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/Transforms/Scalar/DCE.h"
10 #include "llvm/Instruction.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 "Support/STLExtras.h"
17 #include "Support/DepthFirstIterator.h"
24 //===----------------------------------------------------------------------===//
27 // This class does all of the work of Agressive Dead Code Elimination.
28 // It's public interface consists of a constructor and a doADCE() method.
31 Method *M; // The method that we are working on...
32 std::vector<Instruction*> WorkList; // Instructions that just became live
33 std::set<Instruction*> LiveSet; // The set of live instructions
36 //===--------------------------------------------------------------------===//
37 // The public interface for this class
40 // ADCE Ctor - Save the method to operate on...
41 inline ADCE(Method *m) : M(m), MadeChanges(false) {}
43 // doADCE() - Run the Agressive Dead Code Elimination algorithm, returning
44 // true if the method was modified.
47 //===--------------------------------------------------------------------===//
48 // The implementation of this class
51 inline void markInstructionLive(Instruction *I) {
52 if (LiveSet.count(I)) return;
54 cerr << "Insn Live: " << I;
57 WorkList.push_back(I);
60 inline void markTerminatorLive(const BasicBlock *BB) {
62 cerr << "Terminat Live: " << BB->getTerminator();
64 markInstructionLive((Instruction*)BB->getTerminator());
67 // fixupCFG - Walk the CFG in depth first order, eliminating references to
70 BasicBlock *fixupCFG(BasicBlock *Head, std::set<BasicBlock*> &VisitedBlocks,
71 const std::set<BasicBlock*> &AliveBlocks);
76 // doADCE() - Run the Agressive Dead Code Elimination algorithm, returning
77 // true if the method was modified.
80 // Compute the control dependence graph... Note that this has a side effect
81 // on the CFG: a new return bb is added and all returns are merged here.
83 cfg::DominanceFrontier CDG(cfg::DominatorSet(M, true));
86 cerr << "Method: " << M;
89 // Iterate over all of the instructions in the method, eliminating trivially
90 // dead instructions, and marking instructions live that are known to be
91 // needed. Perform the walk in depth first order so that we avoid marking any
92 // instructions live in basic blocks that are unreachable. These blocks will
93 // be eliminated later, along with the instructions inside.
95 for (df_iterator<Method*> BBI = df_begin(M),
98 BasicBlock *BB = *BBI;
99 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
100 Instruction *I = *II;
102 if (I->hasSideEffects() || I->getOpcode() == Instruction::Ret) {
103 markInstructionLive(I);
105 // Check to see if anything is trivially dead
106 if (I->use_size() == 0 && I->getType() != Type::VoidTy) {
107 // Remove the instruction from it's basic block...
108 delete BB->getInstList().remove(II);
110 continue; // Don't increment the iterator past the current slot
114 ++II; // Increment the inst iterator if the inst wasn't deleted
119 cerr << "Processing work list\n";
122 // AliveBlocks - Set of basic blocks that we know have instructions that are
125 std::set<BasicBlock*> AliveBlocks;
127 // Process the work list of instructions that just became live... if they
128 // became live, then that means that all of their operands are neccesary as
129 // well... make them live as well.
131 while (!WorkList.empty()) {
132 Instruction *I = WorkList.back(); // Get an instruction that became live...
135 BasicBlock *BB = I->getParent();
136 if (AliveBlocks.count(BB) == 0) { // Basic block not alive yet...
137 // Mark the basic block as being newly ALIVE... and mark all branches that
138 // this block is control dependant on as being alive also...
140 AliveBlocks.insert(BB); // Block is now ALIVE!
141 cfg::DominanceFrontier::const_iterator It = CDG.find(BB);
142 if (It != CDG.end()) {
143 // Get the blocks that this node is control dependant on...
144 const cfg::DominanceFrontier::DomSetType &CDB = It->second;
145 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
146 bind_obj(this, &ADCE::markTerminatorLive));
149 // If this basic block is live, then the terminator must be as well!
150 markTerminatorLive(BB);
153 // Loop over all of the operands of the live instruction, making sure that
154 // they are known to be alive as well...
156 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op) {
157 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
158 markInstructionLive(Operand);
163 cerr << "Current Method: X = Live\n";
164 for (Method::inst_iterator IL = M->inst_begin(); IL != M->inst_end(); ++IL) {
165 if (LiveSet.count(*IL)) cerr << "X ";
170 // After the worklist is processed, recursively walk the CFG in depth first
171 // order, patching up references to dead blocks...
