1 //===- DCE.cpp - Code to perform dead code elimination --------------------===//
3 // This file implements dead code elimination and basic block merging.
6 // * removes definitions with no uses
7 // * removes basic blocks with no predecessors
8 // * merges a basic block into its predecessor if there is only one and the
9 // predecessor only has one successor.
10 // * Eliminates PHI nodes for basic blocks with a single predecessor
11 // * Eliminates a basic block that only contains an unconditional branch
12 // * Eliminates method prototypes that are not referenced
14 // TODO: This should REALLY be worklist driven instead of iterative. Right now,
15 // we scan linearly through values, removing unused ones as we go. The problem
16 // is that this may cause other earlier values to become unused. To make sure
17 // that we get them all, we iterate until things stop changing. Instead, when
18 // removing a value, recheck all of its operands to see if they are now unused.
19 // Piece of cake, and more efficient as well.
21 // Note, this is not trivial, because we have to worry about invalidating
24 //===----------------------------------------------------------------------===//
26 #include "llvm/Transforms/Scalar/DCE.h"
27 #include "llvm/Module.h"
28 #include "llvm/GlobalVariable.h"
29 #include "llvm/Method.h"
30 #include "llvm/BasicBlock.h"
31 #include "llvm/iTerminators.h"
32 #include "llvm/iPHINode.h"
33 #include "llvm/Assembly/Writer.h"
34 #include "llvm/Support/CFG.h"
35 #include "llvm/Pass.h"
36 #include "Support/STLExtras.h"
39 // dceInstruction - Inspect the instruction at *BBI and figure out if it's
40 // [trivially] dead. If so, remove the instruction and update the iterator
41 // to point to the instruction that immediately succeeded the original
44 bool dceInstruction(BasicBlock::InstListType &BBIL,
45 BasicBlock::iterator &BBI) {
46 // Look for un"used" definitions...
47 if ((*BBI)->use_empty() && !(*BBI)->hasSideEffects() &&
48 !isa<TerminatorInst>(*BBI)) {
49 delete BBIL.remove(BBI); // Bye bye
55 static inline bool RemoveUnusedDefs(BasicBlock::InstListType &Vals) {
57 for (BasicBlock::InstListType::iterator DI = Vals.begin();
59 if (dceInstruction(Vals, DI))
66 struct DeadInstElimination : public BasicBlockPass {
67 virtual bool runOnBasicBlock(BasicBlock *BB) {
68 return RemoveUnusedDefs(BB->getInstList());
72 Pass *createDeadInstEliminationPass() {
73 return new DeadInstElimination();
76 // RemoveSingularPHIs - This removes PHI nodes from basic blocks that have only
77 // a single predecessor. This means that the PHI node must only have a single
78 // RHS value and can be eliminated.
80 // This routine is very simple because we know that PHI nodes must be the first
81 // things in a basic block, if they are present.
83 static bool RemoveSingularPHIs(BasicBlock *BB) {
84 pred_iterator PI(pred_begin(BB));
85 if (PI == pred_end(BB) || ++PI != pred_end(BB))
86 return false; // More than one predecessor...
88 Instruction *I = BB->front();
89 if (!isa<PHINode>(I)) return false; // No PHI nodes
91 //cerr << "Killing PHIs from " << BB;
92 //cerr << "Pred #0 = " << *pred_begin(BB);
94 //cerr << "Method == " << BB->getParent();
97 PHINode *PN = cast<PHINode>(I);
98 assert(PN->getNumOperands() == 2 && "PHI node should only have one value!");
99 Value *V = PN->getOperand(0);
101 PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
102 delete BB->getInstList().remove(BB->begin());
105 } while (isa<PHINode>(I));
107 return true; // Yes, we nuked at least one phi node
110 static void ReplaceUsesWithConstant(Instruction *I) {
111 Constant *CPV = Constant::getNullConstant(I->getType());
113 // Make all users of this instruction reference the constant instead
114 I->replaceAllUsesWith(CPV);
117 // PropogatePredecessors - This gets "Succ" ready to have the predecessors from
118 // "BB". This is a little tricky because "Succ" has PHI nodes, which need to
119 // have extra slots added to them to hold the merge edges from BB's
120 // predecessors. This function returns true (failure) if the Succ BB already
121 // has a predecessor that is a predecessor of BB.
