IPConstantPropagation.cpp revision 551ccae044b0ff658fe629dd67edd5ffe75d10e8 
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 "llvm/ADT/Statistic.h"
24using namespace llvm;
25
26namespace {
27  Statistic<> NumArgumentsProped("ipconstprop",
28                                 "Number of args turned into constants");
29
30  /// IPCP - The interprocedural constant propagation pass
31  ///
32  struct IPCP : public Pass {
33    bool run(Module &M);
34  private:
35    bool processFunction(Function &F);
36  };
37  RegisterOpt<IPCP> X("ipconstprop", "Interprocedural constant propagation");
38}
39
40Pass *llvm::createIPConstantPropagationPass() { return new IPCP(); }
41
42bool IPCP::run(Module &M) {
43  bool Changed = false;
44  bool LocalChange = true;
45
46  // FIXME: instead of using smart algorithms, we just iterate until we stop
47  // making changes.
48  while (LocalChange) {
49    LocalChange = false;
50    for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
51      if (!I->isExternal() && I->hasInternalLinkage())
52        LocalChange |= processFunction(*I);
53    Changed |= LocalChange;
54  }
55  return Changed;
56}
57
58/// processFunction - Look at all uses of the specified function.  If all uses
59/// are direct call sites, and all pass a particular constant in for an
60/// argument, propagate that constant in as the argument.
61///
62bool IPCP::processFunction(Function &F) {
63  if (F.aempty() || F.use_empty()) return false;  // No arguments?  Early exit.
64
65  std::vector<std::pair<Constant*, bool> > ArgumentConstants;
66  ArgumentConstants.resize(F.asize());
67
68  unsigned NumNonconstant = 0;
69
70  for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I)
71    if (!isa<Instruction>(*I))
72      return false;  // Used by a non-instruction, do not transform
73    else {
74      CallSite CS = CallSite::get(cast<Instruction>(*I));
75      if (CS.getInstruction() == 0 ||
76          CS.getCalledFunction() != &F)
77        return false;  // Not a direct call site?
78
79      // Check out all of the potentially constant arguments
80      CallSite::arg_iterator AI = CS.arg_begin();
81      for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) {
82        if (*AI == &F) return false;  // Passes the function into itself
83
84        if (!ArgumentConstants[i].second) {
85          if (Constant *C = dyn_cast<Constant>(*AI)) {
86            if (!ArgumentConstants[i].first)
87              ArgumentConstants[i].first = C;
88            else if (ArgumentConstants[i].first != C) {
89              // Became non-constant
90              ArgumentConstants[i].second = true;
91              ++NumNonconstant;
92              if (NumNonconstant == ArgumentConstants.size()) return false;
93            }
94          } else {
95            // This is not a constant argument.  Mark the argument as
96            // non-constant.
97            ArgumentConstants[i].second = true;
98            ++NumNonconstant;
99            if (NumNonconstant == ArgumentConstants.size()) return false;
100          }
101        }
102      }
103    }
104
105  // If we got to this point, there is a constant argument!
106  assert(NumNonconstant != ArgumentConstants.size());
107  Function::aiterator AI = F.abegin();
108  bool MadeChange = false;
109  for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI)
110    // Do we have a constant argument!?
111    if (!ArgumentConstants[i].second && !AI->use_empty()) {
112      assert(ArgumentConstants[i].first && "Unknown constant value!");
113      Value *V = ArgumentConstants[i].first;
114      AI->replaceAllUsesWith(V);
115      ++NumArgumentsProped;
116      MadeChange = true;
117    }
118  return MadeChange;
119}
120
121