1 //===- CorrelatedValuePropagation.cpp - Propagate CFG-derived info --------===//
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 file implements the Correlated Value Propagation pass.
12 //===----------------------------------------------------------------------===//
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/IR/Module.h"
23 #include "llvm/Pass.h"
24 #include "llvm/Support/Debug.h"
25 #include "llvm/Support/raw_ostream.h"
26 #include "llvm/Transforms/Utils/Local.h"
29 #define DEBUG_TYPE "correlated-value-propagation"
31 STATISTIC(NumPhis, "Number of phis propagated");
32 STATISTIC(NumSelects, "Number of selects propagated");
33 STATISTIC(NumMemAccess, "Number of memory access targets propagated");
34 STATISTIC(NumCmps, "Number of comparisons propagated");
35 STATISTIC(NumDeadCases, "Number of switch cases removed");
38 class CorrelatedValuePropagation : public FunctionPass {
41 bool processSelect(SelectInst *SI);
42 bool processPHI(PHINode *P);
43 bool processMemAccess(Instruction *I);
44 bool processCmp(CmpInst *C);
45 bool processSwitch(SwitchInst *SI);
49 CorrelatedValuePropagation(): FunctionPass(ID) {
50 initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry());
53 bool runOnFunction(Function &F) override;
55 void getAnalysisUsage(AnalysisUsage &AU) const override {
56 AU.addRequired<LazyValueInfo>();
61 char CorrelatedValuePropagation::ID = 0;
62 INITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation",
63 "Value Propagation", false, false)
64 INITIALIZE_PASS_DEPENDENCY(LazyValueInfo)
65 INITIALIZE_PASS_END(CorrelatedValuePropagation, "correlated-propagation",
66 "Value Propagation", false, false)
68 // Public interface to the Value Propagation pass
69 Pass *llvm::createCorrelatedValuePropagationPass() {
70 return new CorrelatedValuePropagation();
73 bool CorrelatedValuePropagation::processSelect(SelectInst *S) {
74 if (S->getType()->isVectorTy()) return false;
75 if (isa<Constant>(S->getOperand(0))) return false;
77 Constant *C = LVI->getConstant(S->getOperand(0), S->getParent(), S);
80 ConstantInt *CI = dyn_cast<ConstantInt>(C);
81 if (!CI) return false;
83 Value *ReplaceWith = S->getOperand(1);
84 Value *Other = S->getOperand(2);
85 if (!CI->isOne()) std::swap(ReplaceWith, Other);
86 if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType());
88 S->replaceAllUsesWith(ReplaceWith);
96 bool CorrelatedValuePropagation::processPHI(PHINode *P) {
99 BasicBlock *BB = P->getParent();
100 for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
101 Value *Incoming = P->getIncomingValue(i);
102 if (isa<Constant>(Incoming)) continue;
104 Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB, P);
106 // Look if the incoming value is a select with a scalar condition for which
107 // LVI can tells us the value. In that case replace the incoming value with
108 // the appropriate value of the select. This often allows us to remove the
111 SelectInst *SI = dyn_cast<SelectInst>(Incoming);
114 Value *Condition = SI->getCondition();
115 if (!Condition->getType()->isVectorTy()) {
116 if (Constant *C = LVI->getConstantOnEdge(
117 Condition, P->getIncomingBlock(i), BB, P)) {
118 if (C->isOneValue()) {
119 V = SI->getTrueValue();
120 } else if (C->isZeroValue()) {
121 V = SI->getFalseValue();
123 // Once LVI learns to handle vector types, we could also add support
124 // for vector type constants that are not all zeroes or all ones.
128 // Look if the select has a constant but LVI tells us that the incoming
129 // value can never be that constant. In that case replace the incoming
130 // value with the other value of the select. This often allows us to
131 // remove the select later.
133 Constant *C = dyn_cast<Constant>(SI->getFalseValue());
136 if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C,
137 P->getIncomingBlock(i), BB, P) !=
138 LazyValueInfo::False)
140 V = SI->getTrueValue();
143 DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n');
146 P->setIncomingValue(i, V);
150 // FIXME: Provide TLI, DT, AT to SimplifyInstruction.
151 const DataLayout &DL = BB->getModule()->getDataLayout();
152 if (Value *V = SimplifyInstruction(P, DL)) {
153 P->replaceAllUsesWith(V);
154 P->eraseFromParent();
164 bool CorrelatedValuePropagation::processMemAccess(Instruction *I) {
165 Value *Pointer = nullptr;
166 if (LoadInst *L = dyn_cast<LoadInst>(I))
167 Pointer = L->getPointerOperand();
169 Pointer = cast<StoreInst>(I)->getPointerOperand();
171 if (isa<Constant>(Pointer)) return false;
173 Constant *C = LVI->getConstant(Pointer, I->getParent(), I);
174 if (!C) return false;
177 I->replaceUsesOfWith(Pointer, C);
181 /// processCmp - If the value of this comparison could be determined locally,
182 /// constant propagation would already have figured it out. Instead, walk
183 /// the predecessors and statically evaluate the comparison based on information
184 /// available on that edge. If a given static evaluation is true on ALL
185 /// incoming edges, then it's true universally and we can simplify the compare.
