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/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, unsigned &MaxAlign); 82 void AssignProtectedObjSet(const StackObjSet &UnassignedObjs, 83 SmallSet<int, 16> &ProtectedObjs, 84 MachineFrameInfo &MFI, bool StackGrowsDown, 85 int64_t &Offset, unsigned &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 (!TRI->requiresVirtualBaseRegisters(MF) || LocalObjectCount == 0) 120 return true; 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, 143 int FrameIdx, int64_t &Offset, 144 bool StackGrowsDown, 145 unsigned &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 unsigned Align = MFI.getObjectAlignment(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, Align); 155 156 // Adjust to alignment boundary. 157 Offset = (Offset + Align - 1) / Align * Align; 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(const StackObjSet &UnassignedObjs, 176 SmallSet<int, 16> &ProtectedObjs, 177 MachineFrameInfo &MFI, 178 bool StackGrowsDown, int64_t &Offset, 179 unsigned &MaxAlign) { 180 for (StackObjSet::const_iterator I = UnassignedObjs.begin(), 181 E = UnassignedObjs.end(); I != E; ++I) { 182 int i = *I; 183 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign); 184 ProtectedObjs.insert(i); 185 } 186 } 187 188 /// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the 189 /// abstract stack objects. 190 void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) { 191 // Loop over all of the stack objects, assigning sequential addresses... 192 MachineFrameInfo &MFI = Fn.getFrameInfo(); 193 const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering(); 194 bool StackGrowsDown = 195 TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown; 196 int64_t Offset = 0; 197 unsigned MaxAlign = 0; 198 199 // Make sure that the stack protector comes before the local variables on the 200 // stack. 201 SmallSet<int, 16> ProtectedObjs; 202 if (MFI.hasStackProtectorIndex()) { 203 int StackProtectorFI = MFI.getStackProtectorIndex(); 204 205 // We need to make sure we didn't pre-allocate the stack protector when 206 // doing this. 207 // If we already have a stack protector, this will re-assign it to a slot 208 // that is **not** covering the protected objects. 209 assert(!MFI.isObjectPreAllocated(StackProtectorFI) && 210 "Stack protector pre-allocated in LocalStackSlotAllocation"); 211 212 StackObjSet LargeArrayObjs; 213 StackObjSet SmallArrayObjs; 214 StackObjSet AddrOfObjs; 215 216 AdjustStackOffset(MFI, StackProtectorFI, Offset, StackGrowsDown, 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; 224 225 switch (MFI.getObjectSSPLayout(i)) { 226 case MachineFrameInfo::SSPLK_None: 227 continue; 228 case MachineFrameInfo::SSPLK_SmallArray: 229 SmallArrayObjs.insert(i); 230 continue; 231 case MachineFrameInfo::SSPLK_AddrOf: 232 AddrOfObjs.insert(i); 233 continue; 234 case MachineFrameInfo::SSPLK_LargeArray: 235 LargeArrayObjs.insert(i); 236 continue; 237 } 238 llvm_unreachable("Unexpected SSPLayoutKind."); 239 } 240 241 AssignProtectedObjSet(LargeArrayObjs, ProtectedObjs, MFI, StackGrowsDown, 242 Offset, MaxAlign); 243 AssignProtectedObjSet(SmallArrayObjs, ProtectedObjs, MFI, StackGrowsDown, 244 Offset, MaxAlign); 245 AssignProtectedObjSet(AddrOfObjs, ProtectedObjs, MFI, StackGrowsDown, 246 Offset, MaxAlign); 247 } 248 249 // Then assign frame offsets to stack objects that are not used to spill 250 // callee saved registers. 