1 //===- LocalStackSlotAllocation.cpp - Pre-allocate locals to stack slots --===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass assigns local frame indices to stack slots relative to one another
11 // and allocates additional base registers to access them when the target
12 // estimates they are likely to be out of range of stack pointer and frame
13 // pointer relative addressing.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/ADT/SetVector.h"
18 #include "llvm/ADT/SmallSet.h"
19 #include "llvm/ADT/SmallVector.h"
20 #include "llvm/ADT/Statistic.h"
21 #include "llvm/CodeGen/MachineBasicBlock.h"
22 #include "llvm/CodeGen/MachineFrameInfo.h"
23 #include "llvm/CodeGen/MachineFunction.h"
24 #include "llvm/CodeGen/MachineFunctionPass.h"
25 #include "llvm/CodeGen/MachineInstr.h"
26 #include "llvm/CodeGen/MachineOperand.h"
27 #include "llvm/CodeGen/MachineRegisterInfo.h"
28 #include "llvm/CodeGen/StackProtector.h"
29 #include "llvm/CodeGen/TargetFrameLowering.h"
30 #include "llvm/CodeGen/TargetOpcodes.h"
31 #include "llvm/CodeGen/TargetRegisterInfo.h"
32 #include "llvm/CodeGen/TargetSubtargetInfo.h"
33 #include "llvm/Pass.h"
34 #include "llvm/Support/Debug.h"
35 #include "llvm/Support/ErrorHandling.h"
36 #include "llvm/Support/raw_ostream.h"
44 #define DEBUG_TYPE "localstackalloc"
46 STATISTIC(NumAllocations, "Number of frame indices allocated into local block");
47 STATISTIC(NumBaseRegisters, "Number of virtual frame base registers allocated");
48 STATISTIC(NumReplacements, "Number of frame indices references replaced");
53 MachineBasicBlock::iterator MI; // Instr referencing the frame
54 int64_t LocalOffset; // Local offset of the frame idx referenced
55 int FrameIdx; // The frame index
57 // Order reference instruction appears in program. Used to ensure
58 // deterministic order when multiple instructions may reference the same
63 FrameRef(MachineInstr *I, int64_t Offset, int Idx, unsigned Ord) :
64 MI(I), LocalOffset(Offset), FrameIdx(Idx), Order(Ord) {}
66 bool operator<(const FrameRef &RHS) const {
67 return std::tie(LocalOffset, FrameIdx, Order) <
68 std::tie(RHS.LocalOffset, RHS.FrameIdx, RHS.Order);
71 MachineBasicBlock::iterator getMachineInstr() const { return MI; }
72 int64_t getLocalOffset() const { return LocalOffset; }
73 int getFrameIndex() const { return FrameIdx; }
76 class LocalStackSlotPass: public MachineFunctionPass {
77 SmallVector<int64_t, 16> LocalOffsets;
79 /// StackObjSet - A set of stack object indexes
80 using StackObjSet = SmallSetVector<int, 8>;
82 void AdjustStackOffset(MachineFrameInfo &MFI, int FrameIdx, int64_t &Offset,
83 bool StackGrowsDown, unsigned &MaxAlign);
84 void AssignProtectedObjSet(const StackObjSet &UnassignedObjs,
85 SmallSet<int, 16> &ProtectedObjs,
86 MachineFrameInfo &MFI, bool StackGrowsDown,
87 int64_t &Offset, unsigned &MaxAlign);
88 void calculateFrameObjectOffsets(MachineFunction &Fn);
89 bool insertFrameReferenceRegisters(MachineFunction &Fn);
92 static char ID; // Pass identification, replacement for typeid
94 explicit LocalStackSlotPass() : MachineFunctionPass(ID) {
95 initializeLocalStackSlotPassPass(*PassRegistry::getPassRegistry());
98 bool runOnMachineFunction(MachineFunction &MF) override;
100 void getAnalysisUsage(AnalysisUsage &AU) const override {
101 AU.setPreservesCFG();
102 AU.addRequired<StackProtector>();
103 MachineFunctionPass::getAnalysisUsage(AU);
107 } // end anonymous namespace
109 char LocalStackSlotPass::ID = 0;
111 char &llvm::LocalStackSlotAllocationID = LocalStackSlotPass::ID;
113 INITIALIZE_PASS_BEGIN(LocalStackSlotPass, DEBUG_TYPE,
114 "Local Stack Slot Allocation", false, false)
115 INITIALIZE_PASS_DEPENDENCY(StackProtector)
116 INITIALIZE_PASS_END(LocalStackSlotPass, DEBUG_TYPE,
117 "Local Stack Slot Allocation", false, false)
119 bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
120 MachineFrameInfo &MFI = MF.getFrameInfo();
121 const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
122 unsigned LocalObjectCount = MFI.getObjectIndexEnd();
124 // If the target doesn't want/need this pass, or if there are no locals
125 // to consider, early exit.
