1//===- LocalStackSlotAllocation.cpp - Pre-allocate locals to stack slots --===//
2//
3// The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
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.
14//
15//===----------------------------------------------------------------------===//
16
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/ADT/STLExtras.h"
28#include "llvm/CodeGen/MachineFrameInfo.h"
29#include "llvm/CodeGen/MachineFunction.h"
30#include "llvm/CodeGen/MachineFunctionPass.h"
31#include "llvm/CodeGen/MachineRegisterInfo.h"
32#include "llvm/CodeGen/Passes.h"
33#include "llvm/Support/Debug.h"
34#include "llvm/Support/ErrorHandling.h"
35#include "llvm/Support/raw_ostream.h"
36#include "llvm/Target/TargetRegisterInfo.h"
37#include "llvm/Target/TargetFrameLowering.h"
38
39using namespace llvm;
40
41STATISTIC(NumAllocations, "Number of frame indices allocated into local block");
42STATISTIC(NumBaseRegisters, "Number of virtual frame base registers allocated");
43STATISTIC(NumReplacements, "Number of frame indices references replaced");
44
45namespace {
46 class FrameRef {
47 MachineBasicBlock::iterator MI; // Instr referencing the frame
48 int64_t LocalOffset; // Local offset of the frame idx referenced
49 public:
50 FrameRef(MachineBasicBlock::iterator I, int64_t Offset) :
51 MI(I), LocalOffset(Offset) {}
52 bool operator<(const FrameRef &RHS) const {
53 return LocalOffset < RHS.LocalOffset;
54 }
55 MachineBasicBlock::iterator getMachineInstr() { return MI; }
56 };
57
58 class LocalStackSlotPass: public MachineFunctionPass {
59 SmallVector<int64_t,16> LocalOffsets;
60
61 void AdjustStackOffset(MachineFrameInfo *MFI, int FrameIdx, int64_t &Offset,
62 bool StackGrowsDown, unsigned &MaxAlign);
63 void calculateFrameObjectOffsets(MachineFunction &Fn);
64 bool insertFrameReferenceRegisters(MachineFunction &Fn);
65 public:
66 static char ID; // Pass identification, replacement for typeid
67 explicit LocalStackSlotPass() : MachineFunctionPass(ID) { }
68 bool runOnMachineFunction(MachineFunction &MF);
69
70 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
71 AU.setPreservesCFG();
72 MachineFunctionPass::getAnalysisUsage(AU);
73 }
74
75 private:
76 };
77} // end anonymous namespace
78
79char LocalStackSlotPass::ID = 0;
80char &llvm::LocalStackSlotAllocationID = LocalStackSlotPass::ID;
81INITIALIZE_PASS(LocalStackSlotPass, "localstackalloc",
82 "Local Stack Slot Allocation", false, false)
83
84bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
85 MachineFrameInfo *MFI = MF.getFrameInfo();
86 const TargetRegisterInfo *TRI = MF.getTarget().getRegisterInfo();
87 unsigned LocalObjectCount = MFI->getObjectIndexEnd();
88
89 // If the target doesn't want/need this pass, or if there are no locals
90 // to consider, early exit.
91 if (!TRI->requiresVirtualBaseRegisters(MF) || LocalObjectCount == 0)
92 return true;
93
94 // Make sure we have enough space to store the local offsets.
95 LocalOffsets.resize(MFI->getObjectIndexEnd());
96
97 // Lay out the local blob.
98 calculateFrameObjectOffsets(MF);
99
100 // Insert virtual base registers to resolve frame index references.
101 bool UsedBaseRegs = insertFrameReferenceRegisters(MF);
102
103 // Tell MFI whether any base registers were allocated. PEI will only
104 // want to use the local block allocations from this pass if there were any.
105 // Otherwise, PEI can do a bit better job of getting the alignment right
106 // without a hole at the start since it knows the alignment of the stack
107 // at the start of local allocation, and this pass doesn't.
108 MFI->setUseLocalStackAllocationBlock(UsedBaseRegs);
109
110 return true;
111}
112
113/// AdjustStackOffset - Helper function used to adjust the stack frame offset.
114void LocalStackSlotPass::AdjustStackOffset(MachineFrameInfo *MFI,
115 int FrameIdx, int64_t &Offset,
116 bool StackGrowsDown,
117 unsigned &MaxAlign) {
118 // If the stack grows down, add the object size to find the lowest address.
119 if (StackGrowsDown)
120 Offset += MFI->getObjectSize(FrameIdx);
121
122 unsigned Align = MFI->getObjectAlignment(FrameIdx);
123
124 // If the alignment of this object is greater than that of the stack, then
125 // increase the stack alignment to match.