251 for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) { 252 if (MFI.isDeadObjectIndex(i)) 253 continue; 254 if (MFI.getStackProtectorIndex() == (int)i) 255 continue; 256 if (ProtectedObjs.count(i)) 257 continue; 258 259 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign); 260 } 261 262 // Remember how big this blob of stack space is 263 MFI.setLocalFrameSize(Offset); 264 MFI.setLocalFrameMaxAlign(assumeAligned(MaxAlign)); 265 } 266 267 static inline bool 268 lookupCandidateBaseReg(unsigned BaseReg, 269 int64_t BaseOffset, 270 int64_t FrameSizeAdjust, 271 int64_t LocalFrameOffset, 272 const MachineInstr &MI, 273 const TargetRegisterInfo *TRI) { 274 // Check if the relative offset from the where the base register references 275 // to the target address is in range for the instruction. 276 int64_t Offset = FrameSizeAdjust + LocalFrameOffset - BaseOffset; 277 return TRI->isFrameOffsetLegal(&MI, BaseReg, Offset); 278 } 279 280 bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) { 281 // Scan the function's instructions looking for frame index references. 282 // For each, ask the target if it wants a virtual base register for it 283 // based on what we can tell it about where the local will end up in the 284 // stack frame. If it wants one, re-use a suitable one we've previously 285 // allocated, or if there isn't one that fits the bill, allocate a new one 286 // and ask the target to create a defining instruction for it. 287 bool UsedBaseReg = false; 288 289 MachineFrameInfo &MFI = Fn.getFrameInfo(); 290 const TargetRegisterInfo *TRI = Fn.getSubtarget().getRegisterInfo(); 291 const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering(); 292 bool StackGrowsDown = 293 TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown; 294 295 // Collect all of the instructions in the block that reference 296 // a frame index. Also store the frame index referenced to ease later 297 // lookup. (For any insn that has more than one FI reference, we arbitrarily 298 // choose the first one). 299 SmallVector<FrameRef, 64> FrameReferenceInsns; 300 301 unsigned Order = 0; 302 303 for (MachineBasicBlock &BB : Fn) { 304 for (MachineInstr &MI : BB) { 305 // Debug value, stackmap and patchpoint instructions can't be out of 306 // range, so they don't need any updates. 307 if (MI.isDebugInstr() || MI.getOpcode() == TargetOpcode::STATEPOINT || 308 MI.getOpcode() == TargetOpcode::STACKMAP || 309 MI.getOpcode() == TargetOpcode::PATCHPOINT) 310 continue; 311 312 // For now, allocate the base register(s) within the basic block 313 // where they're used, and don't try to keep them around outside 314 // of that. It may be beneficial to try sharing them more broadly 315 // than that, but the increased register pressure makes that a 316 // tricky thing to balance. Investigate if re-materializing these 317 // becomes an issue. 318 for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) { 319 // Consider replacing all frame index operands that reference 320 // an object allocated in the local block. 321 if (MI.getOperand(i).isFI()) { 322 // Don't try this with values not in the local block. 323 if (!MFI.isObjectPreAllocated(MI.getOperand(i).getIndex())) 324 break; 325 int Idx = MI.getOperand(i).getIndex(); 326 int64_t LocalOffset = LocalOffsets[Idx]; 327 if (!TRI->needsFrameBaseReg(&MI, LocalOffset)) 328 break; 329 FrameReferenceInsns.push_back(FrameRef(&MI, LocalOffset, Idx, Order++)); 330 break; 331 } 332 } 333 } 334 } 335 336 // Sort the frame references by local offset. 337 // Use frame index as a tie-breaker in case MI's have the same offset. 338 llvm::sort(FrameReferenceInsns); 339 340 MachineBasicBlock *Entry = &Fn.front(); 341 342 unsigned BaseReg = 0; 343 int64_t BaseOffset = 0; 344 345 // Loop through the frame references and allocate for them as necessary. 