1 //===- LocalStackSlotAllocation.cpp - Pre-allocate locals to stack slots --===//
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
7 //===----------------------------------------------------------------------===//
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.
14 //===----------------------------------------------------------------------===//
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"
43 #define DEBUG_TYPE "localstackalloc"
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");
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
56 // Order reference instruction appears in program. Used to ensure
57 // deterministic order when multiple instructions may reference the same
62 FrameRef(MachineInstr *I, int64_t Offset, int Idx, unsigned Ord) :
63 MI(I), LocalOffset(Offset), FrameIdx(Idx), Order(Ord) {}
65 bool operator<(const FrameRef &RHS) const {
66 return std::tie(LocalOffset, FrameIdx, Order) <
67 std::tie(RHS.LocalOffset, RHS.FrameIdx, RHS.Order);
70 MachineBasicBlock::iterator getMachineInstr() const { return MI; }
71 int64_t getLocalOffset() const { return LocalOffset; }
72 int getFrameIndex() const { return FrameIdx; }
75 class LocalStackSlotPass: public MachineFunctionPass {
76 SmallVector<int64_t, 16> LocalOffsets;
78 /// StackObjSet - A set of stack object indexes
79 using StackObjSet = SmallSetVector<int, 8>;
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);
91 static char ID; // Pass identification, replacement for typeid
93 explicit LocalStackSlotPass() : MachineFunctionPass(ID) {
94 initializeLocalStackSlotPassPass(*PassRegistry::getPassRegistry());
97 bool runOnMachineFunction(MachineFunction &MF) override;
99 void getAnalysisUsage(AnalysisUsage &AU) const override {
100 AU.setPreservesCFG();
101 MachineFunctionPass::getAnalysisUsage(AU);
105 } // end anonymous namespace
107 char LocalStackSlotPass::ID = 0;
109 char &llvm::LocalStackSlotAllocationID = LocalStackSlotPass::ID;
110 INITIALIZE_PASS(LocalStackSlotPass, DEBUG_TYPE,
111 "Local Stack Slot Allocation", false, false)
113 bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
114 MachineFrameInfo &MFI = MF.getFrameInfo();
115 const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
116 unsigned LocalObjectCount = MFI.getObjectIndexEnd();
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)
123 // Make sure we have enough space to store the local offsets.
124 LocalOffsets.resize(MFI.getObjectIndexEnd());
126 // Lay out the local blob.
127 calculateFrameObjectOffsets(MF);
129 // Insert virtual base registers to resolve frame index references.
130 bool UsedBaseRegs = insertFrameReferenceRegisters(MF);
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);
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,
146 // If the stack grows down, add the object size to find the lowest address.
148 Offset += MFI.getObjectSize(FrameIdx);
150 Align Alignment = MFI.getObjectAlign(FrameIdx);
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);
156 // Adjust to alignment boundary.
157 Offset = alignTo(Offset, Alignment);
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);
168 Offset += MFI.getObjectSize(FrameIdx);
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,
179 for (StackObjSet::const_iterator I = UnassignedObjs.begin(),
180 E = UnassignedObjs.end(); I != E; ++I) {
182 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
183 ProtectedObjs.insert(i);
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;
198 // Make sure that the stack protector comes before the local variables on the
200 SmallSet<int, 16> ProtectedObjs;
201 if (MFI.hasStackProtectorIndex()) {
202 int StackProtectorFI = MFI.getStackProtectorIndex();
204 // We need to make sure we didn't pre-allocate the stack protector when
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");
211 StackObjSet LargeArrayObjs;
212 StackObjSet SmallArrayObjs;
213 StackObjSet AddrOfObjs;
215 AdjustStackOffset(MFI, StackProtectorFI, Offset, StackGrowsDown, MaxAlign);
217 // Assign large stack objects first.
