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223e47cc LB |
1 | //===- CorrelatedValuePropagation.cpp - Propagate CFG-derived info --------===// |
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 file implements the Correlated Value Propagation pass. | |
11 | // | |
12 | //===----------------------------------------------------------------------===// | |
13 | ||
223e47cc | 14 | #include "llvm/Transforms/Scalar.h" |
970d7e83 | 15 | #include "llvm/ADT/Statistic.h" |
223e47cc LB |
16 | #include "llvm/Analysis/InstructionSimplify.h" |
17 | #include "llvm/Analysis/LazyValueInfo.h" | |
1a4d82fc | 18 | #include "llvm/IR/CFG.h" |
970d7e83 LB |
19 | #include "llvm/IR/Constants.h" |
20 | #include "llvm/IR/Function.h" | |
21 | #include "llvm/IR/Instructions.h" | |
22 | #include "llvm/Pass.h" | |
970d7e83 LB |
23 | #include "llvm/Support/Debug.h" |
24 | #include "llvm/Support/raw_ostream.h" | |
223e47cc | 25 | #include "llvm/Transforms/Utils/Local.h" |
223e47cc LB |
26 | using namespace llvm; |
27 | ||
1a4d82fc JJ |
28 | #define DEBUG_TYPE "correlated-value-propagation" |
29 | ||
223e47cc LB |
30 | STATISTIC(NumPhis, "Number of phis propagated"); |
31 | STATISTIC(NumSelects, "Number of selects propagated"); | |
32 | STATISTIC(NumMemAccess, "Number of memory access targets propagated"); | |
33 | STATISTIC(NumCmps, "Number of comparisons propagated"); | |
34 | STATISTIC(NumDeadCases, "Number of switch cases removed"); | |
35 | ||
36 | namespace { | |
37 | class CorrelatedValuePropagation : public FunctionPass { | |
38 | LazyValueInfo *LVI; | |
39 | ||
40 | bool processSelect(SelectInst *SI); | |
41 | bool processPHI(PHINode *P); | |
42 | bool processMemAccess(Instruction *I); | |
43 | bool processCmp(CmpInst *C); | |
44 | bool processSwitch(SwitchInst *SI); | |
45 | ||
46 | public: | |
47 | static char ID; | |
48 | CorrelatedValuePropagation(): FunctionPass(ID) { | |
49 | initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry()); | |
50 | } | |
51 | ||
1a4d82fc | 52 | bool runOnFunction(Function &F) override; |
223e47cc | 53 | |
1a4d82fc | 54 | void getAnalysisUsage(AnalysisUsage &AU) const override { |
223e47cc LB |
55 | AU.addRequired<LazyValueInfo>(); |
56 | } | |
57 | }; | |
58 | } | |
59 | ||
60 | char CorrelatedValuePropagation::ID = 0; | |
61 | INITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation", | |
62 | "Value Propagation", false, false) | |
63 | INITIALIZE_PASS_DEPENDENCY(LazyValueInfo) | |
64 | INITIALIZE_PASS_END(CorrelatedValuePropagation, "correlated-propagation", | |
65 | "Value Propagation", false, false) | |
66 | ||
67 | // Public interface to the Value Propagation pass | |
68 | Pass *llvm::createCorrelatedValuePropagationPass() { | |
69 | return new CorrelatedValuePropagation(); | |
70 | } | |
71 | ||
72 | bool CorrelatedValuePropagation::processSelect(SelectInst *S) { | |
73 | if (S->getType()->isVectorTy()) return false; | |
74 | if (isa<Constant>(S->getOperand(0))) return false; | |
75 | ||
1a4d82fc | 76 | Constant *C = LVI->getConstant(S->getOperand(0), S->getParent(), S); |
223e47cc LB |
77 | if (!C) return false; |
78 | ||
79 | ConstantInt *CI = dyn_cast<ConstantInt>(C); | |
80 | if (!CI) return false; | |
81 | ||
82 | Value *ReplaceWith = S->getOperand(1); | |
83 | Value *Other = S->getOperand(2); | |
84 | if (!CI->isOne()) std::swap(ReplaceWith, Other); | |
85 | if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType()); | |
86 | ||
87 | S->replaceAllUsesWith(ReplaceWith); | |
88 | S->eraseFromParent(); | |
89 | ||
90 | ++NumSelects; | |
91 | ||
92 | return true; | |
93 | } | |
94 | ||
95 | bool CorrelatedValuePropagation::processPHI(PHINode *P) { | |
96 | bool Changed = false; | |
97 | ||
98 | BasicBlock *BB = P->getParent(); | |
99 | for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) { | |
100 | Value *Incoming = P->getIncomingValue(i); | |
101 | if (isa<Constant>(Incoming)) continue; | |
102 | ||
1a4d82fc | 103 | Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB, P); |
970d7e83 LB |
104 | |
105 | // Look if the incoming value is a select with a constant but LVI tells us | |
106 | // that the incoming value can never be that constant. In that case replace | |
107 | // the incoming value with the other value of the select. This often allows | |
108 | // us to remove the select later. | |
