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/Compiler.h"
  38 #include "llvm/Support/raw_ostream.h"
  39 using namespace llvm;
  40
  41 STATISTIC(NumRejectedSRETUses , "Number of sret rejected due to unexpected uses");
  42 STATISTIC(NumSRET , "Number of sret promoted");
  43 namespace {
  44   /// SRETPromotion - This pass removes sret parameter and updates
  45   /// function to use multiple return value.
  46   ///
  47   struct VISIBILITY_HIDDEN SRETPromotion : public CallGraphSCCPass {
  48     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  49       CallGraphSCCPass::getAnalysisUsage(AU);
  50     }
  51
  52     virtual bool runOnSCC(std::vector<CallGraphNode *> &SCC);
  53     static char ID; // Pass identification, replacement for typeid
  54     SRETPromotion() : CallGraphSCCPass(&ID) {}
  55
  56   private:
  57     CallGraphNode *PromoteReturn(CallGraphNode *CGN);
  58     bool isSafeToUpdateAllCallers(Function *F);
  59     Function *cloneFunctionBody(Function *F, const StructType *STy);
  60     CallGraphNode *updateCallSites(Function *F, Function *NF);
  61     bool nestedStructType(const StructType *STy);
  62   };
  63 }
  64
  65 char SRETPromotion::ID = 0;
  66 static RegisterPass<SRETPromotion>
  67 X("sretpromotion", "Promote sret arguments to multiple ret values");
  68
  69 Pass *llvm::createStructRetPromotionPass() {
  70   return new SRETPromotion();
  71 }
  72
  73 bool SRETPromotion::runOnSCC(std::vector<CallGraphNode *> &SCC) {
  74   bool Changed = false;
  75
  76   for (unsigned i = 0, e = SCC.size(); i != e; ++i)
  77     if (CallGraphNode *NewNode = PromoteReturn(SCC[i])) {
  78       SCC[i] = NewNode;
  79       Changed = true;
  80     }
  81
  82   return Changed;
  83 }
  84
  85 /// PromoteReturn - This method promotes function that uses StructRet paramater
  86 /// into a function that uses multiple return values.
  87 CallGraphNode *SRETPromotion::PromoteReturn(CallGraphNode *CGN) {
  88   Function *F = CGN->getFunction();
  89
  90   if (!F || F->isDeclaration() || !F->hasLocalLinkage())
  91     return 0;
  92
  93   // Make sure that function returns struct.
  94   if (F->arg_size() == 0 || !F->hasStructRetAttr() || F->doesNotReturn())
  95     return 0;
  96
  97   DEBUG(errs() << "SretPromotion: Looking at sret function "
  98         << F->getName() << "\n");
  99
 100   assert(F->getReturnType() == Type::getVoidTy(F->getContext()) &&
 101          "Invalid function return type");
 102   Function::arg_iterator AI = F->arg_begin();
 103   const llvm::PointerType *FArgType = dyn_cast<PointerType>(AI->getType());
 104   assert(FArgType && "Invalid sret parameter type");
 105   const llvm::StructType *STy =
 106     dyn_cast<StructType>(FArgType->getElementType());
 107   assert(STy && "Invalid sret parameter element type");
 108
 109   // Check if it is ok to perform this promotion.
 110   if (isSafeToUpdateAllCallers(F) == false) {
 111     DEBUG(errs() << "SretPromotion: Not all callers can be updated\n");
 112     NumRejectedSRETUses++;
 113     return 0;
 114   }
 115
 116   DEBUG(errs() << "SretPromotion: sret argument will be promoted\n");
 117   NumSRET++;
 118   // [1] Replace use of sret parameter
 119   AllocaInst *TheAlloca = new AllocaInst(STy, NULL, "mrv",
 120                                          F->getEntryBlock().begin());
 121   Value *NFirstArg = F->arg_begin();
 122   NFirstArg->replaceAllUsesWith(TheAlloca);
 123
 124   // [2] Find and replace ret instructions
 125   for (Function::iterator FI = F->begin(), FE = F->end();  FI != FE; ++FI)
 126     for(BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) {
 127       Instruction *I = BI;
 128       ++BI;
 129       if (isa<ReturnInst>(I)) {
 130         Value *NV = new LoadInst(TheAlloca, "mrv.ld", I);
 131         ReturnInst *NR = ReturnInst::Create(F->getContext(), NV, I);
 132         I->replaceAllUsesWith(NR);
 133         I->eraseFromParent();
 134       }
 135     }
 136
 137   // [3] Create the new function body and insert it into the module.
