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.
45 bool doADCE(cfg::DominanceFrontier &CDG);
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.
79 bool ADCE::doADCE(cfg::DominanceFrontier &CDG) {
81 cerr << "Method: " << M;
84 // Iterate over all of the instructions in the method, eliminating trivially
85 // dead instructions, and marking instructions live that are known to be
86 // needed. Perform the walk in depth first order so that we avoid marking any
87 // instructions live in basic blocks that are unreachable. These blocks will
88 // be eliminated later, along with the instructions inside.
90 for (df_iterator<Method*> BBI = df_begin(M),
93 BasicBlock *BB = *BBI;
94 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
97 if (I->hasSideEffects() || I->getOpcode() == Instruction::Ret) {
98 markInstructionLive(I);
100 // Check to see if anything is trivially dead
101 if (I->use_size() == 0 && I->getType() != Type::VoidTy) {
102 // Remove the instruction from it's basic block...
103 delete BB->getInstList().remove(II);
105 continue; // Don't increment the iterator past the current slot
109 ++II; // Increment the inst iterator if the inst wasn't deleted
114 cerr << "Processing work list\n";
117 // AliveBlocks - Set of basic blocks that we know have instructions that are
120 std::set<BasicBlock*> AliveBlocks;
122 // Process the work list of instructions that just became live... if they
123 // became live, then that means that all of their operands are neccesary as
124 // well... make them live as well.
126 while (!WorkList.empty()) {
127 Instruction *I = WorkList.back(); // Get an instruction that became live...
130 BasicBlock *BB = I->getParent();
131 if (AliveBlocks.count(BB) == 0) { // Basic block not alive yet...
132 // Mark the basic block as being newly ALIVE... and mark all branches that
133 // this block is control dependant on as being alive also...
135 AliveBlocks.insert(BB); // Block is now ALIVE!
136 cfg::DominanceFrontier::const_iterator It = CDG.find(BB);
137 if (It != CDG.end()) {
138 // Get the blocks that this node is control dependant on...
139 const cfg::DominanceFrontier::DomSetType &CDB = It->second;
140 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
141 bind_obj(this, &ADCE::markTerminatorLive));
144 // If this basic block is live, then the terminator must be as well!
145 markTerminatorLive(BB);
148 // Loop over all of the operands of the live instruction, making sure that
149 // they are known to be alive as well...
151 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op) {
152 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
153 markInstructionLive(Operand);
158 cerr << "Current Method: X = Live\n";
159 for (Method::inst_iterator IL = M->inst_begin(); IL != M->inst_end(); ++IL) {
160 if (LiveSet.count(*IL)) cerr << "X ";
165 // After the worklist is processed, recursively walk the CFG in depth first
166 // order, patching up references to dead blocks...
168 std::set<BasicBlock*> VisitedBlocks;
169 BasicBlock *EntryBlock = fixupCFG(M->front(), VisitedBlocks, AliveBlocks);
170 if (EntryBlock && EntryBlock != M->front()) {
171 if (isa<PHINode>(EntryBlock->front())) {
172 // Cannot make the first block be a block with a PHI node in it! Instead,
173 // strip the first basic block of the method to contain no instructions,
174 // then add a simple branch to the "real" entry node...
176 BasicBlock *E = M->front();
177 if (!isa<TerminatorInst>(E->front()) || // Check for an actual change...
178 cast<TerminatorInst>(E->front())->getNumSuccessors() != 1 ||
179 cast<TerminatorInst>(E->front())->getSuccessor(0) != EntryBlock) {
180 E->getInstList().delete_all(); // Delete all instructions in block
181 E->getInstList().push_back(new BranchInst(EntryBlock));
184 AliveBlocks.insert(E);
186 // Next we need to change any PHI nodes in the entry block to refer to the
187 // new predecessor node...
191 // We need to move the new entry block to be the first bb of the method.