109 | if (!V) { | |
110 | SelectInst *SI = dyn_cast<SelectInst>(Incoming); | |
111 | if (!SI) continue; | |
112 | ||
113 | Constant *C = dyn_cast<Constant>(SI->getFalseValue()); | |
114 | if (!C) continue; | |
115 | ||
116 | if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C, | |
1a4d82fc | 117 | P->getIncomingBlock(i), BB, P) != |
970d7e83 LB |
118 | LazyValueInfo::False) |
119 | continue; | |
120 | ||
121 | DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n'); | |
122 | V = SI->getTrueValue(); | |
123 | } | |
223e47cc | 124 | |
970d7e83 | 125 | P->setIncomingValue(i, V); |
223e47cc LB |
126 | Changed = true; |
127 | } | |
128 | ||
1a4d82fc | 129 | // FIXME: Provide DL, TLI, DT, AT to SimplifyInstruction. |
223e47cc LB |
130 | if (Value *V = SimplifyInstruction(P)) { |
131 | P->replaceAllUsesWith(V); | |
132 | P->eraseFromParent(); | |
133 | Changed = true; | |
134 | } | |
135 | ||
136 | if (Changed) | |
137 | ++NumPhis; | |
138 | ||
139 | return Changed; | |
140 | } | |
141 | ||
142 | bool CorrelatedValuePropagation::processMemAccess(Instruction *I) { | |
1a4d82fc | 143 | Value *Pointer = nullptr; |
223e47cc LB |
144 | if (LoadInst *L = dyn_cast<LoadInst>(I)) |
145 | Pointer = L->getPointerOperand(); | |
146 | else | |
147 | Pointer = cast<StoreInst>(I)->getPointerOperand(); | |
148 | ||
149 | if (isa<Constant>(Pointer)) return false; | |
150 | ||
1a4d82fc | 151 | Constant *C = LVI->getConstant(Pointer, I->getParent(), I); |
223e47cc LB |
152 | if (!C) return false; |
153 | ||
154 | ++NumMemAccess; | |
155 | I->replaceUsesOfWith(Pointer, C); | |
156 | return true; | |
157 | } | |
158 | ||
159 | /// processCmp - If the value of this comparison could be determined locally, | |
160 | /// constant propagation would already have figured it out. Instead, walk | |
161 | /// the predecessors and statically evaluate the comparison based on information | |
162 | /// available on that edge. If a given static evaluation is true on ALL | |
163 | /// incoming edges, then it's true universally and we can simplify the compare. | |
164 | bool CorrelatedValuePropagation::processCmp(CmpInst *C) { | |
165 | Value *Op0 = C->getOperand(0); | |
166 | if (isa<Instruction>(Op0) && | |
167 | cast<Instruction>(Op0)->getParent() == C->getParent()) | |
168 | return false; | |
169 | ||
170 | Constant *Op1 = dyn_cast<Constant>(C->getOperand(1)); | |
171 | if (!Op1) return false; | |
172 | ||
173 | pred_iterator PI = pred_begin(C->getParent()), PE = pred_end(C->getParent()); | |
174 | if (PI == PE) return false; | |
175 | ||
176 | LazyValueInfo::Tristate Result = LVI->getPredicateOnEdge(C->getPredicate(), | |
1a4d82fc JJ |
177 | C->getOperand(0), Op1, *PI, |
178 | C->getParent(), C); | |
223e47cc LB |
179 | if (Result == LazyValueInfo::Unknown) return false; |
180 | ||
181 | ++PI; | |
182 | while (PI != PE) { | |
183 | LazyValueInfo::Tristate Res = LVI->getPredicateOnEdge(C->getPredicate(), | |
1a4d82fc JJ |
184 | C->getOperand(0), Op1, *PI, |
185 | C->getParent(), C); | |
223e47cc LB |
186 | if (Res != Result) return false; |
187 | ++PI; | |
188 | } | |
189 | ||
190 | ++NumCmps; | |
191 | ||
192 | if (Result == LazyValueInfo::True) | |
193 | C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext())); | |
194 | else | |
195 | C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext())); | |
196 | ||
197 | C->eraseFromParent(); | |
198 | ||
199 | return true; | |
200 | } | |
201 | ||
202 | /// processSwitch - Simplify a switch instruction by removing cases which can | |
203 | /// never fire. If the uselessness of a case could be determined locally then | |
204 | /// constant propagation would already have figured it out. Instead, walk the | |
205 | /// predecessors and statically evaluate cases based on information available | |
206 | /// on that edge. Cases that cannot fire no matter what the incoming edge can | |
207 | /// safely be removed. If a case fires on every incoming edge then the entire | |
208 | /// switch can be removed and replaced with a branch to the case destination. | |
209 | bool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) { | |
210 | Value *Cond = SI->getCondition(); | |
211 | BasicBlock *BB = SI->getParent(); | |
212 | ||
213 | // If the condition was defined in same block as the switch then LazyValueInfo | |
