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"
17 #include "llvm/iOther.h"
23 //===----------------------------------------------------------------------===//
26 // This class does all of the work of Agressive Dead Code Elimination.
27 // It's public interface consists of a constructor and a doADCE() method.
30 Method *M; // The method that we are working on...
31 vector<Instruction*> WorkList; // Instructions that just became live
32 set<Instruction*> LiveSet; // The set of live instructions
35 //===--------------------------------------------------------------------===//
36 // The public interface for this class
39 // ADCE Ctor - Save the method to operate on...
40 inline ADCE(Method *m) : M(m), MadeChanges(false) {}
42 // doADCE() - Run the Agressive Dead Code Elimination algorithm, returning
43 // true if the method was modified.
46 //===--------------------------------------------------------------------===//
47 // The implementation of this class
50 inline void markInstructionLive(Instruction *I) {
51 if (LiveSet.count(I)) return;
53 cerr << "Insn Live: " << I;
56 WorkList.push_back(I);
59 inline void markTerminatorLive(const BasicBlock *BB) {
61 cerr << "Terminat Live: " << BB->getTerminator();
63 markInstructionLive((Instruction*)BB->getTerminator());
66 // fixupCFG - Walk the CFG in depth first order, eliminating references to
69 BasicBlock *fixupCFG(BasicBlock *Head, set<BasicBlock*> &VisitedBlocks,
70 const set<BasicBlock*> &AliveBlocks);
75 // doADCE() - Run the Agressive Dead Code Elimination algorithm, returning
76 // true if the method was modified.
79 // Compute the control dependence graph... Note that this has a side effect
80 // on the CFG: a new return bb is added and all returns are merged here.
82 cfg::DominanceFrontier CDG(cfg::DominatorSet(M, true));
85 cerr << "Method: " << M;
88 // Iterate over all of the instructions in the method, eliminating trivially
89 // dead instructions, and marking instructions live that are known to be
90 // needed. Perform the walk in depth first order so that we avoid marking any
91 // instructions live in basic blocks that are unreachable. These blocks will
92 // be eliminated later, along with the instructions inside.
94 for (df_iterator<Method*> BBI = df_begin(M),
97 BasicBlock *BB = *BBI;
98 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
101 if (I->hasSideEffects() || I->getOpcode() == Instruction::Ret) {
102 markInstructionLive(I);
104 // Check to see if anything is trivially dead
105 if (I->use_size() == 0 && I->getType() != Type::VoidTy) {
106 // Remove the instruction from it's basic block...
107 delete BB->getInstList().remove(II);
109 continue; // Don't increment the iterator past the current slot
113 ++II; // Increment the inst iterator if the inst wasn't deleted
118 cerr << "Processing work list\n";
121 // AliveBlocks - Set of basic blocks that we know have instructions that are
124 set<BasicBlock*> AliveBlocks;
126 // Process the work list of instructions that just became live... if they
127 // became live, then that means that all of their operands are neccesary as
128 // well... make them live as well.
130 while (!WorkList.empty()) {
131 Instruction *I = WorkList.back(); // Get an instruction that became live...
134 BasicBlock *BB = I->getParent();
135 if (AliveBlocks.count(BB) == 0) { // Basic block not alive yet...
136 // Mark the basic block as being newly ALIVE... and mark all branches that
137 // this block is control dependant on as being alive also...
139 AliveBlocks.insert(BB); // Block is now ALIVE!
140 cfg::DominanceFrontier::const_iterator It = CDG.find(BB);
141 if (It != CDG.end()) {
142 // Get the blocks that this node is control dependant on...
143 const cfg::DominanceFrontier::DomSetType &CDB = It->second;
144 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
145 bind_obj(this, &ADCE::markTerminatorLive));
148 // If this basic block is live, then the terminator must be as well!
149 markTerminatorLive(BB);
152 // Loop over all of the operands of the live instruction, making sure that
153 // they are known to be alive as well...
155 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op) {
156 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
157 markInstructionLive(Operand);
162 cerr << "Current Method: X = Live\n";
163 for (Method::inst_iterator IL = M->inst_begin(); IL != M->inst_end(); ++IL) {
164 if (LiveSet.count(*IL)) cerr << "X ";
169 // After the worklist is processed, recursively walk the CFG in depth first
170 // order, patching up references to dead blocks...
