lib/CodeGen/GlobalMerge.cpp - platform/external/llvm - Git at Google

 //===-- GlobalMerge.cpp - Internal globals merging  -----------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 // This pass merges globals with internal linkage into one. This way all the
 // globals which were merged into a biggest one can be addressed using offsets
 // from the same base pointer (no need for separate base pointer for each of the
 // global). Such a transformation can significantly reduce the register pressure
 // when many globals are involved.
 //
 // For example, consider the code which touches several global variables at
 // once:
 //
 // static int foo[N], bar[N], baz[N];
 //
 // for (i = 0; i < N; ++i) {
 //    foo[i] = bar[i] * baz[i];
 // }
 //
 //  On ARM the addresses of 3 arrays should be kept in the registers, thus
 //  this code has quite large register pressure (loop body):
 //
 //  ldr     r1, [r5], #4
 //  ldr     r2, [r6], #4
 //  mul     r1, r2, r1
 //  str     r1, [r0], #4
 //
 //  Pass converts the code to something like:
 //
 //  static struct {
 //    int foo[N];
 //    int bar[N];
 //    int baz[N];
 //  } merged;
 //
 //  for (i = 0; i < N; ++i) {
 //    merged.foo[i] = merged.bar[i] * merged.baz[i];
 //  }
 //
 //  and in ARM code this becomes:
 //
 //  ldr     r0, [r5, #40]
 //  ldr     r1, [r5, #80]
 //  mul     r0, r1, r0
 //  str     r0, [r5], #4
 //
 //  note that we saved 2 registers here almostly "for free".
 // ===---------------------------------------------------------------------===//

 #include "llvm/Transforms/Scalar.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/Module.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Target/TargetLowering.h"
 #include "llvm/Target/TargetLoweringObjectFile.h"
 #include "llvm/Target/TargetSubtargetInfo.h"
 using namespace llvm;

 #define DEBUG_TYPE "global-merge"

 // FIXME: This is only useful as a last-resort way to disable the pass.
 cl::opt<bool>
 EnableGlobalMerge("enable-global-merge", cl::Hidden,
                   cl::desc("Enable the global merge pass"),
                   cl::init(true));

 static cl::opt<bool>
 EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
                          cl::desc("Enable global merge pass on constants"),
                          cl::init(false));

 // FIXME: this could be a transitional option, and we probably need to remove
 // it if only we are sure this optimization could always benefit all targets.
 static cl::opt<bool>
 EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
      cl::desc("Enable global merge pass on external linkage"),
      cl::init(false));

 STATISTIC(NumMerged, "Number of globals merged");
 namespace {
   class GlobalMerge : public FunctionPass {
     const TargetMachine *TM;
     const DataLayout *DL;
     // FIXME: Infer the maximum possible offset depending on the actual users
     // (these max offsets are different for the users inside Thumb or ARM
     // functions), see the code that passes in the offset in the ARM backend
     // for more information.
     unsigned MaxOffset;

     bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
                  Module &M, bool isConst, unsigned AddrSpace) const;

     /// \brief Check if the given variable has been identified as must keep
     /// \pre setMustKeepGlobalVariables must have been called on the Module that
     ///      contains GV
     bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
       return MustKeepGlobalVariables.count(GV);
     }

     /// Collect every variables marked as "used" or used in a landing pad
     /// instruction for this Module.
     void setMustKeepGlobalVariables(Module &M);

     /// Collect every variables marked as "used"
     void collectUsedGlobalVariables(Module &M);

