LLVM API Documentation

PartialInlining.cpp
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00001 //===- PartialInlining.cpp - Inline parts of functions --------------------===//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file is distributed under the University of Illinois Open Source
00006 // License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This pass performs partial inlining, typically by inlining an if statement
00011 // that surrounds the body of the function.
00012 //
00013 //===----------------------------------------------------------------------===//
00014 
00015 #include "llvm/Transforms/IPO.h"
00016 #include "llvm/ADT/Statistic.h"
00017 #include "llvm/IR/CFG.h"
00018 #include "llvm/IR/Dominators.h"
00019 #include "llvm/IR/Instructions.h"
00020 #include "llvm/IR/Module.h"
00021 #include "llvm/Pass.h"
00022 #include "llvm/Transforms/Utils/Cloning.h"
00023 #include "llvm/Transforms/Utils/CodeExtractor.h"
00024 using namespace llvm;
00025 
00026 #define DEBUG_TYPE "partialinlining"
00027 
00028 STATISTIC(NumPartialInlined, "Number of functions partially inlined");
00029 
00030 namespace {
00031   struct PartialInliner : public ModulePass {
00032     void getAnalysisUsage(AnalysisUsage &AU) const override { }
00033     static char ID; // Pass identification, replacement for typeid
00034     PartialInliner() : ModulePass(ID) {
00035       initializePartialInlinerPass(*PassRegistry::getPassRegistry());
00036     }
00037 
00038     bool runOnModule(Module& M) override;
00039 
00040   private:
00041     Function* unswitchFunction(Function* F);
00042   };
00043 }
00044 
00045 char PartialInliner::ID = 0;
00046 INITIALIZE_PASS(PartialInliner, "partial-inliner",
00047                 "Partial Inliner", false, false)
00048 
00049 ModulePass* llvm::createPartialInliningPass() { return new PartialInliner(); }
00050 
00051 Function* PartialInliner::unswitchFunction(Function* F) {
00052   // First, verify that this function is an unswitching candidate...
00053   BasicBlock* entryBlock = F->begin();
00054   BranchInst *BR = dyn_cast<BranchInst>(entryBlock->getTerminator());
00055   if (!BR || BR->isUnconditional())
00056     return 0;
00057   
00058   BasicBlock* returnBlock = 0;
00059   BasicBlock* nonReturnBlock = 0;
00060   unsigned returnCount = 0;
00061   for (succ_iterator SI = succ_begin(entryBlock), SE = succ_end(entryBlock);
00062        SI != SE; ++SI)
00063     if (isa<ReturnInst>((*SI)->getTerminator())) {
00064       returnBlock = *SI;
00065       returnCount++;
00066     } else
00067       nonReturnBlock = *SI;
00068   
00069   if (returnCount != 1)
00070     return 0;
00071   
00072   // Clone the function, so that we can hack away on it.
00073   ValueToValueMapTy VMap;
00074   Function* duplicateFunction = CloneFunction(F, VMap,
00075                                               /*ModuleLevelChanges=*/false);
00076   duplicateFunction->setLinkage(GlobalValue::InternalLinkage);
00077   F->getParent()->getFunctionList().push_back(duplicateFunction);
00078   BasicBlock* newEntryBlock = cast<BasicBlock>(VMap[entryBlock]);
00079   BasicBlock* newReturnBlock = cast<BasicBlock>(VMap[returnBlock]);
00080   BasicBlock* newNonReturnBlock = cast<BasicBlock>(VMap[nonReturnBlock]);
00081   
00082   // Go ahead and update all uses to the duplicate, so that we can just
00083   // use the inliner functionality when we're done hacking.
00084   F->replaceAllUsesWith(duplicateFunction);
00085   
00086   // Special hackery is needed with PHI nodes that have inputs from more than
00087   // one extracted block.  For simplicity, just split the PHIs into a two-level
00088   // sequence of PHIs, some of which will go in the extracted region, and some
00089   // of which will go outside.
