lib/Transforms/IPO/StructRetPromotion.cpp

   1 //===-- StructRetPromotion.cpp - Promote sret arguments ------------------===//
   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 //
  10 // This pass finds functions that return a struct (using a pointer to the struct
  11 // as the first argument of the function, marked with the 'sret' attribute) and
  12 // replaces them with a new function that simply returns each of the elements of
  13 // that struct (using multiple return values).
  14 //
  15 // This pass works under a number of conditions:
  16 //  1. The returned struct must not contain other structs
  17 //  2. The returned struct must only be used to load values from
  18 //  3. The placeholder struct passed in is the result of an alloca
  19 //
  20 //===----------------------------------------------------------------------===//
  21
  22 #define DEBUG_TYPE "sretpromotion"
  23 #include "llvm/Transforms/IPO.h"
  24 #include "llvm/Constants.h"
  25 #include "llvm/DerivedTypes.h"
  26 #include "llvm/LLVMContext.h"
  27 #include "llvm/Module.h"
  28 #include "llvm/CallGraphSCCPass.h"
  29 #include "llvm/Instructions.h"
  30 #include "llvm/Analysis/CallGraph.h"
  31 #include "llvm/Support/CallSite.h"
  32 #include "llvm/Support/CFG.h"
  33 #include "llvm/Support/Debug.h"
  34 #include "llvm/ADT/Statistic.h"
  35 #include "llvm/ADT/SmallVector.h"
  36 #include "llvm/ADT/Statistic.h"
  37 #include "llvm/Support/raw_ostream.h"
  38 using namespace llvm;
  39
  40 STATISTIC(NumRejectedSRETUses , "Number of sret rejected due to unexpected uses");
  41 STATISTIC(NumSRET , "Number of sret promoted");
  42 namespace {
  43   /// SRETPromotion - This pass removes sret parameter and updates
  44   /// function to use multiple return value.
  45   ///
  46   struct SRETPromotion : public CallGraphSCCPass {
  47     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  48       CallGraphSCCPass::getAnalysisUsage(AU);
  49     }
  50
  51     virtual bool runOnSCC(std::vector<CallGraphNode *> &SCC);
  52     static char ID; // Pass identification, replacement for typeid
  53     SRETPromotion() : CallGraphSCCPass(&ID) {}
  54
  55   private:
  56     CallGraphNode *PromoteReturn(CallGraphNode *CGN);
  57     bool isSafeToUpdateAllCallers(Function *F);
  58     Function *cloneFunctionBody(Function *F, const StructType *STy);
  59     CallGraphNode *updateCallSites(Function *F, Function *NF);
  60     bool nestedStructType(const StructType *STy);
  61   };
  62 }
  63
  64 char SRETPromotion::ID = 0;
  65 static RegisterPass<SRETPromotion>
  66 X("sretpromotion", "Promote sret arguments to multiple ret values");
  67
  68 Pass *llvm::createStructRetPromotionPass() {
  69   return new SRETPromotion();
  70 }
  71
  72 bool SRETPromotion::runOnSCC(std::vector<CallGraphNode *> &SCC) {
  73   bool Changed = false;
  74
  75   for (unsigned i = 0, e = SCC.size(); i != e; ++i)
  76     if (CallGraphNode *NewNode = PromoteReturn(SCC[i])) {
  77       SCC[i] = NewNode;
  78       Changed = true;
  79     }
  80
  81   return Changed;
  82 }
  83
  84 /// PromoteReturn - This method promotes function that uses StructRet paramater
  85 /// into a function that uses multiple return values.
  86 CallGraphNode *SRETPromotion::PromoteReturn(CallGraphNode *CGN) {
  87   Function *F = CGN->getFunction();
  88
  89   if (!F || F->isDeclaration() || !F->hasLocalLinkage())
  90     return 0;
  91
  92   // Make sure that function returns struct.
