| //===-- 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" |
| |
| static cl::opt<bool> |
| EnableGlobalMerge("enable-global-merge", cl::Hidden, |
| cl::desc("Enable 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; |
| |
| 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) |
| : FunctionPass(ID), TM(TM) { |
| 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_TM_PASS(GlobalMerge, "global-merge", "Merge global variables", |
| false, false) |
| |
| bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals, |
| Module &M, bool isConst, unsigned AddrSpace) const { |
| const TargetLowering *TLI = TM->getSubtargetImpl()->getTargetLowering(); |
| const DataLayout *DL = TLI->getDataLayout(); |
| |
| // FIXME: Infer the maximum possible offset depending on the actual users |
| // (these max offsets are different for the users inside Thumb or ARM |
| // functions) |
| unsigned MaxOffset = TLI->getMaximalGlobalOffset(); |
| |
| // FIXME: Find better heuristics |
| std::stable_sort(Globals.begin(), Globals.end(), |
| [DL](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(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; |
| const TargetLowering *TLI = TM->getSubtargetImpl()->getTargetLowering(); |
| const DataLayout *DL = TLI->getDataLayout(); |
| unsigned MaxOffset = TLI->getMaximalGlobalOffset(); |
| 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) { |
| return new GlobalMerge(TM); |
| } |