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, unsigned &MaxAlign); 83 void AssignProtectedObjSet(const StackObjSet &UnassignedObjs, 84 SmallSet<int, 16> &ProtectedObjs, 85 MachineFrameInfo &MFI, bool StackGrowsDown, 86 int64_t &Offset, unsigned &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 (!TRI->requiresVirtualBaseRegisters(MF) || LocalObjectCount == 0) 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, 144 int FrameIdx, int64_t &Offset, 145 bool StackGrowsDown, 146 unsigned &MaxAlign) { 147 // If the stack grows down, add the object size to find the lowest address. 148 if (StackGrowsDown) 149 Offset += MFI.getObjectSize(FrameIdx); 150 151 unsigned Align = MFI.getObjectAlignment(FrameIdx); 152 153 // If the alignment of this object is greater than that of the stack, then 154 // increase the stack alignment to match. 155 MaxAlign = std::max(MaxAlign, Align); 156 157 // Adjust to alignment boundary. 158 Offset = (Offset + Align - 1) / Align * Align; 159 160 int64_t LocalOffset = StackGrowsDown ? -Offset : Offset; 161 LLVM_DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset " 162 << LocalOffset << "\n"); 163 // Keep the offset available for base register allocation 164 LocalOffsets[FrameIdx] = LocalOffset; 165 // And tell MFI about it for PEI to use later 166 MFI.mapLocalFrameObject(FrameIdx, LocalOffset); 167 168 if (!StackGrowsDown) 169 Offset += MFI.getObjectSize(FrameIdx); 170 171 ++NumAllocations; 172 } 173 174 /// AssignProtectedObjSet - Helper function to assign large stack objects (i.e., 175 /// those required to be close to the Stack Protector) to stack offsets. 176 void LocalStackSlotPass::AssignProtectedObjSet(const StackObjSet &UnassignedObjs, 177 SmallSet<int, 16> &ProtectedObjs, 178 MachineFrameInfo &MFI, 179 bool StackGrowsDown, int64_t &Offset, 180 unsigned &MaxAlign) { 181 for (StackObjSet::const_iterator I = UnassignedObjs.begin(), 182 E = UnassignedObjs.end(); I != E; ++I) { 183 int i = *I; 184 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign); 185 ProtectedObjs.insert(i); 186 } 187 } 188 189 /// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the 190 /// abstract stack objects. 191 void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) { 192 // Loop over all of the stack objects, assigning sequential addresses... 193 MachineFrameInfo &MFI = Fn.getFrameInfo(); 194 const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering(); 195 bool StackGrowsDown = 196 TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown; 197 int64_t Offset = 0; 198 unsigned MaxAlign = 0; 199 200 // Make sure that the stack protector comes before the local variables on the 201 // stack. 202 SmallSet<int, 16> ProtectedObjs; 203 if (MFI.hasStackProtectorIndex()) { 204 int StackProtectorFI = MFI.getStackProtectorIndex(); 205 206 // We need to make sure we didn't pre-allocate the stack protector when 207 // doing this. 208 // If we already have a stack protector, this will re-assign it to a slot 209 // that is **not** covering the protected objects. 210 assert(!MFI.isObjectPreAllocated(StackProtectorFI) && 211 "Stack protector pre-allocated in LocalStackSlotAllocation"); 212 213 StackObjSet LargeArrayObjs; 214 StackObjSet SmallArrayObjs; 215 StackObjSet AddrOfObjs; 216 217 AdjustStackOffset(MFI, StackProtectorFI, Offset, StackGrowsDown, MaxAlign); 218 219 // Assign large stack objects first. 