IPConstantPropagation.cpp revision ff1529b6cf3b4babad13a39587a20f41b4b46577
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 ModulePass { 33 bool runOnModule(Module &M); 34 private: 35 bool processFunction(Function &F); 36 }; 37 RegisterOpt<IPCP> X("ipconstprop", "Interprocedural constant propagation"); 38} 39 40ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); } 41 42bool IPCP::runOnModule(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 // Delete any klingons. 66 F.removeDeadConstantUsers(); 67 68 std::vector<std::pair<Constant*, bool> > ArgumentConstants; 69 ArgumentConstants.resize(F.asize()); 70 71 unsigned NumNonconstant = 0; 72 73 for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I) 74 if (!isa<Instruction>(*I)) 75 return false; // Used by a non-instruction, do not transform 76 else { 77 CallSite CS = CallSite::get(cast<Instruction>(*I)); 78 if (CS.getInstruction() == 0 || 79 CS.getCalledFunction() != &F) 80 return false; // Not a direct call site? 81 82 // Check out all of the potentially constant arguments 83 CallSite::arg_iterator AI = CS.arg_begin(); 84 Function::aiterator Arg = F.abegin(); 85 for (unsigned i = 0, e = ArgumentConstants.size(); i != e; 86 ++i, ++AI, ++Arg) { 87 if (*AI == &F) return false; // Passes the function into itself 88 89 if (!ArgumentConstants[i].second) { 90 if (Constant *C = dyn_cast<Constant>(*AI)) { 91 if (!ArgumentConstants[i].first) 92 ArgumentConstants[i].first = C; 93 else if (ArgumentConstants[i].first != C) { 94 // Became non-constant 95 ArgumentConstants[i].second = true; 96 ++NumNonconstant; 97 if (NumNonconstant == ArgumentConstants.size()) return false; 98 } 99 } else if (*AI != &*Arg) { // Ignore recursive calls with same arg 100 // This is not a constant argument. Mark the argument as 101 // non-constant. 102 ArgumentConstants[i].second = true; 103 ++NumNonconstant; 104 if (NumNonconstant == ArgumentConstants.size()) return false; 105 } 106 } 107 } 108 } 109 110 // If we got to this point, there is a constant argument! 111 assert(NumNonconstant != ArgumentConstants.size()); 112 Function::aiterator AI = F.abegin(); 113 bool MadeChange = false; 114 for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) 115 // Do we have a constant argument!? 116 if (!ArgumentConstants[i].second && !AI->use_empty()) { 117 assert(ArgumentConstants[i].first && "Unknown constant value!"); 118 Value *V = ArgumentConstants[i].first; 119 AI->replaceAllUsesWith(V); 120 ++NumArgumentsProped; 121 MadeChange = true; 122 } 123 return MadeChange; 124} 125 126