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/TargetFrameLowering.h" 27 #include "llvm/CodeGen/TargetOpcodes.h" 28 #include "llvm/CodeGen/TargetRegisterInfo.h" 29 #include "llvm/CodeGen/TargetSubtargetInfo.h" 30 #include "llvm/InitializePasses.h" 31 #include "llvm/Pass.h" 32 #include "llvm/Support/Debug.h" 33 #include "llvm/Support/ErrorHandling.h" 34 #include "llvm/Support/raw_ostream.h" 35 #include <algorithm> 36 #include <cassert> 37 #include <cstdint> 38 #include <tuple> 39 40 using namespace llvm; 41 42 #define DEBUG_TYPE "localstackalloc" 43 44 STATISTIC(NumAllocations, "Number of frame indices allocated into local block"); 45 STATISTIC(NumBaseRegisters, "Number of virtual frame base registers allocated"); 46 STATISTIC(NumReplacements, "Number of frame indices references replaced"); 47 48 namespace { 49 50 class FrameRef { 51 MachineBasicBlock::iterator MI; // Instr referencing the frame 52 int64_t LocalOffset; // Local offset of the frame idx referenced 53 int FrameIdx; // The frame index 54 55 // Order reference instruction appears in program. Used to ensure 56 // deterministic order when multiple instructions may reference the same 57 // location. 58 unsigned Order; 59 60 public: 61 FrameRef(MachineInstr *I, int64_t Offset, int Idx, unsigned Ord) : 62 MI(I), LocalOffset(Offset), FrameIdx(Idx), Order(Ord) {} 63 64 bool operator<(const FrameRef &RHS) const { 65 return std::tie(LocalOffset, FrameIdx, Order) < 66 std::tie(RHS.LocalOffset, RHS.FrameIdx, RHS.Order); 67 } 68 69 MachineBasicBlock::iterator getMachineInstr() const { return MI; } 70 int64_t getLocalOffset() const { return LocalOffset; } 71 int getFrameIndex() const { return FrameIdx; } 72 }; 73 74 class LocalStackSlotPass: public MachineFunctionPass { 75 SmallVector<int64_t, 16> LocalOffsets; 76 77 /// StackObjSet - A set of stack object indexes 78 using StackObjSet = SmallSetVector<int, 8>; 79 80 void AdjustStackOffset(MachineFrameInfo &MFI, int FrameIdx, int64_t &Offset, 81 bool StackGrowsDown, Align &MaxAlign); 82 void AssignProtectedObjSet(const StackObjSet &UnassignedObjs, 83 SmallSet<int, 16> &ProtectedObjs, 84 MachineFrameInfo &MFI, bool StackGrowsDown, 85 int64_t &Offset, Align &MaxAlign); 86 void calculateFrameObjectOffsets(MachineFunction &Fn); 87 bool insertFrameReferenceRegisters(MachineFunction &Fn); 88 89 public: 90 static char ID; // Pass identification, replacement for typeid 91 92 explicit LocalStackSlotPass() : MachineFunctionPass(ID) { 93 initializeLocalStackSlotPassPass(*PassRegistry::getPassRegistry()); 94 } 95 96 bool runOnMachineFunction(MachineFunction &MF) override; 97 98 void getAnalysisUsage(AnalysisUsage &AU) const override { 99 AU.setPreservesCFG(); 100 MachineFunctionPass::getAnalysisUsage(AU); 101 } 102 }; 103 104 } // end anonymous namespace 105 106 char LocalStackSlotPass::ID = 0; 107 108 char &llvm::LocalStackSlotAllocationID = LocalStackSlotPass::ID; 109 INITIALIZE_PASS(LocalStackSlotPass, DEBUG_TYPE, 110 "Local Stack Slot Allocation", false, false) 111 112 bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) { 113 MachineFrameInfo &MFI = MF.getFrameInfo(); 114 const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo(); 115 unsigned LocalObjectCount = MFI.getObjectIndexEnd(); 116 117 // If the target doesn't want/need this pass, or if there are no locals 118 // to consider, early exit. 119 if (LocalObjectCount == 0 || !TRI->requiresVirtualBaseRegisters(MF)) 120 return false; 121 122 // Make sure we have enough space to store the local offsets. 123 LocalOffsets.resize(MFI.getObjectIndexEnd()); 124 125 // Lay out the local blob. 126 calculateFrameObjectOffsets(MF); 127 128 // Insert virtual base registers to resolve frame index references. 129 bool UsedBaseRegs = insertFrameReferenceRegisters(MF); 130 131 // Tell MFI whether any base registers were allocated. PEI will only 132 // want to use the local block allocations from this pass if there were any. 133 // Otherwise, PEI can do a bit better job of getting the alignment right 134 // without a hole at the start since it knows the alignment of the stack 135 // at the start of local allocation, and this pass doesn't. 136 MFI.setUseLocalStackAllocationBlock(UsedBaseRegs); 137 138 return true; 139 } 140 141 /// AdjustStackOffset - Helper function used to adjust the stack frame offset. 