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      1 //===- PartialInlining.cpp - Inline parts of functions --------------------===//
      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 performs partial inlining, typically by inlining an if statement
     11 // that surrounds the body of the function.
     12 //
     13 //===----------------------------------------------------------------------===//
     14 
     15 #include "llvm/Transforms/IPO.h"
     16 #include "llvm/ADT/Statistic.h"
     17 #include "llvm/IR/CFG.h"
     18 #include "llvm/IR/Dominators.h"
     19 #include "llvm/IR/Instructions.h"
     20 #include "llvm/IR/Module.h"
     21 #include "llvm/Pass.h"
     22 #include "llvm/Transforms/Utils/Cloning.h"
     23 #include "llvm/Transforms/Utils/CodeExtractor.h"
     24 using namespace llvm;
     25 
     26 #define DEBUG_TYPE "partialinlining"
     27 
     28 STATISTIC(NumPartialInlined, "Number of functions partially inlined");
     29 
     30 namespace {
     31   struct PartialInliner : public ModulePass {
     32     void getAnalysisUsage(AnalysisUsage &AU) const override { }
     33     static char ID; // Pass identification, replacement for typeid
     34     PartialInliner() : ModulePass(ID) {
     35       initializePartialInlinerPass(*PassRegistry::getPassRegistry());
     36     }
     37 
     38     bool runOnModule(Module& M) override;
     39 
     40   private:
     41     Function* unswitchFunction(Function* F);
     42   };
     43 }
     44 
     45 char PartialInliner::ID = 0;
     46 INITIALIZE_PASS(PartialInliner, "partial-inliner",
     47                 "Partial Inliner", false, false)
     48 
     49 ModulePass* llvm::createPartialInliningPass() { return new PartialInliner(); }
     50 
     51 Function* PartialInliner::unswitchFunction(Function* F) {
     52   // First, verify that this function is an unswitching candidate...
     53   BasicBlock* entryBlock = F->begin();
     54   BranchInst *BR = dyn_cast<BranchInst>(entryBlock->getTerminator());
     55   if (!BR || BR->isUnconditional())
     56     return nullptr;
     57 
     58   BasicBlock* returnBlock = nullptr;
     59   BasicBlock* nonReturnBlock = nullptr;
     60   unsigned returnCount = 0;
     61   for (BasicBlock *BB : successors(entryBlock)) {
     62     if (isa<ReturnInst>(BB->getTerminator())) {
     63       returnBlock = BB;
     64       returnCount++;
     65     } else
     66       nonReturnBlock = BB;
     67   }
     68 
     69   if (returnCount != 1)
     70     return nullptr;
     71 
     72   // Clone the function, so that we can hack away on it.
     73   ValueToValueMapTy VMap;
     74   Function* duplicateFunction = CloneFunction(F, VMap,
     75                                               /*ModuleLevelChanges=*/false);
     76   duplicateFunction->setLinkage(GlobalValue::InternalLinkage);
     77   F->getParent()->getFunctionList().push_back(duplicateFunction);
     78   BasicBlock* newEntryBlock = cast<BasicBlock>(VMap[entryBlock]);
     79   BasicBlock* newReturnBlock = cast<BasicBlock>(VMap[returnBlock]);
     80   BasicBlock* newNonReturnBlock = cast<BasicBlock>(VMap[nonReturnBlock]);
     81 
     82   // Go ahead and update all uses to the duplicate, so that we can just
     83   // use the inliner functionality when we're done hacking.
     84   F->replaceAllUsesWith(duplicateFunction);
     85 
     86   // Special hackery is needed with PHI nodes that have inputs from more than
     87   // one extracted block.  For simplicity, just split the PHIs into a two-level
     88   // sequence of PHIs, some of which will go in the extracted region, and some
     89   // of which will go outside.
