1 //===---- LiveRangeCalc.cpp - Calculate live ranges -----------------------===//
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 // Implementation of the LiveRangeCalc class.
11 //
12 //===----------------------------------------------------------------------===//
13
14 #define DEBUG_TYPE "regalloc"
15 #include "LiveRangeCalc.h"
16 #include "llvm/CodeGen/MachineDominators.h"
17 #include "llvm/CodeGen/MachineRegisterInfo.h"
18
19 using namespace llvm;
20
reset(const MachineFunction * MF,SlotIndexes * SI,MachineDominatorTree * MDT,VNInfo::Allocator * VNIA)21 void LiveRangeCalc::reset(const MachineFunction *MF,
22 SlotIndexes *SI,
23 MachineDominatorTree *MDT,
24 VNInfo::Allocator *VNIA) {
25 MRI = &MF->getRegInfo();
26 Indexes = SI;
27 DomTree = MDT;
28 Alloc = VNIA;
29
30 unsigned N = MF->getNumBlockIDs();
31 Seen.clear();
32 Seen.resize(N);
33 LiveOut.resize(N);
34 LiveIn.clear();
35 }
36
37
createDeadDefs(LiveInterval * LI,unsigned Reg)38 void LiveRangeCalc::createDeadDefs(LiveInterval *LI, unsigned Reg) {
39 assert(MRI && Indexes && "call reset() first");
40
41 // Visit all def operands. If the same instruction has multiple defs of Reg,
42 // LI->createDeadDef() will deduplicate.
43 for (MachineRegisterInfo::def_iterator
44 I = MRI->def_begin(Reg), E = MRI->def_end(); I != E; ++I) {
45 const MachineInstr *MI = &*I;
46 // Find the corresponding slot index.
47 SlotIndex Idx;
48 if (MI->isPHI())
49 // PHI defs begin at the basic block start index.
50 Idx = Indexes->getMBBStartIdx(MI->getParent());
51 else
52 // Instructions are either normal 'r', or early clobber 'e'.
53 Idx = Indexes->getInstructionIndex(MI)
54 .getRegSlot(I.getOperand().isEarlyClobber());
55
56 // Create the def in LI. This may find an existing def.
57 LI->createDeadDef(Idx, *Alloc);
58 }
59 }
60
61
extendToUses(LiveInterval * LI,unsigned Reg)62 void LiveRangeCalc::extendToUses(LiveInterval *LI, unsigned Reg) {
63 assert(MRI && Indexes && "call reset() first");
64
65 // Visit all operands that read Reg. This may include partial defs.
66 for (MachineRegisterInfo::reg_nodbg_iterator I = MRI->reg_nodbg_begin(Reg),
67 E = MRI->reg_nodbg_end(); I != E; ++I) {
68 MachineOperand &MO = I.getOperand();
69 // Clear all kill flags. They will be reinserted after register allocation
70 // by LiveIntervalAnalysis::addKillFlags().
71 if (MO.isUse())
72 MO.setIsKill(false);
73 if (!MO.readsReg())
74 continue;
75 // MI is reading Reg. We may have visited MI before if it happens to be
76 // reading Reg multiple times. That is OK, extend() is idempotent.
77 const MachineInstr *MI = &*I;
78
79 // Find the SlotIndex being read.
80 SlotIndex Idx;
81 if (MI->isPHI()) {
82 assert(!MO.isDef() && "Cannot handle PHI def of partial register.");
83 // PHI operands are paired: (Reg, PredMBB).
84 // Extend the live range to be live-out from PredMBB.
85 Idx = Indexes->getMBBEndIdx(MI->getOperand(I.getOperandNo()+1).getMBB());
86 } else {
87 // This is a normal instruction.
88 Idx = Indexes->getInstructionIndex(MI).getRegSlot();
89 // Check for early-clobber redefs.
90 unsigned DefIdx;
91 if (MO.isDef()) {
92 if (MO.isEarlyClobber())
93 Idx = Idx.getRegSlot(true);
94 } else if (MI->isRegTiedToDefOperand(I.getOperandNo(), &DefIdx)) {
95 // FIXME: This would be a lot easier if tied early-clobber uses also
96 // had an early-clobber flag.
97 if (MI->getOperand(DefIdx).isEarlyClobber())
98 Idx = Idx.getRegSlot(true);
99 }
100 }
101 extend(LI, Idx, Reg);
102 }
103 }
104
105
106 // Transfer information from the LiveIn vector to the live ranges.
updateLiveIns(VNInfo * OverrideVNI)107 void LiveRangeCalc::updateLiveIns(VNInfo *OverrideVNI) {
108 for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(),
109 E = LiveIn.end(); I != E; ++I) {
110 if (!I->DomNode)
111 continue;
112 MachineBasicBlock *MBB = I->DomNode->getBlock();
113
114 VNInfo *VNI = OverrideVNI ? OverrideVNI : I->Value;
115 assert(VNI && "No live-in value found");
116
117 SlotIndex Start, End;
118 tie(Start, End) = Indexes->getMBBRange(MBB);
119
120 if (I->Kill.isValid())
121 I->LI->addRange(LiveRange(Start, I->Kill, VNI));
122 else {
123 I->LI->addRange(LiveRange(Start, End, VNI));
124 // The value is live-through, update LiveOut as well. Defer the Domtree
125 // lookup until it is needed.