126 MaxAlign = std::max(MaxAlign, Align);
127
128 // Adjust to alignment boundary.
129 Offset = (Offset + Align - 1) / Align * Align;
130
131 int64_t LocalOffset = StackGrowsDown ? -Offset : Offset;
132 DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset "
133 << LocalOffset << "\n");
134 // Keep the offset available for base register allocation
135 LocalOffsets[FrameIdx] = LocalOffset;
136 // And tell MFI about it for PEI to use later
137 MFI->mapLocalFrameObject(FrameIdx, LocalOffset);
138
139 if (!StackGrowsDown)
140 Offset += MFI->getObjectSize(FrameIdx);
141
142 ++NumAllocations;
143}
144
145/// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
146/// abstract stack objects.
147///
148void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
149 // Loop over all of the stack objects, assigning sequential addresses...
150 MachineFrameInfo *MFI = Fn.getFrameInfo();
151 const TargetFrameLowering &TFI = *Fn.getTarget().getFrameLowering();
152 bool StackGrowsDown =
153 TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
154 int64_t Offset = 0;
155 unsigned MaxAlign = 0;
156
157 // Make sure that the stack protector comes before the local variables on the
158 // stack.
159 SmallSet<int, 16> LargeStackObjs;
160 if (MFI->getStackProtectorIndex() >= 0) {
161 AdjustStackOffset(MFI, MFI->getStackProtectorIndex(), Offset,
162 StackGrowsDown, MaxAlign);
163
164 // Assign large stack objects first.
165 for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
166 if (MFI->isDeadObjectIndex(i))
167 continue;
168 if (MFI->getStackProtectorIndex() == (int)i)
169 continue;
170 if (!MFI->MayNeedStackProtector(i))
171 continue;
172
173 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
174 LargeStackObjs.insert(i);
175 }
176 }
177
178 // Then assign frame offsets to stack objects that are not used to spill
179 // callee saved registers.
180 for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
181 if (MFI->isDeadObjectIndex(i))
182 continue;
183 if (MFI->getStackProtectorIndex() == (int)i)
184 continue;
185 if (LargeStackObjs.count(i))
186 continue;
187
188 AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
189 }
190
191 // Remember how big this blob of stack space is
192 MFI->setLocalFrameSize(Offset);
193 MFI->setLocalFrameMaxAlign(MaxAlign);
194}
195
196static inline bool
197lookupCandidateBaseReg(const SmallVector<std::pair<unsigned, int64_t>, 8> &Regs,
198 std::pair<unsigned, int64_t> &RegOffset,
199 int64_t FrameSizeAdjust,
200 int64_t LocalFrameOffset,
201 const MachineInstr *MI,
202 const TargetRegisterInfo *TRI) {
203 unsigned e = Regs.size();
204 for (unsigned i = 0; i < e; ++i) {
205 RegOffset = Regs[i];
206 // Check if the relative offset from the where the base register references
207 // to the target address is in range for the instruction.
208 int64_t Offset = FrameSizeAdjust + LocalFrameOffset - RegOffset.second;
209 if (TRI->isFrameOffsetLegal(MI, Offset))
210 return true;
211 }
212 return false;
213}
214
215bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
216 // Scan the function's instructions looking for frame index references.
217 // For each, ask the target if it wants a virtual base register for it
218 // based on what we can tell it about where the local will end up in the
219 // stack frame. If it wants one, re-use a suitable one we've previously
220 // allocated, or if there isn't one that fits the bill, allocate a new one
221 // and ask the target to create a defining instruction for it.
222 bool UsedBaseReg = false;
223
224 MachineFrameInfo *MFI = Fn.getFrameInfo();
225 const TargetRegisterInfo *TRI = Fn.getTarget().getRegisterInfo();
226 const TargetFrameLowering &TFI = *Fn.getTarget().getFrameLowering();
227 bool StackGrowsDown =
228 TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
229
230 // Collect all of the instructions in the block that reference
231 // a frame index. Also store the frame index referenced to ease later
232 // lookup. (For any insn that has more than one FI reference, we arbitrarily
233 // choose the first one).
234 SmallVector<FrameRef, 64> FrameReferenceInsns;
235
236 // A base register definition is a register + offset pair.
237 SmallVector<std::pair<unsigned, int64_t>, 8> BaseRegisters;
238
239 for (MachineFunction::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
240 for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
241 MachineInstr *MI = I;
242
243 // Debug value instructions can't be out of range, so they don't need
244 // any updates.
