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/LLVMContext.h"
  30 #include "llvm/Module.h"
  31 #include "llvm/Attributes.h"
  32 #include "llvm/Support/CFG.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 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, LLVMContext &Context) {
  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->getContext(), 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                             LLVMContext &Context) {
 100
 101   SmallVector<BasicBlock*, 128> Worklist;
 102   Worklist.push_back(BB);
 103   bool Changed = false;
 104   while (!Worklist.empty()) {
 105     BB = Worklist.back();
 106     Worklist.pop_back();
 107
 108     if (!Reachable.insert(BB))
 109       continue;
 110
 111     // Do a quick scan of the basic block, turning any obviously unreachable
 112     // instructions into LLVM unreachable insts.  The instruction combining pass
 113     // canonicalizes unreachable insts into stores to null or undef.
 114     for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
 115       if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
 116         if (CI->doesNotReturn()) {
 117           // If we found a call to a no-return function, insert an unreachable
 118           // instruction after it.  Make sure there isn't *already* one there
 119           // though.
 120           ++BBI;
 121           if (!isa<UnreachableInst>(BBI)) {
 122             ChangeToUnreachable(BBI, Context);
 123             Changed = true;
 124           }
 125           break;
 126         }
 127       }
 128
 129       if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
 130         Value *Ptr = SI->getOperand(1);
 131
 132         if (isa<UndefValue>(Ptr) ||
 133             (isa<ConstantPointerNull>(Ptr) &&
 134              SI->getPointerAddressSpace() == 0)) {
 135           ChangeToUnreachable(SI, Context);
 136           Changed = true;
 137           break;
 138         }
 139       }
 140     }
 141
 142     // Turn invokes that call 'nounwind' functions into ordinary calls.
 143     if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
 144       if (II->doesNotThrow()) {
 145         ChangeToCall(II);
 146         Changed = true;
 147       }
 148
 149     Changed |= ConstantFoldTerminator(BB);
 150     for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
 151       Worklist.push_back(*SI);
 152   }
 153   return Changed;
 154 }
 155
 156 /// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
 157 /// if they are in a dead cycle.  Return true if a change was made, false
 158 /// otherwise.
 159 static bool RemoveUnreachableBlocksFromFn(Function &F) {
 160   SmallPtrSet<BasicBlock*, 128> Reachable;
 161   bool Changed = MarkAliveBlocks(F.begin(), Reachable, F.getContext());
 162
 163   // If there are unreachable blocks in the CFG...
 164   if (Reachable.size() == F.size())
 165     return Changed;
 166
 167   assert(Reachable.size() < F.size());
 168   NumSimpl += F.size()-Reachable.size();
 169
 170   // Loop over all of the basic blocks that are not reachable, dropping all of
 171   // their internal references...
 172   for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
 173     if (Reachable.count(BB))
 174       continue;
 175
 176     for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
 177       if (Reachable.count(*SI))
 178         (*SI)->removePredecessor(BB);
 179     BB->dropAllReferences();
 180   }
 181
 182   for (Function::iterator I = ++F.begin(); I != F.end();)
 183     if (!Reachable.count(I))
 184       I = F.getBasicBlockList().erase(I);
 185     else
 186       ++I;
 187
 188   return true;
 189 }
 190
 191 /// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
 192 /// iterating until no more changes are made.
 193 static bool IterativeSimplifyCFG(Function &F) {
 194   bool Changed = false;
 195   bool LocalChange = true;
 196   while (LocalChange) {
 197     LocalChange = false;
 198
 199     // Loop over all of the basic blocks (except the first one) and remove them
 200     // if they are unneeded...
 201     //
 202     for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
 203       if (SimplifyCFG(BBIt++)) {
 204         LocalChange = true;
 205         ++NumSimpl;
 206       }
 207     }
 208     Changed |= LocalChange;
 209   }
 210   return Changed;
 211 }
 212
 213 // It is possible that we may require multiple passes over the code to fully
 214 // simplify the CFG.
 215 //
 216 bool CFGSimplifyPass::runOnFunction(Function &F) {
 217   bool EverChanged = RemoveUnreachableBlocksFromFn(F);
 218   EverChanged |= IterativeSimplifyCFG(F);
 219
 220   // If neither pass changed anything, we're done.
 221   if (!EverChanged) return false;
 222
 223   // IterativeSimplifyCFG can (rarely) make some loops dead.  If this happens,
 224   // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
 225   // iterate between the two optimizations.  We structure the code like this to
 226   // avoid reruning IterativeSimplifyCFG if the second pass of
 227   // RemoveUnreachableBlocksFromFn doesn't do anything.
 228   if (!RemoveUnreachableBlocksFromFn(F))
 229     return true;
 230
 231   do {
 232     EverChanged = IterativeSimplifyCFG(F);
 233     EverChanged |= RemoveUnreachableBlocksFromFn(F);
 234   } while (EverChanged);
 235
 236   return true;
 237 }