1 //===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements "aggressive" dead code elimination. ADCE is DCe where
11 // values are assumed to be dead until proven otherwise. This is similar to
12 // SCCP, except applied to the liveness of values.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Transforms/Scalar.h"
17 #include "llvm/Transforms/Utils/Local.h"
18 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
19 #include "llvm/Type.h"
20 #include "llvm/Analysis/PostDominators.h"
21 #include "llvm/iTerminators.h"
22 #include "llvm/iPHINode.h"
23 #include "llvm/Constant.h"
24 #include "llvm/Support/CFG.h"
25 #include "Support/Debug.h"
26 #include "Support/DepthFirstIterator.h"
27 #include "Support/Statistic.h"
28 #include "Support/STLExtras.h"
32 Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed");
33 Statistic<> NumInstRemoved ("adce", "Number of instructions removed");
35 //===----------------------------------------------------------------------===//
38 // This class does all of the work of Aggressive Dead Code Elimination.
39 // It's public interface consists of a constructor and a doADCE() method.
41 class ADCE : public FunctionPass {
42 Function *Func; // The function that we are working on
43 std::vector<Instruction*> WorkList; // Instructions that just became live
44 std::set<Instruction*> LiveSet; // The set of live instructions
46 //===--------------------------------------------------------------------===//
47 // The public interface for this class
50 // Execute the Aggressive Dead Code Elimination Algorithm
52 virtual bool runOnFunction(Function &F) {
54 bool Changed = doADCE();
55 assert(WorkList.empty());
59 // getAnalysisUsage - We require post dominance frontiers (aka Control
61 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
62 AU.addRequired<PostDominatorTree>();
63 AU.addRequired<PostDominanceFrontier>();
67 //===--------------------------------------------------------------------===//
68 // The implementation of this class
71 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
72 // true if the function was modified.
76 void markBlockAlive(BasicBlock *BB);
79 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
80 // instructions in the specified basic block, dropping references on
81 // instructions that are dead according to LiveSet.
82 bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB);
84 TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
86 inline void markInstructionLive(Instruction *I) {
87 if (LiveSet.count(I)) return;
88 DEBUG(std::cerr << "Insn Live: " << I);
90 WorkList.push_back(I);
93 inline void markTerminatorLive(const BasicBlock *BB) {
94 DEBUG(std::cerr << "Terminator Live: " << BB->getTerminator());
95 markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
99 RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
100 } // End of anonymous namespace
102 Pass *createAggressiveDCEPass() { return new ADCE(); }
104 void ADCE::markBlockAlive(BasicBlock *BB) {
105 // Mark the basic block as being newly ALIVE... and mark all branches that
106 // this block is control dependent on as being alive also...
108 PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
110 PostDominanceFrontier::const_iterator It = CDG.find(BB);
111 if (It != CDG.end()) {
112 // Get the blocks that this node is control dependent on...
113 const PostDominanceFrontier::DomSetType &CDB = It->second;
114 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
115 bind_obj(this, &ADCE::markTerminatorLive));
118 // If this basic block is live, and it ends in an unconditional branch, then
119 // the branch is alive as well...
120 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
121 if (BI->isUnconditional())
122 markTerminatorLive(BB);
125 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
126 // instructions in the specified basic block, dropping references on
127 // instructions that are dead according to LiveSet.
128 bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) {
129 bool Changed = false;
130 for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; )
131 if (!LiveSet.count(I)) { // Is this instruction alive?
132 I->dropAllReferences(); // Nope, drop references...
133 if (PHINode *PN = dyn_cast<PHINode>(I)) {
134 // We don't want to leave PHI nodes in the program that have
135 // #arguments != #predecessors, so we remove them now.
137 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
139 // Delete the instruction...
140 I = BB->getInstList().erase(I);
152 /// convertToUnconditionalBranch - Transform this conditional terminator
153 /// instruction into an unconditional branch because we don't care which of the
154 /// successors it goes to. This eliminate a use of the condition as well.
156 TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
157 BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
158 BasicBlock *BB = TI->getParent();
160 // Remove entries from PHI nodes to avoid confusing ourself later...
161 for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
162 TI->getSuccessor(i)->removePredecessor(BB);
164 // Delete the old branch itself...
165 BB->getInstList().erase(TI);
170 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
171 // true if the function was modified.
173 bool ADCE::doADCE() {
174 bool MadeChanges = false;
176 // Iterate over all of the instructions in the function, eliminating trivially
177 // dead instructions, and marking instructions live that are known to be
178 // needed. Perform the walk in depth first order so that we avoid marking any
179 // instructions live in basic blocks that are unreachable. These blocks will
180 // be eliminated later, along with the instructions inside.