220 for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) { 221 if (MFI.isDeadObjectIndex(i)) 222 continue; 223 if (StackProtectorFI == (int)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; 259 260 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign); 261 } 262 263 // Remember how big this blob of stack space is 264 MFI.setLocalFrameSize(Offset); 265 MFI.setLocalFrameMaxAlign(assumeAligned(MaxAlign)); 266 } 267 268 static inline bool 269 lookupCandidateBaseReg(unsigned BaseReg, 270 int64_t BaseOffset, 271 int64_t FrameSizeAdjust, 272 int64_t LocalFrameOffset, 273 const MachineInstr &MI, 274 const TargetRegisterInfo *TRI) { 275 // Check if the relative offset from the where the base register references 276 // to the target address is in range for the instruction. 277 int64_t Offset = FrameSizeAdjust + LocalFrameOffset - BaseOffset; 278 return TRI->isFrameOffsetLegal(&MI, BaseReg, Offset); 279 } 280 281 bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) { 282 // Scan the function's instructions looking for frame index references. 283 // For each, ask the target if it wants a virtual base register for it 284 // based on what we can tell it about where the local will end up in the 285 // stack frame. If it wants one, re-use a suitable one we've previously 286 // allocated, or if there isn't one that fits the bill, allocate a new one 287 // and ask the target to create a defining instruction for it. 288 bool UsedBaseReg = false; 289 290 MachineFrameInfo &MFI = Fn.getFrameInfo(); 291 const TargetRegisterInfo *TRI = Fn.getSubtarget().getRegisterInfo(); 292 const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering(); 293 bool StackGrowsDown = 294 TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown; 295 296 // Collect all of the instructions in the block that reference 297 // a frame index. Also store the frame index referenced to ease later 298 // lookup. (For any insn that has more than one FI reference, we arbitrarily 299 // choose the first one). 300 SmallVector<FrameRef, 64> FrameReferenceInsns; 301 302 unsigned Order = 0; 303 304 for (MachineBasicBlock &BB : Fn) { 305 for (MachineInstr &MI : BB) { 306 // Debug value, stackmap and patchpoint instructions can't be out of 307 // range, so they don't need any updates. 308 if (MI.isDebugInstr() || MI.getOpcode() == TargetOpcode::STATEPOINT || 309 MI.getOpcode() == TargetOpcode::STACKMAP || 310 MI.getOpcode() == TargetOpcode::PATCHPOINT) 311 continue; 312 313 // For now, allocate the base register(s) within the basic block 314 // where they're used, and don't try to keep them around outside 315 // of that. It may be beneficial to try sharing them more broadly 316 // than that, but the increased register pressure makes that a 317 // tricky thing to balance. Investigate if re-materializing these 318 // becomes an issue. 319 for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) { 320 // Consider replacing all frame index operands that reference 321 // an object allocated in the local block. 322 if (MI.getOperand(i).isFI()) { 323 // Don't try this with values not in the local block. 324 if (!MFI.isObjectPreAllocated(MI.getOperand(i).getIndex())) 325 break; 326 int Idx = MI.getOperand(i).getIndex(); 327 int64_t LocalOffset = LocalOffsets[Idx]; 328 if (!TRI->needsFrameBaseReg(&MI, LocalOffset)) 329 break; 330 FrameReferenceInsns.push_back(FrameRef(&MI, LocalOffset, Idx, Order++)); 331 break; 332 } 333 } 334 } 335 } 336 337 // Sort the frame references by local offset. 338 // Use frame index as a tie-breaker in case MI's have the same offset. 339 llvm::sort(FrameReferenceInsns); 340 341 MachineBasicBlock *Entry = &Fn.front(); 342 343 unsigned BaseReg = 0; 344 int64_t BaseOffset = 0; 345 346 // Loop through the frame references and allocate for them as necessary. 