126 assert(Seen.test(MBB->getNumber()));
127 LiveOut[MBB] = LiveOutPair(VNI, (MachineDomTreeNode *)0);
128 }
129 }
130 LiveIn.clear();
131 }
132
133
extend(LiveInterval * LI,SlotIndex Kill,unsigned PhysReg)134 void LiveRangeCalc::extend(LiveInterval *LI,
135 SlotIndex Kill,
136 unsigned PhysReg) {
137 assert(LI && "Missing live range");
138 assert(Kill.isValid() && "Invalid SlotIndex");
139 assert(Indexes && "Missing SlotIndexes");
140 assert(DomTree && "Missing dominator tree");
141
142 MachineBasicBlock *KillMBB = Indexes->getMBBFromIndex(Kill.getPrevSlot());
143 assert(KillMBB && "No MBB at Kill");
144
145 // Is there a def in the same MBB we can extend?
146 if (LI->extendInBlock(Indexes->getMBBStartIdx(KillMBB), Kill))
147 return;
148
149 // Find the single reaching def, or determine if Kill is jointly dominated by
150 // multiple values, and we may need to create even more phi-defs to preserve
151 // VNInfo SSA form. Perform a search for all predecessor blocks where we
152 // know the dominating VNInfo.
153 VNInfo *VNI = findReachingDefs(LI, KillMBB, Kill, PhysReg);
154
155 // When there were multiple different values, we may need new PHIs.
156 if (!VNI)
157 updateSSA();
158
159 updateLiveIns(VNI);
160 }
161
162
163 // This function is called by a client after using the low-level API to add
164 // live-out and live-in blocks. The unique value optimization is not
165 // available, SplitEditor::transferValues handles that case directly anyway.
calculateValues()166 void LiveRangeCalc::calculateValues() {
167 assert(Indexes && "Missing SlotIndexes");
168 assert(DomTree && "Missing dominator tree");
169 updateSSA();
170 updateLiveIns(0);
171 }
172
173
findReachingDefs(LiveInterval * LI,MachineBasicBlock * KillMBB,SlotIndex Kill,unsigned PhysReg)174 VNInfo *LiveRangeCalc::findReachingDefs(LiveInterval *LI,
175 MachineBasicBlock *KillMBB,
176 SlotIndex Kill,
177 unsigned PhysReg) {
178 // Blocks where LI should be live-in.
179 SmallVector<MachineBasicBlock*, 16> WorkList(1, KillMBB);
180
181 // Remember if we have seen more than one value.
182 bool UniqueVNI = true;
183 VNInfo *TheVNI = 0;
184
185 // Using Seen as a visited set, perform a BFS for all reaching defs.
186 for (unsigned i = 0; i != WorkList.size(); ++i) {
187 MachineBasicBlock *MBB = WorkList[i];
188
189 #ifndef NDEBUG
190 if (MBB->pred_empty()) {
191 MBB->getParent()->verify();
192 llvm_unreachable("Use not jointly dominated by defs.");
193 }
194
195 if (TargetRegisterInfo::isPhysicalRegister(PhysReg) &&
196 !MBB->isLiveIn(PhysReg)) {
197 MBB->getParent()->verify();
198 errs() << "The register needs to be live in to BB#" << MBB->getNumber()
199 << ", but is missing from the live-in list.\n";
200 llvm_unreachable("Invalid global physical register");
201 }
202 #endif
203
204 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
205 PE = MBB->pred_end(); PI != PE; ++PI) {
206 MachineBasicBlock *Pred = *PI;
207
208 // Is this a known live-out block?
209 if (Seen.test(Pred->getNumber())) {
210 if (VNInfo *VNI = LiveOut[Pred].first) {
211 if (TheVNI && TheVNI != VNI)
212 UniqueVNI = false;
213 TheVNI = VNI;
214 }
215 continue;
216 }
217
218 SlotIndex Start, End;
219 tie(Start, End) = Indexes->getMBBRange(Pred);
220
221 // First time we see Pred. Try to determine the live-out value, but set
222 // it as null if Pred is live-through with an unknown value.