     /// Keep track of the GlobalVariable that must not be merged away
     SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;

   public:
     static char ID;             // Pass identification, replacement for typeid.
     explicit GlobalMerge(const TargetMachine *TM = nullptr,
                          unsigned MaximalOffset = 0)
         : FunctionPass(ID), TM(TM), DL(TM->getDataLayout()),
           MaxOffset(MaximalOffset) {
       initializeGlobalMergePass(*PassRegistry::getPassRegistry());
     }

     bool doInitialization(Module &M) override;
     bool runOnFunction(Function &F) override;
     bool doFinalization(Module &M) override;

     const char *getPassName() const override {
       return "Merge internal globals";
     }

     void getAnalysisUsage(AnalysisUsage &AU) const override {
       AU.setPreservesCFG();
       FunctionPass::getAnalysisUsage(AU);
     }
   };
 } // end anonymous namespace

 char GlobalMerge::ID = 0;
 INITIALIZE_PASS_BEGIN(GlobalMerge, "global-merge", "Merge global variables",
                       false, false)
 INITIALIZE_PASS_END(GlobalMerge, "global-merge", "Merge global variables",
                     false, false)

 bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
                           Module &M, bool isConst, unsigned AddrSpace) const {
   // FIXME: Find better heuristics
   std::stable_sort(Globals.begin(), Globals.end(),
                    [this](const GlobalVariable *GV1, const GlobalVariable *GV2) {
     Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType();
     Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType();

     return (DL->getTypeAllocSize(Ty1) < DL->getTypeAllocSize(Ty2));
   });

   Type *Int32Ty = Type::getInt32Ty(M.getContext());

   assert(Globals.size() > 1);

   // FIXME: This simple solution merges globals all together as maximum as
   // possible. However, with this solution it would be hard to remove dead
   // global symbols at link-time. An alternative solution could be checking
   // global symbols references function by function, and make the symbols
   // being referred in the same function merged and we would probably need
   // to introduce heuristic algorithm to solve the merge conflict from
   // different functions.
   for (size_t i = 0, e = Globals.size(); i != e; ) {
     size_t j = 0;
     uint64_t MergedSize = 0;
     std::vector<Type*> Tys;
     std::vector<Constant*> Inits;

     bool HasExternal = false;
     GlobalVariable *TheFirstExternal = 0;
     for (j = i; j != e; ++j) {
       Type *Ty = Globals[j]->getType()->getElementType();
       MergedSize += DL->getTypeAllocSize(Ty);
       if (MergedSize > MaxOffset) {
         break;
       }
       Tys.push_back(Ty);
       Inits.push_back(Globals[j]->getInitializer());

       if (Globals[j]->hasExternalLinkage() && !HasExternal) {
         HasExternal = true;
         TheFirstExternal = Globals[j];
       }
     }

     // If merged variables doesn't have external linkage, we needn't to expose
     // the symbol after merging.
     GlobalValue::LinkageTypes Linkage = HasExternal
                                             ? GlobalValue::ExternalLinkage
                                             : GlobalValue::InternalLinkage;

     StructType *MergedTy = StructType::get(M.getContext(), Tys);
     Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);

     // If merged variables have external linkage, we use symbol name of the
     // first variable merged as the suffix of global symbol name. This would
     // be able to avoid the link-time naming conflict for globalm symbols.
     GlobalVariable *MergedGV = new GlobalVariable(
         M, MergedTy, isConst, Linkage, MergedInit,
         HasExternal ? "_MergedGlobals_" + TheFirstExternal->getName()
                     : "_MergedGlobals",
         nullptr, GlobalVariable::NotThreadLocal, AddrSpace);

     for (size_t k = i; k < j; ++k) {
       GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
       std::string Name = Globals[k]->getName();

       Constant *Idx[2] = {
         ConstantInt::get(Int32Ty, 0),
         ConstantInt::get(Int32Ty, k-i)
       };
       Constant *GEP =
           ConstantExpr::getInBoundsGetElementPtr(MergedTy, MergedGV, Idx);
       Globals[k]->replaceAllUsesWith(GEP);
       Globals[k]->eraseFromParent();

       if (Linkage != GlobalValue::InternalLinkage) {
         // Generate a new alias...
         auto *PTy = cast<PointerType>(GEP->getType());
         GlobalAlias::create(PTy->getElementType(), PTy->getAddressSpace(),
                             Linkage, Name, GEP, &M);
       }

