1 //===- ADCE.cpp - Code to perform agressive dead code elimination ---------===//
3 // This file implements "agressive" 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/Optimizations/DCE.h"
10 #include "llvm/Instruction.h"
11 #include "llvm/Type.h"
12 #include "llvm/Analysis/Dominators.h"
13 #include "llvm/Support/STLExtras.h"
14 #include "llvm/Support/DepthFirstIterator.h"
15 #include "llvm/Analysis/Writer.h"
16 #include "llvm/iTerminators.h"
22 //===----------------------------------------------------------------------===//
25 // This class does all of the work of Agressive Dead Code Elimination.
26 // It's public interface consists of a constructor and a doADCE() method.
29 Method *M; // The method that we are working on...
30 vector<Instruction*> WorkList; // Instructions that just became live
31 set<Instruction*> LiveSet; // The set of live instructions
34 //===--------------------------------------------------------------------===//
35 // The public interface for this class
38 // ADCE Ctor - Save the method to operate on...
39 inline ADCE(Method *m) : M(m), MadeChanges(false) {}
41 // doADCE() - Run the Agressive Dead Code Elimination algorithm, returning
42 // true if the method was modified.
45 //===--------------------------------------------------------------------===//
46 // The implementation of this class
49 inline void markInstructionLive(Instruction *I) {
50 if (LiveSet.count(I)) return;
52 cerr << "Insn Live: " << I;
55 WorkList.push_back(I);
58 inline void markTerminatorLive(const BasicBlock *BB) {
60 cerr << "Terminat Live: " << BB->getTerminator();
62 markInstructionLive((Instruction*)BB->getTerminator());
65 // fixupCFG - Walk the CFG in depth first order, eliminating references to
68 BasicBlock *fixupCFG(BasicBlock *Head, set<BasicBlock*> &VisitedBlocks,
69 const set<BasicBlock*> &AliveBlocks);
74 // doADCE() - Run the Agressive Dead Code Elimination algorithm, returning
75 // true if the method was modified.
78 // Compute the control dependence graph... Note that this has a side effect
79 // on the CFG: a new return bb is added and all returns are merged here.
81 cfg::DominanceFrontier CDG(cfg::DominatorSet(M, true));
84 cerr << "Method: " << M;
87 // Iterate over all of the instructions in the method, eliminating trivially
88 // dead instructions, and marking instructions live that are known to be
89 // needed. Perform the walk in depth first order so that we avoid marking any
90 // instructions live in basic blocks that are unreachable. These blocks will
91 // be eliminated later, along with the instructions inside.
93 for (df_iterator<Method*> BBI = df_begin(M),
96 BasicBlock *BB = *BBI;
97 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
100 if (I->hasSideEffects() || I->getOpcode() == Instruction::Ret) {
101 markInstructionLive(I);
103 // Check to see if anything is trivially dead
104 if (I->use_size() == 0 && I->getType() != Type::VoidTy) {
105 // Remove the instruction from it's basic block...
106 delete BB->getInstList().remove(II);
108 continue; // Don't increment the iterator past the current slot
112 ++II; // Increment the inst iterator if the inst wasn't deleted
117 cerr << "Processing work list\n";
120 // AliveBlocks - Set of basic blocks that we know have instructions that are
123 set<BasicBlock*> AliveBlocks;
125 // Process the work list of instructions that just became live... if they
126 // became live, then that means that all of their operands are neccesary as
127 // well... make them live as well.
129 while (!WorkList.empty()) {
130 Instruction *I = WorkList.back(); // Get an instruction that became live...
133 BasicBlock *BB = I->getParent();
134 if (AliveBlocks.count(BB) == 0) { // Basic block not alive yet...
135 // Mark the basic block as being newly ALIVE... and mark all branches that
136 // this block is control dependant on as being alive also...
138 AliveBlocks.insert(BB); // Block is now ALIVE!
139 cfg::DominanceFrontier::const_iterator It = CDG.find(BB);
140 if (It != CDG.end()) {
141 // Get the blocks that this node is control dependant on...
142 const cfg::DominanceFrontier::DomSetType &CDB = It->second;
143 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
144 bind_obj(this, &ADCE::markTerminatorLive));
147 // If this basic block is live, then the terminator must be as well!
148 markTerminatorLive(BB);
151 // Loop over all of the operands of the live instruction, making sure that
152 // they are known to be alive as well...
154 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op) {
155 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
156 markInstructionLive(Operand);
161 cerr << "Current Method: X = Live\n";
162 for (Method::inst_iterator IL = M->inst_begin(); IL != M->inst_end(); ++IL) {
163 if (LiveSet.count(*IL)) cerr << "X ";
168 // After the worklist is processed, recursively walk the CFG in depth first
169 // order, patching up references to dead blocks...
