1//===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
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 file implements dead code elimination and basic block merging, along
11// with a collection of other peephole control flow optimizations.  For example:
12//
13//   * Removes basic blocks with no predecessors.
14//   * Merges a basic block into its predecessor if there is only one and the
15//     predecessor only has one successor.
16//   * Eliminates PHI nodes for basic blocks with a single predecessor.
17//   * Eliminates a basic block that only contains an unconditional branch.
18//   * Changes invoke instructions to nounwind functions to be calls.
19//   * Change things like "if (x) if (y)" into "if (x&y)".
20//   * etc..
21//
22//===----------------------------------------------------------------------===//
23
24#include "llvm/ADT/SmallPtrSet.h"
25#include "llvm/ADT/SmallVector.h"
26#include "llvm/ADT/Statistic.h"
27#include "llvm/Analysis/AssumptionCache.h"
28#include "llvm/Analysis/CFG.h"
29#include "llvm/Analysis/GlobalsModRef.h"
30#include "llvm/Analysis/TargetTransformInfo.h"
31#include "llvm/IR/Attributes.h"
32#include "llvm/IR/CFG.h"
33#include "llvm/IR/Constants.h"
34#include "llvm/IR/DataLayout.h"
35#include "llvm/IR/Instructions.h"
36#include "llvm/IR/IntrinsicInst.h"
37#include "llvm/IR/Module.h"
38#include "llvm/Pass.h"
39#include "llvm/Support/CommandLine.h"
40#include "llvm/Transforms/Scalar.h"
41#include "llvm/Transforms/Scalar/SimplifyCFG.h"
42#include "llvm/Transforms/Utils/Local.h"
43#include <utility>
44using namespace llvm;
45
46#define DEBUG_TYPE "simplifycfg"
47
48static cl::opt<unsigned>
49UserBonusInstThreshold("bonus-inst-threshold", cl::Hidden, cl::init(1),
50   cl::desc("Control the number of bonus instructions (default = 1)"));
51
52STATISTIC(NumSimpl, "Number of blocks simplified");
53
54/// If we have more than one empty (other than phi node) return blocks,
55/// merge them together to promote recursive block merging.
56static bool mergeEmptyReturnBlocks(Function &F) {
57  bool Changed = false;
58
59  BasicBlock *RetBlock = nullptr;
60
61  // Scan all the blocks in the function, looking for empty return blocks.
62  for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
63    BasicBlock &BB = *BBI++;
64
65    // Only look at return blocks.
66    ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
67    if (!Ret) continue;
68
69    // Only look at the block if it is empty or the only other thing in it is a
70    // single PHI node that is the operand to the return.
71    if (Ret != &BB.front()) {
72      // Check for something else in the block.
73      BasicBlock::iterator I(Ret);
74      --I;
75      // Skip over debug info.
76      while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
77        --I;
78      if (!isa<DbgInfoIntrinsic>(I) &&
79          (!isa<PHINode>(I) || I != BB.begin() || Ret->getNumOperands() == 0 ||
80           Ret->getOperand(0) != &*I))
81        continue;
82    }
83
84    // If this is the first returning block, remember it and keep going.
85    if (!RetBlock) {
86      RetBlock = &BB;
87      continue;
88    }
89
90    // Otherwise, we found a duplicate return block.  Merge the two.
91    Changed = true;
92
93    // Case when there is no input to the return or when the returned values
94    // agree is trivial.  Note that they can't agree if there are phis in the
95    // blocks.
96    if (Ret->getNumOperands() == 0 ||
97        Ret->getOperand(0) ==
98          cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
99      BB.replaceAllUsesWith(RetBlock);
100      BB.eraseFromParent();
101      continue;
102    }
103
104    // If the canonical return block has no PHI node, create one now.
105    PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
106    if (!RetBlockPHI) {
107      Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
108      pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
109      RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
110                                    std::distance(PB, PE), "merge",
111                                    &RetBlock->front());
112
113      for (pred_iterator PI = PB; PI != PE; ++PI)
114        RetBlockPHI->addIncoming(InVal, *PI);
115      RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
116    }
117
118    // Turn BB into a block that just unconditionally branches to the return
119    // block.  This handles the case when the two return blocks have a common
120    // predecessor but that return different things.