     90   BasicBlock* preReturn = newReturnBlock;
     91   newReturnBlock = newReturnBlock->splitBasicBlock(
     92                                               newReturnBlock->getFirstNonPHI());
     93   BasicBlock::iterator I = preReturn->begin();
     94   BasicBlock::iterator Ins = newReturnBlock->begin();
     95   while (I != preReturn->end()) {
     96     PHINode* OldPhi = dyn_cast<PHINode>(I);
     97     if (!OldPhi) break;
     98 
     99     PHINode* retPhi = PHINode::Create(OldPhi->getType(), 2, "", Ins);
    100     OldPhi->replaceAllUsesWith(retPhi);
    101     Ins = newReturnBlock->getFirstNonPHI();
    102 
    103     retPhi->addIncoming(I, preReturn);
    104     retPhi->addIncoming(OldPhi->getIncomingValueForBlock(newEntryBlock),
    105                         newEntryBlock);
    106     OldPhi->removeIncomingValue(newEntryBlock);
    107 
    108     ++I;
    109   }
    110   newEntryBlock->getTerminator()->replaceUsesOfWith(preReturn, newReturnBlock);
    111 
    112   // Gather up the blocks that we're going to extract.
    113   std::vector<BasicBlock*> toExtract;
    114   toExtract.push_back(newNonReturnBlock);
    115   for (Function::iterator FI = duplicateFunction->begin(),
    116        FE = duplicateFunction->end(); FI != FE; ++FI)
    117     if (&*FI != newEntryBlock && &*FI != newReturnBlock &&
    118         &*FI != newNonReturnBlock)
    119       toExtract.push_back(FI);
    120 
    121   // The CodeExtractor needs a dominator tree.
    122   DominatorTree DT;
    123   DT.recalculate(*duplicateFunction);
    124 
    125   // Extract the body of the if.
    126   Function* extractedFunction
    127     = CodeExtractor(toExtract, &DT).extractCodeRegion();
    128 
    129   InlineFunctionInfo IFI;
    130 
    131   // Inline the top-level if test into all callers.
    132   std::vector<User *> Users(duplicateFunction->user_begin(),
    133                             duplicateFunction->user_end());
    134   for (std::vector<User*>::iterator UI = Users.begin(), UE = Users.end();
    135        UI != UE; ++UI)
    136     if (CallInst *CI = dyn_cast<CallInst>(*UI))
    137       InlineFunction(CI, IFI);
    138     else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI))
    139       InlineFunction(II, IFI);
    140 
    141   // Ditch the duplicate, since we're done with it, and rewrite all remaining
    142   // users (function pointers, etc.) back to the original function.
    143   duplicateFunction->replaceAllUsesWith(F);
    144   duplicateFunction->eraseFromParent();
    145 
    146   ++NumPartialInlined;
    147 
    148   return extractedFunction;
    149 }
    150 
    151 bool PartialInliner::runOnModule(Module& M) {
    152   std::vector<Function*> worklist;
    153   worklist.reserve(M.size());
    154   for (Module::iterator FI = M.begin(), FE = M.end(); FI != FE; ++FI)
    155     if (!FI->use_empty() && !FI->isDeclaration())
    156       worklist.push_back(&*FI);
    157 
    158   bool changed = false;
    159   while (!worklist.empty()) {
    160     Function* currFunc = worklist.back();
    161     worklist.pop_back();
    162 
    163     if (currFunc->use_empty()) continue;
    164 
    165     bool recursive = false;
    166     for (User *U : currFunc->users())
    167       if (Instruction* I = dyn_cast<Instruction>(U))
    168         if (I->getParent()->getParent() == currFunc) {
    169           recursive = true;
    170           break;
    171         }
    172     if (recursive) continue;
    173 
    174 
    175     if (Function* newFunc = unswitchFunction(currFunc)) {
    176       worklist.push_back(newFunc);
    177       changed = true;
    178     }
    179 
    180   }
    181 
    182   return changed;
    183 }
    184