1 //===- LocalStackSlotAllocation.cpp - Pre-allocate locals to stack slots --===// 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 assigns local frame indices to stack slots relative to one another 10 // and allocates additional base registers to access them when the target 11 // estimates they are likely to be out of range of stack pointer and frame 12 // pointer relative addressing. 13 // 14 //===----------------------------------------------------------------------===// 15 16 #include "llvm/ADT/SetVector.h" 17 #include "llvm/ADT/SmallSet.h" 18 #include "llvm/ADT/SmallVector.h" 19 #include "llvm/ADT/Statistic.h" 20 #include "llvm/CodeGen/MachineBasicBlock.h" 21 #include "llvm/CodeGen/MachineFrameInfo.h" 22 #include "llvm/CodeGen/MachineFunction.h" 23 #include "llvm/CodeGen/MachineFunctionPass.h" 24 #include "llvm/CodeGen/MachineInstr.h" 25 #include "llvm/CodeGen/MachineOperand.h" 26 #include "llvm/CodeGen/MachineRegisterInfo.h" 27 #include "llvm/CodeGen/TargetFrameLowering.h" 28 #include "llvm/CodeGen/TargetOpcodes.h" 29 #include "llvm/CodeGen/TargetRegisterInfo.h" 30 #include "llvm/CodeGen/TargetSubtargetInfo.h" 31 #include "llvm/InitializePasses.h" 32 #include "llvm/Pass.h" 33 #include "llvm/Support/Debug.h" 34 #include "llvm/Support/ErrorHandling.h" 35 #include "llvm/Support/raw_ostream.h" 36 #include <algorithm> 37 #include <cassert> 38 #include <cstdint> 39 #include <tuple> 40 41 using namespace llvm; 42 43 #define DEBUG_TYPE "localstackalloc" 44 45 STATISTIC(NumAllocations, "Number of frame indices allocated into local block"); 46 STATISTIC(NumBaseRegisters, "Number of virtual frame base registers allocated"); 47 STATISTIC(NumReplacements, "Number of frame indices references replaced"); 48 49 namespace { 50 51 class FrameRef { 52 MachineBasicBlock::iterator MI; // Instr referencing the frame 53 int64_t LocalOffset; // Local offset of the frame idx referenced 54 int FrameIdx; // The frame index 55 56 // Order reference instruction appears in program. Used to ensure 57 // deterministic order when multiple instructions may reference the same 58 // location. 59 unsigned Order; 60 61 public: 62 FrameRef(MachineInstr *I, int64_t Offset, int Idx, unsigned Ord) : 63 MI(I), LocalOffset(Offset), FrameIdx(Idx), Order(Ord) {} 64 65 bool operator<(const FrameRef &RHS) const { 66 return std::tie(LocalOffset, FrameIdx, Order) < 67 std::tie(RHS.LocalOffset, RHS.FrameIdx, RHS.Order); 68 } 69 70 MachineBasicBlock::iterator getMachineInstr() const { return MI; } 71 int64_t getLocalOffset() const { return LocalOffset; } 72 int getFrameIndex() const { return FrameIdx; } 73 }; 74 75 class LocalStackSlotPass: public MachineFunctionPass { 76 SmallVector<int64_t, 16> LocalOffsets; 77 78 /// StackObjSet - A set of stack object indexes 79 using StackObjSet = SmallSetVector<int, 8>; 80 81 void AdjustStackOffset(MachineFrameInfo &MFI, int FrameIdx, int64_t &Offset, 82 bool StackGrowsDown, Align &MaxAlign); 83 void AssignProtectedObjSet(const StackObjSet &UnassignedObjs, 84 SmallSet<int, 16> &ProtectedObjs, 85 MachineFrameInfo &MFI, bool StackGrowsDown, 86 int64_t &Offset, Align &MaxAlign); 87 void calculateFrameObjectOffsets(MachineFunction &Fn); 88 bool insertFrameReferenceRegisters(MachineFunction &Fn); 89 90 public: 91 static char ID; // Pass identification, replacement for typeid 92 93 explicit LocalStackSlotPass() : MachineFunctionPass(ID) { 94 initializeLocalStackSlotPassPass(*PassRegistry::getPassRegistry()); 95 } 96 97 bool runOnMachineFunction(MachineFunction &MF) override; 98 99 void getAnalysisUsage(AnalysisUsage &AU) const override { 100 AU.setPreservesCFG(); 101 MachineFunctionPass::getAnalysisUsage(AU); 102 } 103 }; 104 105 } // end anonymous namespace 106 107 char LocalStackSlotPass::ID = 0; 108 109 char &llvm::LocalStackSlotAllocationID = LocalStackSlotPass::ID; 110 INITIALIZE_PASS(LocalStackSlotPass, DEBUG_TYPE, 111 "Local Stack Slot Allocation", false, false) 112 113 bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) { 114 MachineFrameInfo &MFI = MF.getFrameInfo(); 115 const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo(); 116 unsigned LocalObjectCount = MFI.getObjectIndexEnd(); 117 118 // If the target doesn't want/need this pass, or if there are no locals 119 // to consider, early exit. 120 if (LocalObjectCount == 0 || !TRI->requiresVirtualBaseRegisters(MF)) 121 return true; 122 123 // Make sure we have enough space to store the local offsets. 