IPConstantPropagation.cpp revision fc74abfba5128544a750fce22fdf13eb0403e3ce
1//===-- IPConstantPropagation.cpp - Propagate constants through calls -----===//
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 pass implements an _extremely_ simple interprocedural constant
11// propagation pass.  It could certainly be improved in many different ways,
12// like using a worklist.  This pass makes arguments dead, but does not remove
13// them.  The existing dead argument elimination pass should be run after this
14// to clean up the mess.
15//
16//===----------------------------------------------------------------------===//
17
18#define DEBUG_TYPE "ipconstprop"
19#include "llvm/Transforms/IPO.h"
20#include "llvm/Constants.h"
21#include "llvm/Instructions.h"
22#include "llvm/Module.h"
23#include "llvm/Pass.h"
24#include "llvm/Analysis/ValueTracking.h"
25#include "llvm/Support/CallSite.h"
26#include "llvm/Support/Compiler.h"
27#include "llvm/ADT/Statistic.h"
28#include "llvm/ADT/SmallVector.h"
29using namespace llvm;
30
31STATISTIC(NumArgumentsProped, "Number of args turned into constants");
32STATISTIC(NumReturnValProped, "Number of return values turned into constants");
33
34namespace {
35  /// IPCP - The interprocedural constant propagation pass
36  ///
37  struct VISIBILITY_HIDDEN IPCP : public ModulePass {
38    static char ID; // Pass identification, replacement for typeid
39    IPCP() : ModulePass((intptr_t)&ID) {}
40
41    bool runOnModule(Module &M);
42  private:
43    bool PropagateConstantsIntoArguments(Function &F);
44    bool PropagateConstantReturn(Function &F);
45  };
46}
47
48char IPCP::ID = 0;
49static RegisterPass<IPCP>
50X("ipconstprop", "Interprocedural constant propagation");
51
52ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); }
53
54bool IPCP::runOnModule(Module &M) {
55  bool Changed = false;
56  bool LocalChange = true;
57
58  // FIXME: instead of using smart algorithms, we just iterate until we stop
59  // making changes.
60  while (LocalChange) {
61    LocalChange = false;
62    for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
63      if (!I->isDeclaration()) {
64        // Delete any klingons.
65        I->removeDeadConstantUsers();
66        if (I->hasInternalLinkage())
67          LocalChange |= PropagateConstantsIntoArguments(*I);
68        Changed |= PropagateConstantReturn(*I);
69      }
70    Changed |= LocalChange;
71  }
72  return Changed;
73}
74
75/// PropagateConstantsIntoArguments - Look at all uses of the specified
76/// function.  If all uses are direct call sites, and all pass a particular
77/// constant in for an argument, propagate that constant in as the argument.
78///
79bool IPCP::PropagateConstantsIntoArguments(Function &F) {
80  if (F.arg_empty() || F.use_empty()) return false; // No arguments? Early exit.
81
82  // For each argument, keep track of its constant value and whether it is a
83  // constant or not.  The bool is driven to true when found to be non-constant.
84  SmallVector<std::pair<Constant*, bool>, 16> ArgumentConstants;
85  ArgumentConstants.resize(F.arg_size());
86
87  unsigned NumNonconstant = 0;
88  for (Value::use_iterator UI = F.use_begin(), E = F.use_end(); UI != E; ++UI) {
89    // Used by a non-instruction, or not the callee of a function, do not
90    // transform.
91    if (UI.getOperandNo() != 0 ||
92        (!isa<CallInst>(*UI) && !isa<InvokeInst>(*UI)))
93      return false;
94
95    CallSite CS = CallSite::get(cast<Instruction>(*UI));
96
97    // Check out all of the potentially constant arguments.  Note that we don't
98    // inspect varargs here.
99    CallSite::arg_iterator AI = CS.arg_begin();
100    Function::arg_iterator Arg = F.arg_begin();
101    for (unsigned i = 0, e = ArgumentConstants.size(); i != e;
102         ++i, ++AI, ++Arg) {
103
104      // If this argument is known non-constant, ignore it.
105      if (ArgumentConstants[i].second)
106        continue;
107
108      Constant *C = dyn_cast<Constant>(*AI);
109      if (C && ArgumentConstants[i].first == 0) {
110        ArgumentConstants[i].first = C;   // First constant seen.
111      } else if (C && ArgumentConstants[i].first == C) {
112        // Still the constant value we think it is.
113      } else if (*AI == &*Arg) {
114        // Ignore recursive calls passing argument down.
115      } else {
116        // Argument became non-constant.  If all arguments are non-constant now,
117        // give up on this function.
118        if (++NumNonconstant == ArgumentConstants.size())
119          return false;
120        ArgumentConstants[i].second = true;
121      }
122    }
123  }
124
125  // If we got to this point, there is a constant argument!
126  assert(NumNonconstant != ArgumentConstants.size());
127  bool MadeChange = false;
128  Function::arg_iterator AI = F.arg_begin();
129  for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) {
130    // Do we have a constant argument?