192 Method::iterator EBI = find(M->begin(), M->end(), EntryBlock);
193 std::swap(*EBI, *M->begin());// Exchange old location with start of method
198 // Now go through and tell dead blocks to drop all of their references so they
199 // can be safely deleted.
201 for (Method::iterator BI = M->begin(), BE = M->end(); BI != BE; ++BI) {
202 BasicBlock *BB = *BI;
203 if (!AliveBlocks.count(BB)) {
204 BB->dropAllReferences();
208 // Now loop through all of the blocks and delete them. We can safely do this
209 // now because we know that there are no references to dead blocks (because
210 // they have dropped all of their references...
212 for (Method::iterator BI = M->begin(); BI != M->end();) {
213 if (!AliveBlocks.count(*BI)) {
214 delete M->getBasicBlocks().remove(BI);
216 continue; // Don't increment iterator
218 ++BI; // Increment iterator...
225 // fixupCFG - Walk the CFG in depth first order, eliminating references to
227 // If the BB is alive (in AliveBlocks):
228 // 1. Eliminate all dead instructions in the BB
229 // 2. Recursively traverse all of the successors of the BB:
230 // - If the returned successor is non-null, update our terminator to
231 // reference the returned BB
232 // 3. Return 0 (no update needed)
234 // If the BB is dead (not in AliveBlocks):
235 // 1. Add the BB to the dead set
236 // 2. Recursively traverse all of the successors of the block:
237 // - Only one shall return a nonnull value (or else this block should have
238 // been in the alive set).
239 // 3. Return the nonnull child, or 0 if no non-null children.
241 BasicBlock *ADCE::fixupCFG(BasicBlock *BB, std::set<BasicBlock*> &VisitedBlocks,
242 const std::set<BasicBlock*> &AliveBlocks) {
243 if (VisitedBlocks.count(BB)) return 0; // Revisiting a node? No update.
244 VisitedBlocks.insert(BB); // We have now visited this node!
247 cerr << "Fixing up BB: " << BB;
250 if (AliveBlocks.count(BB)) { // Is the block alive?
251 // Yes it's alive: loop through and eliminate all dead instructions in block
252 for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; ) {
253 Instruction *I = *II;
254 if (!LiveSet.count(I)) { // Is this instruction alive?
255 // Nope... remove the instruction from it's basic block...
256 delete BB->getInstList().remove(II);
258 continue; // Don't increment II
263 // Recursively traverse successors of this basic block.
264 BasicBlock::succ_iterator SI = BB->succ_begin(), SE = BB->succ_end();
265 for (; SI != SE; ++SI) {
266 BasicBlock *Succ = *SI;
267 BasicBlock *Repl = fixupCFG(Succ, VisitedBlocks, AliveBlocks);
268 if (Repl && Repl != Succ) { // We have to replace the successor
269 Succ->replaceAllUsesWith(Repl);
274 } else { // Otherwise the block is dead...
275 BasicBlock *ReturnBB = 0; // Default to nothing live down here
277 // Recursively traverse successors of this basic block.
278 BasicBlock::succ_iterator SI = BB->succ_begin(), SE = BB->succ_end();
279 for (; SI != SE; ++SI) {
280 BasicBlock *RetBB = fixupCFG(*SI, VisitedBlocks, AliveBlocks);
282 assert(ReturnBB == 0 && "One one live child allowed!");
286 return ReturnBB; // Return the result of traversal
292 // doADCE - Execute the Agressive Dead Code Elimination Algorithm
294 bool AgressiveDCE::runOnMethod(Method *M) {
295 if (M->isExternal()) return false;
298 getAnalysis<cfg::DominanceFrontier>(cfg::DominanceFrontier::PostDomID));
302 // getAnalysisUsageInfo - We require post dominance frontiers (aka Control
305 void AgressiveDCE::getAnalysisUsageInfo(Pass::AnalysisSet &Requires,
306 Pass::AnalysisSet &Destroyed,
307 Pass::AnalysisSet &Provided) {
308 Requires.push_back(cfg::DominanceFrontier::PostDomID);