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"
26using namespace llvm;
27
28#define DEBUG_TYPE "correlated-value-propagation"
29
30STATISTIC(NumPhis,      "Number of phis propagated");
31STATISTIC(NumSelects,   "Number of selects propagated");
32STATISTIC(NumMemAccess, "Number of memory access targets propagated");
33STATISTIC(NumCmps,      "Number of comparisons propagated");
34STATISTIC(NumDeadCases, "Number of switch cases removed");
35
36namespace {
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
52    bool runOnFunction(Function &F) override;
53
54    void getAnalysisUsage(AnalysisUsage &AU) const override {
55      AU.addRequired<LazyValueInfo>();
56    }
57  };
58}
59
60char CorrelatedValuePropagation::ID = 0;
61INITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation",
62                "Value Propagation", false, false)
63INITIALIZE_PASS_DEPENDENCY(LazyValueInfo)
64INITIALIZE_PASS_END(CorrelatedValuePropagation, "correlated-propagation",
65                "Value Propagation", false, false)
66
67// Public interface to the Value Propagation pass
68Pass *llvm::createCorrelatedValuePropagationPass() {
69  return new CorrelatedValuePropagation();
70}
71
72bool 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
95bool 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
141bool 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.
163bool 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.
206bool 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
284bool 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