  93   if (F->arg_size() == 0 || !F->hasStructRetAttr() || F->doesNotReturn())
  94     return 0;
  95
  96   DEBUG(errs() << "SretPromotion: Looking at sret function "
  97         << F->getName() << "\n");
  98
  99   assert(F->getReturnType() == Type::getVoidTy(F->getContext()) &&
 100          "Invalid function return type");
 101   Function::arg_iterator AI = F->arg_begin();
 102   const llvm::PointerType *FArgType = dyn_cast<PointerType>(AI->getType());
 103   assert(FArgType && "Invalid sret parameter type");
 104   const llvm::StructType *STy =
 105     dyn_cast<StructType>(FArgType->getElementType());
 106   assert(STy && "Invalid sret parameter element type");
 107
 108   // Check if it is ok to perform this promotion.
 109   if (isSafeToUpdateAllCallers(F) == false) {
 110     DEBUG(errs() << "SretPromotion: Not all callers can be updated\n");
 111     NumRejectedSRETUses++;
 112     return 0;
 113   }
 114
 115   DEBUG(errs() << "SretPromotion: sret argument will be promoted\n");
 116   NumSRET++;
 117   // [1] Replace use of sret parameter
 118   AllocaInst *TheAlloca = new AllocaInst(STy, NULL, "mrv",
 119                                          F->getEntryBlock().begin());
 120   Value *NFirstArg = F->arg_begin();
 121   NFirstArg->replaceAllUsesWith(TheAlloca);
 122
 123   // [2] Find and replace ret instructions
 124   for (Function::iterator FI = F->begin(), FE = F->end();  FI != FE; ++FI)
 125     for(BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) {
 126       Instruction *I = BI;
 127       ++BI;
 128       if (isa<ReturnInst>(I)) {
 129         Value *NV = new LoadInst(TheAlloca, "mrv.ld", I);
 130         ReturnInst *NR = ReturnInst::Create(F->getContext(), NV, I);
 131         I->replaceAllUsesWith(NR);
 132         I->eraseFromParent();
 133       }
 134     }
 135
 136   // [3] Create the new function body and insert it into the module.
 137   Function *NF = cloneFunctionBody(F, STy);
 138
 139   // [4] Update all call sites to use new function
 140   CallGraphNode *NF_CFN = updateCallSites(F, NF);
 141
 142   CallGraph &CG = getAnalysis<CallGraph>();
 143   NF_CFN->stealCalledFunctionsFrom(CG[F]);
 144
 145   delete CG.removeFunctionFromModule(F);
 146   return NF_CFN;
 147 }
 148
 149 // Check if it is ok to perform this promotion.
 150 bool SRETPromotion::isSafeToUpdateAllCallers(Function *F) {
 151
 152   if (F->use_empty())
 153     // No users. OK to modify signature.
 154     return true;
 155
 156   for (Value::use_iterator FnUseI = F->use_begin(), FnUseE = F->use_end();
 157        FnUseI != FnUseE; ++FnUseI) {
 158     // The function is passed in as an argument to (possibly) another function,
 159     // we can't change it!
 160     CallSite CS = CallSite::get(*FnUseI);
 161     Instruction *Call = CS.getInstruction();
 162     // The function is used by something else than a call or invoke instruction,
 163     // we can't change it!
 164     if (!Call || !CS.isCallee(FnUseI))
 165       return false;
 166     CallSite::arg_iterator AI = CS.arg_begin();
 167     Value *FirstArg = *AI;
 168
 169     if (!isa<AllocaInst>(FirstArg))
 170       return false;
 171
 172     // Check FirstArg's users.
 173     for (Value::use_iterator ArgI = FirstArg->use_begin(),
 174            ArgE = FirstArg->use_end(); ArgI != ArgE; ++ArgI) {
 175
 176       // If FirstArg user is a CallInst that does not correspond to current
 177       // call site then this function F is not suitable for sret promotion.
