llvm/lib/CodeGen/LocalStackSlotAllocation.cpp

   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 }