124 LocalOffsets.resize(MFI.getObjectIndexEnd()); 125 126 // Lay out the local blob. 127 calculateFrameObjectOffsets(MF); 128 129 // Insert virtual base registers to resolve frame index references. 130 bool UsedBaseRegs = insertFrameReferenceRegisters(MF); 131 132 // Tell MFI whether any base registers were allocated. PEI will only 133 // want to use the local block allocations from this pass if there were any. 134 // Otherwise, PEI can do a bit better job of getting the alignment right 135 // without a hole at the start since it knows the alignment of the stack 136 // at the start of local allocation, and this pass doesn't. 137 MFI.setUseLocalStackAllocationBlock(UsedBaseRegs); 138 139 return true; 140 } 141 142 /// AdjustStackOffset - Helper function used to adjust the stack frame offset. 143 void LocalStackSlotPass::AdjustStackOffset(MachineFrameInfo &MFI, int FrameIdx, 144 int64_t &Offset, bool StackGrowsDown, 145 Align &MaxAlign) { 146 // If the stack grows down, add the object size to find the lowest address. 147 if (StackGrowsDown) 148 Offset += MFI.getObjectSize(FrameIdx); 149 150 Align Alignment = MFI.getObjectAlign(FrameIdx); 151 152 // If the alignment of this object is greater than that of the stack, then 153 // increase the stack alignment to match. 154 MaxAlign = std::max(MaxAlign, Alignment); 155 156 // Adjust to alignment boundary. 157 Offset = alignTo(Offset, Alignment); 158 159 int64_t LocalOffset = StackGrowsDown ? -Offset : Offset; 160 LLVM_DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset " 161 << LocalOffset << "\n"); 162 // Keep the offset available for base register allocation 163 LocalOffsets[FrameIdx] = LocalOffset; 164 // And tell MFI about it for PEI to use later 165 MFI.mapLocalFrameObject(FrameIdx, LocalOffset); 166 167 if (!StackGrowsDown) 168 Offset += MFI.getObjectSize(FrameIdx); 169 170 ++NumAllocations; 171 } 172 173 /// AssignProtectedObjSet - Helper function to assign large stack objects (i.e., 174 /// those required to be close to the Stack Protector) to stack offsets. 175 void LocalStackSlotPass::AssignProtectedObjSet( 176 const StackObjSet &UnassignedObjs, SmallSet<int, 16> &ProtectedObjs, 177 MachineFrameInfo &MFI, bool StackGrowsDown, int64_t &Offset, 178 Align &MaxAlign) { 179 for (StackObjSet::const_iterator I = UnassignedObjs.begin(), 180 E = UnassignedObjs.end(); I != E; ++I) { 181 int i = *I; 182 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign); 183 ProtectedObjs.insert(i); 184 } 185 } 186 187 /// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the 188 /// abstract stack objects. 189 void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) { 190 // Loop over all of the stack objects, assigning sequential addresses... 191 MachineFrameInfo &MFI = Fn.getFrameInfo(); 192 const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering(); 193 bool StackGrowsDown = 194 TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown; 195 int64_t Offset = 0; 196 Align MaxAlign; 197 198 // Make sure that the stack protector comes before the local variables on the 199 // stack. 200 SmallSet<int, 16> ProtectedObjs; 201 if (MFI.hasStackProtectorIndex()) { 202 int StackProtectorFI = MFI.getStackProtectorIndex(); 203 204 // We need to make sure we didn't pre-allocate the stack protector when 205 // doing this. 206 // If we already have a stack protector, this will re-assign it to a slot 207 // that is **not** covering the protected objects. 208 assert(!MFI.isObjectPreAllocated(StackProtectorFI) && 209 "Stack protector pre-allocated in LocalStackSlotAllocation"); 210 211 StackObjSet LargeArrayObjs; 212 StackObjSet SmallArrayObjs; 213 StackObjSet AddrOfObjs; 214 215 AdjustStackOffset(MFI, StackProtectorFI, Offset, StackGrowsDown, MaxAlign); 216 217 // Assign large stack objects first. 