00090   BasicBlock* preReturn = newReturnBlock;
00091   newReturnBlock = newReturnBlock->splitBasicBlock(
00092                                               newReturnBlock->getFirstNonPHI());
00093   BasicBlock::iterator I = preReturn->begin();
00094   BasicBlock::iterator Ins = newReturnBlock->begin();
00095   while (I != preReturn->end()) {
00096     PHINode* OldPhi = dyn_cast<PHINode>(I);
00097     if (!OldPhi) break;
00098     
00099     PHINode* retPhi = PHINode::Create(OldPhi->getType(), 2, "", Ins);
00100     OldPhi->replaceAllUsesWith(retPhi);
00101     Ins = newReturnBlock->getFirstNonPHI();
00102     
00103     retPhi->addIncoming(I, preReturn);
00104     retPhi->addIncoming(OldPhi->getIncomingValueForBlock(newEntryBlock),
00105                         newEntryBlock);
00106     OldPhi->removeIncomingValue(newEntryBlock);
00107     
00108     ++I;
00109   }
00110   newEntryBlock->getTerminator()->replaceUsesOfWith(preReturn, newReturnBlock);
00111   
00112   // Gather up the blocks that we're going to extract.
00113   std::vector<BasicBlock*> toExtract;
00114   toExtract.push_back(newNonReturnBlock);
00115   for (Function::iterator FI = duplicateFunction->begin(),
00116        FE = duplicateFunction->end(); FI != FE; ++FI)
00117     if (&*FI != newEntryBlock && &*FI != newReturnBlock &&
00118         &*FI != newNonReturnBlock)
00119       toExtract.push_back(FI);
00120       
00121   // The CodeExtractor needs a dominator tree.
00122   DominatorTree DT;
00123   DT.recalculate(*duplicateFunction);
00124 
00125   // Extract the body of the if.
00126   Function* extractedFunction
00127     = CodeExtractor(toExtract, &DT).extractCodeRegion();
00128   
00129   InlineFunctionInfo IFI;
00130   
00131   // Inline the top-level if test into all callers.
00132   std::vector<User *> Users(duplicateFunction->user_begin(),
00133                             duplicateFunction->user_end());
00134   for (std::vector<User*>::iterator UI = Users.begin(), UE = Users.end();
00135        UI != UE; ++UI)
00136     if (CallInst *CI = dyn_cast<CallInst>(*UI))
00137       InlineFunction(CI, IFI);
00138     else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI))
00139       InlineFunction(II, IFI);
00140   
00141   // Ditch the duplicate, since we're done with it, and rewrite all remaining
00142   // users (function pointers, etc.) back to the original function.
00143   duplicateFunction->replaceAllUsesWith(F);
00144   duplicateFunction->eraseFromParent();
00145   
00146   ++NumPartialInlined;
00147   
00148   return extractedFunction;
00149 }
00150 
00151 bool PartialInliner::runOnModule(Module& M) {
00152   std::vector<Function*> worklist;
00153   worklist.reserve(M.size());
00154   for (Module::iterator FI = M.begin(), FE = M.end(); FI != FE; ++FI)
00155     if (!FI->use_empty() && !FI->isDeclaration())
00156       worklist.push_back(&*FI);
00157     
00158   bool changed = false;
00159   while (!worklist.empty()) {
00160     Function* currFunc = worklist.back();
00161     worklist.pop_back();
00162   
00163     if (currFunc->use_empty()) continue;
00164     
00165     bool recursive = false;
00166     for (User *U : currFunc->users())
00167       if (Instruction* I = dyn_cast<Instruction>(U))
00168         if (I->getParent()->getParent() == currFunc) {
00169           recursive = true;
00170           break;
00171         }
00172     if (recursive) continue;
00173           
00174     
00175     if (Function* newFunc = unswitchFunction(currFunc)) {
00176       worklist.push_back(newFunc);
00177       changed = true;
00178     }
00179     
00180   }
00181   
00182   return changed;
00183 }