IPConstantPropagation.cpp revision a990d94330141404158a2ae6ba25661be4468b25 
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#include "llvm/Transforms/IPO.h"
19#include "llvm/Constants.h"
20#include "llvm/Instructions.h"
21#include "llvm/Module.h"
22#include "llvm/Pass.h"
23#include "llvm/Support/CallSite.h"
24#include "llvm/ADT/Statistic.h"
25using namespace llvm;
26
27namespace {
28  Statistic<> NumArgumentsProped("ipconstprop",
29                                 "Number of args turned into constants");
30  Statistic<> NumReturnValProped("ipconstprop",
31                                 "Number of return values turned into constants");
32
33  /// IPCP - The interprocedural constant propagation pass
34  ///
35  struct IPCP : public ModulePass {
36    bool runOnModule(Module &M);
37  private:
38    bool PropagateConstantsIntoArguments(Function &F);
39    bool PropagateConstantReturn(Function &F);
40  };
41  RegisterOpt<IPCP> X("ipconstprop", "Interprocedural constant propagation");
42}
43
44ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); }
45
46bool IPCP::runOnModule(Module &M) {
47  bool Changed = false;
48  bool LocalChange = true;
49
50  // FIXME: instead of using smart algorithms, we just iterate until we stop
51  // making changes.
52  while (LocalChange) {
53    LocalChange = false;
54    for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
55      if (!I->isExternal()) {
56        // Delete any klingons.
57        I->removeDeadConstantUsers();
58        if (I->hasInternalLinkage())
59          LocalChange |= PropagateConstantsIntoArguments(*I);
60        Changed |= PropagateConstantReturn(*I);
61      }
62    Changed |= LocalChange;
63  }
64  return Changed;
65}
66
67/// PropagateConstantsIntoArguments - Look at all uses of the specified
68/// function.  If all uses are direct call sites, and all pass a particular
69/// constant in for an argument, propagate that constant in as the argument.
70///
71bool IPCP::PropagateConstantsIntoArguments(Function &F) {
72  if (F.aempty() || F.use_empty()) return false;  // No arguments?  Early exit.
73
74  std::vector<std::pair<Constant*, bool> > ArgumentConstants;
75  ArgumentConstants.resize(F.asize());
76
77  unsigned NumNonconstant = 0;
78
79  for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I)
80    if (!isa<Instruction>(*I))
81      return false;  // Used by a non-instruction, do not transform
82    else {
83      CallSite CS = CallSite::get(cast<Instruction>(*I));
84      if (CS.getInstruction() == 0 ||
85          CS.getCalledFunction() != &F)
86        return false;  // Not a direct call site?
87
88      // Check out all of the potentially constant arguments
89      CallSite::arg_iterator AI = CS.arg_begin();
90      Function::aiterator Arg = F.abegin();
91      for (unsigned i = 0, e = ArgumentConstants.size(); i != e;
92           ++i, ++AI, ++Arg) {
93        if (*AI == &F) return false;  // Passes the function into itself
94
95        if (!ArgumentConstants[i].second) {
96          if (Constant *C = dyn_cast<Constant>(*AI)) {
97            if (!ArgumentConstants[i].first)
98              ArgumentConstants[i].first = C;
99            else if (ArgumentConstants[i].first != C) {
100              // Became non-constant
101              ArgumentConstants[i].second = true;
102              ++NumNonconstant;
103              if (NumNonconstant == ArgumentConstants.size()) return false;
104            }
105          } else if (*AI != &*Arg) {    // Ignore recursive calls with same arg
106            // This is not a constant argument.  Mark the argument as
107            // non-constant.
108            ArgumentConstants[i].second = true;
109            ++NumNonconstant;
110            if (NumNonconstant == ArgumentConstants.size()) return false;
111          }
112        }
113      }
114    }
115
116  // If we got to this point, there is a constant argument!
117  assert(NumNonconstant != ArgumentConstants.size());
118  Function::aiterator AI = F.abegin();
119  bool MadeChange = false;
120  for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI)
121    // Do we have a constant argument!?
122    if (!ArgumentConstants[i].second && !AI->use_empty()) {
123      Value *V = ArgumentConstants[i].first;
124      if (V == 0) V = UndefValue::get(AI->getType());
125      AI->replaceAllUsesWith(V);
126      ++NumArgumentsProped;
127      MadeChange = true;
128    }
129  return MadeChange;
130}
131
132
133// Check to see if this function returns a constant.  If so, replace all callers
134// that user the return value with the returned valued.  If we can replace ALL
135// callers,
136bool IPCP::PropagateConstantReturn(Function &F) {
137  if (F.getReturnType() == Type::VoidTy)
138    return false; // No return value.
139
140  // Check to see if this function returns a constant.
141  Value *RetVal = 0;
142  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
143    if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
144      if (isa<UndefValue>(RI->getOperand(0))) {
145        // Ignore.
146      } else if (Constant *C = dyn_cast<Constant>(RI->getOperand(0))) {
147        if (RetVal == 0)
148          RetVal = C;
149        else if (RetVal != C)
150          return false;  // Does not return the same constant.
151      } else {
152        return false;  // Does not return a constant.
153      }
154
155  if (RetVal == 0) RetVal = UndefValue::get(F.getReturnType());
156
157  // If we got here, the function returns a constant value.  Loop over all
158  // users, replacing any uses of the return value with the returned constant.
159  bool ReplacedAllUsers = true;
160  bool MadeChange = false;
161  for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I)
162    if (!isa<Instruction>(*I))
163      ReplacedAllUsers = false;
164    else {
165      CallSite CS = CallSite::get(cast<Instruction>(*I));
166      if (CS.getInstruction() == 0 ||
167          CS.getCalledFunction() != &F) {
168        ReplacedAllUsers = false;
169      } else {
170        if (!CS.getInstruction()->use_empty()) {
171          CS.getInstruction()->replaceAllUsesWith(RetVal);
172          MadeChange = true;
173        }
174      }
175    }
176
177  // If we replace all users with the returned constant, and there can be no
178  // other callers of the function, replace the constant being returned in the
179  // function with an undef value.
180  if (ReplacedAllUsers && F.hasInternalLinkage() && !isa<UndefValue>(RetVal)) {
181    Value *RV = UndefValue::get(RetVal->getType());
182    for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
183      if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
184        RI->setOperand(0, RV);
185    MadeChange = true;
186  }
187
188  if (MadeChange) ++NumReturnValProped;
189
190  // FIXME: DAE should remove dead return values if the result is an undef
191  // value... or if it is never used.
192
193  return MadeChange;
194}
195