IPConstantPropagation.cpp revision 1e2385b941242f2f96398dc62767420622856149
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" 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 (isa<Constant>(*AI) || isa<GlobalValue>(*AI)) { 86 Constant *C = dyn_cast<Constant>(*AI); 87 if (!C) C = ConstantPointerRef::get(cast<GlobalValue>(*AI)); 88 89 if (!ArgumentConstants[i].first) 90 ArgumentConstants[i].first = C; 91 else if (ArgumentConstants[i].first != C) { 92 // Became non-constant 93 ArgumentConstants[i].second = true; 94 ++NumNonconstant; 95 if (NumNonconstant == ArgumentConstants.size()) return false; 96 } 97 } else { 98 // This is not a constant argument. Mark the argument as 99 // non-constant. 100 ArgumentConstants[i].second = true; 101 ++NumNonconstant; 102 if (NumNonconstant == ArgumentConstants.size()) return false; 103 } 104 } 105 } 106 } 107 108 // If we got to this point, there is a constant argument! 109 assert(NumNonconstant != ArgumentConstants.size()); 110 Function::aiterator AI = F.abegin(); 111 bool MadeChange = false; 112 for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) 113 // Do we have a constant argument!? 114 if (!ArgumentConstants[i].second && !AI->use_empty()) { 115 assert(ArgumentConstants[i].first && "Unknown constant value!"); 116 Value *V = ArgumentConstants[i].first; 117 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V)) 118 V = CPR->getValue(); 119 120 AI->replaceAllUsesWith(V); 121 ++NumArgumentsProped; 122 MadeChange = true; 123 } 124 return MadeChange; 125} 126 127