214 | // currently won't say anything useful about it, though in theory it could. | |
215 | if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB) | |
216 | return false; | |
217 | ||
218 | // If the switch is unreachable then trying to improve it is a waste of time. | |
219 | pred_iterator PB = pred_begin(BB), PE = pred_end(BB); | |
220 | if (PB == PE) return false; | |
221 | ||
222 | // Analyse each switch case in turn. This is done in reverse order so that | |
223 | // removing a case doesn't cause trouble for the iteration. | |
224 | bool Changed = false; | |
225 | for (SwitchInst::CaseIt CI = SI->case_end(), CE = SI->case_begin(); CI-- != CE; | |
226 | ) { | |
227 | ConstantInt *Case = CI.getCaseValue(); | |
228 | ||
229 | // Check to see if the switch condition is equal to/not equal to the case | |
230 | // value on every incoming edge, equal/not equal being the same each time. | |
231 | LazyValueInfo::Tristate State = LazyValueInfo::Unknown; | |
232 | for (pred_iterator PI = PB; PI != PE; ++PI) { | |
233 | // Is the switch condition equal to the case value? | |
234 | LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ, | |
1a4d82fc JJ |
235 | Cond, Case, *PI, |
236 | BB, SI); | |
223e47cc LB |
237 | // Give up on this case if nothing is known. |
238 | if (Value == LazyValueInfo::Unknown) { | |
239 | State = LazyValueInfo::Unknown; | |
240 | break; | |
241 | } | |
242 | ||
243 | // If this was the first edge to be visited, record that all other edges | |
244 | // need to give the same result. | |
245 | if (PI == PB) { | |
246 | State = Value; | |
247 | continue; | |
248 | } | |
249 | ||
250 | // If this case is known to fire for some edges and known not to fire for | |
251 | // others then there is nothing we can do - give up. | |
252 | if (Value != State) { | |
253 | State = LazyValueInfo::Unknown; | |
254 | break; | |
255 | } | |
256 | } | |
257 | ||
258 | if (State == LazyValueInfo::False) { | |
259 | // This case never fires - remove it. | |
260 | CI.getCaseSuccessor()->removePredecessor(BB); | |
261 | SI->removeCase(CI); // Does not invalidate the iterator. | |
262 | ||
263 | // The condition can be modified by removePredecessor's PHI simplification | |
264 | // logic. | |
265 | Cond = SI->getCondition(); | |
266 | ||
267 | ++NumDeadCases; | |
268 | Changed = true; | |
269 | } else if (State == LazyValueInfo::True) { | |
270 | // This case always fires. Arrange for the switch to be turned into an | |
271 | // unconditional branch by replacing the switch condition with the case | |
272 | // value. | |
273 | SI->setCondition(Case); | |
274 | NumDeadCases += SI->getNumCases(); | |
275 | Changed = true; | |
276 | break; | |
277 | } | |
278 | } | |
279 | ||
280 | if (Changed) | |
281 | // If the switch has been simplified to the point where it can be replaced | |
282 | // by a branch then do so now. | |
283 | ConstantFoldTerminator(BB); | |
284 | ||
285 | return Changed; | |
286 | } | |
287 | ||
288 | bool CorrelatedValuePropagation::runOnFunction(Function &F) { | |
1a4d82fc JJ |
289 | if (skipOptnoneFunction(F)) |
290 | return false; | |
291 | ||
223e47cc LB |
292 | LVI = &getAnalysis<LazyValueInfo>(); |
293 | ||
294 | bool FnChanged = false; | |
295 | ||
296 | for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) { | |
297 | bool BBChanged = false; | |
298 | for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) { | |
299 | Instruction *II = BI++; | |
300 | switch (II->getOpcode()) { | |
301 | case Instruction::Select: | |
302 | BBChanged |= processSelect(cast<SelectInst>(II)); | |
303 | break; | |
304 | case Instruction::PHI: | |
305 | BBChanged |= processPHI(cast<PHINode>(II)); | |
306 | break; | |
307 | case Instruction::ICmp: | |
308 | case Instruction::FCmp: | |
309 | BBChanged |= processCmp(cast<CmpInst>(II)); | |
310 | break; | |
311 | case Instruction::Load: | |
312 | case Instruction::Store: | |
313 | BBChanged |= processMemAccess(II); | |
314 | break; | |
315 | } | |
316 | } | |
317 | ||
318 | Instruction *Term = FI->getTerminator(); | |
319 | switch (Term->getOpcode()) { | |
320 | case Instruction::Switch: | |
321 | BBChanged |= processSwitch(cast<SwitchInst>(Term)); | |
322 | break; | |
323 | } | |
324 | ||
325 | FnChanged |= BBChanged; | |
326 | } | |
327 | ||
328 | return FnChanged; | |
329 | } |