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83 lines
2.9 KiB
C++
83 lines
2.9 KiB
C++
//===- GlobalConstifier.cpp - Mark read-only globals constant -------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass loops over the non-constant internal global variables in the
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// program. If it can prove that they are never written to, it marks them
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// constant.
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//
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// NOTE: this should eventually use the alias analysis interfaces to do the
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// transformation, but for now we just stick with a simple solution. DSA in
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// particular could give a much more accurate answer to the mod/ref query, but
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// it's not quite ready for this.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/IPO.h"
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#include "llvm/Constants.h"
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#include "llvm/Instructions.h"
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#include "llvm/Module.h"
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#include "llvm/Pass.h"
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#include "Support/Debug.h"
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#include "Support/Statistic.h"
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using namespace llvm;
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namespace {
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Statistic<> NumMarked("constify", "Number of globals marked constant");
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struct Constifier : public Pass {
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bool run(Module &M);
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};
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RegisterOpt<Constifier> X("constify", "Global Constifier");
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}
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Pass *llvm::createGlobalConstifierPass() { return new Constifier(); }
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/// isStoredThrough - Return false if the specified pointer is provably never
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/// stored through. If we can't tell, we must conservatively assume it might.
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///
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static bool isStoredThrough(Value *V) {
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for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
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if (Constant *C = dyn_cast<Constant>(*UI)) {
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if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
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if (isStoredThrough(CE))
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return true;
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} else if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C)) {
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if (isStoredThrough(CPR)) return true;
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} else {
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// Must be an element of a constant array or something.
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return true;
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}
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} else if (Instruction *I = dyn_cast<Instruction>(*UI)) {
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if (I->getOpcode() == Instruction::GetElementPtr) {
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if (isStoredThrough(I)) return true;
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} else if (!isa<LoadInst>(*UI) && !isa<SetCondInst>(*UI))
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return true; // Any other non-load instruction might store!
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} else {
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// Otherwise must be a global or some other user.
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return true;
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}
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return false;
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}
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bool Constifier::run(Module &M) {
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bool Changed = false;
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for (Module::giterator GV = M.gbegin(), E = M.gend(); GV != E; ++GV)
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if (!GV->isConstant() && GV->hasInternalLinkage() && GV->hasInitializer()) {
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if (!isStoredThrough(GV)) {
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DEBUG(std::cerr << "MARKING CONSTANT: " << *GV << "\n");
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GV->setConstant(true);
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++NumMarked;
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Changed = true;
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}
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}
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return Changed;
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}
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