(KilledProdRegKill); 255 MI.getOperand(3).setIsKill(AddRegKill); 256 257 MI.getOperand(1).setIsUndef(KilledProdRegUndef); 258 MI.getOperand(3).setIsUndef(AddRegUndef); 259 260 MI.setDesc(TII->get(AltOpc)); 261 262 // If the addend is also a multiplicand, replace it with the addend 263 // source in both places. 264 if (OtherProdReg == AddendMI->getOperand(0).getReg()) { 265 MI.getOperand(2).setReg(AddendSrcReg); 266 MI.getOperand(2).setSubReg(AddSubReg); 267 MI.getOperand(2).setIsKill(AddRegKill); 268 MI.getOperand(2).setIsUndef(AddRegUndef); 269 } else { 270 MI.getOperand(2).setReg(OtherProdReg); 271 MI.getOperand(2).setSubReg(OtherProdSubReg); 272 MI.getOperand(2).setIsKill(OtherProdRegKill); 273 MI.getOperand(2).setIsUndef(OtherProdRegUndef); 274 } 275 276 LLVM_DEBUG(dbgs() << " -> " << MI); 277 278 // The killed product operand was killed here, so we can reuse it now 279 // for the result of the fma. 280 281 LiveInterval &FMAInt = LIS->getInterval(OldFMAReg); 282 VNInfo *FMAValNo = FMAInt.getVNInfoAt(FMAIdx.getRegSlot()); 283 for (auto UI = MRI.reg_nodbg_begin(OldFMAReg), UE = MRI.reg_nodbg_end(); 284 UI != UE;) { 285 MachineOperand &UseMO = *UI; 286 MachineInstr *UseMI = UseMO.getParent(); 287 ++UI; 288 289 // Don't replace the result register of the copy we're about to erase. 290 if (UseMI == AddendMI) 291 continue; 292 293 UseMO.substVirtReg(KilledProdReg, KilledProdSubReg, *TRI); 294 } 295 296 // Extend the live intervals of the killed product operand to hold the 297 // fma result. 298 299 LiveInterval &NewFMAInt = LIS->getInterval(KilledProdReg); 300 for (auto &AI : FMAInt) { 301 // Don't add the segment that corresponds to the original copy. 302 if (AI.valno == AddendValNo) 303 continue; 304 305 VNInfo *NewFMAValNo = 306 NewFMAInt.getNextValue(AI.start, LIS->getVNInfoAllocator()); 307 308 NewFMAInt.addSegment( 309 LiveInterval::Segment(AI.start, AI.end, NewFMAValNo)); 310 } 311 LLVM_DEBUG(dbgs() << " extended: " << NewFMAInt << '\n'); 312 313 // Extend the live interval of the addend source (it might end at the 314 // copy to be removed, or somewhere in between there and here). This 315 // is necessary only if it is a physical register. 316 if (!AddendSrcReg.isVirtual()) 317 for (MCRegUnit Unit : TRI->regunits(AddendSrcReg.asMCReg())) { 318 LiveRange &AddendSrcRange = LIS->getRegUnit(Unit); 319 AddendSrcRange.extendInBlock(LIS->getMBBStartIdx(&MBB), 320 FMAIdx.getRegSlot()); 321 LLVM_DEBUG(dbgs() << " extended: " << AddendSrcRange << '\n'); 322 } 323 324 FMAInt.removeValNo(FMAValNo); 325 LLVM_DEBUG(dbgs() << " trimmed: " << FMAInt << '\n'); 326 327 // Remove the (now unused) copy. 328 329 LLVM_DEBUG(dbgs() << " removing: " << *AddendMI << '\n'); 330 LIS->RemoveMachineInstrFromMaps(*AddendMI); 331 AddendMI->eraseFromParent(); 332 333 Changed = true; 334 } 335 336 return Changed; 337 } 338 339 public: 340 bool runOnMachineFunction(MachineFunction &MF) override { 341 if (skipFunction(MF.getFunction())) 342 return false; 343 344 // If we don't have VSX then go ahead and return without doing 345 // anything. 346 const PPCSubtarget &STI = MF.getSubtarget<PPCSubtarget>(); 347 if (!STI.hasVSX()) 348 return false; 349 350 LIS = &getAnalysis<LiveIntervals>(); 351 352 TII = STI.getInstrInfo(); 353 354 bool Changed = false; 355 356 if (DisableVSXFMAMutate) 357 return Changed; 358 359 for (MachineBasicBlock &B : llvm::make_early_inc_range(MF)) 360 if (processBlock(B)) 361 Changed = true; 362 363 return Changed; 364 } 365 366 void getAnalysisUsage(AnalysisUsage &AU) const override { 367 AU.addRequired<LiveIntervals>(); 368 AU.addPreserved<LiveIntervals>(); 369 AU.addRequired<SlotIndexes>(); 370 AU.addPreserved<SlotIndexes>(); 371 AU.addRequired<MachineDominatorTree>(); 372 AU.addPreserved<MachineDominatorTree>(); 373 MachineFunctionPass::getAnalysisUsage(AU); 374 } 375 }; 376 } 377 378 INITIALIZE_PASS_BEGIN(PPCVSXFMAMutate, DEBUG_TYPE, 379 "PowerPC VSX FMA Mutation", false, false) 380 INITIALIZE_PASS_DEPENDENCY(LiveIntervals) 381 INITIALIZE_PASS_DEPENDENCY(SlotIndexes) 382 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree) 383 INITIALIZE_PASS_END(PPCVSXFMAMutate, DEBUG_TYPE, 384 "PowerPC VSX FMA Mutation", false, false) 385 386 char &llvm::PPCVSXFMAMutateID = PPCVSXFMAMutate::ID; 387 388 char PPCVSXFMAMutate::ID = 0; 389 FunctionPass *llvm::createPPCVSXFMAMutatePass() { 390 return new PPCVSXFMAMutate(); 391 } 392