173 std::set<BasicBlock*> VisitedBlocks;
174 BasicBlock *EntryBlock = fixupCFG(M->front(), VisitedBlocks, AliveBlocks);
175 if (EntryBlock && EntryBlock != M->front()) {
176 if (isa<PHINode>(EntryBlock->front())) {
177 // Cannot make the first block be a block with a PHI node in it! Instead,
178 // strip the first basic block of the method to contain no instructions,
179 // then add a simple branch to the "real" entry node...
181 BasicBlock *E = M->front();
182 if (!isa<TerminatorInst>(E->front()) || // Check for an actual change...
183 cast<TerminatorInst>(E->front())->getNumSuccessors() != 1 ||
184 cast<TerminatorInst>(E->front())->getSuccessor(0) != EntryBlock) {
185 E->getInstList().delete_all(); // Delete all instructions in block
186 E->getInstList().push_back(new BranchInst(EntryBlock));
189 AliveBlocks.insert(E);
191 // Next we need to change any PHI nodes in the entry block to refer to the
192 // new predecessor node...
196 // We need to move the new entry block to be the first bb of the method.
197 Method::iterator EBI = find(M->begin(), M->end(), EntryBlock);
198 std::swap(*EBI, *M->begin());// Exchange old location with start of method
203 // Now go through and tell dead blocks to drop all of their references so they
204 // can be safely deleted.
206 for (Method::iterator BI = M->begin(), BE = M->end(); BI != BE; ++BI) {
207 BasicBlock *BB = *BI;
208 if (!AliveBlocks.count(BB)) {
209 BB->dropAllReferences();
213 // Now loop through all of the blocks and delete them. We can safely do this
214 // now because we know that there are no references to dead blocks (because
215 // they have dropped all of their references...
217 for (Method::iterator BI = M->begin(); BI != M->end();) {
218 if (!AliveBlocks.count(*BI)) {
219 delete M->getBasicBlocks().remove(BI);
221 continue; // Don't increment iterator
223 ++BI; // Increment iterator...
230 // fixupCFG - Walk the CFG in depth first order, eliminating references to
232 // If the BB is alive (in AliveBlocks):
233 // 1. Eliminate all dead instructions in the BB
234 // 2. Recursively traverse all of the successors of the BB:
235 // - If the returned successor is non-null, update our terminator to
236 // reference the returned BB
237 // 3. Return 0 (no update needed)
239 // If the BB is dead (not in AliveBlocks):
240 // 1. Add the BB to the dead set
241 // 2. Recursively traverse all of the successors of the block:
242 // - Only one shall return a nonnull value (or else this block should have
243 // been in the alive set).
244 // 3. Return the nonnull child, or 0 if no non-null children.
246 BasicBlock *ADCE::fixupCFG(BasicBlock *BB, std::set<BasicBlock*> &VisitedBlocks,
247 const std::set<BasicBlock*> &AliveBlocks) {
248 if (VisitedBlocks.count(BB)) return 0; // Revisiting a node? No update.
249 VisitedBlocks.insert(BB); // We have now visited this node!
252 cerr << "Fixing up BB: " << BB;
255 if (AliveBlocks.count(BB)) { // Is the block alive?
256 // Yes it's alive: loop through and eliminate all dead instructions in block
257 for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; ) {
258 Instruction *I = *II;
259 if (!LiveSet.count(I)) { // Is this instruction alive?
260 // Nope... remove the instruction from it's basic block...
261 delete BB->getInstList().remove(II);
263 continue; // Don't increment II
268 // Recursively traverse successors of this basic block.
269 BasicBlock::succ_iterator SI = BB->succ_begin(), SE = BB->succ_end();
270 for (; SI != SE; ++SI) {
271 BasicBlock *Succ = *SI;
272 BasicBlock *Repl = fixupCFG(Succ, VisitedBlocks, AliveBlocks);
273 if (Repl && Repl != Succ) { // We have to replace the successor
274 Succ->replaceAllUsesWith(Repl);
279 } else { // Otherwise the block is dead...
280 BasicBlock *ReturnBB = 0; // Default to nothing live down here
282 // Recursively traverse successors of this basic block.
283 BasicBlock::succ_iterator SI = BB->succ_begin(), SE = BB->succ_end();
284 for (; SI != SE; ++SI) {
285 BasicBlock *RetBB = fixupCFG(*SI, VisitedBlocks, AliveBlocks);
287 assert(ReturnBB == 0 && "One one live child allowed!");
291 return ReturnBB; // Return the result of traversal
297 // doADCE - Execute the Agressive Dead Code Elimination Algorithm
299 bool AgressiveDCE::doADCE(Method *M) {
300 if (M->isExternal()) return false;