 178       if (CallInst *CI = dyn_cast<CallInst>(ArgI)) {
 179         if (CI != Call)
 180           return false;
 181       }
 182       // If FirstArg user is a GEP whose all users are not LoadInst then
 183       // this function F is not suitable for sret promotion.
 184       else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(ArgI)) {
 185         // TODO : Use dom info and insert PHINodes to collect get results
 186         // from multiple call sites for this GEP.
 187         if (GEP->getParent() != Call->getParent())
 188           return false;
 189         for (Value::use_iterator GEPI = GEP->use_begin(), GEPE = GEP->use_end();
 190              GEPI != GEPE; ++GEPI)
 191           if (!isa<LoadInst>(GEPI))
 192             return false;
 193       }
 194       // Any other FirstArg users make this function unsuitable for sret
 195       // promotion.
 196       else
 197         return false;
 198     }
 199   }
 200
 201   return true;
 202 }
 203
 204 /// cloneFunctionBody - Create a new function based on F and
 205 /// insert it into module. Remove first argument. Use STy as
 206 /// the return type for new function.
 207 Function *SRETPromotion::cloneFunctionBody(Function *F,
 208                                            const StructType *STy) {
 209
 210   const FunctionType *FTy = F->getFunctionType();
 211   std::vector<const Type*> Params;
 212
 213   // Attributes - Keep track of the parameter attributes for the arguments.
 214   SmallVector<AttributeWithIndex, 8> AttributesVec;
 215   const AttrListPtr &PAL = F->getAttributes();
 216
 217   // Add any return attributes.
 218   if (Attributes attrs = PAL.getRetAttributes())
 219     AttributesVec.push_back(AttributeWithIndex::get(0, attrs));
 220
 221   // Skip first argument.
 222   Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
 223   ++I;
 224   // 0th parameter attribute is reserved for return type.
 225   // 1th parameter attribute is for first 1st sret argument.
 226   unsigned ParamIndex = 2;
 227   while (I != E) {
 228     Params.push_back(I->getType());
 229     if (Attributes Attrs = PAL.getParamAttributes(ParamIndex))
 230       AttributesVec.push_back(AttributeWithIndex::get(ParamIndex - 1, Attrs));
 231     ++I;
 232     ++ParamIndex;
 233   }
 234
 235   // Add any fn attributes.
 236   if (Attributes attrs = PAL.getFnAttributes())
 237     AttributesVec.push_back(AttributeWithIndex::get(~0, attrs));
 238
 239
 240   FunctionType *NFTy = FunctionType::get(STy, Params, FTy->isVarArg());
 241   Function *NF = Function::Create(NFTy, F->getLinkage());
 242   NF->takeName(F);
 243   NF->copyAttributesFrom(F);
 244   NF->setAttributes(AttrListPtr::get(AttributesVec.begin(), AttributesVec.end()));
 245   F->getParent()->getFunctionList().insert(F, NF);
 246   NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
 247
 248   // Replace arguments
 249   I = F->arg_begin();
 250   E = F->arg_end();
 251   Function::arg_iterator NI = NF->arg_begin();
 252   ++I;
 253   while (I != E) {
 254     I->replaceAllUsesWith(NI);
 255     NI->takeName(I);
 256     ++I;
 257     ++NI;
 258   }
 259
 260   return NF;
 261 }
 262
 263 /// updateCallSites - Update all sites that call F to use NF.
 264 CallGraphNode *SRETPromotion::updateCallSites(Function *F, Function *NF) {
 265   CallGraph &CG = getAnalysis<CallGraph>();
 266   SmallVector<Value*, 16> Args;
 267
 268   // Attributes - Keep track of the parameter attributes for the arguments.
 269   SmallVector<AttributeWithIndex, 8> ArgAttrsVec;
 270
 271   // Get a new callgraph node for NF.
 272   CallGraphNode *NF_CGN = CG.getOrInsertFunction(NF);
 273
 274   while (!F->use_empty()) {
 275     CallSite CS = CallSite::get(*F->use_begin());
 276     Instruction *Call = CS.getInstruction();
 277
 278     const AttrListPtr &PAL = F->getAttributes();
 279     // Add any return attributes.