218 for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
219 if (MFI.isDeadObjectIndex(i))
221 if (StackProtectorFI == (int)i)
223 if (!TFI.isStackIdSafeForLocalArea(MFI.getStackID(i)))
226 switch (MFI.getObjectSSPLayout(i)) {
227 case MachineFrameInfo::SSPLK_None:
229 case MachineFrameInfo::SSPLK_SmallArray:
230 SmallArrayObjs.insert(i);
232 case MachineFrameInfo::SSPLK_AddrOf:
233 AddrOfObjs.insert(i);
235 case MachineFrameInfo::SSPLK_LargeArray:
236 LargeArrayObjs.insert(i);
239 llvm_unreachable("Unexpected SSPLayoutKind.");
242 AssignProtectedObjSet(LargeArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
244 AssignProtectedObjSet(SmallArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
246 AssignProtectedObjSet(AddrOfObjs, ProtectedObjs, MFI, StackGrowsDown,
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))
255 if (MFI.getStackProtectorIndex() == (int)i)
257 if (ProtectedObjs.count(i))
259 if (!TFI.isStackIdSafeForLocalArea(MFI.getStackID(i)))
262 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
265 // Remember how big this blob of stack space is
266 MFI.setLocalFrameSize(Offset);
267 MFI.setLocalFrameMaxAlign(MaxAlign);
271 lookupCandidateBaseReg(unsigned BaseReg,
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);
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;
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;
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;
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)
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
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()))
328 int Idx = MI.getOperand(i).getIndex();
329 int64_t LocalOffset = LocalOffsets[Idx];
330 if (!TRI->needsFrameBaseReg(&MI, LocalOffset))
332 FrameReferenceInsns.push_back(FrameRef(&MI, LocalOffset, Idx, Order++));
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);
343 MachineBasicBlock *Entry = &Fn.front();
345 unsigned BaseReg = 0;
346 int64_t BaseOffset = 0;
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!");
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())
365 LLVM_DEBUG(dbgs() << "Considering: " << MI);
368 for (unsigned f = MI.getNumOperands(); idx != f; ++idx) {
369 if (!MI.getOperand(idx).isFI())
372 if (FrameIdx == MI.getOperand(idx).getIndex())
376 assert(idx < MI.getNumOperands() && "Cannot find FI operand");
379 int64_t FrameSizeAdjust = StackGrowsDown ? MFI.getLocalFrameSize() : 0;
381 LLVM_DEBUG(dbgs() << " Replacing FI in: " << MI);
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
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;
395 // No previously defined register was in range, so create a new one.
396 int64_t InstrOffset = TRI->getFrameIndexInstrOffset(&MI, idx);
398 int64_t PrevBaseOffset = BaseOffset;
399 BaseOffset = FrameSizeAdjust + LocalOffset + InstrOffset;
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.
407 !lookupCandidateBaseReg(
408 BaseReg, BaseOffset, FrameSizeAdjust,
409 FrameReferenceInsns[ref + 1].getLocalOffset(),
410 *FrameReferenceInsns[ref + 1].getMachineInstr(), TRI)) {
411 BaseOffset = PrevBaseOffset;
415 const MachineFunction *MF = MI.getMF();
416 const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF);
417 BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
419 LLVM_DEBUG(dbgs() << " Materializing base register " << BaseReg
420 << " at frame local offset "
421 << LocalOffset + InstrOffset << "\n");
423 // Tell the target to insert the instruction to initialize
424 // the base register.
425 // MachineBasicBlock::iterator InsertionPt = Entry->begin();
426 TRI->materializeFrameBaseRegister(Entry, BaseReg, FrameIdx,
429 // The base register already includes any offset specified
430 // by the instruction, so account for that so it doesn't get
432 Offset = -InstrOffset;
437 assert(BaseReg != 0 && "Unable to allocate virtual base register!");
439 // Modify the instruction to use the new base register rather
440 // than the frame index operand.
441 TRI->resolveFrameIndex(MI, BaseReg, Offset);
442 LLVM_DEBUG(dbgs() << "Resolved: " << MI);