126 if (!TRI->requiresVirtualBaseRegisters(MF) || LocalObjectCount == 0)
129 // Make sure we have enough space to store the local offsets.
130 LocalOffsets.resize(MFI.getObjectIndexEnd());
132 // Lay out the local blob.
133 calculateFrameObjectOffsets(MF);
135 // Insert virtual base registers to resolve frame index references.
136 bool UsedBaseRegs = insertFrameReferenceRegisters(MF);
138 // Tell MFI whether any base registers were allocated. PEI will only
139 // want to use the local block allocations from this pass if there were any.
140 // Otherwise, PEI can do a bit better job of getting the alignment right
141 // without a hole at the start since it knows the alignment of the stack
142 // at the start of local allocation, and this pass doesn't.
143 MFI.setUseLocalStackAllocationBlock(UsedBaseRegs);
148 /// AdjustStackOffset - Helper function used to adjust the stack frame offset.
149 void LocalStackSlotPass::AdjustStackOffset(MachineFrameInfo &MFI,
150 int FrameIdx, int64_t &Offset,
152 unsigned &MaxAlign) {
153 // If the stack grows down, add the object size to find the lowest address.
155 Offset += MFI.getObjectSize(FrameIdx);
157 unsigned Align = MFI.getObjectAlignment(FrameIdx);
159 // If the alignment of this object is greater than that of the stack, then
160 // increase the stack alignment to match.
161 MaxAlign = std::max(MaxAlign, Align);
163 // Adjust to alignment boundary.
164 Offset = (Offset + Align - 1) / Align * Align;
166 int64_t LocalOffset = StackGrowsDown ? -Offset : Offset;
167 DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset "
168 << LocalOffset << "\n");
169 // Keep the offset available for base register allocation
170 LocalOffsets[FrameIdx] = LocalOffset;
171 // And tell MFI about it for PEI to use later
172 MFI.mapLocalFrameObject(FrameIdx, LocalOffset);
175 Offset += MFI.getObjectSize(FrameIdx);
180 /// AssignProtectedObjSet - Helper function to assign large stack objects (i.e.,
181 /// those required to be close to the Stack Protector) to stack offsets.
182 void LocalStackSlotPass::AssignProtectedObjSet(const StackObjSet &UnassignedObjs,
183 SmallSet<int, 16> &ProtectedObjs,
184 MachineFrameInfo &MFI,
185 bool StackGrowsDown, int64_t &Offset,
186 unsigned &MaxAlign) {
187 for (StackObjSet::const_iterator I = UnassignedObjs.begin(),
188 E = UnassignedObjs.end(); I != E; ++I) {
190 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
191 ProtectedObjs.insert(i);
195 /// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
196 /// abstract stack objects.
197 void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
198 // Loop over all of the stack objects, assigning sequential addresses...
199 MachineFrameInfo &MFI = Fn.getFrameInfo();
200 const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering();
201 bool StackGrowsDown =
202 TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
204 unsigned MaxAlign = 0;
205 StackProtector *SP = &getAnalysis<StackProtector>();
207 // Make sure that the stack protector comes before the local variables on the
209 SmallSet<int, 16> ProtectedObjs;
210 if (MFI.getStackProtectorIndex() >= 0) {
211 StackObjSet LargeArrayObjs;
212 StackObjSet SmallArrayObjs;
213 StackObjSet AddrOfObjs;
215 AdjustStackOffset(MFI, MFI.getStackProtectorIndex(), Offset,
216 StackGrowsDown, MaxAlign);
218 // Assign large stack objects first.