 280     if (Attributes attrs = PAL.getRetAttributes())
 281       ArgAttrsVec.push_back(AttributeWithIndex::get(0, attrs));
 282
 283     // Copy arguments, however skip first one.
 284     CallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
 285     Value *FirstCArg = *AI;
 286     ++AI;
 287     // 0th parameter attribute is reserved for return type.
 288     // 1th parameter attribute is for first 1st sret argument.
 289     unsigned ParamIndex = 2;
 290     while (AI != AE) {
 291       Args.push_back(*AI);
 292       if (Attributes Attrs = PAL.getParamAttributes(ParamIndex))
 293         ArgAttrsVec.push_back(AttributeWithIndex::get(ParamIndex - 1, Attrs));
 294       ++ParamIndex;
 295       ++AI;
 296     }
 297
 298     // Add any function attributes.
 299     if (Attributes attrs = PAL.getFnAttributes())
 300       ArgAttrsVec.push_back(AttributeWithIndex::get(~0, attrs));
 301
 302     AttrListPtr NewPAL = AttrListPtr::get(ArgAttrsVec.begin(), ArgAttrsVec.end());
 303
 304     // Build new call instruction.
 305     Instruction *New;
 306     if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
 307       New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
 308                                Args.begin(), Args.end(), "", Call);
 309       cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
 310       cast<InvokeInst>(New)->setAttributes(NewPAL);
 311     } else {
 312       New = CallInst::Create(NF, Args.begin(), Args.end(), "", Call);
 313       cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
 314       cast<CallInst>(New)->setAttributes(NewPAL);
 315       if (cast<CallInst>(Call)->isTailCall())
 316         cast<CallInst>(New)->setTailCall();
 317     }
 318     Args.clear();
 319     ArgAttrsVec.clear();
 320     New->takeName(Call);
 321
 322     // Update the callgraph to know that the callsite has been transformed.
 323     CallGraphNode *CalleeNode = CG[Call->getParent()->getParent()];
 324     CalleeNode->removeCallEdgeFor(Call);
 325     CalleeNode->addCalledFunction(New, NF_CGN);
 326
 327     // Update all users of sret parameter to extract value using extractvalue.
 328     for (Value::use_iterator UI = FirstCArg->use_begin(),
 329            UE = FirstCArg->use_end(); UI != UE; ) {
 330       User *U2 = *UI++;
 331       CallInst *C2 = dyn_cast<CallInst>(U2);
 332       if (C2 && (C2 == Call))
 333         continue;
 334
 335       GetElementPtrInst *UGEP = cast<GetElementPtrInst>(U2);
 336       ConstantInt *Idx = cast<ConstantInt>(UGEP->getOperand(2));
 337       Value *GR = ExtractValueInst::Create(New, Idx->getZExtValue(),
 338                                            "evi", UGEP);
 339       while(!UGEP->use_empty()) {
 340         // isSafeToUpdateAllCallers has checked that all GEP uses are
 341         // LoadInsts
 342         LoadInst *L = cast<LoadInst>(*UGEP->use_begin());
 343         L->replaceAllUsesWith(GR);
 344         L->eraseFromParent();
 345       }
 346       UGEP->eraseFromParent();
 347       continue;
 348     }
 349     Call->eraseFromParent();
 350   }
 351
 352   return NF_CGN;
 353 }
 354
 355 /// nestedStructType - Return true if STy includes any
 356 /// other aggregate types
 357 bool SRETPromotion::nestedStructType(const StructType *STy) {
 358   unsigned Num = STy->getNumElements();
 359   for (unsigned i = 0; i < Num; i++) {
 360     const Type *Ty = STy->getElementType(i);
 361     if (!Ty->isSingleValueType() && Ty != Type::getVoidTy(STy->getContext()))
 362       return true;
 363   }
 364   return false;
 365 }