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 the are likely to be out of range of stack pointer and frame
13 // pointer relative addressing.
15 //===----------------------------------------------------------------------===//
17 #define DEBUG_TYPE "localstackalloc"
18 #include "llvm/Constants.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Instructions.h"
21 #include "llvm/Intrinsics.h"
22 #include "llvm/LLVMContext.h"
23 #include "llvm/Module.h"
24 #include "llvm/Pass.h"
25 #include "llvm/ADT/SmallSet.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/CodeGen/MachineFrameInfo.h"
28 #include "llvm/CodeGen/MachineFunction.h"
29 #include "llvm/CodeGen/MachineFunctionPass.h"
30 #include "llvm/CodeGen/MachineRegisterInfo.h"
31 #include "llvm/CodeGen/Passes.h"
32 #include "llvm/Support/Debug.h"
33 #include "llvm/Support/ErrorHandling.h"
34 #include "llvm/Support/raw_ostream.h"
35 #include "llvm/Target/TargetRegisterInfo.h"
36 #include "llvm/Target/TargetFrameInfo.h"
40 STATISTIC(NumAllocations, "Number of frame indices allocated into local block");
41 STATISTIC(NumBaseRegisters, "Number of virtual frame base registers allocated");
42 STATISTIC(NumReplacements, "Number of frame indices references replaced");
45 class LocalStackSlotPass: public MachineFunctionPass {
46 SmallVector<int64_t,16> LocalOffsets;
48 void AdjustStackOffset(MachineFrameInfo *MFI, int FrameIdx, int64_t &Offset,
49 bool StackGrowsDown, unsigned &MaxAlign);
50 void calculateFrameObjectOffsets(MachineFunction &Fn);
51 bool insertFrameReferenceRegisters(MachineFunction &Fn);
53 static char ID; // Pass identification, replacement for typeid
54 explicit LocalStackSlotPass() : MachineFunctionPass(ID) { }
55 bool runOnMachineFunction(MachineFunction &MF);
57 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
59 MachineFunctionPass::getAnalysisUsage(AU);
61 const char *getPassName() const {
62 return "Local Stack Slot Allocation";
67 } // end anonymous namespace
69 char LocalStackSlotPass::ID = 0;
71 FunctionPass *llvm::createLocalStackSlotAllocationPass() {
72 return new LocalStackSlotPass();
75 bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
76 MachineFrameInfo *MFI = MF.getFrameInfo();
77 const TargetRegisterInfo *TRI = MF.getTarget().getRegisterInfo();
78 unsigned LocalObjectCount = MFI->getObjectIndexEnd();
80 // If the target doesn't want/need this pass, or if there are no locals
81 // to consider, early exit.
82 if (!TRI->requiresVirtualBaseRegisters(MF) || LocalObjectCount == 0)
85 // Make sure we have enough space to store the local offsets.
86 LocalOffsets.resize(MFI->getObjectIndexEnd());
88 // Lay out the local blob.
89 calculateFrameObjectOffsets(MF);
91 // Insert virtual base registers to resolve frame index references.
92 bool UsedBaseRegs = insertFrameReferenceRegisters(MF);
94 // Tell MFI whether any base registers were allocated. PEI will only
95 // want to use the local block allocations from this pass if there were any.
96 // Otherwise, PEI can do a bit better job of getting the alignment right
97 // without a hole at the start since it knows the alignment of the stack
98 // at the start of local allocation, and this pass doesn't.
99 MFI->setUseLocalStackAllocationBlock(UsedBaseRegs);
104 /// AdjustStackOffset - Helper function used to adjust the stack frame offset.
105 void LocalStackSlotPass::AdjustStackOffset(MachineFrameInfo *MFI,
106 int FrameIdx, int64_t &Offset,
108 unsigned &MaxAlign) {
109 // If the stack grows down, add the object size to find the lowest address.
111 Offset += MFI->getObjectSize(FrameIdx);
113 unsigned Align = MFI->getObjectAlignment(FrameIdx);
115 // If the alignment of this object is greater than that of the stack, then
116 // increase the stack alignment to match.
117 MaxAlign = std::max(MaxAlign, Align);
119 // Adjust to alignment boundary.
120 Offset = (Offset + Align - 1) / Align * Align;
122 int64_t LocalOffset = StackGrowsDown ? -Offset : Offset;
123 DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset "
124 << LocalOffset << "\n");
125 // Keep the offset available for base register allocation
126 LocalOffsets[FrameIdx] = LocalOffset;
127 // And tell MFI about it for PEI to use later
128 MFI->mapLocalFrameObject(FrameIdx, LocalOffset);
131 Offset += MFI->getObjectSize(FrameIdx);
136 /// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
137 /// abstract stack objects.
139 void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
140 // Loop over all of the stack objects, assigning sequential addresses...
141 MachineFrameInfo *MFI = Fn.getFrameInfo();
142 const TargetFrameInfo &TFI = *Fn.getTarget().getFrameInfo();
143 bool StackGrowsDown =
144 TFI.getStackGrowthDirection() == TargetFrameInfo::StackGrowsDown;
146 unsigned MaxAlign = 0;
148 // Make sure that the stack protector comes before the local variables on the
150 SmallSet<int, 16> LargeStackObjs;
151 if (MFI->getStackProtectorIndex() >= 0) {
152 AdjustStackOffset(MFI, MFI->getStackProtectorIndex(), Offset,
153 StackGrowsDown, MaxAlign);
155 // Assign large stack objects first.