 138   Function *NF = cloneFunctionBody(F, STy);
 139
 140   // [4] Update all call sites to use new function
 141   CallGraphNode *NF_CFN = updateCallSites(F, NF);
 142
 143   CallGraph &CG = getAnalysis<CallGraph>();
 144   NF_CFN->stealCalledFunctionsFrom(CG[F]);
 145
 146   delete CG.removeFunctionFromModule(F);
 147   return NF_CFN;
 148 }
 149
 150 // Check if it is ok to perform this promotion.
 151 bool SRETPromotion::isSafeToUpdateAllCallers(Function *F) {
 152
 153   if (F->use_empty())
 154     // No users. OK to modify signature.
 155     return true;
 156
 157   for (Value::use_iterator FnUseI = F->use_begin(), FnUseE = F->use_end();
 158        FnUseI != FnUseE; ++FnUseI) {
 159     // The function is passed in as an argument to (possibly) another function,
 160     // we can't change it!
 161     CallSite CS = CallSite::get(*FnUseI);
 162     Instruction *Call = CS.getInstruction();
 163     // The function is used by something else than a call or invoke instruction,
 164     // we can't change it!
 165     if (!Call || !CS.isCallee(FnUseI))
 166       return false;
 167     CallSite::arg_iterator AI = CS.arg_begin();
 168     Value *FirstArg = *AI;
 169
 170     if (!isa<AllocaInst>(FirstArg))
 171       return false;
 172
 173     // Check FirstArg's users.
 174     for (Value::use_iterator ArgI = FirstArg->use_begin(),
 175            ArgE = FirstArg->use_end(); ArgI != ArgE; ++ArgI) {
 176
 177       // If FirstArg user is a CallInst that does not correspond to current
 178       // call site then this function F is not suitable for sret promotion.
 179       if (CallInst *CI = dyn_cast<CallInst>(ArgI)) {
 180         if (CI != Call)
 181           return false;
 182       }
 183       // If FirstArg user is a GEP whose all users are not LoadInst then
 184       // this function F is not suitable for sret promotion.
 185       else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(ArgI)) {
 186         // TODO : Use dom info and insert PHINodes to collect get results
 187         // from multiple call sites for this GEP.
 188         if (GEP->getParent() != Call->getParent())
 189           return false;
 190         for (Value::use_iterator GEPI = GEP->use_begin(), GEPE = GEP->use_end();
 191              GEPI != GEPE; ++GEPI)
 192           if (!isa<LoadInst>(GEPI))
 193             return false;
 194       }
 195       // Any other FirstArg users make this function unsuitable for sret
 196       // promotion.
 197       else
 198         return false;
 199     }
 200   }
 201
 202   return true;
 203 }
 204
 205 /// cloneFunctionBody - Create a new function based on F and
 206 /// insert it into module. Remove first argument. Use STy as
 207 /// the return type for new function.
 208 Function *SRETPromotion::cloneFunctionBody(Function *F,
 209                                            const StructType *STy) {
 210
 211   const FunctionType *FTy = F->getFunctionType();
 212   std::vector<const Type*> Params;
 213
 214   // Attributes - Keep track of the parameter attributes for the arguments.
 215   SmallVector<AttributeWithIndex, 8> AttributesVec;
 216   const AttrListPtr &PAL = F->getAttributes();
 217
 218   // Add any return attributes.
 219   if (Attributes attrs = PAL.getRetAttributes())
 220     AttributesVec.push_back(AttributeWithIndex::get(0, attrs));
 221
 222   // Skip first argument.
 223   Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
 224   ++I;
 225   // 0th parameter attribute is reserved for return type.
 226   // 1th parameter attribute is for first 1st sret argument.
 227   unsigned ParamIndex = 2;
 228   while (I != E) {
 229     Params.push_back(I->getType());
 230     if (Attributes Attrs = PAL.getParamAttributes(ParamIndex))
 231       AttributesVec.push_back(AttributeWithIndex::get(ParamIndex - 1, Attrs));
 232     ++I;
 233     ++ParamIndex;
 234   }
 235
 236   // Add any fn attributes.
 237   if (Attributes attrs = PAL.getFnAttributes())
 238     AttributesVec.push_back(AttributeWithIndex::get(~0, attrs));
 239
 240
 241   FunctionType *NFTy = FunctionType::get(STy, Params, FTy->isVarArg());
 242   Function *NF = Function::Create(NFTy, F->getLinkage());
 243   NF->takeName(F);
 244   NF->copyAttributesFrom(F);
 245   NF->setAttributes(AttrListPtr::get(AttributesVec.begin(), AttributesVec.end()));
 246   F->getParent()->getFunctionList().insert(F, NF);
 247   NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
 248
 249   // Replace arguments
 250   I = F->arg_begin();
 251   E = F->arg_end();
 252   Function::arg_iterator NI = NF->arg_begin();
 253   ++I;
 254   while (I != E) {
 255     I->replaceAllUsesWith(NI);
 256     NI->takeName(I);
 257     ++I;
 258     ++NI;
 259   }
 260
 261   return NF;
 262 }
 263
 264 /// updateCallSites - Update all sites that call F to use NF.