172 set<BasicBlock*> VisitedBlocks;
173 BasicBlock *EntryBlock = fixupCFG(M->front(), VisitedBlocks, AliveBlocks);
174 if (EntryBlock && EntryBlock != M->front()) {
175 if (isa<PHINode>(EntryBlock->front())) {
176 // Cannot make the first block be a block with a PHI node in it! Instead,
177 // strip the first basic block of the method to contain no instructions,
178 // then add a simple branch to the "real" entry node...
180 BasicBlock *E = M->front();
181 if (!isa<TerminatorInst>(E->front()) || // Check for an actual change...
182 cast<TerminatorInst>(E->front())->getNumSuccessors() != 1 ||
183 cast<TerminatorInst>(E->front())->getSuccessor(0) != EntryBlock) {
184 E->getInstList().delete_all(); // Delete all instructions in block
185 E->getInstList().push_back(new BranchInst(EntryBlock));
188 AliveBlocks.insert(E);
190 // Next we need to change any PHI nodes in the entry block to refer to the
191 // new predecessor node...
195 // We need to move the new entry block to be the first bb of the method.
196 Method::iterator EBI = find(M->begin(), M->end(), EntryBlock);
197 swap(*EBI, *M->begin()); // Exchange old location with start of method
202 // Now go through and tell dead blocks to drop all of their references so they
203 // can be safely deleted.
205 for (Method::iterator BI = M->begin(), BE = M->end(); BI != BE; ++BI) {
206 BasicBlock *BB = *BI;
207 if (!AliveBlocks.count(BB)) {
208 BB->dropAllReferences();
212 // Now loop through all of the blocks and delete them. We can safely do this
213 // now because we know that there are no references to dead blocks (because
214 // they have dropped all of their references...
216 for (Method::iterator BI = M->begin(); BI != M->end();) {
217 if (!AliveBlocks.count(*BI)) {
218 delete M->getBasicBlocks().remove(BI);
220 continue; // Don't increment iterator
222 ++BI; // Increment iterator...
229 // fixupCFG - Walk the CFG in depth first order, eliminating references to
231 // If the BB is alive (in AliveBlocks):
232 // 1. Eliminate all dead instructions in the BB
233 // 2. Recursively traverse all of the successors of the BB:
234 // - If the returned successor is non-null, update our terminator to
235 // reference the returned BB
236 // 3. Return 0 (no update needed)
238 // If the BB is dead (not in AliveBlocks):
239 // 1. Add the BB to the dead set
240 // 2. Recursively traverse all of the successors of the block:
241 // - Only one shall return a nonnull value (or else this block should have
242 // been in the alive set).
243 // 3. Return the nonnull child, or 0 if no non-null children.
245 BasicBlock *ADCE::fixupCFG(BasicBlock *BB, set<BasicBlock*> &VisitedBlocks,
246 const set<BasicBlock*> &AliveBlocks) {
247 if (VisitedBlocks.count(BB)) return 0; // Revisiting a node? No update.
248 VisitedBlocks.insert(BB); // We have now visited this node!
251 cerr << "Fixing up BB: " << BB;
254 if (AliveBlocks.count(BB)) { // Is the block alive?
255 // Yes it's alive: loop through and eliminate all dead instructions in block
256 for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; ) {
257 Instruction *I = *II;
258 if (!LiveSet.count(I)) { // Is this instruction alive?
259 // Nope... remove the instruction from it's basic block...
260 delete BB->getInstList().remove(II);
262 continue; // Don't increment II
267 // Recursively traverse successors of this basic block.
268 BasicBlock::succ_iterator SI = BB->succ_begin(), SE = BB->succ_end();
269 for (; SI != SE; ++SI) {
270 BasicBlock *Succ = *SI;
271 BasicBlock *Repl = fixupCFG(Succ, VisitedBlocks, AliveBlocks);
272 if (Repl && Repl != Succ) { // We have to replace the successor
273 Succ->replaceAllUsesWith(Repl);
278 } else { // Otherwise the block is dead...
279 BasicBlock *ReturnBB = 0; // Default to nothing live down here
281 // Recursively traverse successors of this basic block.
282 BasicBlock::succ_iterator SI = BB->succ_begin(), SE = BB->succ_end();
283 for (; SI != SE; ++SI) {
284 BasicBlock *RetBB = fixupCFG(*SI, VisitedBlocks, AliveBlocks);
286 assert(ReturnBB == 0 && "One one live child allowed!");
290 return ReturnBB; // Return the result of traversal
296 // doADCE - Execute the Agressive Dead Code Elimination Algorithm
298 bool opt::AgressiveDCE::doADCE(Method *M) {
299 if (M->isExternal()) return false;