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