1 //===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
3 // This file implements "aggressive" dead code elimination. ADCE is DCe where
4 // values are assumed to be dead until proven otherwise. This is similar to
5 // SCCP, except applied to the liveness of values.
7 //===----------------------------------------------------------------------===//
9 #include "llvm/Transforms/Scalar.h"
10 #include "llvm/Transforms/Utils/Local.h"
11 #include "llvm/Type.h"
12 #include "llvm/Analysis/Dominators.h"
13 #include "llvm/Analysis/Writer.h"
14 #include "llvm/iTerminators.h"
15 #include "llvm/iPHINode.h"
16 #include "llvm/Support/CFG.h"
17 #include "Support/STLExtras.h"
18 #include "Support/DepthFirstIterator.h"
19 #include "Support/StatisticReporter.h"
26 //===----------------------------------------------------------------------===//
29 // This class does all of the work of Aggressive Dead Code Elimination.
30 // It's public interface consists of a constructor and a doADCE() method.
32 class ADCE : public FunctionPass {
33 Function *Func; // The function that we are working on
34 std::vector<Instruction*> WorkList; // Instructions that just became live
35 std::set<Instruction*> LiveSet; // The set of live instructions
38 //===--------------------------------------------------------------------===//
39 // The public interface for this class
42 const char *getPassName() const { return "Aggressive Dead Code Elimination"; }
44 // doADCE - Execute the Aggressive Dead Code Elimination Algorithm
46 virtual bool runOnFunction(Function *F) {
47 Func = F; MadeChanges = false;
48 doADCE(getAnalysis<DominanceFrontier>(DominanceFrontier::PostDomID));
49 assert(WorkList.empty());
53 // getAnalysisUsage - We require post dominance frontiers (aka Control
55 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
56 AU.addRequired(DominanceFrontier::PostDomID);
60 //===--------------------------------------------------------------------===//
61 // The implementation of this class
64 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
65 // true if the function was modified.
67 void doADCE(DominanceFrontier &CDG);
69 inline void markInstructionLive(Instruction *I) {
70 if (LiveSet.count(I)) return;
71 DEBUG(cerr << "Insn Live: " << I);
73 WorkList.push_back(I);
76 inline void markTerminatorLive(const BasicBlock *BB) {
77 DEBUG(cerr << "Terminat Live: " << BB->getTerminator());
78 markInstructionLive((Instruction*)BB->getTerminator());
81 // fixupCFG - Walk the CFG in depth first order, eliminating references to
84 BasicBlock *fixupCFG(BasicBlock *Head, std::set<BasicBlock*> &VisitedBlocks,
85 const std::set<BasicBlock*> &AliveBlocks);
88 } // End of anonymous namespace
90 Pass *createAggressiveDCEPass() {
95 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
96 // true if the function was modified.
98 void ADCE::doADCE(DominanceFrontier &CDG) {
99 DEBUG(cerr << "Function: " << Func);
101 // Iterate over all of the instructions in the function, eliminating trivially
102 // dead instructions, and marking instructions live that are known to be
103 // needed. Perform the walk in depth first order so that we avoid marking any
104 // instructions live in basic blocks that are unreachable. These blocks will
105 // be eliminated later, along with the instructions inside.
107 for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
109 BasicBlock *BB = *BBI;
110 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
111 Instruction *I = *II;
113 if (I->hasSideEffects() || I->getOpcode() == Instruction::Ret) {
114 markInstructionLive(I);
115 ++II; // Increment the inst iterator if the inst wasn't deleted
116 } else if (isInstructionTriviallyDead(I)) {
117 // Remove the instruction from it's basic block...
118 delete BB->getInstList().remove(II);
121 ++II; // Increment the inst iterator if the inst wasn't deleted
126 DEBUG(cerr << "Processing work list\n");
128 // AliveBlocks - Set of basic blocks that we know have instructions that are
131 std::set<BasicBlock*> AliveBlocks;
133 // Process the work list of instructions that just became live... if they
134 // became live, then that means that all of their operands are neccesary as
135 // well... make them live as well.
137 while (!WorkList.empty()) {
138 Instruction *I = WorkList.back(); // Get an instruction that became live...
141 BasicBlock *BB = I->getParent();
142 if (AliveBlocks.count(BB) == 0) { // Basic block not alive yet...
143 // Mark the basic block as being newly ALIVE... and mark all branches that
144 // this block is control dependant on as being alive also...
146 AliveBlocks.insert(BB); // Block is now ALIVE!
147 DominanceFrontier::const_iterator It = CDG.find(BB);
148 if (It != CDG.end()) {
149 // Get the blocks that this node is control dependant on...
150 const DominanceFrontier::DomSetType &CDB = It->second;
151 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
152 bind_obj(this, &ADCE::markTerminatorLive));
155 // If this basic block is live, then the terminator must be as well!
156 markTerminatorLive(BB);
159 // Loop over all of the operands of the live instruction, making sure that
160 // they are known to be alive as well...