121    RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
122    BB.getTerminator()->eraseFromParent();
123    BranchInst::Create(RetBlock, &BB);
124  }
125
126  return Changed;
127}
128
129/// Call SimplifyCFG on all the blocks in the function,
130/// iterating until no more changes are made.
131static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
132                                   AssumptionCache *AC,
133                                   unsigned BonusInstThreshold) {
134  bool Changed = false;
135  bool LocalChange = true;
136
137  SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges;
138  FindFunctionBackedges(F, Edges);
139  SmallPtrSet<BasicBlock *, 16> LoopHeaders;
140  for (unsigned i = 0, e = Edges.size(); i != e; ++i)
141    LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second));
142
143  while (LocalChange) {
144    LocalChange = false;
145
146    // Loop over all of the basic blocks and remove them if they are unneeded.
147    for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
148      if (SimplifyCFG(&*BBIt++, TTI, BonusInstThreshold, AC, &LoopHeaders)) {
149        LocalChange = true;
150        ++NumSimpl;
151      }
152    }
153    Changed |= LocalChange;
154  }
155  return Changed;
156}
157
158static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
159                                AssumptionCache *AC, int BonusInstThreshold) {
160  bool EverChanged = removeUnreachableBlocks(F);
161  EverChanged |= mergeEmptyReturnBlocks(F);
162  EverChanged |= iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold);
163
164  // If neither pass changed anything, we're done.
165  if (!EverChanged) return false;
166
167  // iterativelySimplifyCFG can (rarely) make some loops dead.  If this happens,
168  // removeUnreachableBlocks is needed to nuke them, which means we should
169  // iterate between the two optimizations.  We structure the code like this to
170  // avoid rerunning iterativelySimplifyCFG if the second pass of
171  // removeUnreachableBlocks doesn't do anything.
172  if (!removeUnreachableBlocks(F))
173    return true;
174
175  do {
176    EverChanged = iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold);
177    EverChanged |= removeUnreachableBlocks(F);
178  } while (EverChanged);
179
180  return true;
181}
182
183SimplifyCFGPass::SimplifyCFGPass()
184    : BonusInstThreshold(UserBonusInstThreshold) {}
185
186SimplifyCFGPass::SimplifyCFGPass(int BonusInstThreshold)
187    : BonusInstThreshold(BonusInstThreshold) {}
188
189PreservedAnalyses SimplifyCFGPass::run(Function &F,
190                                       AnalysisManager<Function> &AM) {
191  auto &TTI = AM.getResult<TargetIRAnalysis>(F);
192  auto &AC = AM.getResult<AssumptionAnalysis>(F);
193
194  if (!simplifyFunctionCFG(F, TTI, &AC, BonusInstThreshold))
195    return PreservedAnalyses::all();
196  PreservedAnalyses PA;
197  PA.preserve<GlobalsAA>();
198  return PA;
199}
200
201namespace {
202struct CFGSimplifyPass : public FunctionPass {
203  static char ID; // Pass identification, replacement for typeid
204  unsigned BonusInstThreshold;
205  std::function<bool(const Function &)> PredicateFtor;
206
207  CFGSimplifyPass(int T = -1,
208                  std::function<bool(const Function &)> Ftor = nullptr)
209      : FunctionPass(ID), PredicateFtor(std::move(Ftor)) {
210    BonusInstThreshold = (T == -1) ? UserBonusInstThreshold : unsigned(T);
211    initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
212  }
213  bool runOnFunction(Function &F) override {
214    if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F)))
215      return false;
216
217    AssumptionCache *AC =
218        &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
219    const TargetTransformInfo &TTI =
220        getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
221    return simplifyFunctionCFG(F, TTI, AC, BonusInstThreshold);
222  }
223
224  void getAnalysisUsage(AnalysisUsage &AU) const override {
225    AU.addRequired<AssumptionCacheTracker>();
226    AU.addRequired<TargetTransformInfoWrapperPass>();
227    AU.addPreserved<GlobalsAAWrapperPass>();
228  }
229};
230}
231
232char CFGSimplifyPass::ID = 0;
233INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
234                      false)
235INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
236INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
237INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
238                    false)
239
240// Public interface to the CFGSimplification pass
241FunctionPass *
242llvm::createCFGSimplificationPass(int Threshold,
243                                  std::function<bool(const Function &)> Ftor) {
244  return new CFGSimplifyPass(Threshold, std::move(Ftor));
245}
246