346 for (int ref = 0, e = FrameReferenceInsns.size(); ref < e ; ++ref) { 347 FrameRef &FR = FrameReferenceInsns[ref]; 348 MachineInstr &MI = *FR.getMachineInstr(); 349 int64_t LocalOffset = FR.getLocalOffset(); 350 int FrameIdx = FR.getFrameIndex(); 351 assert(MFI.isObjectPreAllocated(FrameIdx) && 352 "Only pre-allocated locals expected!"); 353 354 // We need to keep the references to the stack protector slot through frame 355 // index operands so that it gets resolved by PEI rather than this pass. 356 // This avoids accesses to the stack protector though virtual base 357 // registers, and forces PEI to address it using fp/sp/bp. 358 if (MFI.hasStackProtectorIndex() && 359 FrameIdx == MFI.getStackProtectorIndex()) 360 continue; 361 362 LLVM_DEBUG(dbgs() << "Considering: " << MI); 363 364 unsigned idx = 0; 365 for (unsigned f = MI.getNumOperands(); idx != f; ++idx) { 366 if (!MI.getOperand(idx).isFI()) 367 continue; 368 369 if (FrameIdx == MI.getOperand(idx).getIndex()) 370 break; 371 } 372 373 assert(idx < MI.getNumOperands() && "Cannot find FI operand"); 374 375 int64_t Offset = 0; 376 int64_t FrameSizeAdjust = StackGrowsDown ? MFI.getLocalFrameSize() : 0; 377 378 LLVM_DEBUG(dbgs() << " Replacing FI in: " << MI); 379 380 // If we have a suitable base register available, use it; otherwise 381 // create a new one. Note that any offset encoded in the 382 // instruction itself will be taken into account by the target, 383 // so we don't have to adjust for it here when reusing a base 384 // register. 385 if (UsedBaseReg && 386 lookupCandidateBaseReg(BaseReg, BaseOffset, FrameSizeAdjust, 387 LocalOffset, MI, TRI)) { 388 LLVM_DEBUG(dbgs() << " Reusing base register " << BaseReg << "\n"); 389 // We found a register to reuse. 390 Offset = FrameSizeAdjust + LocalOffset - BaseOffset; 391 } else { 392 // No previously defined register was in range, so create a new one. 393 int64_t InstrOffset = TRI->getFrameIndexInstrOffset(&MI, idx); 394 395 int64_t PrevBaseOffset = BaseOffset; 396 BaseOffset = FrameSizeAdjust + LocalOffset + InstrOffset; 397 398 // We'd like to avoid creating single-use virtual base registers. 399 // Because the FrameRefs are in sorted order, and we've already 400 // processed all FrameRefs before this one, just check whether or not 401 // the next FrameRef will be able to reuse this new register. If not, 402 // then don't bother creating it. 403 if (ref + 1 >= e || 404 !lookupCandidateBaseReg( 405 BaseReg, BaseOffset, FrameSizeAdjust, 406 FrameReferenceInsns[ref + 1].getLocalOffset(), 407 *FrameReferenceInsns[ref + 1].getMachineInstr(), TRI)) { 408 BaseOffset = PrevBaseOffset; 409 continue; 410 } 411 412 const MachineFunction *MF = MI.getMF(); 413 const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF); 414 BaseReg = Fn.getRegInfo().createVirtualRegister(RC); 415 416 LLVM_DEBUG(dbgs() << " Materializing base register " << BaseReg 417 << " at frame local offset " 418 << LocalOffset + InstrOffset << "\n"); 419 420 // Tell the target to insert the instruction to initialize 421 // the base register. 422 // MachineBasicBlock::iterator InsertionPt = Entry->begin(); 423 TRI->materializeFrameBaseRegister(Entry, BaseReg, FrameIdx, 424 InstrOffset); 425 426 // The base register already includes any offset specified 427 // by the instruction, so account for that so it doesn't get 428 // applied twice. 429 Offset = -InstrOffset; 430 431 ++NumBaseRegisters; 432 UsedBaseReg = true; 433 } 434 assert(BaseReg != 0 && "Unable to allocate virtual base register!"); 435 436 // Modify the instruction to use the new base register rather 437 // than the frame index operand. 438 TRI->resolveFrameIndex(MI, BaseReg, Offset); 439 LLVM_DEBUG(dbgs() << "Resolved: " << MI); 440 441 ++NumReplacements; 442 } 443 444 return UsedBaseReg; 445 } 446