142 void LocalStackSlotPass::AdjustStackOffset(MachineFrameInfo &MFI, int FrameIdx, 143 int64_t &Offset, bool StackGrowsDown, 144 Align &MaxAlign) { 145 // If the stack grows down, add the object size to find the lowest address. 146 if (StackGrowsDown) 147 Offset += MFI.getObjectSize(FrameIdx); 148 149 Align Alignment = MFI.getObjectAlign(FrameIdx); 150 151 // If the alignment of this object is greater than that of the stack, then 152 // increase the stack alignment to match. 153 MaxAlign = std::max(MaxAlign, Alignment); 154 155 // Adjust to alignment boundary. 156 Offset = alignTo(Offset, Alignment); 157 158 int64_t LocalOffset = StackGrowsDown ? -Offset : Offset; 159 LLVM_DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset " 160 << LocalOffset << "\n"); 161 // Keep the offset available for base register allocation 162 LocalOffsets[FrameIdx] = LocalOffset; 163 // And tell MFI about it for PEI to use later 164 MFI.mapLocalFrameObject(FrameIdx, LocalOffset); 165 166 if (!StackGrowsDown) 167 Offset += MFI.getObjectSize(FrameIdx); 168 169 ++NumAllocations; 170 } 171 172 /// AssignProtectedObjSet - Helper function to assign large stack objects (i.e., 173 /// those required to be close to the Stack Protector) to stack offsets. 174 void LocalStackSlotPass::AssignProtectedObjSet( 175 const StackObjSet &UnassignedObjs, SmallSet<int, 16> &ProtectedObjs, 176 MachineFrameInfo &MFI, bool StackGrowsDown, int64_t &Offset, 177 Align &MaxAlign) { 178 for (int i : UnassignedObjs) { 179 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign); 180 ProtectedObjs.insert(i); 181 } 182 } 183 184 /// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the 185 /// abstract stack objects. 186 void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) { 187 // Loop over all of the stack objects, assigning sequential addresses... 188 MachineFrameInfo &MFI = Fn.getFrameInfo(); 189 const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering(); 190 bool StackGrowsDown = 191 TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown; 192 int64_t Offset = 0; 193 Align MaxAlign; 194 195 // Make sure that the stack protector comes before the local variables on the 196 // stack. 197 SmallSet<int, 16> ProtectedObjs; 198 if (MFI.hasStackProtectorIndex()) { 199 int StackProtectorFI = MFI.getStackProtectorIndex(); 200 201 // We need to make sure we didn't pre-allocate the stack protector when 202 // doing this. 203 // If we already have a stack protector, this will re-assign it to a slot 204 // that is **not** covering the protected objects. 205 assert(!MFI.isObjectPreAllocated(StackProtectorFI) && 206 "Stack protector pre-allocated in LocalStackSlotAllocation"); 207 208 StackObjSet LargeArrayObjs; 209 StackObjSet SmallArrayObjs; 210 StackObjSet AddrOfObjs; 211 212 // Only place the stack protector in the local stack area if the target 213 // allows it. 214 if (TFI.isStackIdSafeForLocalArea(MFI.getStackID(StackProtectorFI))) 215 AdjustStackOffset(MFI, StackProtectorFI, Offset, StackGrowsDown, 216 MaxAlign); 217 218 // Assign large stack objects first. 219 for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) { 220 if (MFI.isDeadObjectIndex(i)) 221 continue; 222 if (StackProtectorFI == (int)i) 223 continue; 224 if (!TFI.isStackIdSafeForLocalArea(MFI.getStackID(i))) 225 continue; 226 227 switch (MFI.getObjectSSPLayout(i)) { 228 case MachineFrameInfo::SSPLK_None: 229 continue; 230 case MachineFrameInfo::SSPLK_SmallArray: 231 SmallArrayObjs.insert(i); 232 continue; 233 case MachineFrameInfo::SSPLK_AddrOf: 234 AddrOfObjs.insert(i); 235 continue; 236 case MachineFrameInfo::SSPLK_LargeArray: 237 LargeArrayObjs.insert(i); 238 continue; 239 } 240 llvm_unreachable("Unexpected SSPLayoutKind."); 241 } 242 243 AssignProtectedObjSet(LargeArrayObjs, ProtectedObjs, MFI, StackGrowsDown, 244 Offset, MaxAlign); 245 AssignProtectedObjSet(SmallArrayObjs, ProtectedObjs, MFI, StackGrowsDown, 246 Offset, MaxAlign); 247 AssignProtectedObjSet(AddrOfObjs, ProtectedObjs, MFI, StackGrowsDown, 248 Offset, MaxAlign); 249 } 250 251 // Then assign frame offsets to stack objects that are not used to spill 252 // callee saved registers. 