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
14 #include "llvm/Transforms/Scalar.h"
15 #include "llvm/ADT/Statistic.h"
16 #include "llvm/Analysis/InstructionSimplify.h"
17 #include "llvm/Analysis/LazyValueInfo.h"
18 #include "llvm/IR/CFG.h"
19 #include "llvm/IR/Constants.h"
20 #include "llvm/IR/Function.h"
21 #include "llvm/IR/Instructions.h"
22 #include "llvm/Pass.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/Support/raw_ostream.h"
25 #include "llvm/Transforms/Utils/Local.h"
26 using namespace llvm;
27
28 #define DEBUG_TYPE "correlated-value-propagation"
29
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;
CorrelatedValuePropagation()48 CorrelatedValuePropagation(): FunctionPass(ID) {
49 initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry());
50 }
51
52 bool runOnFunction(Function &F) override;
53
getAnalysisUsage(AnalysisUsage & AU) const54 void getAnalysisUsage(AnalysisUsage &AU) const override {
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)
INITIALIZE_PASS_DEPENDENCY(LazyValueInfo)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
processSelect(SelectInst * S)72 bool CorrelatedValuePropagation::processSelect(SelectInst *S) {
73 if (S->getType()->isVectorTy()) return false;
74 if (isa<Constant>(S->getOperand(0))) return false;
75
76 Constant *C = LVI->getConstant(S->getOperand(0), S->getParent());
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
processPHI(PHINode * P)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
103 Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB);
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,
117 P->getIncomingBlock(i), BB) !=
118 LazyValueInfo::False)
119 continue;
120
121 DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n');
122 V = SI->getTrueValue();
123 }
124
125 P->setIncomingValue(i, V);
126 Changed = true;
127 }
128
129 if (Value *V = SimplifyInstruction(P)) {
130 P->replaceAllUsesWith(V);
131 P->eraseFromParent();
132 Changed = true;
133 }
134
135 if (Changed)
136 ++NumPhis;
137
138 return Changed;
139 }
140
processMemAccess(Instruction * I)141 bool CorrelatedValuePropagation::processMemAccess(Instruction *I) {
142 Value *Pointer = nullptr;
143 if (LoadInst *L = dyn_cast<LoadInst>(I))
144 Pointer = L->getPointerOperand();
145 else
146 Pointer = cast<StoreInst>(I)->getPointerOperand();
147
148 if (isa<Constant>(Pointer)) return false;
149
150 Constant *C = LVI->getConstant(Pointer, I->getParent());
151 if (!C) return false;
152
153 ++NumMemAccess;
154 I->replaceUsesOfWith(Pointer, C);
155 return true;
156 }
157
158 /// processCmp - If the value of this comparison could be determined locally,
159 /// constant propagation would already have figured it out. Instead, walk
160 /// the predecessors and statically evaluate the comparison based on information
161 /// available on that edge. If a given static evaluation is true on ALL
162 /// incoming edges, then it's true universally and we can simplify the compare.
processCmp(CmpInst * C)163 bool CorrelatedValuePropagation::processCmp(CmpInst *C) {
164 Value *Op0 = C->getOperand(0);
165 if (isa<Instruction>(Op0) &&
166 cast<Instruction>(Op0)->getParent() == C->getParent())
167 return false;
168
169 Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
170 if (!Op1) return false;
171
172 pred_iterator PI = pred_begin(C->getParent()), PE = pred_end(C->getParent());
173 if (PI == PE) return false;
174
175 LazyValueInfo::Tristate Result = LVI->getPredicateOnEdge(C->getPredicate(),
176 C->getOperand(0), Op1, *PI, C->getParent());
177 if (Result == LazyValueInfo::Unknown) return false;
178
179 ++PI;
180 while (PI != PE) {
181 LazyValueInfo::Tristate Res = LVI->getPredicateOnEdge(C->getPredicate(),
182 C->getOperand(0), Op1, *PI, C->getParent());
183 if (Res != Result) return false;
184 ++PI;
185 }
186
187 ++NumCmps;
188
189 if (Result == LazyValueInfo::True)
190 C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext()));
191 else
192 C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext()));
193
194 C->eraseFromParent();
195
196 return true;
197 }
198
199 /// processSwitch - Simplify a switch instruction by removing cases which can
200 /// never fire. If the uselessness of a case could be determined locally then
201 /// constant propagation would already have figured it out. Instead, walk the
202 /// predecessors and statically evaluate cases based on information available
203 /// on that edge. Cases that cannot fire no matter what the incoming edge can
204 /// safely be removed. If a case fires on every incoming edge then the entire
205 /// switch can be removed and replaced with a branch to the case destination.