218 for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) { 219 if (MFI.isDeadObjectIndex(i)) 220 continue; 221 if (StackProtectorFI == (int)i) 222 continue; 223 if (!TFI.isStackIdSafeForLocalArea(MFI.getStackID(i))) 224 continue; 225 226 switch (MFI.getObjectSSPLayout(i)) { 227 case MachineFrameInfo::SSPLK_None: 228 continue; 229 case MachineFrameInfo::SSPLK_SmallArray: 230 SmallArrayObjs.insert(i); 231 continue; 232 case MachineFrameInfo::SSPLK_AddrOf: 233 AddrOfObjs.insert(i); 234 continue; 235 case MachineFrameInfo::SSPLK_LargeArray: 236 LargeArrayObjs.insert(i); 237 continue; 238 } 239 llvm_unreachable("Unexpected SSPLayoutKind."); 240 } 241 242 AssignProtectedObjSet(LargeArrayObjs, ProtectedObjs, MFI, StackGrowsDown, 243 Offset, MaxAlign); 244 AssignProtectedObjSet(SmallArrayObjs, ProtectedObjs, MFI, StackGrowsDown, 245 Offset, MaxAlign); 246 AssignProtectedObjSet(AddrOfObjs, ProtectedObjs, MFI, StackGrowsDown, 247 Offset, MaxAlign); 248 } 249 250 // Then assign frame offsets to stack objects that are not used to spill 251 // callee saved registers. 252 for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) { 253 if (MFI.isDeadObjectIndex(i)) 254 continue; 255 if (MFI.getStackProtectorIndex() == (int)i) 256 continue; 257 if (ProtectedObjs.count(i)) 258 continue; 259 if (!TFI.isStackIdSafeForLocalArea(MFI.getStackID(i))) 260 continue; 261 262 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign); 263 } 264 265 // Remember how big this blob of stack space is 266 MFI.setLocalFrameSize(Offset); 267 MFI.setLocalFrameMaxAlign(MaxAlign); 268 } 269 270 static inline bool 271 lookupCandidateBaseReg(unsigned BaseReg, 272 int64_t BaseOffset, 273 int64_t FrameSizeAdjust, 274 int64_t LocalFrameOffset, 275 const MachineInstr &MI, 276 const TargetRegisterInfo *TRI) { 277 // Check if the relative offset from the where the base register references 278 // to the target address is in range for the instruction. 279 int64_t Offset = FrameSizeAdjust + LocalFrameOffset - BaseOffset; 280 return TRI->isFrameOffsetLegal(&MI, BaseReg, Offset); 281 } 282 283 bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) { 284 // Scan the function's instructions looking for frame index references. 285 // For each, ask the target if it wants a virtual base register for it 286 // based on what we can tell it about where the local will end up in the 287 // stack frame. If it wants one, re-use a suitable one we've previously 288 // allocated, or if there isn't one that fits the bill, allocate a new one 289 // and ask the target to create a defining instruction for it. 290 bool UsedBaseReg = false; 291 292 MachineFrameInfo &MFI = Fn.getFrameInfo(); 293 const TargetRegisterInfo *TRI = Fn.getSubtarget().getRegisterInfo(); 294 const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering(); 295 bool StackGrowsDown = 296 TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown; 297 298 // Collect all of the instructions in the block that reference 299 // a frame index. Also store the frame index referenced to ease later 300 // lookup. (For any insn that has more than one FI reference, we arbitrarily 301 // choose the first one). 302 SmallVector<FrameRef, 64> FrameReferenceInsns; 303 304 unsigned Order = 0; 305 306 for (MachineBasicBlock &BB : Fn) { 307 for (MachineInstr &MI : BB) { 308 // Debug value, stackmap and patchpoint instructions can't be out of 309 // range, so they don't need any updates. 310 if (MI.isDebugInstr() || MI.getOpcode() == TargetOpcode::STATEPOINT || 311 MI.getOpcode() == TargetOpcode::STACKMAP || 312 MI.getOpcode() == TargetOpcode::PATCHPOINT) 313 continue; 314 315 // For now, allocate the base register(s) within the basic block 316 // where they're used, and don't try to keep them around outside 317 // of that. It may be beneficial to try sharing them more broadly 318 // than that, but the increased register pressure makes that a 319 // tricky thing to balance. Investigate if re-materializing these 320 // becomes an issue. 321 for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) { 322 // Consider replacing all frame index operands that reference 323 // an object allocated in the local block. 324 if (MI.getOperand(i).isFI()) { 325 // Don't try this with values not in the local block. 326 if (!MFI.isObjectPreAllocated(MI.getOperand(i).getIndex())) 327 break; 328 int Idx = MI.getOperand(i).getIndex(); 329 int64_t LocalOffset = LocalOffsets[Idx]; 330 if (!TRI->needsFrameBaseReg(&MI, LocalOffset)) 331 break; 332 FrameReferenceInsns.push_back(FrameRef(&MI, LocalOffset, Idx, Order++)); 333 break; 334 } 335 } 336 } 337 } 338 339 // Sort the frame references by local offset. 