SimplifyCFGPass.cpp revision be04929f7fd76a921540e9901f24563e51dc1219 
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/ADT/SmallPtrSet.h"
27#include "llvm/ADT/SmallVector.h"
28#include "llvm/ADT/Statistic.h"
29#include "llvm/Analysis/TargetTransformInfo.h"
30#include "llvm/IR/Attributes.h"
31#include "llvm/IR/Constants.h"
32#include "llvm/IR/DataLayout.h"
33#include "llvm/IR/Instructions.h"
34#include "llvm/IR/IntrinsicInst.h"
35#include "llvm/IR/Module.h"
36#include "llvm/Pass.h"
37#include "llvm/Support/CFG.h"
38#include "llvm/Transforms/Utils/Local.h"
39using namespace llvm;
40
41STATISTIC(NumSimpl, "Number of blocks simplified");
42
43namespace {
44  struct CFGSimplifyPass : public FunctionPass {
45    static char ID; // Pass identification, replacement for typeid
46    CFGSimplifyPass() : FunctionPass(ID) {
47      initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
48    }
49
50    virtual bool runOnFunction(Function &F);
51  };
52}
53
54char CFGSimplifyPass::ID = 0;
55INITIALIZE_PASS(CFGSimplifyPass, "simplifycfg",
56                "Simplify the CFG", false, false)
57
58// Public interface to the CFGSimplification pass
59FunctionPass *llvm::createCFGSimplificationPass() {
60  return new CFGSimplifyPass();
61}
62
63/// changeToUnreachable - Insert an unreachable instruction before the specified
64/// instruction, making it and the rest of the code in the block dead.
65static void changeToUnreachable(Instruction *I, bool UseLLVMTrap) {
66  BasicBlock *BB = I->getParent();
67  // Loop over all of the successors, removing BB's entry from any PHI
68  // nodes.
69  for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
70    (*SI)->removePredecessor(BB);
71
72  // Insert a call to llvm.trap right before this.  This turns the undefined
73  // behavior into a hard fail instead of falling through into random code.
74  if (UseLLVMTrap) {
75    Function *TrapFn =
76      Intrinsic::getDeclaration(BB->getParent()->getParent(), Intrinsic::trap);
77    CallInst *CallTrap = CallInst::Create(TrapFn, "", I);
78    CallTrap->setDebugLoc(I->getDebugLoc());
79  }
80  new UnreachableInst(I->getContext(), I);
81
82  // All instructions after this are dead.
83  BasicBlock::iterator BBI = I, BBE = BB->end();
84  while (BBI != BBE) {
85    if (!BBI->use_empty())
86      BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
87    BB->getInstList().erase(BBI++);
88  }
89}
90
91/// changeToCall - Convert the specified invoke into a normal call.
92static void changeToCall(InvokeInst *II) {
93  SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
94  CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args, "", II);
95  NewCall->takeName(II);
96  NewCall->setCallingConv(II->getCallingConv());
97  NewCall->setAttributes(II->getAttributes());
98  NewCall->setDebugLoc(II->getDebugLoc());
99  II->replaceAllUsesWith(NewCall);
100
101  // Follow the call by a branch to the normal destination.
102  BranchInst::Create(II->getNormalDest(), II);
103
104  // Update PHI nodes in the unwind destination
105  II->getUnwindDest()->removePredecessor(II->getParent());
106  II->eraseFromParent();
107}
108
109static bool markAliveBlocks(BasicBlock *BB,
110                            SmallPtrSet<BasicBlock*, 128> &Reachable) {
111
112  SmallVector<BasicBlock*, 128> Worklist;
113  Worklist.push_back(BB);
114  Reachable.insert(BB);
115  bool Changed = false;
116  do {
117    BB = Worklist.pop_back_val();
118
119    // Do a quick scan of the basic block, turning any obviously unreachable
120    // instructions into LLVM unreachable insts.  The instruction combining pass
121    // canonicalizes unreachable insts into stores to null or undef.
122    for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
123      if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
124        if (CI->doesNotReturn()) {
125          // If we found a call to a no-return function, insert an unreachable
126          // instruction after it.  Make sure there isn't *already* one there
127          // though.
128          ++BBI;
129          if (!isa<UnreachableInst>(BBI)) {
130            // Don't insert a call to llvm.trap right before the unreachable.
131            changeToUnreachable(BBI, false);
132            Changed = true;
133          }
134          break;
135        }
136      }
137
138      // Store to undef and store to null are undefined and used to signal that
139      // they should be changed to unreachable by passes that can't modify the
140      // CFG.
141      if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
142        // Don't touch volatile stores.
143        if (SI->isVolatile()) continue;
144
145        Value *Ptr = SI->getOperand(1);
146
147        if (isa<UndefValue>(Ptr) ||
148            (isa<ConstantPointerNull>(Ptr) &&
149             SI->getPointerAddressSpace() == 0)) {
150          changeToUnreachable(SI, true);
151          Changed = true;
152          break;
153        }
154      }
155    }
156
157    // Turn invokes that call 'nounwind' functions into ordinary calls.
158    if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator())) {
159      Value *Callee = II->getCalledValue();
160      if (isa<ConstantPointerNull>(Callee) || isa<UndefValue>(Callee)) {
161        changeToUnreachable(II, true);
162        Changed = true;
163      } else if (II->doesNotThrow()) {
164        if (II->use_empty() && II->onlyReadsMemory()) {
165          // jump to the normal destination branch.