171 set<BasicBlock*> VisitedBlocks;
172 BasicBlock *EntryBlock = fixupCFG(M->front(), VisitedBlocks, AliveBlocks);
173 if (EntryBlock && EntryBlock != M->front()) {
174 if (EntryBlock->front()->isPHINode()) {
175 // Cannot make the first block be a block with a PHI node in it! Instead,
176 // strip the first basic block of the method to contain no instructions,
177 // then add a simple branch to the "real" entry node...
179 BasicBlock *E = M->front();
180 if (!E->front()->isTerminator() || // Check for an actual change...
181 ((TerminatorInst*)E->front())->getNumSuccessors() != 1 ||
182 ((TerminatorInst*)E->front())->getSuccessor(0) != EntryBlock) {
183 E->getInstList().delete_all(); // Delete all instructions in block
184 E->getInstList().push_back(new BranchInst(EntryBlock));
187 AliveBlocks.insert(E);
189 // Next we need to change any PHI nodes in the entry block to refer to the
190 // new predecessor node...
194 // We need to move the new entry block to be the first bb of the method.
195 Method::iterator EBI = find(M->begin(), M->end(), EntryBlock);
196 swap(*EBI, *M->begin()); // Exchange old location with start of method
201 // Now go through and tell dead blocks to drop all of their references so they
202 // can be safely deleted.
204 for (Method::iterator BI = M->begin(), BE = M->end(); BI != BE; ++BI) {
205 BasicBlock *BB = *BI;
206 if (!AliveBlocks.count(BB)) {
207 BB->dropAllReferences();
211 // Now loop through all of the blocks and delete them. We can safely do this
212 // now because we know that there are no references to dead blocks (because
213 // they have dropped all of their references...
215 for (Method::iterator BI = M->begin(); BI != M->end();) {
216 if (!AliveBlocks.count(*BI)) {
217 delete M->getBasicBlocks().remove(BI);
219 continue; // Don't increment iterator
221 ++BI; // Increment iterator...
228 // fixupCFG - Walk the CFG in depth first order, eliminating references to
230 // If the BB is alive (in AliveBlocks):
231 // 1. Eliminate all dead instructions in the BB
232 // 2. Recursively traverse all of the successors of the BB:
233 // - If the returned successor is non-null, update our terminator to
234 // reference the returned BB
235 // 3. Return 0 (no update needed)
237 // If the BB is dead (not in AliveBlocks):
238 // 1. Add the BB to the dead set
239 // 2. Recursively traverse all of the successors of the block:
240 // - Only one shall return a nonnull value (or else this block should have
241 // been in the alive set).
242 // 3. Return the nonnull child, or 0 if no non-null children.
244 BasicBlock *ADCE::fixupCFG(BasicBlock *BB, set<BasicBlock*> &VisitedBlocks,
245 const set<BasicBlock*> &AliveBlocks) {
246 if (VisitedBlocks.count(BB)) return 0; // Revisiting a node? No update.
247 VisitedBlocks.insert(BB); // We have now visited this node!
250 cerr << "Fixing up BB: " << BB;
253 if (AliveBlocks.count(BB)) { // Is the block alive?
254 // Yes it's alive: loop through and eliminate all dead instructions in block
255 for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; ) {
256 Instruction *I = *II;
257 if (!LiveSet.count(I)) { // Is this instruction alive?
258 // Nope... remove the instruction from it's basic block...
259 delete BB->getInstList().remove(II);
261 continue; // Don't increment II
266 // Recursively traverse successors of this basic block.
267 BasicBlock::succ_iterator SI = BB->succ_begin(), SE = BB->succ_end();
268 for (; SI != SE; ++SI) {
269 BasicBlock *Succ = *SI;
270 BasicBlock *Repl = fixupCFG(Succ, VisitedBlocks, AliveBlocks);
271 if (Repl && Repl != Succ) { // We have to replace the successor
272 Succ->replaceAllUsesWith(Repl);
277 } else { // Otherwise the block is dead...
278 BasicBlock *ReturnBB = 0; // Default to nothing live down here
280 // Recursively traverse successors of this basic block.
281 BasicBlock::succ_iterator SI = BB->succ_begin(), SE = BB->succ_end();
282 for (; SI != SE; ++SI) {
283 BasicBlock *RetBB = fixupCFG(*SI, VisitedBlocks, AliveBlocks);
285 assert(ReturnBB == 0 && "One one live child allowed!");
289 return ReturnBB; // Return the result of traversal
295 // DoADCE - Execute the Agressive Dead Code Elimination Algorithm
297 bool opt::DoADCE(Method *M) {
298 if (M->isExternal()) return false;