223 VNInfo *VNI = LI->extendInBlock(Start, End);
224 setLiveOutValue(Pred, VNI);
225 if (VNI) {
226 if (TheVNI && TheVNI != VNI)
227 UniqueVNI = false;
228 TheVNI = VNI;
229 continue;
230 }
231
232 // No, we need a live-in value for Pred as well
233 if (Pred != KillMBB)
234 WorkList.push_back(Pred);
235 else
236 // Loopback to KillMBB, so value is really live through.
237 Kill = SlotIndex();
238 }
239 }
240
241 // Transfer WorkList to LiveInBlocks in reverse order.
242 // This ordering works best with updateSSA().
243 LiveIn.clear();
244 LiveIn.reserve(WorkList.size());
245 while(!WorkList.empty())
246 addLiveInBlock(LI, DomTree->getNode(WorkList.pop_back_val()));
247
248 // The kill block may not be live-through.
249 assert(LiveIn.back().DomNode->getBlock() == KillMBB);
250 LiveIn.back().Kill = Kill;
251
252 return UniqueVNI ? TheVNI : 0;
253 }
254
255
256 // This is essentially the same iterative algorithm that SSAUpdater uses,
257 // except we already have a dominator tree, so we don't have to recompute it.
updateSSA()258 void LiveRangeCalc::updateSSA() {
259 assert(Indexes && "Missing SlotIndexes");
260 assert(DomTree && "Missing dominator tree");
261
262 // Interate until convergence.
263 unsigned Changes;
264 do {
265 Changes = 0;
266 // Propagate live-out values down the dominator tree, inserting phi-defs
267 // when necessary.
268 for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(),
269 E = LiveIn.end(); I != E; ++I) {
270 MachineDomTreeNode *Node = I->DomNode;
271 // Skip block if the live-in value has already been determined.
272 if (!Node)
273 continue;
274 MachineBasicBlock *MBB = Node->getBlock();
275 MachineDomTreeNode *IDom = Node->getIDom();
276 LiveOutPair IDomValue;
277
278 // We need a live-in value to a block with no immediate dominator?
279 // This is probably an unreachable block that has survived somehow.
280 bool needPHI = !IDom || !Seen.test(IDom->getBlock()->getNumber());
281
282 // IDom dominates all of our predecessors, but it may not be their
283 // immediate dominator. Check if any of them have live-out values that are
284 // properly dominated by IDom. If so, we need a phi-def here.
285 if (!needPHI) {
286 IDomValue = LiveOut[IDom->getBlock()];
287
288 // Cache the DomTree node that defined the value.
289 if (IDomValue.first && !IDomValue.second)
290 LiveOut[IDom->getBlock()].second = IDomValue.second =
291 DomTree->getNode(Indexes->getMBBFromIndex(IDomValue.first->def));
292
293 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
294 PE = MBB->pred_end(); PI != PE; ++PI) {
295 LiveOutPair &Value = LiveOut[*PI];
296 if (!Value.first || Value.first == IDomValue.first)
297 continue;
298
299 // Cache the DomTree node that defined the value.
300 if (!Value.second)
301 Value.second =
302 DomTree->getNode(Indexes->getMBBFromIndex(Value.first->def));
303
304 // This predecessor is carrying something other than IDomValue.
305 // It could be because IDomValue hasn't propagated yet, or it could be
306 // because MBB is in the dominance frontier of that value.
307 if (DomTree->dominates(IDom, Value.second)) {
308 needPHI = true;
309 break;
310 }
311 }
312 }
313
314 // The value may be live-through even if Kill is set, as can happen when
315 // we are called from extendRange. In that case LiveOutSeen is true, and
316 // LiveOut indicates a foreign or missing value.
317 LiveOutPair &LOP = LiveOut[MBB];
318
319 // Create a phi-def if required.
320 if (needPHI) {
321 ++Changes;
322 assert(Alloc && "Need VNInfo allocator to create PHI-defs");
323 SlotIndex Start, End;
324 tie(Start, End) = Indexes->getMBBRange(MBB);
325 VNInfo *VNI = I->LI->getNextValue(Start, *Alloc);
326 I->Value = VNI;
327 // This block is done, we know the final value.
328 I->DomNode = 0;
329
330 // Add liveness since updateLiveIns now skips this node.
331 if (I->Kill.isValid())
332 I->LI->addRange(LiveRange(Start, I->Kill, VNI));
333 else {
334 I->LI->addRange(LiveRange(Start, End, VNI));
335 LOP = LiveOutPair(VNI, Node);
336 }
337 } else if (IDomValue.first) {
338 // No phi-def here. Remember incoming value.
339 I->Value = IDomValue.first;
340
341 // If the IDomValue is killed in the block, don't propagate through.
342 if (I->Kill.isValid())
343 continue;
344
345 // Propagate IDomValue if it isn't killed:
346 // MBB is live-out and doesn't define its own value.
347 if (LOP.first == IDomValue.first)
348 continue;
349 ++Changes;
350 LOP = IDomValue;
351 }
352 }
353 } while (Changes);
354 }
355