IPConstantPropagation.cpp revision 794fd75c67a2cdc128d67342c6d88a504d186896 
1//===-- IPConstantPropagation.cpp - Propagate constants through calls -----===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by the LLVM research group and is distributed under
6// the University of Illinois Open Source 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/Support/CallSite.h"
25#include "llvm/Support/Compiler.h"
26#include "llvm/ADT/Statistic.h"
27using namespace llvm;
28
29STATISTIC(NumArgumentsProped, "Number of args turned into constants");
30STATISTIC(NumReturnValProped, "Number of return values turned into constants");
31
32namespace {
33  /// IPCP - The interprocedural constant propagation pass
34  ///
35  struct VISIBILITY_HIDDEN IPCP : public ModulePass {
36    static const int ID; // Pass identifcation, replacement for typeid
37    IPCP() : ModulePass((intptr_t)&ID) {}
38
39    bool runOnModule(Module &M);
40  private:
41    bool PropagateConstantsIntoArguments(Function &F);
42    bool PropagateConstantReturn(Function &F);
43  };
44  const int IPCP::ID = 0;
45  RegisterPass<IPCP> X("ipconstprop", "Interprocedural constant propagation");
46}
47
48ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); }
49
50bool IPCP::runOnModule(Module &M) {
51  bool Changed = false;
52  bool LocalChange = true;
53
54  // FIXME: instead of using smart algorithms, we just iterate until we stop
55  // making changes.
56  while (LocalChange) {
57    LocalChange = false;
58    for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
59      if (!I->isDeclaration()) {
60        // Delete any klingons.
61        I->removeDeadConstantUsers();
62        if (I->hasInternalLinkage())
63          LocalChange |= PropagateConstantsIntoArguments(*I);
64        Changed |= PropagateConstantReturn(*I);
65      }
66    Changed |= LocalChange;
67  }
68  return Changed;
69}
70
71/// PropagateConstantsIntoArguments - Look at all uses of the specified
72/// function.  If all uses are direct call sites, and all pass a particular
73/// constant in for an argument, propagate that constant in as the argument.
74///
75bool IPCP::PropagateConstantsIntoArguments(Function &F) {
76  if (F.arg_empty() || F.use_empty()) return false; // No arguments? Early exit.
77
78  std::vector<std::pair<Constant*, bool> > ArgumentConstants;
79  ArgumentConstants.resize(F.arg_size());
80
81  unsigned NumNonconstant = 0;
82
83  for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I)
84    if (!isa<Instruction>(*I))
85      return false;  // Used by a non-instruction, do not transform
86    else {
87      CallSite CS = CallSite::get(cast<Instruction>(*I));
88      if (CS.getInstruction() == 0 ||
89          CS.getCalledFunction() != &F)
90        return false;  // Not a direct call site?
91
92      // Check out all of the potentially constant arguments
93      CallSite::arg_iterator AI = CS.arg_begin();
94      Function::arg_iterator Arg = F.arg_begin();
95      for (unsigned i = 0, e = ArgumentConstants.size(); i != e;
96           ++i, ++AI, ++Arg) {
97        if (*AI == &F) return false;  // Passes the function into itself
98
99        if (!ArgumentConstants[i].second) {
100          if (Constant *C = dyn_cast<Constant>(*AI)) {
101            if (!ArgumentConstants[i].first)
102              ArgumentConstants[i].first = C;
103            else if (ArgumentConstants[i].first != C) {
104              // Became non-constant
105              ArgumentConstants[i].second = true;
106              ++NumNonconstant;
107              if (NumNonconstant == ArgumentConstants.size()) return false;
108            }
109          } else if (*AI != &*Arg) {    // Ignore recursive calls with same arg
110            // This is not a constant argument.  Mark the argument as
111            // non-constant.
112            ArgumentConstants[i].second = true;
113            ++NumNonconstant;
114            if (NumNonconstant == ArgumentConstants.size()) return false;
115          }
116        }
117      }
118    }
119
120  // If we got to this point, there is a constant argument!
121  assert(NumNonconstant != ArgumentConstants.size());
122  Function::arg_iterator AI = F.arg_begin();
123  bool MadeChange = false;
124  for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI)
125    // Do we have a constant argument!?
126    if (!ArgumentConstants[i].second && !AI->use_empty()) {
127      Value *V = ArgumentConstants[i].first;
128      if (V == 0) V = UndefValue::get(AI->getType());
129      AI->replaceAllUsesWith(V);
130      ++NumArgumentsProped;
131      MadeChange = true;
132    }
133  return MadeChange;
134}
135
136
137// Check to see if this function returns a constant.  If so, replace all callers
138// that user the return value with the returned valued.  If we can replace ALL
139// callers,
140bool IPCP::PropagateConstantReturn(Function &F) {
141  if (F.getReturnType() == Type::VoidTy)
142    return false; // No return value.
143
144  // Check to see if this function returns a constant.
145  Value *RetVal = 0;
146  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
147    if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
148      if (isa<UndefValue>(RI->getOperand(0))) {
149        // Ignore.
150      } else if (Constant *C = dyn_cast<Constant>(RI->getOperand(0))) {
151        if (RetVal == 0)
152          RetVal = C;
153        else if (RetVal != C)
154          return false;  // Does not return the same constant.
155      } else {
156        return false;  // Does not return a constant.
157      }
158
159  if (RetVal == 0) RetVal = UndefValue::get(F.getReturnType());
160
161  // If we got here, the function returns a constant value.  Loop over all
162  // users, replacing any uses of the return value with the returned constant.
163  bool ReplacedAllUsers = true;
164  bool MadeChange = false;
165  for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I)
166    if (!isa<Instruction>(*I))
167      ReplacedAllUsers = false;
168    else {
169      CallSite CS = CallSite::get(cast<Instruction>(*I));
170      if (CS.getInstruction() == 0 ||
171          CS.getCalledFunction() != &F) {
172        ReplacedAllUsers = false;
173      } else {
174        if (!CS.getInstruction()->use_empty()) {
175          CS.getInstruction()->replaceAllUsesWith(RetVal);
176          MadeChange = true;
177        }
178      }
179    }
180
181  // If we replace all users with the returned constant, and there can be no
182  // other callers of the function, replace the constant being returned in the
183  // function with an undef value.
184  if (ReplacedAllUsers && F.hasInternalLinkage() && !isa<UndefValue>(RetVal)) {
185    Value *RV = UndefValue::get(RetVal->getType());
186    for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
187      if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
188        if (RI->getOperand(0) != RV) {
189          RI->setOperand(0, RV);
190          MadeChange = true;
191        }
192      }
193  }
194
195  if (MadeChange) ++NumReturnValProped;
196  return MadeChange;
197}
198