131    if (ArgumentConstants[i].second || AI->use_empty())
132      continue;
133
134    Value *V = ArgumentConstants[i].first;
135    if (V == 0) V = UndefValue::get(AI->getType());
136    AI->replaceAllUsesWith(V);
137    ++NumArgumentsProped;
138    MadeChange = true;
139  }
140  return MadeChange;
141}
142
143
144// Check to see if this function returns one or more constants. If so, replace
145// all callers that use those return values with the constant value. This will
146// leave in the actual return values and instructions, but deadargelim will
147// clean that up.
148//
149// Additionally if a function always returns one of its arguments directly,
150// callers will be updated to use the value they pass in directly instead of
151// using the return value.
152bool IPCP::PropagateConstantReturn(Function &F) {
153  if (F.getReturnType() == Type::VoidTy)
154    return false; // No return value.
155
156  // If this function could be overridden later in the link stage, we can't
157  // propagate information about its results into callers.
158  if (F.hasLinkOnceLinkage() || F.hasWeakLinkage())
159    return false;
160
161  // Check to see if this function returns a constant.
162  SmallVector<Value *,4> RetVals;
163  const StructType *STy = dyn_cast<StructType>(F.getReturnType());
164  if (STy)
165    for (unsigned i = 0, e = STy->getNumElements(); i < e; ++i)
166      RetVals.push_back(UndefValue::get(STy->getElementType(i)));
167  else
168    RetVals.push_back(UndefValue::get(F.getReturnType()));
169
170  unsigned NumNonConstant = 0;
171  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
172    if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
173      // Return type does not match operand type, this is an old style multiple
174      // return
175      bool OldReturn = (F.getReturnType() != RI->getOperand(0)->getType());
176
177      for (unsigned i = 0, e = RetVals.size(); i != e; ++i) {
178        // Already found conflicting return values?
179        Value *RV = RetVals[i];
180        if (!RV)
181          continue;
182
183        // Find the returned value
184        Value *V;
185        if (!STy || OldReturn)
186          V = RI->getOperand(i);
187        else
188          V = FindInsertedValue(RI->getOperand(0), i);
189
190        if (V) {
191          // Ignore undefs, we can change them into anything
192          if (isa<UndefValue>(V))
193            continue;
194
195          // Try to see if all the rets return the same constant or argument.
196          if (isa<Constant>(V) || isa<Argument>(V)) {
197            if (isa<UndefValue>(RV)) {
198              // No value found yet? Try the current one.
199              RetVals[i] = V;
200              continue;
201            }
202            // Returning the same value? Good.
203            if (RV == V)
204              continue;
205          }
206        }
207        // Different or no known return value? Don't propagate this return
208        // value.
209        RetVals[i] = 0;
210        // All values non constant? Stop looking.
211        if (++NumNonConstant == RetVals.size())
212          return false;
213      }
214    }
215
216  // If we got here, the function returns at least one constant value.  Loop
217  // over all users, replacing any uses of the return value with the returned
218  // constant.
219  bool MadeChange = false;
220  for (Value::use_iterator UI = F.use_begin(), E = F.use_end(); UI != E; ++UI) {
221    CallSite CS = CallSite::get(*UI);
222    Instruction* Call = CS.getInstruction();
223
224    // Not a call instruction or a call instruction that's not calling F
225    // directly?
226    if (!Call || UI.getOperandNo() != 0)
227      continue;
228
229    // Call result not used?
230    if (Call->use_empty())
231      continue;
232
233    MadeChange = true;
234
235    if (STy == 0) {
236      Value* New = RetVals[0];
237      if (Argument *A = dyn_cast<Argument>(New))
238        // Was an argument returned? Then find the corresponding argument in
239        // the call instruction and use that.
240        New = CS.getArgument(A->getArgNo());
241      Call->replaceAllUsesWith(New);
242      continue;
243    }
244
245    for (Value::use_iterator I = Call->use_begin(), E = Call->use_end();
246         I != E;) {
247      Instruction *Ins = dyn_cast<Instruction>(*I);
248
249      // Increment now, so we can remove the use
250      ++I;
251
252      // Not an instruction? Ignore
253      if (!Ins)
254        continue;
255
256      // Find the index of the retval to replace with
257      int index = -1;
258      if (ExtractValueInst *EV = dyn_cast<ExtractValueInst>(Ins))
259        if (EV->hasIndices())
260          index = *EV->idx_begin();
261
262      // If this use uses a specific return value, and we have a replacement,
263      // replace it.
264      if (index != -1) {
265        Value *New = RetVals[index];
266        if (New) {
267          if (Argument *A = dyn_cast<Argument>(New))
268            // Was an argument returned? Then find the corresponding argument in
269            // the call instruction and use that.
270            New = CS.getArgument(A->getArgNo());
271          Ins->replaceAllUsesWith(New);
272          Ins->eraseFromParent();
273        }
274      }
275    }
276  }
277
278  if (MadeChange) ++NumReturnValProped;
279  return MadeChange;
280}
281