340 // Use frame index as a tie-breaker in case MI's have the same offset. 341 llvm::sort(FrameReferenceInsns); 342 343 MachineBasicBlock *Entry = &Fn.front(); 344 345 unsigned BaseReg = 0; 346 int64_t BaseOffset = 0; 347 348 // Loop through the frame references and allocate for them as necessary. 349 for (int ref = 0, e = FrameReferenceInsns.size(); ref < e ; ++ref) { 350 FrameRef &FR = FrameReferenceInsns[ref]; 351 MachineInstr &MI = *FR.getMachineInstr(); 352 int64_t LocalOffset = FR.getLocalOffset(); 353 int FrameIdx = FR.getFrameIndex(); 354 assert(MFI.isObjectPreAllocated(FrameIdx) && 355 "Only pre-allocated locals expected!"); 356 357 // We need to keep the references to the stack protector slot through frame 358 // index operands so that it gets resolved by PEI rather than this pass. 359 // This avoids accesses to the stack protector though virtual base 360 // registers, and forces PEI to address it using fp/sp/bp. 361 if (MFI.hasStackProtectorIndex() && 362 FrameIdx == MFI.getStackProtectorIndex()) 363 continue; 364 365 LLVM_DEBUG(dbgs() << "Considering: " << MI); 366 367 unsigned idx = 0; 368 for (unsigned f = MI.getNumOperands(); idx != f; ++idx) { 369 if (!MI.getOperand(idx).isFI()) 370 continue; 371 372 if (FrameIdx == MI.getOperand(idx).getIndex()) 373 break; 374 } 375 376 assert(idx < MI.getNumOperands() && "Cannot find FI operand"); 377 378 int64_t Offset = 0; 379 int64_t FrameSizeAdjust = StackGrowsDown ? MFI.getLocalFrameSize() : 0; 380 381 LLVM_DEBUG(dbgs() << " Replacing FI in: " << MI); 382 383 // If we have a suitable base register available, use it; otherwise 384 // create a new one. Note that any offset encoded in the 385 // instruction itself will be taken into account by the target, 386 // so we don't have to adjust for it here when reusing a base 387 // register. 388 if (UsedBaseReg && 389 lookupCandidateBaseReg(BaseReg, BaseOffset, FrameSizeAdjust, 390 LocalOffset, MI, TRI)) { 391 LLVM_DEBUG(dbgs() << " Reusing base register " << BaseReg << "\n"); 392 // We found a register to reuse. 393 Offset = FrameSizeAdjust + LocalOffset - BaseOffset; 394 } else { 395 // No previously defined register was in range, so create a new one. 396 int64_t InstrOffset = TRI->getFrameIndexInstrOffset(&MI, idx); 397 398 int64_t PrevBaseOffset = BaseOffset; 399 BaseOffset = FrameSizeAdjust + LocalOffset + InstrOffset; 400 401 // We'd like to avoid creating single-use virtual base registers. 402 // Because the FrameRefs are in sorted order, and we've already 403 // processed all FrameRefs before this one, just check whether or not 404 // the next FrameRef will be able to reuse this new register. If not, 405 // then don't bother creating it. 406 if (ref + 1 >= e || 407 !lookupCandidateBaseReg( 408 BaseReg, BaseOffset, FrameSizeAdjust, 409 FrameReferenceInsns[ref + 1].getLocalOffset(), 410 *FrameReferenceInsns[ref + 1].getMachineInstr(), TRI)) { 411 BaseOffset = PrevBaseOffset; 412 continue; 413 } 414 415 const MachineFunction *MF = MI.getMF(); 416 const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF); 417 BaseReg = Fn.getRegInfo().createVirtualRegister(RC); 418 419 LLVM_DEBUG(dbgs() << " Materializing base register" 420 << " at frame local offset " 421 << LocalOffset + InstrOffset); 422 423 // Tell the target to insert the instruction to initialize 424 // the base register. 425 // MachineBasicBlock::iterator InsertionPt = Entry->begin(); 426 BaseReg = TRI->materializeFrameBaseRegister(Entry, FrameIdx, InstrOffset); 427 428 LLVM_DEBUG(dbgs() << " into " << printReg(BaseReg, TRI) << '\n'); 429 430 // The base register already includes any offset specified 431 // by the instruction, so account for that so it doesn't get 432 // applied twice. 433 Offset = -InstrOffset; 434 435 ++NumBaseRegisters; 436 UsedBaseReg = true; 437 } 438 assert(BaseReg != 0 && "Unable to allocate virtual base register!"); 439 440 // Modify the instruction to use the new base register rather 441 // than the frame index operand. 442 TRI->resolveFrameIndex(MI, BaseReg, Offset); 443 LLVM_DEBUG(dbgs() << "Resolved: " << MI); 444 445 ++NumReplacements; 446 } 447 448 return UsedBaseReg; 449 } 450