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 #define DEBUG_TYPE "simplifycfg"
  25 #include "llvm/Transforms/Scalar.h"
  26 #include "llvm/Transforms/Utils/Local.h"
  27 #include "llvm/Constants.h"
  28 #include "llvm/Instructions.h"
  29 #include "llvm/Module.h"
  30 #include "llvm/Attributes.h"
  31 #include "llvm/Support/CFG.h"
  32 #include "llvm/Support/Compiler.h"
  33 #include "llvm/Pass.h"
  34 #include "llvm/ADT/SmallVector.h"
  35 #include "llvm/ADT/SmallPtrSet.h"
  36 #include "llvm/ADT/Statistic.h"
  37 using namespace llvm;
  38
  39 STATISTIC(NumSimpl, "Number of blocks simplified");
  40
  41 namespace {
  42   struct VISIBILITY_HIDDEN CFGSimplifyPass : public FunctionPass {
  43     static char ID; // Pass identification, replacement for typeid
  44     CFGSimplifyPass() : FunctionPass(&ID) {}
  45
  46     virtual bool runOnFunction(Function &F);
  47   };
  48 }
  49
  50 char CFGSimplifyPass::ID = 0;
  51 static RegisterPass<CFGSimplifyPass> X("simplifycfg", "Simplify the CFG");
  52
  53 // Public interface to the CFGSimplification pass
  54 FunctionPass *llvm::createCFGSimplificationPass() {
  55   return new CFGSimplifyPass();
  56 }
  57
  58 /// ChangeToUnreachable - Insert an unreachable instruction before the specified
  59 /// instruction, making it and the rest of the code in the block dead.
  60 static void ChangeToUnreachable(Instruction *I) {
  61   BasicBlock *BB = I->getParent();
  62   // Loop over all of the successors, removing BB's entry from any PHI
  63   // nodes.
  64   for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
  65     (*SI)->removePredecessor(BB);
  66
  67   new UnreachableInst(I);
  68
  69   // All instructions after this are dead.
  70   BasicBlock::iterator BBI = I, BBE = BB->end();
  71   while (BBI != BBE) {
  72     if (!BBI->use_empty())
  73       BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
  74     BB->getInstList().erase(BBI++);
  75   }
  76 }
  77
  78 /// ChangeToCall - Convert the specified invoke into a normal call.
  79 static void ChangeToCall(InvokeInst *II) {
  80   BasicBlock *BB = II->getParent();
  81   SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
  82   CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args.begin(),
  83                                        Args.end(), "", II);
  84   NewCall->takeName(II);
  85   NewCall->setCallingConv(II->getCallingConv());
  86   NewCall->setAttributes(II->getAttributes());
  87   II->replaceAllUsesWith(NewCall);
  88
  89   // Follow the call by a branch to the normal destination.
  90   BranchInst::Create(II->getNormalDest(), II);
  91
  92   // Update PHI nodes in the unwind destination
  93   II->getUnwindDest()->removePredecessor(BB);
  94   BB->getInstList().erase(II);
  95 }
  96
  97 static bool MarkAliveBlocks(BasicBlock *BB,
  98                             SmallPtrSet<BasicBlock*, 128> &Reachable) {
  99
 100   SmallVector<BasicBlock*, 128> Worklist;
 101   Worklist.push_back(BB);
 102   bool Changed = false;
 103   while (!Worklist.empty()) {
 104     BB = Worklist.back();
 105     Worklist.pop_back();
 106
 107     if (!Reachable.insert(BB))
 108       continue;
 109
 110     // Do a quick scan of the basic block, turning any obviously unreachable
 111     // instructions into LLVM unreachable insts.  The instruction combining pass
 112     // canonicalizes unreachable insts into stores to null or undef.
 113     for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
 114       if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
 115         if (CI->doesNotReturn()) {
 116           // If we found a call to a no-return function, insert an unreachable
 117           // instruction after it.  Make sure there isn't *already* one there
 118           // though.
 119           ++BBI;
 120           if (!isa<UnreachableInst>(BBI)) {
 121             ChangeToUnreachable(BBI);
 122             Changed = true;
 123           }
 124           break;
 125         }
 126       }
 127
 128       if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
 129         if (isa<ConstantPointerNull>(SI->getOperand(1)) ||
 130             isa<UndefValue>(SI->getOperand(1))) {
 131           ChangeToUnreachable(SI);
 132           Changed = true;
 133           break;
 134         }
 135     }
 136
 137     // Turn invokes that call 'nounwind' functions into ordinary calls.
 138     if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
 139       if (II->doesNotThrow()) {
 140         ChangeToCall(II);
 141         Changed = true;
 142       }
 143
 144     Changed |= ConstantFoldTerminator(BB);
 145     for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
 146       Worklist.push_back(*SI);
 147   }
 148   return Changed;
 149 }
 150
 151 /// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
 152 /// if they are in a dead cycle.  Return true if a change was made, false
 153 /// otherwise.
 154 static bool RemoveUnreachableBlocksFromFn(Function &F) {
 155   SmallPtrSet<BasicBlock*, 128> Reachable;
 156   bool Changed = MarkAliveBlocks(F.begin(), Reachable);
 157
 158   // If there are unreachable blocks in the CFG...
 159   if (Reachable.size() == F.size())
 160     return Changed;
 161
 162   assert(Reachable.size() < F.size());
 163   NumSimpl += F.size()-Reachable.size();
 164
 165   // Loop over all of the basic blocks that are not reachable, dropping all of
 166   // their internal references...
 167   for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
 168     if (Reachable.count(BB))
 169       continue;
 170
 171     for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
 172       if (Reachable.count(*SI))
 173         (*SI)->removePredecessor(BB);
 174     BB->dropAllReferences();
 175   }
 176
 177   for (Function::iterator I = ++F.begin(); I != F.end();)
 178     if (!Reachable.count(I))
 179       I = F.getBasicBlockList().erase(I);
 180     else
 181       ++I;
 182
 183   return true;
 184 }
 185
 186 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
 187 /// iterating until no more changes are made.
 188 static bool IterativeSimplifyCFG(Function &F) {
 189   bool Changed = false;
 190   bool LocalChange = true;
 191   while (LocalChange) {
 192     LocalChange = false;
 193
 194     // Loop over all of the basic blocks (except the first one) and remove them
 195     // if they are unneeded...
 196     //
 197     for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
 198       if (SimplifyCFG(BBIt++)) {
 199         LocalChange = true;
 200         ++NumSimpl;
 201       }
 202     }
 203     Changed |= LocalChange;
 204   }
 205   return Changed;
 206 }
 207
 208 // It is possible that we may require multiple passes over the code to fully
 209 // simplify the CFG.
 210 //
 211 bool CFGSimplifyPass::runOnFunction(Function &F) {
 212   bool EverChanged = RemoveUnreachableBlocksFromFn(F);
 213   EverChanged |= IterativeSimplifyCFG(F);
 214
 215   // If neither pass changed anything, we're done.
 216   if (!EverChanged) return false;
 217
 218   // IterativeSimplifyCFG can (rarely) make some loops dead.  If this happens,
 219   // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
 220   // iterate between the two optimizations.  We structure the code like this to
 221   // avoid reruning IterativeSimplifyCFG if the second pass of
 222   // RemoveUnreachableBlocksFromFn doesn't do anything.
 223   if (!RemoveUnreachableBlocksFromFn(F))
 224     return true;
 225
 226   do {
 227     EverChanged = IterativeSimplifyCFG(F);
 228     EverChanged |= RemoveUnreachableBlocksFromFn(F);
 229   } while (EverChanged);
 230
 231   return true;
 232 }