156 for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
157 if (MFI->isDeadObjectIndex(i))
159 if (MFI->getStackProtectorIndex() == (int)i)
161 if (!MFI->MayNeedStackProtector(i))
164 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
165 LargeStackObjs.insert(i);
169 // Then assign frame offsets to stack objects that are not used to spill
170 // callee saved registers.
171 for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
172 if (MFI->isDeadObjectIndex(i))
174 if (MFI->getStackProtectorIndex() == (int)i)
176 if (LargeStackObjs.count(i))
179 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
182 // Remember how big this blob of stack space is
183 MFI->setLocalFrameSize(Offset);
184 MFI->setLocalFrameMaxAlign(MaxAlign);
188 lookupCandidateBaseReg(const SmallVector<std::pair<unsigned, int64_t>, 8> &Regs,
189 std::pair<unsigned, int64_t> &RegOffset,
190 int64_t FrameSizeAdjust,
191 int64_t LocalFrameOffset,
192 const MachineInstr *MI,
193 const TargetRegisterInfo *TRI) {
194 unsigned e = Regs.size();
195 for (unsigned i = 0; i < e; ++i) {
197 // Check if the relative offset from the where the base register references
198 // to the target address is in range for the instruction.
199 int64_t Offset = FrameSizeAdjust + LocalFrameOffset - RegOffset.second;
200 if (TRI->isFrameOffsetLegal(MI, Offset))
206 bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
207 // Scan the function's instructions looking for frame index references.
208 // For each, ask the target if it wants a virtual base register for it
209 // based on what we can tell it about where the local will end up in the
210 // stack frame. If it wants one, re-use a suitable one we've previously
211 // allocated, or if there isn't one that fits the bill, allocate a new one
212 // and ask the target to create a defining instruction for it.
213 bool UsedBaseReg = false;
215 MachineFrameInfo *MFI = Fn.getFrameInfo();
216 const TargetRegisterInfo *TRI = Fn.getTarget().getRegisterInfo();
217 const TargetFrameInfo &TFI = *Fn.getTarget().getFrameInfo();
218 bool StackGrowsDown =
219 TFI.getStackGrowthDirection() == TargetFrameInfo::StackGrowsDown;
221 for (MachineFunction::iterator BB = Fn.begin(),
222 E = Fn.end(); BB != E; ++BB) {
223 // A base register definition is a register+offset pair.
224 SmallVector<std::pair<unsigned, int64_t>, 8> BaseRegisters;
226 for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
227 MachineInstr *MI = I;
228 // Debug value instructions can't be out of range, so they don't need
230 // FIXME: When we extend this stuff to handle functions with both
231 // VLAs and dynamic realignment, we should update the debug values
232 // to reference the new base pointer when possible.
233 if (MI->isDebugValue())
236 // For now, allocate the base register(s) within the basic block
237 // where they're used, and don't try to keep them around outside
238 // of that. It may be beneficial to try sharing them more broadly
239 // than that, but the increased register pressure makes that a
240 // tricky thing to balance. Investigate if re-materializing these
242 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
243 // Consider replacing all frame index operands that reference
244 // an object allocated in the local block.
245 if (MI->getOperand(i).isFI()) {
246 int FrameIdx = MI->getOperand(i).getIndex();
248 // Don't try this with values not in the local block.
249 if (!MFI->isObjectPreAllocated(FrameIdx))
252 DEBUG(dbgs() << "Considering: " << *MI);
253 if (TRI->needsFrameBaseReg(MI, LocalOffsets[FrameIdx])) {
254 unsigned BaseReg = 0;
256 int64_t FrameSizeAdjust = StackGrowsDown ? MFI->getLocalFrameSize()
259 DEBUG(dbgs() << " Replacing FI in: " << *MI);
261 // If we have a suitable base register available, use it; otherwise
262 // create a new one. Note that any offset encoded in the
263 // instruction itself will be taken into account by the target,
264 // so we don't have to adjust for it here when reusing a base
266 std::pair<unsigned, int64_t> RegOffset;
267 if (lookupCandidateBaseReg(BaseRegisters, RegOffset,
269 LocalOffsets[FrameIdx],
271 DEBUG(dbgs() << " Reusing base register " <<
272 RegOffset.first << "\n");
273 // We found a register to reuse.
274 BaseReg = RegOffset.first;
275 Offset = FrameSizeAdjust + LocalOffsets[FrameIdx] -
278 // No previously defined register was in range, so create a
280 int64_t InstrOffset = TRI->getFrameIndexInstrOffset(MI, i);
281 const TargetRegisterClass *RC = TRI->getPointerRegClass();
282 BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
284 DEBUG(dbgs() << " Materializing base register " << BaseReg <<
285 " at frame local offset " <<
286 LocalOffsets[FrameIdx] + InstrOffset << "\n");
287 // Tell the target to insert the instruction to initialize
288 // the base register.
289 TRI->materializeFrameBaseRegister(I, BaseReg, FrameIdx,
292 // The base register already includes any offset specified
293 // by the instruction, so account for that so it doesn't get
295 Offset = -InstrOffset;
297 int64_t BaseOffset = FrameSizeAdjust + LocalOffsets[FrameIdx] +
299 BaseRegisters.push_back(
300 std::pair<unsigned, int64_t>(BaseReg, BaseOffset));
304 assert(BaseReg != 0 && "Unable to allocate virtual base register!");
306 // Modify the instruction to use the new base register rather
307 // than the frame index operand.
308 TRI->resolveFrameIndex(I, BaseReg, Offset);
309 DEBUG(dbgs() << "Resolved: " << *MI);