contrib/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp

   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>
  44 using namespace llvm;
  45
  46 #define DEBUG_TYPE "simplifycfg"
  47
  48 static cl::opt<unsigned>
  49 UserBonusInstThreshold("bonus-inst-threshold", cl::Hidden, cl::init(1),
  50    cl::desc("Control the number of bonus instructions (default = 1)"));
  51
  52 STATISTIC(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.
  56 static 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.
 131 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
 132                                    AssumptionCache *AC,
 133                                    unsigned BonusInstThreshold,
 134                                    bool LateSimplifyCFG) {
 135   bool Changed = false;
 136   bool LocalChange = true;
 137
 138   SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges;
 139   FindFunctionBackedges(F, Edges);
 140   SmallPtrSet<BasicBlock *, 16> LoopHeaders;
 141   for (unsigned i = 0, e = Edges.size(); i != e; ++i)
 142     LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second));
 143
 144   while (LocalChange) {
 145     LocalChange = false;
 146
 147     // Loop over all of the basic blocks and remove them if they are unneeded.
 148     for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
 149       if (SimplifyCFG(&*BBIt++, TTI, BonusInstThreshold, AC, &LoopHeaders, LateSimplifyCFG)) {
 150         LocalChange = true;
 151         ++NumSimpl;
 152       }
 153     }
 154     Changed |= LocalChange;
 155   }
 156   return Changed;
 157 }
 158
 159 static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
 160                                 AssumptionCache *AC, int BonusInstThreshold,
 161                                 bool LateSimplifyCFG) {
 162   bool EverChanged = removeUnreachableBlocks(F);
 163   EverChanged |= mergeEmptyReturnBlocks(F);
 164   EverChanged |= iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold,
 165                                         LateSimplifyCFG);
 166
 167   // If neither pass changed anything, we're done.
 168   if (!EverChanged) return false;
 169
 170   // iterativelySimplifyCFG can (rarely) make some loops dead.  If this happens,
 171   // removeUnreachableBlocks is needed to nuke them, which means we should
 172   // iterate between the two optimizations.  We structure the code like this to
 173   // avoid rerunning iterativelySimplifyCFG if the second pass of
 174   // removeUnreachableBlocks doesn't do anything.
 175   if (!removeUnreachableBlocks(F))
 176     return true;
 177
 178   do {
 179     EverChanged = iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold,
 180                                          LateSimplifyCFG);
 181     EverChanged |= removeUnreachableBlocks(F);
 182   } while (EverChanged);
 183
 184   return true;
 185 }
 186
 187 SimplifyCFGPass::SimplifyCFGPass()
 188     : BonusInstThreshold(UserBonusInstThreshold),
 189       LateSimplifyCFG(true) {}
 190
 191 SimplifyCFGPass::SimplifyCFGPass(int BonusInstThreshold, bool LateSimplifyCFG)
 192     : BonusInstThreshold(BonusInstThreshold),
 193       LateSimplifyCFG(LateSimplifyCFG) {}
 194
 195 PreservedAnalyses SimplifyCFGPass::run(Function &F,
 196                                        FunctionAnalysisManager &AM) {
 197   auto &TTI = AM.getResult<TargetIRAnalysis>(F);
 198   auto &AC = AM.getResult<AssumptionAnalysis>(F);
 199
 200   if (!simplifyFunctionCFG(F, TTI, &AC, BonusInstThreshold, LateSimplifyCFG))
 201     return PreservedAnalyses::all();
 202   PreservedAnalyses PA;
 203   PA.preserve<GlobalsAA>();
 204   return PA;
 205 }
 206
 207 namespace {
 208 struct BaseCFGSimplifyPass : public FunctionPass {
 209   unsigned BonusInstThreshold;
 210   std::function<bool(const Function &)> PredicateFtor;
 211   bool LateSimplifyCFG;
 212
 213   BaseCFGSimplifyPass(int T, bool LateSimplifyCFG,
 214                       std::function<bool(const Function &)> Ftor,
 215                       char &ID)
 216       : FunctionPass(ID), PredicateFtor(std::move(Ftor)),
 217         LateSimplifyCFG(LateSimplifyCFG) {
 218     BonusInstThreshold = (T == -1) ? UserBonusInstThreshold : unsigned(T);
 219   }
 220   bool runOnFunction(Function &F) override {
 221     if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F)))
 222       return false;
 223
 224     AssumptionCache *AC =
 225         &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
 226     const TargetTransformInfo &TTI =
 227         getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
 228     return simplifyFunctionCFG(F, TTI, AC, BonusInstThreshold, LateSimplifyCFG);
 229   }
 230
 231   void getAnalysisUsage(AnalysisUsage &AU) const override {
 232     AU.addRequired<AssumptionCacheTracker>();
 233     AU.addRequired<TargetTransformInfoWrapperPass>();
 234     AU.addPreserved<GlobalsAAWrapperPass>();
 235   }
 236 };
 237
 238 struct CFGSimplifyPass : public BaseCFGSimplifyPass {
 239   static char ID; // Pass identification, replacement for typeid
 240
 241   CFGSimplifyPass(int T = -1,
 242                   std::function<bool(const Function &)> Ftor = nullptr)
 243                   : BaseCFGSimplifyPass(T, false, Ftor, ID) {
 244     initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
 245   }
 246 };
 247
 248 struct LateCFGSimplifyPass : public BaseCFGSimplifyPass {
 249   static char ID; // Pass identification, replacement for typeid
 250
 251   LateCFGSimplifyPass(int T = -1,
 252                       std::function<bool(const Function &)> Ftor = nullptr)
 253                       : BaseCFGSimplifyPass(T, true, Ftor, ID) {
 254     initializeLateCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
 255   }
 256 };
 257 }
 258
 259 char CFGSimplifyPass::ID = 0;
 260 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
 261                       false)
 262 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
 263 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
 264 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
 265                     false)
 266
 267 char LateCFGSimplifyPass::ID = 0;
 268 INITIALIZE_PASS_BEGIN(LateCFGSimplifyPass, "latesimplifycfg",
 269                       "Simplify the CFG more aggressively", false, false)
 270 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
 271 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
 272 INITIALIZE_PASS_END(LateCFGSimplifyPass, "latesimplifycfg",
 273                     "Simplify the CFG more aggressively", false, false)
 274
 275 // Public interface to the CFGSimplification pass
 276 FunctionPass *
 277 llvm::createCFGSimplificationPass(int Threshold,
 278     std::function<bool(const Function &)> Ftor) {
 279   return new CFGSimplifyPass(Threshold, std::move(Ftor));
 280 }
 281
 282 // Public interface to the LateCFGSimplification pass
 283 FunctionPass *
 284 llvm::createLateCFGSimplificationPass(int Threshold,
 285                                   std::function<bool(const Function &)> Ftor) {
 286   return new LateCFGSimplifyPass(Threshold, std::move(Ftor));
 287 }