1 //===-- IPConstantPropagation.cpp - Propagate constants through calls -----===//
3 // The LLVM Compiler Infrastructure
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.
8 //===----------------------------------------------------------------------===//
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.
16 //===----------------------------------------------------------------------===//
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"
27 Statistic<> NumArgumentsProped("ipconstprop",
28 "Number of args turned into constants");
30 /// IPCP - The interprocedural constant propagation pass
32 struct IPCP : public ModulePass {
33 bool runOnModule(Module &M);
35 bool processFunction(Function &F);
37 RegisterOpt<IPCP> X("ipconstprop", "Interprocedural constant propagation");
40 ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); }
42 bool IPCP::runOnModule(Module &M) {
44 bool LocalChange = true;
46 // FIXME: instead of using smart algorithms, we just iterate until we stop
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;
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.
62 bool IPCP::processFunction(Function &F) {
63 if (F.aempty() || F.use_empty()) return false; // No arguments? Early exit.
65 std::vector<std::pair<Constant*, bool> > ArgumentConstants;
66 ArgumentConstants.resize(F.asize());
68 unsigned NumNonconstant = 0;
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
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?
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
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;
92 if (NumNonconstant == ArgumentConstants.size()) return false;
95 // This is not a constant argument. Mark the argument as
97 ArgumentConstants[i].second = true;
99 if (NumNonconstant == ArgumentConstants.size()) return false;
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;