123 // Assumption: Succ is the single successor for BB.
125 static bool PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
126 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
127 assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
129 // If there is more than one predecessor, and there are PHI nodes in
130 // the successor, then we need to add incoming edges for the PHI nodes
132 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
134 // Check to see if one of the predecessors of BB is already a predecessor of
135 // Succ. If so, we cannot do the transformation!
137 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);
139 if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end())
143 BasicBlock::iterator I = Succ->begin();
144 do { // Loop over all of the PHI nodes in the successor BB
145 PHINode *PN = cast<PHINode>(*I);
146 Value *OldVal = PN->removeIncomingValue(BB);
147 assert(OldVal && "No entry in PHI for Pred BB!");
149 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
150 End = BBPreds.end(); PredI != End; ++PredI) {
151 // Add an incoming value for each of the new incoming values...
152 PN->addIncoming(OldVal, *PredI);
156 } while (isa<PHINode>(*I));
161 // SimplifyCFG - This function is used to do simplification of a CFG. For
162 // example, it adjusts branches to branches to eliminate the extra hop, it
163 // eliminates unreachable basic blocks, and does other "peephole" optimization
164 // of the CFG. It returns true if a modification was made, and returns an
165 // iterator that designates the first element remaining after the block that
168 // WARNING: The entry node of a method may not be simplified.
170 bool SimplifyCFG(Method::iterator &BBIt) {
171 BasicBlock *BB = *BBIt;
172 Method *M = BB->getParent();
174 assert(BB && BB->getParent() && "Block not embedded in method!");
175 assert(BB->getTerminator() && "Degenerate basic block encountered!");
176 assert(BB->getParent()->front() != BB && "Can't Simplify entry block!");
179 // Remove basic blocks that have no predecessors... which are unreachable.
180 if (pred_begin(BB) == pred_end(BB) &&
181 !BB->hasConstantReferences()) {
182 //cerr << "Removing BB: \n" << BB;
184 // Loop through all of our successors and make sure they know that one
185 // of their predecessors is going away.
186 for_each(succ_begin(BB), succ_end(BB),
187 std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
189 while (!BB->empty()) {
190 Instruction *I = BB->back();
191 // If this instruction is used, replace uses with an arbitrary
192 // constant value. Because control flow can't get here, we don't care
193 // what we replace the value with. Note that since this block is
194 // unreachable, and all values contained within it must dominate their
195 // uses, that all uses will eventually be removed.
196 if (!I->use_empty()) ReplaceUsesWithConstant(I);
198 // Remove the instruction from the basic block
199 delete BB->getInstList().pop_back();
201 delete M->getBasicBlocks().remove(BBIt);
205 // Check to see if this block has no instructions and only a single
206 // successor. If so, replace block references with successor.
207 succ_iterator SI(succ_begin(BB));
208 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
209 if (BB->front()->isTerminator()) { // Terminator is the only instruction!
210 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
211 //cerr << "Killing Trivial BB: \n" << BB;
213 if (Succ != BB) { // Arg, don't hurt infinite loops!
214 // If our successor has PHI nodes, then we need to update them to
215 // include entries for BB's predecessors, not for BB itself.
216 // Be careful though, if this transformation fails (returns true) then
217 // we cannot do this transformation!
219 if (!isa<PHINode>(Succ->front()) ||
220 !PropogatePredecessorsForPHIs(BB, Succ)) {
222 BB->replaceAllUsesWith(Succ);
223 BB = M->getBasicBlocks().remove(BBIt);
225 if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
226 Succ->setName(BB->getName());
227 delete BB; // Delete basic block
229 //cerr << "Method after removal: \n" << M;
236 // Merge basic blocks into their predecessor if there is only one pred,
237 // and if there is only one successor of the predecessor.
