IPConstantPropagation.cpp revision 2e56dd82b3b3ab9dde8cc55c06df78cec4dbd3b7 
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/Module.h"
20#include "llvm/Pass.h"
21#include "llvm/Constants.h"
22#include "llvm/Support/CallSite.h"
23#include "Support/Statistic.h"
24
25namespace {
26  Statistic<> NumArgumentsProped("ipconstprop",
27                                 "Number of args turned into constants");
28
29  /// IPCP - The interprocedural constant propagation pass
30  ///
31  struct IPCP : public Pass {
32    bool run(Module &M);
33  private:
34    bool processFunction(Function &F);
35  };
36  RegisterOpt<IPCP> X("ipconstprop", "Interprocedural constant propagation");
37}
38
39Pass *createIPConstantPropagationPass() { return new IPCP(); }
40
41bool IPCP::run(Module &M) {
42  bool Changed = false;
43  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
44    if (!I->isExternal() && I->hasInternalLinkage())
45      Changed |= processFunction(*I);
46  return Changed;
47}
48
49/// processFunction - Look at all uses of the specified function.  If all uses
50/// are direct call sites, and all pass a particular constant in for an
51/// argument, propagate that constant in as the argument.
52///
53bool IPCP::processFunction(Function &F) {
54  if (F.aempty() || F.use_empty()) return false;  // No arguments?  Early exit.
55
56  std::vector<std::pair<Constant*, bool> > ArgumentConstants;
57  ArgumentConstants.resize(F.asize());
58
59  unsigned NumNonconstant = 0;
60
61  for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I)
62    if (!isa<Instruction>(*I))
63      return false;  // Used by a non-instruction, do not transform
64    else {
65      CallSite CS = CallSite::get(cast<Instruction>(*I));
66      if (CS.getInstruction() == 0 ||
67          CS.getCalledFunction() != &F)
68        return false;  // Not a direct call site?
69
70      // Check out all of the potentially constant arguments
71      CallSite::arg_iterator AI = CS.arg_begin();
72      for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) {
73        if (*AI == &F) return false;  // Passes the function into itself
74
75        if (!ArgumentConstants[i].second) {
76          if (isa<Constant>(*AI) || isa<GlobalValue>(*AI)) {
77            Constant *C = dyn_cast<Constant>(*AI);
78            if (!C) C = ConstantPointerRef::get(cast<GlobalValue>(*AI));
79
80            if (!ArgumentConstants[i].first)
81              ArgumentConstants[i].first = C;
82            else if (ArgumentConstants[i].first != C) {
83              // Became non-constant
84              ArgumentConstants[i].second = true;
85              ++NumNonconstant;
86              if (NumNonconstant == ArgumentConstants.size()) return false;
87            }
88          } else {
89            // This is not a constant argument.  Mark the argument as
90            // non-constant.
91            ArgumentConstants[i].second = true;
92            ++NumNonconstant;
93            if (NumNonconstant == ArgumentConstants.size()) return false;
94          }
95        }
96      }
97    }
98
99  // If we got to this point, there is a constant argument!
100  assert(NumNonconstant != ArgumentConstants.size());
101  Function::aiterator AI = F.abegin();
102  for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI)
103    // Do we have a constant argument!?
104    if (!ArgumentConstants[i].second) {
105      assert(ArgumentConstants[i].first && "Unknown constant value!");
106      Value *V = ArgumentConstants[i].first;
107      if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V))
108        V = CPR->getValue();
109      AI->replaceAllUsesWith(V);
110      ++NumArgumentsProped;
111    }
112  return true;
113}
114