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//===- GlobalConstifier.cpp - Mark read-only globals constant -------------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass loops over the non-constant internal global variables in the
// program. If it can prove that they are never written to, it marks them
// constant.
//
// NOTE: this should eventually use the alias analysis interfaces to do the
// transformation, but for now we just stick with a simple solution. DSA in
// particular could give a much more accurate answer to the mod/ref query, but
// it's not quite ready for this.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "Support/Debug.h"
#include "Support/Statistic.h"
#include <set>
using namespace llvm;
namespace {
Statistic<> NumMarked("constify", "Number of globals marked constant");
struct Constifier : public Pass {
bool run(Module &M);
};
RegisterOpt<Constifier> X("constify", "Global Constifier");
}
Pass *llvm::createGlobalConstifierPass() { return new Constifier(); }
/// A lot of global constants are stored only in trivially dead setter
/// functions. Because we don't want to cycle between globaldce and this pass,
/// just do a simple check to catch the common case.
static bool ContainingFunctionIsTriviallyDead(Instruction *I) {
Function *F = I->getParent()->getParent();
if (!F->hasInternalLinkage()) return false;
F->removeDeadConstantUsers();
return F->use_empty();
}
/// isStoredThrough - Return false if the specified pointer is provably never
/// stored through. If we can't tell, we must conservatively assume it might.
///
static bool isStoredThrough(Value *V, std::set<PHINode*> &PHIUsers) {
for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(*UI)) {
if (isStoredThrough(CE, PHIUsers))
return true;
} else if (Instruction *I = dyn_cast<Instruction>(*UI)) {
if (!ContainingFunctionIsTriviallyDead(I)) {
if (I->getOpcode() == Instruction::GetElementPtr ||
I->getOpcode() == Instruction::Select) {
if (isStoredThrough(I, PHIUsers)) return true;
} else if (PHINode *PN = dyn_cast<PHINode>(I)) {
// PHI nodes we can check just like select or GEP instructions, but we
// have to be careful about infinite recursion.
if (PHIUsers.insert(PN).second) // Not already visited.
if (isStoredThrough(I, PHIUsers)) return true;
} else if (!isa<LoadInst>(I) && !isa<SetCondInst>(I)) {
return true; // Any other non-load instruction might store!
}
}
} else {
// Otherwise must be a global or some other user.
return true;
}
return false;
}
bool Constifier::run(Module &M) {
bool Changed = false;
std::set<PHINode*> PHIUsers;
for (Module::giterator GV = M.gbegin(), E = M.gend(); GV != E; ++GV)
if (!GV->isConstant() && GV->hasInternalLinkage() && GV->hasInitializer()) {
if (!isStoredThrough(GV, PHIUsers)) {
DEBUG(std::cerr << "MARKING CONSTANT: " << *GV << "\n");
GV->setConstant(true);
++NumMarked;
Changed = true;
}
PHIUsers.clear();
}
return Changed;
}
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