253 for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) { 254 if (MFI.isDeadObjectIndex(i)) 255 continue; 256 if (MFI.getStackProtectorIndex() == (int)i) 257 continue; 258 if (ProtectedObjs.count(i)) 259 continue; 260 if (!TFI.isStackIdSafeForLocalArea(MFI.getStackID(i))) 261 continue; 262 263 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign); 264 } 265 266 // Remember how big this blob of stack space is 267 MFI.setLocalFrameSize(Offset); 268 MFI.setLocalFrameMaxAlign(MaxAlign); 269 } 270 271 static inline bool 272 lookupCandidateBaseReg(unsigned BaseReg, 273 int64_t BaseOffset, 274 int64_t FrameSizeAdjust, 275 int64_t LocalFrameOffset, 276 const MachineInstr &MI, 277 const TargetRegisterInfo *TRI) { 278 // Check if the relative offset from the where the base register references 279 // to the target address is in range for the instruction. 280 int64_t Offset = FrameSizeAdjust + LocalFrameOffset - BaseOffset; 281 return TRI->isFrameOffsetLegal(&MI, BaseReg, Offset); 282 } 283 284 bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) { 285 // Scan the function's instructions looking for frame index references. 286 // For each, ask the target if it wants a virtual base register for it 287 // based on what we can tell it about where the local will end up in the 288 // stack frame. If it wants one, re-use a suitable one we've previously 289 // allocated, or if there isn't one that fits the bill, allocate a new one 290 // and ask the target to create a defining instruction for it. 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 (const MachineOperand &MO : MI.operands()) { 322 // Consider replacing all frame index operands that reference 323 // an object allocated in the local block. 324 if (MO.isFI()) { 325 // Don't try this with values not in the local block. 326 if (!MFI.isObjectPreAllocated(MO.getIndex())) 327 break; 328 int Idx = MO.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 Register BaseReg; 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 (BaseReg.isValid() && 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 CandBaseOffset = FrameSizeAdjust + LocalOffset + InstrOffset; 399 400 // We'd like to avoid creating single-use virtual base registers. 401 // Because the FrameRefs are in sorted order, and we've already 402 // processed all FrameRefs before this one, just check whether or not 403 // the next FrameRef will be able to reuse this new register. If not, 404 // then don't bother creating it. 405 if (ref + 1 >= e || 406 !lookupCandidateBaseReg( 407 BaseReg, CandBaseOffset, FrameSizeAdjust, 408 FrameReferenceInsns[ref + 1].getLocalOffset(), 409 *FrameReferenceInsns[ref + 1].getMachineInstr(), TRI)) 410 continue; 411 412 // Save the base offset. 413 BaseOffset = CandBaseOffset; 414 415 // Tell the target to insert the instruction to initialize 416 // the base register. 417 // MachineBasicBlock::iterator InsertionPt = Entry->begin(); 418 BaseReg = TRI->materializeFrameBaseRegister(Entry, FrameIdx, InstrOffset); 419 420 LLVM_DEBUG(dbgs() << " Materialized base register at frame local offset " 421 << LocalOffset + InstrOffset 422 << " into " << printReg(BaseReg, TRI) << '\n'); 423 424 // The base register already includes any offset specified 425 // by the instruction, so account for that so it doesn't get 426 // applied twice. 427 Offset = -InstrOffset; 428 429 ++NumBaseRegisters; 430 } 431 assert(BaseReg && "Unable to allocate virtual base register!"); 432 433 // Modify the instruction to use the new base register rather 434 // than the frame index operand. 435 TRI->resolveFrameIndex(MI, BaseReg, Offset); 436 LLVM_DEBUG(dbgs() << "Resolved: " << MI); 437 438 ++NumReplacements; 439 } 440 441 return BaseReg.isValid(); 442 } 443