347 for (int ref = 0, e = FrameReferenceInsns.size(); ref < e ; ++ref) { 348 FrameRef &FR = FrameReferenceInsns[ref]; 349 MachineInstr &MI = *FR.getMachineInstr(); 350 int64_t LocalOffset = FR.getLocalOffset(); 351 int FrameIdx = FR.getFrameIndex(); 352 assert(MFI.isObjectPreAllocated(FrameIdx) && 353 "Only pre-allocated locals expected!"); 354 355 // We need to keep the references to the stack protector slot through frame 356 // index operands so that it gets resolved by PEI rather than this pass. 357 // This avoids accesses to the stack protector though virtual base 358 // registers, and forces PEI to address it using fp/sp/bp. 359 if (MFI.hasStackProtectorIndex() && 360 FrameIdx == MFI.getStackProtectorIndex()) 361 continue; 362 363 LLVM_DEBUG(dbgs() << "Considering: " << MI); 364 365 unsigned idx = 0; 366 for (unsigned f = MI.getNumOperands(); idx != f; ++idx) { 367 if (!MI.getOperand(idx).isFI()) 368 continue; 369 370 if (FrameIdx == MI.getOperand(idx).getIndex()) 371 break; 372 } 373 374 assert(idx < MI.getNumOperands() && "Cannot find FI operand"); 375 376 int64_t Offset = 0; 377 int64_t FrameSizeAdjust = StackGrowsDown ? MFI.getLocalFrameSize() : 0; 378 379 LLVM_DEBUG(dbgs() << " Replacing FI in: " << MI); 380 381 // If we have a suitable base register available, use it; otherwise 382 // create a new one. Note that any offset encoded in the 383 // instruction itself will be taken into account by the target, 384 // so we don't have to adjust for it here when reusing a base 385 // register. 386 if (UsedBaseReg && 387 lookupCandidateBaseReg(BaseReg, BaseOffset, FrameSizeAdjust, 388 LocalOffset, MI, TRI)) { 389 LLVM_DEBUG(dbgs() << " Reusing base register " << BaseReg << "\n"); 390 // We found a register to reuse. 391 Offset = FrameSizeAdjust + LocalOffset - BaseOffset; 392 } else { 393 // No previously defined register was in range, so create a new one. 394 int64_t InstrOffset = TRI->getFrameIndexInstrOffset(&MI, idx); 395 396 int64_t PrevBaseOffset = BaseOffset; 397 BaseOffset = FrameSizeAdjust + LocalOffset + InstrOffset; 398 399 // We'd like to avoid creating single-use virtual base registers. 400 // Because the FrameRefs are in sorted order, and we've already 401 // processed all FrameRefs before this one, just check whether or not 402 // the next FrameRef will be able to reuse this new register. If not, 403 // then don't bother creating it. 404 if (ref + 1 >= e || 405 !lookupCandidateBaseReg( 406 BaseReg, BaseOffset, FrameSizeAdjust, 407 FrameReferenceInsns[ref + 1].getLocalOffset(), 408 *FrameReferenceInsns[ref + 1].getMachineInstr(), TRI)) { 409 BaseOffset = PrevBaseOffset; 410 continue; 411 } 412 413 const MachineFunction *MF = MI.getMF(); 414 const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF); 415 BaseReg = Fn.getRegInfo().createVirtualRegister(RC); 416 417 LLVM_DEBUG(dbgs() << " Materializing base register " << BaseReg 418 << " at frame local offset " 419 << LocalOffset + InstrOffset << "\n"); 420 421 // Tell the target to insert the instruction to initialize 422 // the base register. 423 // MachineBasicBlock::iterator InsertionPt = Entry->begin(); 424 TRI->materializeFrameBaseRegister(Entry, BaseReg, FrameIdx, 425 InstrOffset); 426 427 // The base register already includes any offset specified 428 // by the instruction, so account for that so it doesn't get 429 // applied twice. 430 Offset = -InstrOffset; 431 432 ++NumBaseRegisters; 433 UsedBaseReg = true; 434 } 435 assert(BaseReg != 0 && "Unable to allocate virtual base register!"); 436 437 // Modify the instruction to use the new base register rather 438 // than the frame index operand. 439 TRI->resolveFrameIndex(MI, BaseReg, Offset); 440 LLVM_DEBUG(dbgs() << "Resolved: " << MI); 441 442 ++NumReplacements; 443 } 444 445 return UsedBaseReg; 446 } 447