PartialInlining.cpp revision dce4a407a24b04eebc6a376f8e62b41aaa7b071f 
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"
24using namespace llvm;
25
26#define DEBUG_TYPE "partialinlining"
27
28STATISTIC(NumPartialInlined, "Number of functions partially inlined");
29
30namespace {
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
45char PartialInliner::ID = 0;
46INITIALIZE_PASS(PartialInliner, "partial-inliner",
47                "Partial Inliner", false, false)
48
49ModulePass* llvm::createPartialInliningPass() { return new PartialInliner(); }
50
51Function* 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 (succ_iterator SI = succ_begin(entryBlock), SE = succ_end(entryBlock);
62       SI != SE; ++SI)
63    if (isa<ReturnInst>((*SI)->getTerminator())) {
64      returnBlock = *SI;
65      returnCount++;
66    } else
67      nonReturnBlock = *SI;
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
151bool 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