1 //===-- GlobalMerge.cpp - Internal globals merging -----------------------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // This pass merges globals with internal linkage into one. This way all the 10 // globals which were merged into a biggest one can be addressed using offsets 11 // from the same base pointer (no need for separate base pointer for each of the 12 // global). Such a transformation can significantly reduce the register pressure 13 // when many globals are involved. 14 // 15 // For example, consider the code which touches several global variables at 16 // once: 17 // 18 // static int foo[N], bar[N], baz[N]; 19 // 20 // for (i = 0; i < N; ++i) { 21 // foo[i] = bar[i] * baz[i]; 22 // } 23 // 24 // On ARM the addresses of 3 arrays should be kept in the registers, thus 25 // this code has quite large register pressure (loop body): 26 // 27 // ldr r1, [r5], #4 28 // ldr r2, [r6], #4 29 // mul r1, r2, r1 30 // str r1, [r0], #4 31 // 32 // Pass converts the code to something like: 33 // 34 // static struct { 35 // int foo[N]; 36 // int bar[N]; 37 // int baz[N]; 38 // } merged; 39 // 40 // for (i = 0; i < N; ++i) { 41 // merged.foo[i] = merged.bar[i] * merged.baz[i]; 42 // } 43 // 44 // and in ARM code this becomes: 45 // 46 // ldr r0, [r5, #40] 47 // ldr r1, [r5, #80] 48 // mul r0, r1, r0 49 // str r0, [r5], #4 50 // 51 // note that we saved 2 registers here almostly "for free". 52 // ===---------------------------------------------------------------------===// 53 54 #define DEBUG_TYPE "global-merge" 55 #include "llvm/Transforms/Scalar.h" 56 #include "llvm/ADT/SmallPtrSet.h" 57 #include "llvm/ADT/Statistic.h" 58 #include "llvm/IR/Attributes.h" 59 #include "llvm/IR/Constants.h" 60 #include "llvm/IR/DataLayout.h" 61 #include "llvm/IR/DerivedTypes.h" 62 #include "llvm/IR/Function.h" 63 #include "llvm/IR/GlobalVariable.h" 64 #include "llvm/IR/Instructions.h" 65 #include "llvm/IR/Intrinsics.h" 66 #include "llvm/IR/Module.h" 67 #include "llvm/Pass.h" 68 #include "llvm/Support/CommandLine.h" 69 #include "llvm/Target/TargetLowering.h" 70 #include "llvm/Target/TargetLoweringObjectFile.h" 71 using namespace llvm; 72 73 static cl::opt<bool> 74 EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden, 75 cl::desc("Enable global merge pass on constants"), 76 cl::init(false)); 77 78 STATISTIC(NumMerged , "Number of globals merged"); 79 namespace { 80 class GlobalMerge : public FunctionPass { 81 const TargetMachine *TM; 82 83 bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals, 84 Module &M, bool isConst, unsigned AddrSpace) const; 85 86 /// \brief Check if the given variable has been identified as must keep 87 /// \pre setMustKeepGlobalVariables must have been called on the Module that 88 /// contains GV 89 bool isMustKeepGlobalVariable(const GlobalVariable *GV) const { 90 return MustKeepGlobalVariables.count(GV); 91 } 92 93 /// Collect every variables marked as "used" or used in a landing pad 94 /// instruction for this Module. 95 void setMustKeepGlobalVariables(Module &M); 96 97 /// Collect every variables marked as "used" 98 void collectUsedGlobalVariables(Module &M); 99 100 /// Keep track of the GlobalVariable that must not be merged away 101 SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables; 102 103 public: 104 static char ID; // Pass identification, replacement for typeid. 105 explicit GlobalMerge(const TargetMachine *TM = 0) 106 : FunctionPass(ID), TM(TM) { 107 initializeGlobalMergePass(*PassRegistry::getPassRegistry()); 108 } 109 110 virtual bool doInitialization(Module &M); 111 virtual bool runOnFunction(Function &F); 112 virtual bool doFinalization(Module &M); 113 114 const char *getPassName() const { 115 return "Merge internal globals"; 116 } 117 118 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 119 AU.setPreservesCFG(); 120 FunctionPass::getAnalysisUsage(AU); 121 } 122 123 struct GlobalCmp { 124 const DataLayout *TD; 125 126 GlobalCmp(const DataLayout *td) : TD(td) { } 127 128 bool operator()(const GlobalVariable *GV1, const GlobalVariable *GV2) { 129 Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType(); 130 Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType(); 131 132 return (TD->getTypeAllocSize(Ty1) < TD->getTypeAllocSize(Ty2)); 133 } 134 }; 135 }; 136 } // end anonymous namespace 137 138 char GlobalMerge::ID = 0; 139 INITIALIZE_PASS(GlobalMerge, "global-merge", 140 "Global Merge", false, false) 141 142 143 bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals, 144 Module &M, bool isConst, unsigned AddrSpace) const { 145 const TargetLowering *TLI = TM->getTargetLowering(); 146 const DataLayout *TD = TLI->getDataLayout(); 147 148 // FIXME: Infer the maximum possible offset depending on the actual users 149 // (these max offsets are different for the users inside Thumb or ARM 150 // functions) 151 unsigned MaxOffset = TLI->getMaximalGlobalOffset(); 152 153 // FIXME: Find better heuristics 154 std::stable_sort(Globals.begin(), Globals.end(), GlobalCmp(TD)); 155 156 Type *Int32Ty = Type::getInt32Ty(M.getContext()); 157 158 for (size_t i = 0, e = Globals.size(); i != e; ) { 159 size_t j = 0; 160 uint64_t MergedSize = 0; 161 std::vector<Type*> Tys; 162 std::vector<Constant*> Inits; 163 for (j = i; j != e; ++j) { 164 Type *Ty = Globals[j]->getType()->getElementType(); 165 MergedSize += TD->getTypeAllocSize(Ty); 166 if (MergedSize > MaxOffset) { 167 break; 168 } 169 Tys.push_back(Ty); 170 Inits.push_back(Globals[j]->getInitializer()); 171 } 172 173 StructType *MergedTy = StructType::get(M.getContext(), Tys); 174 Constant *MergedInit = ConstantStruct::get(MergedTy, Inits); 175 GlobalVariable *MergedGV = new GlobalVariable(M, MergedTy, isConst, 176 GlobalValue::InternalLinkage, 177 MergedInit, "_MergedGlobals", 178 0, GlobalVariable::NotThreadLocal, 179 AddrSpace); 180 for (size_t k = i; k < j; ++k) { 181 Constant *Idx[2] = { 182 ConstantInt::get(Int32Ty, 0), 183 ConstantInt::get(Int32Ty, k-i) 184 }; 185 Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx); 186 Globals[k]->replaceAllUsesWith(GEP); 187 Globals[k]->eraseFromParent(); 188 NumMerged++; 189 } 190 i = j; 191 } 192 193 return true; 194 } 195 196 void GlobalMerge::collectUsedGlobalVariables(Module &M) { 197 // Extract global variables from llvm.used array 198 const GlobalVariable *GV = M.getGlobalVariable("llvm.used"); 199 if (!GV || !GV->hasInitializer()) return; 200 201 // Should be an array of 'i8*'. 202 const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer()); 203 204 for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i) 205 if (const GlobalVariable *G = 206 dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts())) 207 MustKeepGlobalVariables.insert(G); 208 } 209 210 void GlobalMerge::setMustKeepGlobalVariables(Module &M) { 211 collectUsedGlobalVariables(M); 212 213 for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn; 214 ++IFn) { 215 for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end(); 216 IBB != IEndBB; ++IBB) { 217 // Follow the inwoke link to find the landing pad instruction 218 const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator()); 219 if (!II) continue; 220 221 const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst(); 222 // Look for globals in the clauses of the landing pad instruction 223 for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses(); 224 Idx != NumClauses; ++Idx) 225 if (const GlobalVariable *GV = 226 dyn_cast<GlobalVariable>(LPInst->getClause(Idx) 227 ->stripPointerCasts())) 228 MustKeepGlobalVariables.insert(GV); 229 } 230 } 231 } 232 233 bool GlobalMerge::doInitialization(Module &M) { 234 DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals, 235 BSSGlobals; 236 const TargetLowering *TLI = TM->getTargetLowering(); 237 const DataLayout *TD = TLI->getDataLayout(); 238 unsigned MaxOffset = TLI->getMaximalGlobalOffset(); 239 bool Changed = false; 240 setMustKeepGlobalVariables(M); 241 242 // Grab all non-const globals. 243 for (Module::global_iterator I = M.global_begin(), 244 E = M.global_end(); I != E; ++I) { 245 // Merge is safe for "normal" internal globals only 246 if (!I->hasLocalLinkage() || I->isThreadLocal() || I->hasSection()) 247 continue; 248 249 PointerType *PT = dyn_cast<PointerType>(I->getType()); 250 assert(PT && "Global variable is not a pointer!"); 251 252 unsigned AddressSpace = PT->getAddressSpace(); 253 254 // Ignore fancy-aligned globals for now. 255 unsigned Alignment = TD->getPreferredAlignment(I); 256 Type *Ty = I->getType()->getElementType(); 257 if (Alignment > TD->getABITypeAlignment(Ty)) 258 continue; 259 260 // Ignore all 'special' globals. 261 if (I->getName().startswith("llvm.") || 262 I->getName().startswith(".llvm.")) 263 continue; 264 265 // Ignore all "required" globals: 266 if (isMustKeepGlobalVariable(I)) 267 continue; 268 269 if (TD->getTypeAllocSize(Ty) < MaxOffset) { 270 if (TargetLoweringObjectFile::getKindForGlobal(I, TLI->getTargetMachine()) 271 .isBSSLocal()) 272 BSSGlobals[AddressSpace].push_back(I); 273 else if (I->isConstant()) 274 ConstGlobals[AddressSpace].push_back(I); 275 else 276 Globals[AddressSpace].push_back(I); 277 } 278 } 279 280 for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator 281 I = Globals.begin(), E = Globals.end(); I != E; ++I) 282 if (I->second.size() > 1) 283 Changed |= doMerge(I->second, M, false, I->first); 284 285 for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator 286 I = BSSGlobals.begin(), E = BSSGlobals.end(); I != E; ++I) 287 if (I->second.size() > 1) 288 Changed |= doMerge(I->second, M, false, I->first); 289 290 if (EnableGlobalMergeOnConst) 291 for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator 292 I = ConstGlobals.begin(), E = ConstGlobals.end(); I != E; ++I) 293 if (I->second.size() > 1) 294 Changed |= doMerge(I->second, M, true, I->first); 295 296 return Changed; 297 } 298 299 bool GlobalMerge::runOnFunction(Function &F) { 300 return false; 301 } 302 303 bool GlobalMerge::doFinalization(Module &M) { 304 MustKeepGlobalVariables.clear(); 305 return false; 306 } 307 308 Pass *llvm::createGlobalMergePass(const TargetMachine *TM) { 309 return new GlobalMerge(TM); 310 } 311