162 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
163 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
164 markInstructionLive(Operand);
168 cerr << "Current Function: X = Live\n";
169 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
170 for (BasicBlock::iterator BI = (*I)->begin(), BE = (*I)->end();
172 if (LiveSet.count(*BI)) cerr << "X ";
177 // After the worklist is processed, recursively walk the CFG in depth first
178 // order, patching up references to dead blocks...
180 std::set<BasicBlock*> VisitedBlocks;
181 BasicBlock *EntryBlock = fixupCFG(Func->front(), VisitedBlocks, AliveBlocks);
183 // Now go through and tell dead blocks to drop all of their references so they
184 // can be safely deleted. Also, as we are doing so, if the block has
185 // successors that are still live (and that have PHI nodes in them), remove
186 // the entry for this block from the phi nodes.
188 for (Function::iterator BI = Func->begin(), BE = Func->end(); BI != BE; ++BI){
189 BasicBlock *BB = *BI;
190 if (!AliveBlocks.count(BB)) {
191 // Remove entries from successors PHI nodes if they are still alive...
192 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
193 if (AliveBlocks.count(*SI)) { // Only if the successor is alive...
194 BasicBlock *Succ = *SI;
195 for (BasicBlock::iterator I = Succ->begin();// Loop over all PHI nodes
196 PHINode *PN = dyn_cast<PHINode>(*I); ++I)
197 PN->removeIncomingValue(BB); // Remove value for this block
200 BB->dropAllReferences();
204 cerr << "Before Deleting Blocks: " << Func;
206 // Now loop through all of the blocks and delete them. We can safely do this
207 // now because we know that there are no references to dead blocks (because
208 // they have dropped all of their references...
210 for (Function::iterator BI = Func->begin(); BI != Func->end();) {
211 if (!AliveBlocks.count(*BI)) {
212 delete Func->getBasicBlocks().remove(BI);
214 continue; // Don't increment iterator
216 ++BI; // Increment iterator...
219 if (EntryBlock && EntryBlock != Func->front()) {
220 // We need to move the new entry block to be the first bb of the function
221 Function::iterator EBI = find(Func->begin(), Func->end(), EntryBlock);
222 std::swap(*EBI, *Func->begin()); // Exchange old location with start of fn
225 while (PHINode *PN = dyn_cast<PHINode>(EntryBlock->front())) {
226 assert(PN->getNumIncomingValues() == 1 &&
227 "Can only have a single incoming value at this point...");
228 // The incoming value must be outside of the scope of the function, a
229 // global variable, constant or parameter maybe...
231 PN->replaceAllUsesWith(PN->getIncomingValue(0));
233 // Nuke the phi node...
234 delete EntryBlock->getInstList().remove(EntryBlock->begin());
239 // fixupCFG - Walk the CFG in depth first order, eliminating references to
241 // If the BB is alive (in AliveBlocks):
242 // 1. Eliminate all dead instructions in the BB
243 // 2. Recursively traverse all of the successors of the BB:
244 // - If the returned successor is non-null, update our terminator to
245 // reference the returned BB
246 // 3. Return 0 (no update needed)
248 // If the BB is dead (not in AliveBlocks):
249 // 1. Add the BB to the dead set
250 // 2. Recursively traverse all of the successors of the block:
251 // - Only one shall return a nonnull value (or else this block should have
252 // been in the alive set).
253 // 3. Return the nonnull child, or 0 if no non-null children.
255 BasicBlock *ADCE::fixupCFG(BasicBlock *BB, std::set<BasicBlock*> &VisitedBlocks,
256 const std::set<BasicBlock*> &AliveBlocks) {
257 if (VisitedBlocks.count(BB)) return 0; // Revisiting a node? No update.
258 VisitedBlocks.insert(BB); // We have now visited this node!
260 DEBUG(cerr << "Fixing up BB: " << BB);
262 if (AliveBlocks.count(BB)) { // Is the block alive?
263 // Yes it's alive: loop through and eliminate all dead instructions in block
264 for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; )
265 if (!LiveSet.count(*II)) { // Is this instruction alive?
266 // Nope... remove the instruction from it's basic block...
267 delete BB->getInstList().remove(II);
273 // Recursively traverse successors of this basic block.
274 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI) {
275 BasicBlock *Succ = *SI;
276 BasicBlock *Repl = fixupCFG(Succ, VisitedBlocks, AliveBlocks);
277 if (Repl && Repl != Succ) { // We have to replace the successor
278 Succ->replaceAllUsesWith(Repl);
283 } else { // Otherwise the block is dead...
284 BasicBlock *ReturnBB = 0; // Default to nothing live down here
286 // Recursively traverse successors of this basic block.
287 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI) {
288 BasicBlock *RetBB = fixupCFG(*SI, VisitedBlocks, AliveBlocks);
290 assert(ReturnBB == 0 && "At most one live child allowed!");
294 return ReturnBB; // Return the result of traversal