       NumMerged++;
     }
     i = j;
   }

   return true;
 }

 void GlobalMerge::collectUsedGlobalVariables(Module &M) {
   // Extract global variables from llvm.used array
   const GlobalVariable *GV = M.getGlobalVariable("llvm.used");
   if (!GV || !GV->hasInitializer()) return;

   // Should be an array of 'i8*'.
   const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());

   for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
     if (const GlobalVariable *G =
         dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
       MustKeepGlobalVariables.insert(G);
 }

 void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
   collectUsedGlobalVariables(M);

   for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn;
        ++IFn) {
     for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
          IBB != IEndBB; ++IBB) {
       // Follow the invoke link to find the landing pad instruction
       const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator());
       if (!II) continue;

       const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst();
       // Look for globals in the clauses of the landing pad instruction
       for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses();
            Idx != NumClauses; ++Idx)
         if (const GlobalVariable *GV =
             dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
                                      ->stripPointerCasts()))
           MustKeepGlobalVariables.insert(GV);
     }
   }
 }

 bool GlobalMerge::doInitialization(Module &M) {
   if (!EnableGlobalMerge)
     return false;

   DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
                                                         BSSGlobals;
   bool Changed = false;
   setMustKeepGlobalVariables(M);

   // Grab all non-const globals.
   for (Module::global_iterator I = M.global_begin(),
          E = M.global_end(); I != E; ++I) {
     // Merge is safe for "normal" internal or external globals only
     if (I->isDeclaration() || I->isThreadLocal() || I->hasSection())
       continue;

     if (!(EnableGlobalMergeOnExternal && I->hasExternalLinkage()) &&
         !I->hasInternalLinkage())
       continue;

     PointerType *PT = dyn_cast<PointerType>(I->getType());
     assert(PT && "Global variable is not a pointer!");

     unsigned AddressSpace = PT->getAddressSpace();

     // Ignore fancy-aligned globals for now.
     unsigned Alignment = DL->getPreferredAlignment(I);
     Type *Ty = I->getType()->getElementType();
     if (Alignment > DL->getABITypeAlignment(Ty))
       continue;

     // Ignore all 'special' globals.
     if (I->getName().startswith("llvm.") ||
         I->getName().startswith(".llvm."))
       continue;

     // Ignore all "required" globals:
     if (isMustKeepGlobalVariable(I))
       continue;

     if (DL->getTypeAllocSize(Ty) < MaxOffset) {
       if (TargetLoweringObjectFile::getKindForGlobal(I, *TM).isBSSLocal())
         BSSGlobals[AddressSpace].push_back(I);
       else if (I->isConstant())
         ConstGlobals[AddressSpace].push_back(I);
       else
         Globals[AddressSpace].push_back(I);
     }
   }

   for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
        I = Globals.begin(), E = Globals.end(); I != E; ++I)
     if (I->second.size() > 1)
       Changed |= doMerge(I->second, M, false, I->first);

   for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
        I = BSSGlobals.begin(), E = BSSGlobals.end(); I != E; ++I)
     if (I->second.size() > 1)
       Changed |= doMerge(I->second, M, false, I->first);

   if (EnableGlobalMergeOnConst)
     for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
          I = ConstGlobals.begin(), E = ConstGlobals.end(); I != E; ++I)
       if (I->second.size() > 1)
         Changed |= doMerge(I->second, M, true, I->first);

   return Changed;
 }

 bool GlobalMerge::runOnFunction(Function &F) {
   return false;
 }

 bool GlobalMerge::doFinalization(Module &M) {
   MustKeepGlobalVariables.clear();
   return false;
 }

 Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset) {
   return new GlobalMerge(TM, Offset);
 }
	//===-- GlobalMerge.cpp - Internal globals merging -----------------------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	// This pass merges globals with internal linkage into one. This way all the
	// globals which were merged into a biggest one can be addressed using offsets
	// from the same base pointer (no need for separate base pointer for each of the
	// global). Such a transformation can significantly reduce the register pressure
	// when many globals are involved.
	//
	// For example, consider the code which touches several global variables at
	// once:
	//
	// static int foo[N], bar[N], baz[N];
	//
	// for (i = 0; i < N; ++i) {
	// foo[i] = bar[i] * baz[i];
	// }
	//
	// On ARM the addresses of 3 arrays should be kept in the registers, thus
	// this code has quite large register pressure (loop body):
	//
	// ldr r1, [r5], #4
	// ldr r2, [r6], #4
	// mul r1, r2, r1
	// str r1, [r0], #4
	//
	// Pass converts the code to something like:
	//
	// static struct {
	// int foo[N];
	// int bar[N];
	// int baz[N];
	// } merged;
	//
	// for (i = 0; i < N; ++i) {
	// merged.foo[i] = merged.bar[i] * merged.baz[i];
	// }
	//
	// and in ARM code this becomes:
	//
	// ldr r0, [r5, #40]
	// ldr r1, [r5, #80]
	// mul r0, r1, r0
	// str r0, [r5], #4
	//
	// note that we saved 2 registers here almostly "for free".
	// ===---------------------------------------------------------------------===//

	#include "llvm/Transforms/Scalar.h"
	#include "llvm/ADT/SmallPtrSet.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/CodeGen/Passes.h"
	#include "llvm/IR/Attributes.h"
	#include "llvm/IR/Constants.h"
	#include "llvm/IR/DataLayout.h"
	#include "llvm/IR/DerivedTypes.h"
	#include "llvm/IR/Function.h"
	#include "llvm/IR/GlobalVariable.h"
	#include "llvm/IR/Instructions.h"
	#include "llvm/IR/Intrinsics.h"
	#include "llvm/IR/Module.h"
	#include "llvm/Pass.h"
	#include "llvm/Support/CommandLine.h"
	#include "llvm/Target/TargetLowering.h"
	#include "llvm/Target/TargetLoweringObjectFile.h"
	#include "llvm/Target/TargetSubtargetInfo.h"
	using namespace llvm;

	#define DEBUG_TYPE "global-merge"

	// FIXME: This is only useful as a last-resort way to disable the pass.
	cl::opt<bool>
	EnableGlobalMerge("enable-global-merge", cl::Hidden,
	cl::desc("Enable the global merge pass"),
	cl::init(true));

	static cl::opt<bool>
	EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
	cl::desc("Enable global merge pass on constants"),
	cl::init(false));

	// FIXME: this could be a transitional option, and we probably need to remove
	// it if only we are sure this optimization could always benefit all targets.
	static cl::opt<bool>
	EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
	cl::desc("Enable global merge pass on external linkage"),
	cl::init(false));

	STATISTIC(NumMerged, "Number of globals merged");
	namespace {
	class GlobalMerge : public FunctionPass {
	const TargetMachine *TM;
	const DataLayout *DL;
	// FIXME: Infer the maximum possible offset depending on the actual users
	// (these max offsets are different for the users inside Thumb or ARM
	// functions), see the code that passes in the offset in the ARM backend
	// for more information.
	unsigned MaxOffset;

	bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
	Module &M, bool isConst, unsigned AddrSpace) const;

	/// \brief Check if the given variable has been identified as must keep
	/// \pre setMustKeepGlobalVariables must have been called on the Module that
	/// contains GV
	bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
	return MustKeepGlobalVariables.count(GV);
	}

	/// Collect every variables marked as "used" or used in a landing pad
	/// instruction for this Module.
	void setMustKeepGlobalVariables(Module &M);

	/// Collect every variables marked as "used"
	void collectUsedGlobalVariables(Module &M);