 265 CallGraphNode *SRETPromotion::updateCallSites(Function *F, Function *NF) {
 266   CallGraph &CG = getAnalysis<CallGraph>();
 267   SmallVector<Value*, 16> Args;
 268
 269   // Attributes - Keep track of the parameter attributes for the arguments.
 270   SmallVector<AttributeWithIndex, 8> ArgAttrsVec;
 271
 272   // Get a new callgraph node for NF.
 273   CallGraphNode *NF_CGN = CG.getOrInsertFunction(NF);
 274
 275   while (!F->use_empty()) {
 276     CallSite CS = CallSite::get(*F->use_begin());
 277     Instruction *Call = CS.getInstruction();
 278
 279     const AttrListPtr &PAL = F->getAttributes();
 280     // Add any return attributes.
 281     if (Attributes attrs = PAL.getRetAttributes())
 282       ArgAttrsVec.push_back(AttributeWithIndex::get(0, attrs));
 283
 284     // Copy arguments, however skip first one.
 285     CallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
 286     Value *FirstCArg = *AI;
 287     ++AI;
 288     // 0th parameter attribute is reserved for return type.
 289     // 1th parameter attribute is for first 1st sret argument.
 290     unsigned ParamIndex = 2;
 291     while (AI != AE) {
 292       Args.push_back(*AI);
 293       if (Attributes Attrs = PAL.getParamAttributes(ParamIndex))
 294         ArgAttrsVec.push_back(AttributeWithIndex::get(ParamIndex - 1, Attrs));
 295       ++ParamIndex;
 296       ++AI;
 297     }
 298
 299     // Add any function attributes.
 300     if (Attributes attrs = PAL.getFnAttributes())
 301       ArgAttrsVec.push_back(AttributeWithIndex::get(~0, attrs));
 302
 303     AttrListPtr NewPAL = AttrListPtr::get(ArgAttrsVec.begin(), ArgAttrsVec.end());
 304
 305     // Build new call instruction.
 306     Instruction *New;
 307     if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
 308       New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
 309                                Args.begin(), Args.end(), "", Call);
 310       cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
 311       cast<InvokeInst>(New)->setAttributes(NewPAL);
 312     } else {
 313       New = CallInst::Create(NF, Args.begin(), Args.end(), "", Call);
 314       cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
 315       cast<CallInst>(New)->setAttributes(NewPAL);
 316       if (cast<CallInst>(Call)->isTailCall())
 317         cast<CallInst>(New)->setTailCall();
 318     }
 319     Args.clear();
 320     ArgAttrsVec.clear();
 321     New->takeName(Call);
 322
 323     // Update the callgraph to know that the callsite has been transformed.
 324     CallGraphNode *CalleeNode = CG[Call->getParent()->getParent()];
 325     CalleeNode->removeCallEdgeFor(Call);
 326     CalleeNode->addCalledFunction(New, NF_CGN);
 327
 328     // Update all users of sret parameter to extract value using extractvalue.
 329     for (Value::use_iterator UI = FirstCArg->use_begin(),
 330            UE = FirstCArg->use_end(); UI != UE; ) {
 331       User *U2 = *UI++;
 332       CallInst *C2 = dyn_cast<CallInst>(U2);
 333       if (C2 && (C2 == Call))
 334         continue;
 335
 336       GetElementPtrInst *UGEP = cast<GetElementPtrInst>(U2);
 337       ConstantInt *Idx = cast<ConstantInt>(UGEP->getOperand(2));
 338       Value *GR = ExtractValueInst::Create(New, Idx->getZExtValue(),
 339                                            "evi", UGEP);
 340       while(!UGEP->use_empty()) {
 341         // isSafeToUpdateAllCallers has checked that all GEP uses are
 342         // LoadInsts
 343         LoadInst *L = cast<LoadInst>(*UGEP->use_begin());
 344         L->replaceAllUsesWith(GR);
 345         L->eraseFromParent();
 346       }
 347       UGEP->eraseFromParent();
 348       continue;
 349     }
 350     Call->eraseFromParent();
 351   }
 352
 353   return NF_CGN;
 354 }
 355
 356 /// nestedStructType - Return true if STy includes any
 357 /// other aggregate types
 358 bool SRETPromotion::nestedStructType(const StructType *STy) {
 359   unsigned Num = STy->getNumElements();
 360   for (unsigned i = 0; i < Num; i++) {
 361     const Type *Ty = STy->getElementType(i);
 362     if (!Ty->isSingleValueType() && Ty != Type::getVoidTy(STy->getContext()))
 363       return true;
 364   }
 365   return false;
 366 }