245 if (MI->isDebugValue())
246 continue;
247
248 // For now, allocate the base register(s) within the basic block
249 // where they're used, and don't try to keep them around outside
250 // of that. It may be beneficial to try sharing them more broadly
251 // than that, but the increased register pressure makes that a
252 // tricky thing to balance. Investigate if re-materializing these
253 // becomes an issue.
254 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
255 // Consider replacing all frame index operands that reference
256 // an object allocated in the local block.
257 if (MI->getOperand(i).isFI()) {
258 // Don't try this with values not in the local block.
259 if (!MFI->isObjectPreAllocated(MI->getOperand(i).getIndex()))
260 break;
261 FrameReferenceInsns.
262 push_back(FrameRef(MI, LocalOffsets[MI->getOperand(i).getIndex()]));
263 break;
264 }
265 }
266 }
267 }
268
269 // Sort the frame references by local offset
270 array_pod_sort(FrameReferenceInsns.begin(), FrameReferenceInsns.end());
271
272 MachineBasicBlock *Entry = Fn.begin();
273
274 // Loop through the frame references and allocate for them as necessary.
275 for (int ref = 0, e = FrameReferenceInsns.size(); ref < e ; ++ref) {
276 MachineBasicBlock::iterator I =
277 FrameReferenceInsns[ref].getMachineInstr();
278 MachineInstr *MI = I;
279 for (unsigned idx = 0, e = MI->getNumOperands(); idx != e; ++idx) {
280 // Consider replacing all frame index operands that reference
281 // an object allocated in the local block.
282 if (MI->getOperand(idx).isFI()) {
283 int FrameIdx = MI->getOperand(idx).getIndex();
284
285 assert(MFI->isObjectPreAllocated(FrameIdx) &&
286 "Only pre-allocated locals expected!");
287
288 DEBUG(dbgs() << "Considering: " << *MI);
289 if (TRI->needsFrameBaseReg(MI, LocalOffsets[FrameIdx])) {
290 unsigned BaseReg = 0;
291 int64_t Offset = 0;
292 int64_t FrameSizeAdjust =
293 StackGrowsDown ? MFI->getLocalFrameSize() : 0;
294
295 DEBUG(dbgs() << " Replacing FI in: " << *MI);
296
297 // If we have a suitable base register available, use it; otherwise
298 // create a new one. Note that any offset encoded in the
299 // instruction itself will be taken into account by the target,
300 // so we don't have to adjust for it here when reusing a base
301 // register.
302 std::pair<unsigned, int64_t> RegOffset;
303 if (lookupCandidateBaseReg(BaseRegisters, RegOffset,
304 FrameSizeAdjust,
305 LocalOffsets[FrameIdx],
306 MI, TRI)) {
307 DEBUG(dbgs() << " Reusing base register " <<
308 RegOffset.first << "\n");
309 // We found a register to reuse.
310 BaseReg = RegOffset.first;
311 Offset = FrameSizeAdjust + LocalOffsets[FrameIdx] -
312 RegOffset.second;
313 } else {
314 // No previously defined register was in range, so create a
315 // new one.
316 int64_t InstrOffset = TRI->getFrameIndexInstrOffset(MI, idx);
317 const MachineFunction *MF = MI->getParent()->getParent();
318 const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF);
319 BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
320
321 DEBUG(dbgs() << " Materializing base register " << BaseReg <<
322 " at frame local offset " <<
323 LocalOffsets[FrameIdx] + InstrOffset << "\n");
324
325 // Tell the target to insert the instruction to initialize
326 // the base register.
327 // MachineBasicBlock::iterator InsertionPt = Entry->begin();
328 TRI->materializeFrameBaseRegister(Entry, BaseReg, FrameIdx,
329 InstrOffset);
330
331 // The base register already includes any offset specified
332 // by the instruction, so account for that so it doesn't get
333 // applied twice.
334 Offset = -InstrOffset;
335
336 int64_t BaseOffset = FrameSizeAdjust + LocalOffsets[FrameIdx] +
337 InstrOffset;
338 BaseRegisters.push_back(
339 std::pair<unsigned, int64_t>(BaseReg, BaseOffset));
340 ++NumBaseRegisters;
341 UsedBaseReg = true;
342 }
343 assert(BaseReg != 0 && "Unable to allocate virtual base register!");
344
345 // Modify the instruction to use the new base register rather
346 // than the frame index operand.
347 TRI->resolveFrameIndex(I, BaseReg, Offset);
348 DEBUG(dbgs() << "Resolved: " << *MI);
349
350 ++NumReplacements;
351 }
352 }
353 }
354 }
355 return UsedBaseReg;
356}