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 constant but LVI tells us
107 // that the incoming value can never be that constant. In that case replace
108 // the incoming value with the other value of the select. This often allows
109 // us to remove the select later.
111 SelectInst *SI = dyn_cast<SelectInst>(Incoming);
114 Constant *C = dyn_cast<Constant>(SI->getFalseValue());
117 if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C,
118 P->getIncomingBlock(i), BB, P) !=
119 LazyValueInfo::False)
122 DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n');
123 V = SI->getTrueValue();
126 P->setIncomingValue(i, V);
130 // FIXME: Provide TLI, DT, AT to SimplifyInstruction.
131 const DataLayout &DL = BB->getModule()->getDataLayout();
132 if (Value *V = SimplifyInstruction(P, DL)) {
133 P->replaceAllUsesWith(V);
134 P->eraseFromParent();
144 bool CorrelatedValuePropagation::processMemAccess(Instruction *I) {
145 Value *Pointer = nullptr;
146 if (LoadInst *L = dyn_cast<LoadInst>(I))
147 Pointer = L->getPointerOperand();
149 Pointer = cast<StoreInst>(I)->getPointerOperand();
151 if (isa<Constant>(Pointer)) return false;
153 Constant *C = LVI->getConstant(Pointer, I->getParent(), I);
154 if (!C) return false;
157 I->replaceUsesOfWith(Pointer, C);
161 /// processCmp - If the value of this comparison could be determined locally,
162 /// constant propagation would already have figured it out. Instead, walk
163 /// the predecessors and statically evaluate the comparison based on information
164 /// available on that edge. If a given static evaluation is true on ALL
165 /// incoming edges, then it's true universally and we can simplify the compare.
166 bool CorrelatedValuePropagation::processCmp(CmpInst *C) {
167 Value *Op0 = C->getOperand(0);
168 if (isa<Instruction>(Op0) &&
169 cast<Instruction>(Op0)->getParent() == C->getParent())
172 Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
173 if (!Op1) return false;
175 pred_iterator PI = pred_begin(C->getParent()), PE = pred_end(C->getParent());
176 if (PI == PE) return false;
178 LazyValueInfo::Tristate Result = LVI->getPredicateOnEdge(C->getPredicate(),
179 C->getOperand(0), Op1, *PI,
181 if (Result == LazyValueInfo::Unknown) return false;
185 LazyValueInfo::Tristate Res = LVI->getPredicateOnEdge(C->getPredicate(),
186 C->getOperand(0), Op1, *PI,
188 if (Res != Result) return false;
194 if (Result == LazyValueInfo::True)
195 C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext()));
197 C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext()));
199 C->eraseFromParent();
204 /// processSwitch - Simplify a switch instruction by removing cases which can
205 /// never fire. If the uselessness of a case could be determined locally then
206 /// constant propagation would already have figured it out. Instead, walk the
207 /// predecessors and statically evaluate cases based on information available
208 /// on that edge. Cases that cannot fire no matter what the incoming edge can
209 /// safely be removed. If a case fires on every incoming edge then the entire
210 /// switch can be removed and replaced with a branch to the case destination.
211 bool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) {
212 Value *Cond = SI->getCondition();
213 BasicBlock *BB = SI->getParent();
215 // If the condition was defined in same block as the switch then LazyValueInfo
216 // currently won't say anything useful about it, though in theory it could.
217 if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB)
220 // If the switch is unreachable then trying to improve it is a waste of time.
221 pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
222 if (PB == PE) return false;
224 // Analyse each switch case in turn. This is done in reverse order so that
225 // removing a case doesn't cause trouble for the iteration.
226 bool Changed = false;
227 for (SwitchInst::CaseIt CI = SI->case_end(), CE = SI->case_begin(); CI-- != CE;
229 ConstantInt *Case = CI.getCaseValue();
231 // Check to see if the switch condition is equal to/not equal to the case
232 // value on every incoming edge, equal/not equal being the same each time.
233 LazyValueInfo::Tristate State = LazyValueInfo::Unknown;
234 for (pred_iterator PI = PB; PI != PE; ++PI) {
235 // Is the switch condition equal to the case value?
236 LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ,
239 // Give up on this case if nothing is known.
240 if (Value == LazyValueInfo::Unknown) {
241 State = LazyValueInfo::Unknown;
245 // If this was the first edge to be visited, record that all other edges
246 // need to give the same result.
252 // If this case is known to fire for some edges and known not to fire for
253 // others then there is nothing we can do - give up.
254 if (Value != State) {
255 State = LazyValueInfo::Unknown;
260 if (State == LazyValueInfo::False) {
261 // This case never fires - remove it.
262 CI.getCaseSuccessor()->removePredecessor(BB);
263 SI->removeCase(CI); // Does not invalidate the iterator.
265 // The condition can be modified by removePredecessor's PHI simplification
267 Cond = SI->getCondition();
271 } else if (State == LazyValueInfo::True) {
272 // This case always fires. Arrange for the switch to be turned into an
273 // unconditional branch by replacing the switch condition with the case
275 SI->setCondition(Case);
276 NumDeadCases += SI->getNumCases();
283 // If the switch has been simplified to the point where it can be replaced
284 // by a branch then do so now.
285 ConstantFoldTerminator(BB);
290 bool CorrelatedValuePropagation::runOnFunction(Function &F) {
291 if (skipOptnoneFunction(F))
294 LVI = &getAnalysis<LazyValueInfo>();
296 bool FnChanged = false;
298 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
299 bool BBChanged = false;
300 for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) {
301 Instruction *II = BI++;
302 switch (II->getOpcode()) {
303 case Instruction::Select:
304 BBChanged |= processSelect(cast<SelectInst>(II));
306 case Instruction::PHI:
307 BBChanged |= processPHI(cast<PHINode>(II));
309 case Instruction::ICmp:
310 case Instruction::FCmp:
311 BBChanged |= processCmp(cast<CmpInst>(II));
313 case Instruction::Load:
314 case Instruction::Store:
315 BBChanged |= processMemAccess(II);
320 Instruction *Term = FI->getTerminator();
321 switch (Term->getOpcode()) {
322 case Instruction::Switch:
323 BBChanged |= processSwitch(cast<SwitchInst>(Term));
327 FnChanged |= BBChanged;