GlobalMerge.cpp revision cde25b435a907e7741da0c0d18953850936277c4
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" 71using namespace llvm; 72 73static cl::opt<bool> 74EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden, 75 cl::desc("Enable global merge pass on constants"), 76 cl::init(false)); 77 78STATISTIC(NumMerged , "Number of globals merged"); 79namespace { 80 class GlobalMerge : public FunctionPass { 81 /// TLI - Keep a pointer of a TargetLowering to consult for determining 82 /// target type sizes. 83 const TargetLowering *TLI; 84 85 bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals, 86 Module &M, bool isConst, unsigned AddrSpace) const; 87 88 /// \brief Check if the given variable has been identified as must keep 89 /// \pre setMustKeepGlobalVariables must have been called on the Module that 90 /// contains GV 91 bool isMustKeepGlobalVariable(const GlobalVariable *GV) const { 92 return MustKeepGlobalVariables.count(GV); 93 } 94 95 /// Collect every variables marked as "used" or used in a landing pad 96 /// instruction for this Module. 97 void setMustKeepGlobalVariables(Module &M); 98 99 /// Collect every variables marked as "used" 100 void collectUsedGlobalVariables(Module &M); 101 102 /// Keep track of the GlobalVariable that must not be merged away 103 SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables; 104 105 public: 106 static char ID; // Pass identification, replacement for typeid. 107 explicit GlobalMerge(const TargetLowering *tli = 0) 108 : FunctionPass(ID), TLI(tli) { 109 initializeGlobalMergePass(*PassRegistry::getPassRegistry()); 110 } 111 112 virtual bool doInitialization(Module &M); 113 virtual bool runOnFunction(Function &F); 114 virtual bool doFinalization(Module &M); 115 116 const char *getPassName() const { 117 return "Merge internal globals"; 118 } 119 120 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 121 AU.setPreservesCFG(); 122 FunctionPass::getAnalysisUsage(AU); 123 } 124 125 struct GlobalCmp { 126 const DataLayout *TD; 127 128 GlobalCmp(const DataLayout *td) : TD(td) { } 129 130 bool operator()(const GlobalVariable *GV1, const GlobalVariable *GV2) { 131 Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType(); 132 Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType(); 133 134 return (TD->getTypeAllocSize(Ty1) < TD->getTypeAllocSize(Ty2)); 135 } 136 }; 137 }; 138} // end anonymous namespace 139 140char GlobalMerge::ID = 0; 141INITIALIZE_PASS(GlobalMerge, "global-merge", 142 "Global Merge", false, false) 143 144 145bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals, 146 Module &M, bool isConst, unsigned AddrSpace) const { 147 const DataLayout *TD = TLI->getDataLayout(); 148 149 // FIXME: Infer the maximum possible offset depending on the actual users 150 // (these max offsets are different for the users inside Thumb or ARM 151 // functions) 152 unsigned MaxOffset = TLI->getMaximalGlobalOffset(); 153 154 // FIXME: Find better heuristics 155 std::stable_sort(Globals.begin(), Globals.end(), GlobalCmp(TD)); 156 157 Type *Int32Ty = Type::getInt32Ty(M.getContext()); 158 159 for (size_t i = 0, e = Globals.size(); i != e; ) { 160 size_t j = 0; 161 uint64_t MergedSize = 0; 162 std::vector<Type*> Tys; 163 std::vector<Constant*> Inits; 164 for (j = i; j != e; ++j) { 165 Type *Ty = Globals[j]->getType()->getElementType(); 166 MergedSize += TD->getTypeAllocSize(Ty); 167 if (MergedSize > MaxOffset) { 168 break; 169 } 170 Tys.push_back(Ty); 171 Inits.push_back(Globals[j]->getInitializer()); 172 } 173 174 StructType *MergedTy = StructType::get(M.getContext(), Tys); 175 Constant *MergedInit = ConstantStruct::get(MergedTy, Inits); 176 GlobalVariable *MergedGV = new GlobalVariable(M, MergedTy, isConst, 177 GlobalValue::InternalLinkage, 178 MergedInit, "_MergedGlobals", 179 0, GlobalVariable::NotThreadLocal, 180 AddrSpace); 181 for (size_t k = i; k < j; ++k) { 182 Constant *Idx[2] = { 183 ConstantInt::get(Int32Ty, 0), 184 ConstantInt::get(Int32Ty, k-i) 185 }; 186 Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx); 187 Globals[k]->replaceAllUsesWith(GEP); 188 Globals[k]->eraseFromParent(); 189 NumMerged++; 190 } 191 i = j; 192 } 193 194 return true; 195} 196 197void GlobalMerge::collectUsedGlobalVariables(Module &M) { 198 // Extract global variables from llvm.used array 199 const GlobalVariable *GV = M.getGlobalVariable("llvm.used"); 200 if (!GV || !GV->hasInitializer()) return; 201 202 // Should be an array of 'i8*'. 203 const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer()); 204 205 for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i) 206 if (const GlobalVariable *G = 207 dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts())) 208 MustKeepGlobalVariables.insert(G); 209} 210 211void GlobalMerge::setMustKeepGlobalVariables(Module &M) { 212 collectUsedGlobalVariables(M); 213 214 for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn; 215 ++IFn) { 216 for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end(); 217 IBB != IEndBB; ++IBB) { 218 // Follow the inwoke link to find the landing pad instruction 219 const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator()); 220 if (!II) continue; 221 222 const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst(); 223 // Look for globals in the clauses of the landing pad instruction 224 for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses(); 225 Idx != NumClauses; ++Idx) 226 if (const GlobalVariable *GV = 227 dyn_cast<GlobalVariable>(LPInst->getClause(Idx) 228 ->stripPointerCasts())) 229 MustKeepGlobalVariables.insert(GV); 230 } 231 } 232} 233 234bool GlobalMerge::doInitialization(Module &M) { 235 DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals, 236 BSSGlobals; 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 299bool GlobalMerge::runOnFunction(Function &F) { 300 return false; 301} 302 303bool GlobalMerge::doFinalization(Module &M) { 304 MustKeepGlobalVariables.clear(); 305 return false; 306} 307 308Pass *llvm::createGlobalMergePass(const TargetLowering *tli) { 309 return new GlobalMerge(tli); 310} 311