182 for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
184 BasicBlock *BB = *BBI;
185 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
186 if (II->mayWriteToMemory() || isa<ReturnInst>(II) || isa<UnwindInst>(II)){
187 markInstructionLive(II);
188 ++II; // Increment the inst iterator if the inst wasn't deleted
189 } else if (isInstructionTriviallyDead(II)) {
190 // Remove the instruction from it's basic block...
191 II = BB->getInstList().erase(II);
195 ++II; // Increment the inst iterator if the inst wasn't deleted
200 // Check to ensure we have an exit node for this CFG. If we don't, we won't
201 // have any post-dominance information, thus we cannot perform our
202 // transformations safely.
204 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
205 if (DT[&Func->getEntryBlock()] == 0) {
210 DEBUG(std::cerr << "Processing work list\n");
212 // AliveBlocks - Set of basic blocks that we know have instructions that are
215 std::set<BasicBlock*> AliveBlocks;
217 // Process the work list of instructions that just became live... if they
218 // became live, then that means that all of their operands are necessary as
219 // well... make them live as well.
221 while (!WorkList.empty()) {
222 Instruction *I = WorkList.back(); // Get an instruction that became live...
225 BasicBlock *BB = I->getParent();
226 if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
227 AliveBlocks.insert(BB); // Block is now ALIVE!
228 markBlockAlive(BB); // Make it so now!
231 // PHI nodes are a special case, because the incoming values are actually
232 // defined in the predecessor nodes of this block, meaning that the PHI
233 // makes the predecessors alive.
235 if (PHINode *PN = dyn_cast<PHINode>(I))
236 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
237 if (!AliveBlocks.count(*PI)) {
238 AliveBlocks.insert(BB); // Block is now ALIVE!
242 // Loop over all of the operands of the live instruction, making sure that
243 // they are known to be alive as well...
245 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
246 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
247 markInstructionLive(Operand);
251 std::cerr << "Current Function: X = Live\n";
252 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
253 std::cerr << I->getName() << ":\t"
254 << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
255 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
256 if (LiveSet.count(BI)) std::cerr << "X ";
261 // Find the first postdominator of the entry node that is alive. Make it the
264 if (AliveBlocks.size() == Func->size()) { // No dead blocks?
265 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
266 // Loop over all of the instructions in the function, telling dead
267 // instructions to drop their references. This is so that the next sweep
268 // over the program can safely delete dead instructions without other dead
269 // instructions still referring to them.
271 dropReferencesOfDeadInstructionsInLiveBlock(I);
273 // Check to make sure the terminator instruction is live. If it isn't,
274 // this means that the condition that it branches on (we know it is not an
275 // unconditional branch), is not needed to make the decision of where to
276 // go to, because all outgoing edges go to the same place. We must remove
277 // the use of the condition (because it's probably dead), so we convert
278 // the terminator to a conditional branch.
280 TerminatorInst *TI = I->getTerminator();
281 if (!LiveSet.count(TI))
282 convertToUnconditionalBranch(TI);
285 } else { // If there are some blocks dead...
286 // If the entry node is dead, insert a new entry node to eliminate the entry
287 // node as a special case.
289 if (!AliveBlocks.count(&Func->front())) {
290 BasicBlock *NewEntry = new BasicBlock();
291 NewEntry->getInstList().push_back(new BranchInst(&Func->front()));
292 Func->getBasicBlockList().push_front(NewEntry);
293 AliveBlocks.insert(NewEntry); // This block is always alive!
294 LiveSet.insert(NewEntry->getTerminator()); // The branch is live
297 // Loop over all of the alive blocks in the function. If any successor
298 // blocks are not alive, we adjust the outgoing branches to branch to the
299 // first live postdominator of the live block, adjusting any PHI nodes in
300 // the block to reflect this.
302 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
303 if (AliveBlocks.count(I)) {
305 TerminatorInst *TI = BB->getTerminator();
307 // If the terminator instruction is alive, but the block it is contained
308 // in IS alive, this means that this terminator is a conditional branch
309 // on a condition that doesn't matter. Make it an unconditional branch
310 // to ONE of the successors. This has the side effect of dropping a use
311 // of the conditional value, which may also be dead.
312 if (!LiveSet.count(TI))
313 TI = convertToUnconditionalBranch(TI);
315 // Loop over all of the successors, looking for ones that are not alive.
316 // We cannot save the number of successors in the terminator instruction
317 // here because we may remove them if we don't have a postdominator...
