SimplifyCFGPass.cpp revision 6948897e478cbd66626159776a8017b3c18579b9 
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/Transforms/Scalar/SimplifyCFG.h"
25#include "llvm/ADT/SmallPtrSet.h"
26#include "llvm/ADT/SmallVector.h"
27#include "llvm/ADT/Statistic.h"
28#include "llvm/Analysis/AssumptionCache.h"
29#include "llvm/Analysis/TargetTransformInfo.h"
30#include "llvm/IR/Attributes.h"
31#include "llvm/IR/CFG.h"
32#include "llvm/IR/Constants.h"
33#include "llvm/IR/DataLayout.h"
34#include "llvm/IR/Instructions.h"
35#include "llvm/IR/IntrinsicInst.h"
36#include "llvm/IR/Module.h"
37#include "llvm/Pass.h"
38#include "llvm/Support/CommandLine.h"
39#include "llvm/Transforms/Utils/Local.h"
40#include "llvm/Transforms/Scalar.h"
41using namespace llvm;
42
43#define DEBUG_TYPE "simplifycfg"
44
45static cl::opt<unsigned>
46UserBonusInstThreshold("bonus-inst-threshold", cl::Hidden, cl::init(1),
47   cl::desc("Control the number of bonus instructions (default = 1)"));
48
49STATISTIC(NumSimpl, "Number of blocks simplified");
50
51/// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
52/// node) return blocks, merge them together to promote recursive block merging.
53static bool mergeEmptyReturnBlocks(Function &F) {
54  bool Changed = false;
55
56  BasicBlock *RetBlock = nullptr;
57
58  // Scan all the blocks in the function, looking for empty return blocks.
59  for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
60    BasicBlock &BB = *BBI++;
61
62    // Only look at return blocks.
63    ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
64    if (!Ret) continue;
65
66    // Only look at the block if it is empty or the only other thing in it is a
67    // single PHI node that is the operand to the return.
68    if (Ret != &BB.front()) {
69      // Check for something else in the block.
70      BasicBlock::iterator I = Ret;
71      --I;
72      // Skip over debug info.
73      while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
74        --I;
75      if (!isa<DbgInfoIntrinsic>(I) &&
76          (!isa<PHINode>(I) || I != BB.begin() ||
77           Ret->getNumOperands() == 0 ||
78           Ret->getOperand(0) != I))
79        continue;
80    }
81
82    // If this is the first returning block, remember it and keep going.
83    if (!RetBlock) {
84      RetBlock = &BB;
85      continue;
86    }
87
88    // Otherwise, we found a duplicate return block.  Merge the two.
89    Changed = true;
90
91    // Case when there is no input to the return or when the returned values
92    // agree is trivial.  Note that they can't agree if there are phis in the
93    // blocks.
94    if (Ret->getNumOperands() == 0 ||
95        Ret->getOperand(0) ==
96          cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
97      BB.replaceAllUsesWith(RetBlock);
98      BB.eraseFromParent();
99      continue;
100    }
101
102    // If the canonical return block has no PHI node, create one now.
103    PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
104    if (!RetBlockPHI) {
105      Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
106      pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
107      RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
108                                    std::distance(PB, PE), "merge",
109                                    &RetBlock->front());
110
111      for (pred_iterator PI = PB; PI != PE; ++PI)
112        RetBlockPHI->addIncoming(InVal, *PI);
113      RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
114    }
115
116    // Turn BB into a block that just unconditionally branches to the return
117    // block.  This handles the case when the two return blocks have a common
118    // predecessor but that return different things.
119    RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
120    BB.getTerminator()->eraseFromParent();
121    BranchInst::Create(RetBlock, &BB);
122  }
123
124  return Changed;
125}
126
127/// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
128/// iterating until no more changes are made.
129static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
130                                   AssumptionCache *AC,
131                                   unsigned BonusInstThreshold) {
132  bool Changed = false;
133  bool LocalChange = true;
134  while (LocalChange) {
135    LocalChange = false;
136
137    // Loop over all of the basic blocks and remove them if they are unneeded...
138    //
139    for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
140      if (SimplifyCFG(BBIt++, TTI, BonusInstThreshold, AC)) {
141        LocalChange = true;
142        ++NumSimpl;
143      }
144    }
145    Changed |= LocalChange;
146  }
147  return Changed;
148}
149
150static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
151                                AssumptionCache *AC, int BonusInstThreshold) {
152  bool EverChanged = removeUnreachableBlocks(F);
153  EverChanged |= mergeEmptyReturnBlocks(F);
154  EverChanged |= iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold);
155
156  // If neither pass changed anything, we're done.
157  if (!EverChanged) return false;
158
159  // iterativelySimplifyCFG can (rarely) make some loops dead.  If this happens,
160  // removeUnreachableBlocks is needed to nuke them, which means we should
161  // iterate between the two optimizations.  We structure the code like this to
162  // avoid reruning iterativelySimplifyCFG if the second pass of
163  // removeUnreachableBlocks doesn't do anything.
164  if (!removeUnreachableBlocks(F))
165    return true;
166
167  do {
168    EverChanged = iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold);
169    EverChanged |= removeUnreachableBlocks(F);
170  } while (EverChanged);
171
172  return true;
173}
174
175SimplifyCFGPass::SimplifyCFGPass()
176    : BonusInstThreshold(UserBonusInstThreshold) {}
177
178SimplifyCFGPass::SimplifyCFGPass(int BonusInstThreshold)
179    : BonusInstThreshold(BonusInstThreshold) {}
180
181PreservedAnalyses SimplifyCFGPass::run(Function &F,
182                                       AnalysisManager<Function> *AM) {
183  auto &TTI = AM->getResult<TargetIRAnalysis>(F);
184  auto &AC = AM->getResult<AssumptionAnalysis>(F);
185
186  if (!simplifyFunctionCFG(F, TTI, &AC, BonusInstThreshold))
187    return PreservedAnalyses::none();
188
189  return PreservedAnalyses::all();
190}
191
192namespace {
193struct CFGSimplifyPass : public FunctionPass {
194  static char ID; // Pass identification, replacement for typeid
195  unsigned BonusInstThreshold;
196  std::function<bool(const Function &)> PredicateFtor;
197
198  CFGSimplifyPass(int T = -1,
199                  std::function<bool(const Function &)> Ftor = nullptr)
200      : FunctionPass(ID), PredicateFtor(Ftor) {
201    BonusInstThreshold = (T == -1) ? UserBonusInstThreshold : unsigned(T);
202    initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
203  }
204  bool runOnFunction(Function &F) override {
205    if (PredicateFtor && !PredicateFtor(F))
206      return false;
207
208    if (skipOptnoneFunction(F))
209      return false;
210
211    AssumptionCache *AC =
212        &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
213    const TargetTransformInfo &TTI =
214        getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
215    return simplifyFunctionCFG(F, TTI, AC, BonusInstThreshold);
216  }
217
218  void getAnalysisUsage(AnalysisUsage &AU) const override {
219    AU.addRequired<AssumptionCacheTracker>();
220    AU.addRequired<TargetTransformInfoWrapperPass>();
221  }
222};
223}
224
225char CFGSimplifyPass::ID = 0;
226INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
227                      false)
228INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
229INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
230INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
231                    false)
232
233// Public interface to the CFGSimplification pass
234FunctionPass *
235llvm::createCFGSimplificationPass(int Threshold,
236                                  std::function<bool(const Function &)> Ftor) {
237  return new CFGSimplifyPass(Threshold, Ftor);
238}
239
240