219 for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
220 if (MFI.isDeadObjectIndex(i))
222 if (MFI.getStackProtectorIndex() == (int)i)
225 switch (SP->getSSPLayout(MFI.getObjectAllocation(i))) {
226 case StackProtector::SSPLK_None:
228 case StackProtector::SSPLK_SmallArray:
229 SmallArrayObjs.insert(i);
231 case StackProtector::SSPLK_AddrOf:
232 AddrOfObjs.insert(i);
234 case StackProtector::SSPLK_LargeArray:
235 LargeArrayObjs.insert(i);
238 llvm_unreachable("Unexpected SSPLayoutKind.");
241 AssignProtectedObjSet(LargeArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
243 AssignProtectedObjSet(SmallArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
245 AssignProtectedObjSet(AddrOfObjs, ProtectedObjs, MFI, StackGrowsDown,
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))
254 if (MFI.getStackProtectorIndex() == (int)i)
256 if (ProtectedObjs.count(i))
259 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
262 // Remember how big this blob of stack space is
263 MFI.setLocalFrameSize(Offset);
264 MFI.setLocalFrameMaxAlign(MaxAlign);
268 lookupCandidateBaseReg(unsigned BaseReg,
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);
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;
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;
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;
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.isDebugValue() || MI.getOpcode() == TargetOpcode::STATEPOINT ||
308 MI.getOpcode() == TargetOpcode::STACKMAP ||
309 MI.getOpcode() == TargetOpcode::PATCHPOINT)
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
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()))
325 int Idx = MI.getOperand(i).getIndex();
326 int64_t LocalOffset = LocalOffsets[Idx];
327 if (!TRI->needsFrameBaseReg(&MI, LocalOffset))
329 FrameReferenceInsns.push_back(FrameRef(&MI, LocalOffset, Idx, Order++));
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 std::sort(FrameReferenceInsns.begin(), FrameReferenceInsns.end());
340 MachineBasicBlock *Entry = &Fn.front();
342 unsigned BaseReg = 0;
343 int64_t BaseOffset = 0;
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!");
354 DEBUG(dbgs() << "Considering: " << MI);
357 for (unsigned f = MI.getNumOperands(); idx != f; ++idx) {
358 if (!MI.getOperand(idx).isFI())
361 if (FrameIdx == MI.getOperand(idx).getIndex())
365 assert(idx < MI.getNumOperands() && "Cannot find FI operand");
368 int64_t FrameSizeAdjust = StackGrowsDown ? MFI.getLocalFrameSize() : 0;
370 DEBUG(dbgs() << " Replacing FI in: " << MI);
372 // If we have a suitable base register available, use it; otherwise
373 // create a new one. Note that any offset encoded in the
374 // instruction itself will be taken into account by the target,
375 // so we don't have to adjust for it here when reusing a base
378 lookupCandidateBaseReg(BaseReg, BaseOffset, FrameSizeAdjust,
379 LocalOffset, MI, TRI)) {
380 DEBUG(dbgs() << " Reusing base register " << BaseReg << "\n");
381 // We found a register to reuse.
382 Offset = FrameSizeAdjust + LocalOffset - BaseOffset;
384 // No previously defined register was in range, so create a new one.
385 int64_t InstrOffset = TRI->getFrameIndexInstrOffset(&MI, idx);
387 int64_t PrevBaseOffset = BaseOffset;
388 BaseOffset = FrameSizeAdjust + LocalOffset + InstrOffset;
390 // We'd like to avoid creating single-use virtual base registers.
391 // Because the FrameRefs are in sorted order, and we've already
392 // processed all FrameRefs before this one, just check whether or not
393 // the next FrameRef will be able to reuse this new register. If not,
394 // then don't bother creating it.
396 !lookupCandidateBaseReg(
397 BaseReg, BaseOffset, FrameSizeAdjust,
398 FrameReferenceInsns[ref + 1].getLocalOffset(),
399 *FrameReferenceInsns[ref + 1].getMachineInstr(), TRI)) {
400 BaseOffset = PrevBaseOffset;
404 const MachineFunction *MF = MI.getMF();
405 const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF);
406 BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
408 DEBUG(dbgs() << " Materializing base register " << BaseReg <<
409 " at frame local offset " << LocalOffset + InstrOffset << "\n");
411 // Tell the target to insert the instruction to initialize
412 // the base register.
413 // MachineBasicBlock::iterator InsertionPt = Entry->begin();
414 TRI->materializeFrameBaseRegister(Entry, BaseReg, FrameIdx,
417 // The base register already includes any offset specified
418 // by the instruction, so account for that so it doesn't get
420 Offset = -InstrOffset;
425 assert(BaseReg != 0 && "Unable to allocate virtual base register!");
427 // Modify the instruction to use the new base register rather
428 // than the frame index operand.
429 TRI->resolveFrameIndex(MI, BaseReg, Offset);
430 DEBUG(dbgs() << "Resolved: " << MI);