319 for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
320 if (!AliveBlocks.count(TI->getSuccessor(i))) {
321 // Scan up the postdominator tree, looking for the first
322 // postdominator that is alive, and the last postdominator that is
325 PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
327 // There is a special case here... if there IS no post-dominator for
328 // the block we have no owhere to point our branch to. Instead,
329 // convert it to a return. This can only happen if the code
330 // branched into an infinite loop. Note that this may not be
331 // desirable, because we _are_ altering the behavior of the code.
332 // This is a well known drawback of ADCE, so in the future if we
333 // choose to revisit the decision, this is where it should be.
335 if (LastNode == 0) { // No postdominator!
336 // Call RemoveSuccessor to transmogrify the terminator instruction
337 // to not contain the outgoing branch, or to create a new
338 // terminator if the form fundamentally changes (i.e.,
339 // unconditional branch to return). Note that this will change a
340 // branch into an infinite loop into a return instruction!
342 RemoveSuccessor(TI, i);
344 // RemoveSuccessor may replace TI... make sure we have a fresh
345 // pointer... and e variable.
347 TI = BB->getTerminator();
349 // Rescan this successor...
352 PostDominatorTree::Node *NextNode = LastNode->getIDom();
354 while (!AliveBlocks.count(NextNode->getBlock())) {
356 NextNode = NextNode->getIDom();
359 // Get the basic blocks that we need...
360 BasicBlock *LastDead = LastNode->getBlock();
361 BasicBlock *NextAlive = NextNode->getBlock();
363 // Make the conditional branch now go to the next alive block...
364 TI->getSuccessor(i)->removePredecessor(BB);
365 TI->setSuccessor(i, NextAlive);
367 // If there are PHI nodes in NextAlive, we need to add entries to
368 // the PHI nodes for the new incoming edge. The incoming values
369 // should be identical to the incoming values for LastDead.
371 for (BasicBlock::iterator II = NextAlive->begin();
372 PHINode *PN = dyn_cast<PHINode>(II); ++II)
373 if (LiveSet.count(PN)) { // Only modify live phi nodes
374 // Get the incoming value for LastDead...
375 int OldIdx = PN->getBasicBlockIndex(LastDead);
376 assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
377 Value *InVal = PN->getIncomingValue(OldIdx);
379 // Add an incoming value for BB now...
380 PN->addIncoming(InVal, BB);
385 // Now loop over all of the instructions in the basic block, telling
386 // dead instructions to drop their references. This is so that the next
387 // sweep over the program can safely delete dead instructions without
388 // other dead instructions still referring to them.
390 dropReferencesOfDeadInstructionsInLiveBlock(BB);
394 // We make changes if there are any dead blocks in the function...
395 if (unsigned NumDeadBlocks = Func->size() - AliveBlocks.size()) {
397 NumBlockRemoved += NumDeadBlocks;
400 // Loop over all of the basic blocks in the function, removing control flow
401 // edges to live blocks (also eliminating any entries in PHI functions in
402 // referenced blocks).
404 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
405 if (!AliveBlocks.count(BB)) {
406 // Remove all outgoing edges from this basic block and convert the
407 // terminator into a return instruction.
408 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
410 if (!Succs.empty()) {
411 // Loop over all of the successors, removing this block from PHI node
412 // entries that might be in the block...
413 while (!Succs.empty()) {
414 Succs.back()->removePredecessor(BB);
418 // Delete the old terminator instruction...
419 BB->getInstList().pop_back();
420 const Type *RetTy = Func->getReturnType();
421 BB->getInstList().push_back(new ReturnInst(RetTy != Type::VoidTy ?
422 Constant::getNullValue(RetTy) : 0));
427 // Loop over all of the basic blocks in the function, dropping references of
428 // the dead basic blocks. We must do this after the previous step to avoid
429 // dropping references to PHIs which still have entries...
431 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
432 if (!AliveBlocks.count(BB))
433 BB->dropAllReferences();
435 // Now loop through all of the blocks and delete the dead ones. We can safely
436 // do this now because we know that there are no references to dead blocks
437 // (because they have dropped all of their references... we also remove dead
438 // instructions from alive blocks.
440 for (Function::iterator BI = Func->begin(); BI != Func->end(); )
441 if (!AliveBlocks.count(BI)) { // Delete dead blocks...
442 BI = Func->getBasicBlockList().erase(BI);
443 } else { // Scan alive blocks...
444 for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
445 if (!LiveSet.count(II)) { // Is this instruction alive?
446 // Nope... remove the instruction from it's basic block...
447 II = BI->getInstList().erase(II);
454 ++BI; // Increment iterator...