processSwitch(SwitchInst * SI)206 bool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) {
207 Value *Cond = SI->getCondition();
208 BasicBlock *BB = SI->getParent();
209
210 // If the condition was defined in same block as the switch then LazyValueInfo
211 // currently won't say anything useful about it, though in theory it could.
212 if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB)
213 return false;
214
215 // If the switch is unreachable then trying to improve it is a waste of time.
216 pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
217 if (PB == PE) return false;
218
219 // Analyse each switch case in turn. This is done in reverse order so that
220 // removing a case doesn't cause trouble for the iteration.
221 bool Changed = false;
222 for (SwitchInst::CaseIt CI = SI->case_end(), CE = SI->case_begin(); CI-- != CE;
223 ) {
224 ConstantInt *Case = CI.getCaseValue();
225
226 // Check to see if the switch condition is equal to/not equal to the case
227 // value on every incoming edge, equal/not equal being the same each time.
228 LazyValueInfo::Tristate State = LazyValueInfo::Unknown;
229 for (pred_iterator PI = PB; PI != PE; ++PI) {
230 // Is the switch condition equal to the case value?
231 LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ,
232 Cond, Case, *PI, BB);
233 // Give up on this case if nothing is known.
234 if (Value == LazyValueInfo::Unknown) {
235 State = LazyValueInfo::Unknown;
236 break;
237 }
238
239 // If this was the first edge to be visited, record that all other edges
240 // need to give the same result.
241 if (PI == PB) {
242 State = Value;
243 continue;
244 }
245
246 // If this case is known to fire for some edges and known not to fire for
247 // others then there is nothing we can do - give up.
248 if (Value != State) {
249 State = LazyValueInfo::Unknown;
250 break;
251 }
252 }
253
254 if (State == LazyValueInfo::False) {
255 // This case never fires - remove it.
256 CI.getCaseSuccessor()->removePredecessor(BB);
257 SI->removeCase(CI); // Does not invalidate the iterator.
258
259 // The condition can be modified by removePredecessor's PHI simplification
260 // logic.
261 Cond = SI->getCondition();
262
263 ++NumDeadCases;
264 Changed = true;
265 } else if (State == LazyValueInfo::True) {
266 // This case always fires. Arrange for the switch to be turned into an
267 // unconditional branch by replacing the switch condition with the case
268 // value.
269 SI->setCondition(Case);
270 NumDeadCases += SI->getNumCases();
271 Changed = true;
272 break;
273 }
274 }
275
276 if (Changed)
277 // If the switch has been simplified to the point where it can be replaced
278 // by a branch then do so now.
279 ConstantFoldTerminator(BB);
280
281 return Changed;
282 }
283
runOnFunction(Function & F)284 bool CorrelatedValuePropagation::runOnFunction(Function &F) {
285 if (skipOptnoneFunction(F))
286 return false;
287
288 LVI = &getAnalysis<LazyValueInfo>();
289
290 bool FnChanged = false;
291
292 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
293 bool BBChanged = false;
294 for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) {
295 Instruction *II = BI++;
296 switch (II->getOpcode()) {
297 case Instruction::Select:
298 BBChanged |= processSelect(cast<SelectInst>(II));
299 break;
300 case Instruction::PHI:
301 BBChanged |= processPHI(cast<PHINode>(II));
302 break;
303 case Instruction::ICmp:
304 case Instruction::FCmp:
305 BBChanged |= processCmp(cast<CmpInst>(II));
306 break;
307 case Instruction::Load:
308 case Instruction::Store:
309 BBChanged |= processMemAccess(II);
310 break;
311 }
312 }
313
314 Instruction *Term = FI->getTerminator();
315 switch (Term->getOpcode()) {
316 case Instruction::Switch:
317 BBChanged |= processSwitch(cast<SwitchInst>(Term));
318 break;
319 }
320
321 FnChanged |= BBChanged;
322 }
323
324 return FnChanged;
325 }
326