	/// Keep track of the GlobalVariable that must not be merged away
	SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;

	public:
	static char ID; // Pass identification, replacement for typeid.
	explicit GlobalMerge(const TargetMachine *TM = nullptr,
	unsigned MaximalOffset = 0)
	: FunctionPass(ID), TM(TM), DL(TM->getDataLayout()),
	MaxOffset(MaximalOffset) {
	initializeGlobalMergePass(*PassRegistry::getPassRegistry());
	}

	bool doInitialization(Module &M) override;
	bool runOnFunction(Function &F) override;
	bool doFinalization(Module &M) override;

	const char *getPassName() const override {
	return "Merge internal globals";
	}

	void getAnalysisUsage(AnalysisUsage &AU) const override {
	AU.setPreservesCFG();
	FunctionPass::getAnalysisUsage(AU);
	}
	};
	} // end anonymous namespace

	char GlobalMerge::ID = 0;
	INITIALIZE_PASS_BEGIN(GlobalMerge, "global-merge", "Merge global variables",
	false, false)
	INITIALIZE_PASS_END(GlobalMerge, "global-merge", "Merge global variables",
	false, false)

	bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
	Module &M, bool isConst, unsigned AddrSpace) const {
	// FIXME: Find better heuristics
	std::stable_sort(Globals.begin(), Globals.end(),
	[this](const GlobalVariable GV1, const GlobalVariable GV2) {
	Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType();
	Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType();

	return (DL->getTypeAllocSize(Ty1) < DL->getTypeAllocSize(Ty2));
	});

	Type *Int32Ty = Type::getInt32Ty(M.getContext());

	assert(Globals.size() > 1);

	// FIXME: This simple solution merges globals all together as maximum as
	// possible. However, with this solution it would be hard to remove dead
	// global symbols at link-time. An alternative solution could be checking
	// global symbols references function by function, and make the symbols
	// being referred in the same function merged and we would probably need
	// to introduce heuristic algorithm to solve the merge conflict from
	// different functions.
	for (size_t i = 0, e = Globals.size(); i != e; ) {
	size_t j = 0;
	uint64_t MergedSize = 0;
	std::vector<Type*> Tys;
	std::vector<Constant*> Inits;

	bool HasExternal = false;
	GlobalVariable *TheFirstExternal = 0;
	for (j = i; j != e; ++j) {
	Type *Ty = Globals[j]->getType()->getElementType();
	MergedSize += DL->getTypeAllocSize(Ty);
	if (MergedSize > MaxOffset) {
	break;
	}
	Tys.push_back(Ty);
	Inits.push_back(Globals[j]->getInitializer());

	if (Globals[j]->hasExternalLinkage() && !HasExternal) {
	HasExternal = true;
	TheFirstExternal = Globals[j];
	}
	}

	// If merged variables doesn't have external linkage, we needn't to expose
	// the symbol after merging.
	GlobalValue::LinkageTypes Linkage = HasExternal
	? GlobalValue::ExternalLinkage
	: GlobalValue::InternalLinkage;

	StructType *MergedTy = StructType::get(M.getContext(), Tys);
	Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);

	// If merged variables have external linkage, we use symbol name of the
	// first variable merged as the suffix of global symbol name. This would
	// be able to avoid the link-time naming conflict for globalm symbols.
	GlobalVariable *MergedGV = new GlobalVariable(
	M, MergedTy, isConst, Linkage, MergedInit,
	HasExternal ? "_MergedGlobals_" + TheFirstExternal->getName()
	: "_MergedGlobals",
	nullptr, GlobalVariable::NotThreadLocal, AddrSpace);

	for (size_t k = i; k < j; ++k) {
	GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
	std::string Name = Globals[k]->getName();

	Constant *Idx[2] = {
	ConstantInt::get(Int32Ty, 0),
	ConstantInt::get(Int32Ty, k-i)
	};
	Constant *GEP =
	ConstantExpr::getInBoundsGetElementPtr(MergedTy, MergedGV, Idx);
	Globals[k]->replaceAllUsesWith(GEP);
	Globals[k]->eraseFromParent();

	if (Linkage != GlobalValue::InternalLinkage) {
	// Generate a new alias...
	auto *PTy = cast<PointerType>(GEP->getType());
	GlobalAlias::create(PTy->getElementType(), PTy->getAddressSpace(),
	Linkage, Name, GEP, &M);
	}