166          BranchInst::Create(II->getNormalDest(), II);
167          II->getUnwindDest()->removePredecessor(II->getParent());
168          II->eraseFromParent();
169        } else
170          changeToCall(II);
171        Changed = true;
172      }
173    }
174
175    Changed |= ConstantFoldTerminator(BB, true);
176    for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
177      if (Reachable.insert(*SI))
178        Worklist.push_back(*SI);
179  } while (!Worklist.empty());
180  return Changed;
181}
182
183/// removeUnreachableBlocksFromFn - Remove blocks that are not reachable, even
184/// if they are in a dead cycle.  Return true if a change was made, false
185/// otherwise.
186static bool removeUnreachableBlocksFromFn(Function &F) {
187  SmallPtrSet<BasicBlock*, 128> Reachable;
188  bool Changed = markAliveBlocks(F.begin(), Reachable);
189
190  // If there are unreachable blocks in the CFG...
191  if (Reachable.size() == F.size())
192    return Changed;
193
194  assert(Reachable.size() < F.size());
195  NumSimpl += F.size()-Reachable.size();
196
197  // Loop over all of the basic blocks that are not reachable, dropping all of
198  // their internal references...
199  for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
200    if (Reachable.count(BB))
201      continue;
202
203    for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
204      if (Reachable.count(*SI))
205        (*SI)->removePredecessor(BB);
206    BB->dropAllReferences();
207  }
208
209  for (Function::iterator I = ++F.begin(); I != F.end();)
210    if (!Reachable.count(I))
211      I = F.getBasicBlockList().erase(I);
212    else
213      ++I;
214
215  return true;
216}
217
218/// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
219/// node) return blocks, merge them together to promote recursive block merging.
220static bool mergeEmptyReturnBlocks(Function &F) {
221  bool Changed = false;
222
223  BasicBlock *RetBlock = 0;
224
225  // Scan all the blocks in the function, looking for empty return blocks.
226  for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
227    BasicBlock &BB = *BBI++;
228
229    // Only look at return blocks.
230    ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
231    if (Ret == 0) continue;
232
233    // Only look at the block if it is empty or the only other thing in it is a
234    // single PHI node that is the operand to the return.
235    if (Ret != &BB.front()) {
236      // Check for something else in the block.
237      BasicBlock::iterator I = Ret;
238      --I;
239      // Skip over debug info.
240      while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
241        --I;
242      if (!isa<DbgInfoIntrinsic>(I) &&
243          (!isa<PHINode>(I) || I != BB.begin() ||
244           Ret->getNumOperands() == 0 ||
245           Ret->getOperand(0) != I))
246        continue;
247    }
248
249    // If this is the first returning block, remember it and keep going.
250    if (RetBlock == 0) {
251      RetBlock = &BB;
252      continue;
253    }
254
255    // Otherwise, we found a duplicate return block.  Merge the two.
256    Changed = true;
257
258    // Case when there is no input to the return or when the returned values
259    // agree is trivial.  Note that they can't agree if there are phis in the
260    // blocks.
261    if (Ret->getNumOperands() == 0 ||
262        Ret->getOperand(0) ==
263          cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
264      BB.replaceAllUsesWith(RetBlock);
265      BB.eraseFromParent();
266      continue;
267    }
268
269    // If the canonical return block has no PHI node, create one now.
270    PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
271    if (RetBlockPHI == 0) {
272      Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
273      pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
274      RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
275                                    std::distance(PB, PE), "merge",
276                                    &RetBlock->front());
277
278      for (pred_iterator PI = PB; PI != PE; ++PI)
279        RetBlockPHI->addIncoming(InVal, *PI);
280      RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
281    }
282
283    // Turn BB into a block that just unconditionally branches to the return
284    // block.  This handles the case when the two return blocks have a common
285    // predecessor but that return different things.
286    RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
287    BB.getTerminator()->eraseFromParent();
288    BranchInst::Create(RetBlock, &BB);
289  }
290
291  return Changed;
292}
293
294/// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
295/// iterating until no more changes are made.
296static bool iterativelySimplifyCFG(Function &F, const DataLayout *TD,
297                                   const TargetTransformInfo *TTI) {
298  bool Changed = false;
299  bool LocalChange = true;
300  while (LocalChange) {
301    LocalChange = false;
302
303    // Loop over all of the basic blocks and remove them if they are unneeded...
304    //
305    for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
306      if (SimplifyCFG(BBIt++, TD, TTI)) {
307        LocalChange = true;
308        ++NumSimpl;
309      }
310    }
311    Changed |= LocalChange;
312  }
313  return Changed;
314}
315
316// It is possible that we may require multiple passes over the code to fully
317// simplify the CFG.
318//
319bool CFGSimplifyPass::runOnFunction(Function &F) {
320  const DataLayout *TD = getAnalysisIfAvailable<DataLayout>();
321  const TargetTransformInfo *TTI =
322      getAnalysisIfAvailable<TargetTransformInfo>();
323  bool EverChanged = removeUnreachableBlocksFromFn(F);
324  EverChanged |= mergeEmptyReturnBlocks(F);
325  EverChanged |= iterativelySimplifyCFG(F, TD, TTI);
326
327  // If neither pass changed anything, we're done.
328  if (!EverChanged) return false;
329
330  // iterativelySimplifyCFG can (rarely) make some loops dead.  If this happens,
331  // removeUnreachableBlocksFromFn is needed to nuke them, which means we should
332  // iterate between the two optimizations.  We structure the code like this to
333  // avoid reruning iterativelySimplifyCFG if the second pass of
334  // removeUnreachableBlocksFromFn doesn't do anything.
335  if (!removeUnreachableBlocksFromFn(F))
336    return true;
337
338  do {
339    EverChanged = iterativelySimplifyCFG(F, TD, TTI);
340    EverChanged |= removeUnreachableBlocksFromFn(F);
341  } while (EverChanged);
342
343  return true;
344}
345