186 bool CorrelatedValuePropagation::processCmp(CmpInst *C) {
187 Value *Op0 = C->getOperand(0);
188 if (isa<Instruction>(Op0) &&
189 cast<Instruction>(Op0)->getParent() == C->getParent())
192 Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
193 if (!Op1) return false;
195 pred_iterator PI = pred_begin(C->getParent()), PE = pred_end(C->getParent());
196 if (PI == PE) return false;
198 LazyValueInfo::Tristate Result = LVI->getPredicateOnEdge(C->getPredicate(),
199 C->getOperand(0), Op1, *PI,
201 if (Result == LazyValueInfo::Unknown) return false;
205 LazyValueInfo::Tristate Res = LVI->getPredicateOnEdge(C->getPredicate(),
206 C->getOperand(0), Op1, *PI,
208 if (Res != Result) return false;
214 if (Result == LazyValueInfo::True)
215 C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext()));
217 C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext()));
219 C->eraseFromParent();
224 /// processSwitch - Simplify a switch instruction by removing cases which can
225 /// never fire. If the uselessness of a case could be determined locally then
226 /// constant propagation would already have figured it out. Instead, walk the
227 /// predecessors and statically evaluate cases based on information available
228 /// on that edge. Cases that cannot fire no matter what the incoming edge can
229 /// safely be removed. If a case fires on every incoming edge then the entire
230 /// switch can be removed and replaced with a branch to the case destination.
231 bool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) {
232 Value *Cond = SI->getCondition();
233 BasicBlock *BB = SI->getParent();
235 // If the condition was defined in same block as the switch then LazyValueInfo
236 // currently won't say anything useful about it, though in theory it could.
237 if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB)
240 // If the switch is unreachable then trying to improve it is a waste of time.
241 pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
242 if (PB == PE) return false;
244 // Analyse each switch case in turn. This is done in reverse order so that
245 // removing a case doesn't cause trouble for the iteration.
246 bool Changed = false;
247 for (SwitchInst::CaseIt CI = SI->case_end(), CE = SI->case_begin(); CI-- != CE;
249 ConstantInt *Case = CI.getCaseValue();
251 // Check to see if the switch condition is equal to/not equal to the case
252 // value on every incoming edge, equal/not equal being the same each time.
253 LazyValueInfo::Tristate State = LazyValueInfo::Unknown;
254 for (pred_iterator PI = PB; PI != PE; ++PI) {
255 // Is the switch condition equal to the case value?
256 LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ,
259 // Give up on this case if nothing is known.
260 if (Value == LazyValueInfo::Unknown) {
261 State = LazyValueInfo::Unknown;
265 // If this was the first edge to be visited, record that all other edges
266 // need to give the same result.
272 // If this case is known to fire for some edges and known not to fire for
273 // others then there is nothing we can do - give up.
274 if (Value != State) {
275 State = LazyValueInfo::Unknown;
280 if (State == LazyValueInfo::False) {
281 // This case never fires - remove it.
282 CI.getCaseSuccessor()->removePredecessor(BB);
283 SI->removeCase(CI); // Does not invalidate the iterator.
285 // The condition can be modified by removePredecessor's PHI simplification
287 Cond = SI->getCondition();
291 } else if (State == LazyValueInfo::True) {
292 // This case always fires. Arrange for the switch to be turned into an
293 // unconditional branch by replacing the switch condition with the case
295 SI->setCondition(Case);
296 NumDeadCases += SI->getNumCases();
303 // If the switch has been simplified to the point where it can be replaced
304 // by a branch then do so now.
305 ConstantFoldTerminator(BB);
310 bool CorrelatedValuePropagation::runOnFunction(Function &F) {
311 if (skipOptnoneFunction(F))
314 LVI = &getAnalysis<LazyValueInfo>();
316 bool FnChanged = false;
318 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
319 bool BBChanged = false;
320 for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) {
321 Instruction *II = BI++;
322 switch (II->getOpcode()) {
323 case Instruction::Select:
324 BBChanged |= processSelect(cast<SelectInst>(II));
326 case Instruction::PHI:
327 BBChanged |= processPHI(cast<PHINode>(II));
329 case Instruction::ICmp:
330 case Instruction::FCmp:
331 BBChanged |= processCmp(cast<CmpInst>(II));
333 case Instruction::Load:
334 case Instruction::Store:
335 BBChanged |= processMemAccess(II);
340 Instruction *Term = FI->getTerminator();
341 switch (Term->getOpcode()) {
342 case Instruction::Switch:
343 BBChanged |= processSwitch(cast<SwitchInst>(Term));
347 FnChanged |= BBChanged;