	NumMerged++;
	}
	i = j;
	}

	return true;
	}

	void GlobalMerge::collectUsedGlobalVariables(Module &M) {
	// Extract global variables from llvm.used array
	const GlobalVariable *GV = M.getGlobalVariable("llvm.used");
	if (!GV \|\| !GV->hasInitializer()) return;

	// Should be an array of 'i8*'.
	const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());

	for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
	if (const GlobalVariable *G =
	dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
	MustKeepGlobalVariables.insert(G);
	}

	void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
	collectUsedGlobalVariables(M);

	for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn;
	++IFn) {
	for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
	IBB != IEndBB; ++IBB) {
	// Follow the invoke link to find the landing pad instruction
	const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator());
	if (!II) continue;

	const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst();
	// Look for globals in the clauses of the landing pad instruction
	for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses();
	Idx != NumClauses; ++Idx)
	if (const GlobalVariable *GV =
	dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
	->stripPointerCasts()))
	MustKeepGlobalVariables.insert(GV);
	}
	}
	}

	bool GlobalMerge::doInitialization(Module &M) {
	if (!EnableGlobalMerge)
	return false;

	DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
	BSSGlobals;
	bool Changed = false;
	setMustKeepGlobalVariables(M);

	// Grab all non-const globals.
	for (Module::global_iterator I = M.global_begin(),
	E = M.global_end(); I != E; ++I) {
	// Merge is safe for "normal" internal or external globals only
	if (I->isDeclaration() \|\| I->isThreadLocal() \|\| I->hasSection())
	continue;

	if (!(EnableGlobalMergeOnExternal && I->hasExternalLinkage()) &&
	!I->hasInternalLinkage())
	continue;

	PointerType *PT = dyn_cast<PointerType>(I->getType());
	assert(PT && "Global variable is not a pointer!");

	unsigned AddressSpace = PT->getAddressSpace();

	// Ignore fancy-aligned globals for now.
	unsigned Alignment = DL->getPreferredAlignment(I);
	Type *Ty = I->getType()->getElementType();
	if (Alignment > DL->getABITypeAlignment(Ty))
	continue;

	// Ignore all 'special' globals.
	if (I->getName().startswith("llvm.") \|\|
	I->getName().startswith(".llvm."))
	continue;

	// Ignore all "required" globals:
	if (isMustKeepGlobalVariable(I))
	continue;

	if (DL->getTypeAllocSize(Ty) < MaxOffset) {
	if (TargetLoweringObjectFile::getKindForGlobal(I, *TM).isBSSLocal())
	BSSGlobals[AddressSpace].push_back(I);
	else if (I->isConstant())
	ConstGlobals[AddressSpace].push_back(I);
	else
	Globals[AddressSpace].push_back(I);
	}
	}

	for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
	I = Globals.begin(), E = Globals.end(); I != E; ++I)
	if (I->second.size() > 1)
	Changed \|= doMerge(I->second, M, false, I->first);

	for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
	I = BSSGlobals.begin(), E = BSSGlobals.end(); I != E; ++I)
	if (I->second.size() > 1)
	Changed \|= doMerge(I->second, M, false, I->first);

	if (EnableGlobalMergeOnConst)
	for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
	I = ConstGlobals.begin(), E = ConstGlobals.end(); I != E; ++I)
	if (I->second.size() > 1)
	Changed \|= doMerge(I->second, M, true, I->first);

	return Changed;
	}

	bool GlobalMerge::runOnFunction(Function &F) {
	return false;
	}

	bool GlobalMerge::doFinalization(Module &M) {
	MustKeepGlobalVariables.clear();
	return false;
	}

	Pass llvm::createGlobalMergePass(const TargetMachine TM, unsigned Offset) {
	return new GlobalMerge(TM, Offset);
	}