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