238 pred_iterator PI(pred_begin(BB));
239 if (PI != pred_end(BB) && *PI != BB && // Not empty? Not same BB?
240 ++PI == pred_end(BB) && !BB->hasConstantReferences()) {
241 BasicBlock *Pred = *pred_begin(BB);
242 TerminatorInst *Term = Pred->getTerminator();
243 assert(Term != 0 && "malformed basic block without terminator!");
245 // Does the predecessor block only have a single successor?
246 succ_iterator SI(succ_begin(Pred));
247 if (++SI == succ_end(Pred)) {
248 //cerr << "Merging: " << BB << "into: " << Pred;
250 // Delete the unconditianal branch from the predecessor...
251 BasicBlock::iterator DI = Pred->end();
252 assert(Pred->getTerminator() &&
253 "Degenerate basic block encountered!"); // Empty bb???
254 delete Pred->getInstList().remove(--DI); // Destroy uncond branch
256 // Move all definitions in the succecessor to the predecessor...
257 while (!BB->empty()) {
259 Instruction *Def = BB->getInstList().remove(DI); // Remove from front
260 Pred->getInstList().push_back(Def); // Add to end...
263 // Remove basic block from the method... and advance iterator to the
264 // next valid block...
265 BB = M->getBasicBlocks().remove(BBIt);
267 // Make all PHI nodes that refered to BB now refer to Pred as their
269 BB->replaceAllUsesWith(Pred);
271 // Inherit predecessors name if it exists...
272 if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
274 delete BB; // You ARE the weakest link... goodbye
282 static bool DoDCEPass(Method *M) {
283 Method::iterator BBIt, BBEnd = M->end();
284 if (M->begin() == BBEnd) return false; // Nothing to do
285 bool Changed = false;
287 // Loop through now and remove instructions that have no uses...
288 for (BBIt = M->begin(); BBIt != BBEnd; ++BBIt) {
289 Changed |= RemoveUnusedDefs((*BBIt)->getInstList());
290 Changed |= RemoveSingularPHIs(*BBIt);
293 // Loop over all of the basic blocks (except the first one) and remove them
294 // if they are unneeded...
296 for (BBIt = M->begin(), ++BBIt; BBIt != M->end(); ) {
297 if (SimplifyCFG(BBIt)) {
307 // Remove unused global values - This removes unused global values of no
308 // possible value. This currently includes unused method prototypes and
309 // unitialized global variables.
311 static bool RemoveUnusedGlobalValues(Module *Mod) {
312 bool Changed = false;
314 for (Module::iterator MI = Mod->begin(); MI != Mod->end(); ) {
316 if (Meth->isExternal() && Meth->use_size() == 0) {
317 // No references to prototype?
318 //cerr << "Removing method proto: " << Meth->getName() << endl;
319 delete Mod->getMethodList().remove(MI); // Remove prototype
320 // Remove moves iterator to point to the next one automatically
323 ++MI; // Skip prototype in use.
327 for (Module::giterator GI = Mod->gbegin(); GI != Mod->gend(); ) {
328 GlobalVariable *GV = *GI;
329 if (!GV->hasInitializer() && GV->use_size() == 0) {
330 // No references to uninitialized global variable?
331 //cerr << "Removing global var: " << GV->getName() << endl;
332 delete Mod->getGlobalList().remove(GI);
333 // Remove moves iterator to point to the next one automatically
344 struct DeadCodeElimination : public MethodPass {
347 virtual bool doInitialization(Module *M) {
348 return RemoveUnusedGlobalValues(M);
351 // It is possible that we may require multiple passes over the code to fully
352 // eliminate dead code. Iterate until we are done.
354 virtual bool runOnMethod(Method *M) {
355 bool Changed = false;
356 while (DoDCEPass(M)) Changed = true;
360 virtual bool doFinalization(Module *M) {
361 return RemoveUnusedGlobalValues(M);
366 Pass *createDeadCodeEliminationPass() {
367 return new DeadCodeElimination();