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 (including unused constants)
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/Optimizations/DCE.h"
27 #include "llvm/Support/STLExtras.h"
28 #include "llvm/Module.h"
29 #include "llvm/GlobalVariable.h"
30 #include "llvm/Method.h"
31 #include "llvm/BasicBlock.h"
32 #include "llvm/iTerminators.h"
33 #include "llvm/iOther.h"
34 #include "llvm/Assembly/Writer.h"
37 // dceInstruction - Inspect the instruction at *BBI and figure out if it's
38 // [trivially] dead. If so, remove the instruction and update the iterator
39 // to point to the instruction that immediately succeeded the original
42 bool opt::DeadCodeElimination::dceInstruction(BasicBlock::InstListType &BBIL,
43 BasicBlock::iterator &BBI) {
44 // Look for un"used" definitions...
45 if ((*BBI)->use_empty() && !(*BBI)->hasSideEffects() &&
46 !isa<TerminatorInst>(*BBI)) {
47 delete BBIL.remove(BBI); // Bye bye
53 static inline bool RemoveUnusedDefs(BasicBlock::InstListType &Vals) {
55 for (BasicBlock::InstListType::iterator DI = Vals.begin();
57 if (opt::DeadCodeElimination::dceInstruction(Vals, DI))
64 // RemoveSingularPHIs - This removes PHI nodes from basic blocks that have only
65 // a single predecessor. This means that the PHI node must only have a single
66 // RHS value and can be eliminated.
68 // This routine is very simple because we know that PHI nodes must be the first
69 // things in a basic block, if they are present.
71 static bool RemoveSingularPHIs(BasicBlock *BB) {
72 BasicBlock::pred_iterator PI(BB->pred_begin());
73 if (PI == BB->pred_end() || ++PI != BB->pred_end())
74 return false; // More than one predecessor...
76 Instruction *I = BB->front();
77 if (!isa<PHINode>(I)) return false; // No PHI nodes
79 //cerr << "Killing PHIs from " << BB;
80 //cerr << "Pred #0 = " << *BB->pred_begin();
82 //cerr << "Method == " << BB->getParent();
85 PHINode *PN = cast<PHINode>(I);
86 assert(PN->getNumOperands() == 2 && "PHI node should only have one value!");
87 Value *V = PN->getOperand(0);
89 PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
90 delete BB->getInstList().remove(BB->begin());
93 } while (isa<PHINode>(I));
95 return true; // Yes, we nuked at least one phi node
98 static void ReplaceUsesWithConstant(Instruction *I) {
99 ConstPoolVal *CPV = ConstPoolVal::getNullConstant(I->getType());
101 // Make all users of this instruction reference the constant instead
102 I->replaceAllUsesWith(CPV);
105 // PropogatePredecessors - This gets "Succ" ready to have the predecessors from
106 // "BB". This is a little tricky because "Succ" has PHI nodes, which need to
107 // have extra slots added to them to hold the merge edges from BB's
110 // Assumption: BB is the single predecessor of Succ.
112 static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
113 assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
115 // If there is more than one predecessor, and there are PHI nodes in
116 // the successor, then we need to add incoming edges for the PHI nodes
118 const vector<BasicBlock*> BBPreds(BB->pred_begin(), BB->pred_end());
120 BasicBlock::iterator I = Succ->begin();
121 do { // Loop over all of the PHI nodes in the successor BB
122 PHINode *PN = cast<PHINode>(*I);
123 Value *OldVal = PN->removeIncomingValue(BB);
124 assert(OldVal && "No entry in PHI for Pred BB!");
126 for (vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
127 End = BBPreds.end(); PredI != End; ++PredI) {
128 // Add an incoming value for each of the new incoming values...
129 PN->addIncoming(OldVal, *PredI);
133 } while (isa<PHINode>(*I));
137 // SimplifyCFG - This function is used to do simplification of a CFG. For
138 // example, it adjusts branches to branches to eliminate the extra hop, it
139 // eliminates unreachable basic blocks, and does other "peephole" optimization
140 // of the CFG. It returns true if a modification was made, and returns an
141 // iterator that designates the first element remaining after the block that
144 // WARNING: The entry node of a method may not be simplified.
146 bool opt::SimplifyCFG(Method::iterator &BBIt) {
147 BasicBlock *BB = *BBIt;
148 Method *M = BB->getParent();
150 assert(BB && BB->getParent() && "Block not embedded in method!");
151 assert(BB->getTerminator() && "Degenerate basic block encountered!");
152 assert(BB->getParent()->front() != BB && "Can't Simplify entry block!");
155 // Remove basic blocks that have no predecessors... which are unreachable.
156 if (BB->pred_begin() == BB->pred_end() &&
157 !BB->hasConstantPoolReferences()) {
158 //cerr << "Removing BB: \n" << BB;
160 // Loop through all of our successors and make sure they know that one
161 // of their predecessors is going away.
162 for_each(BB->succ_begin(), BB->succ_end(),
163 std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
165 while (!BB->empty()) {
166 Instruction *I = BB->back();
167 // If this instruction is used, replace uses with an arbitrary
168 // constant value. Because control flow can't get here, we don't care
169 // what we replace the value with. Note that since this block is
170 // unreachable, and all values contained within it must dominate their
171 // uses, that all uses will eventually be removed.
172 if (!I->use_empty()) ReplaceUsesWithConstant(I);
174 // Remove the instruction from the basic block
175 delete BB->getInstList().pop_back();
177 delete M->getBasicBlocks().remove(BBIt);
181 // Check to see if this block has no instructions and only a single
182 // successor. If so, replace block references with successor.
183 BasicBlock::succ_iterator SI(BB->succ_begin());
184 if (SI != BB->succ_end() && ++SI == BB->succ_end()) { // One succ?
185 Instruction *I = BB->front();
186 if (I->isTerminator()) { // Terminator is the only instruction!
187 BasicBlock *Succ = *BB->succ_begin(); // There is exactly one successor
188 //cerr << "Killing Trivial BB: \n" << BB;
190 if (Succ != BB) { // Arg, don't hurt infinite loops!
191 if (isa<PHINode>(Succ->front())) {
192 // If our successor has PHI nodes, then we need to update them to
193 // include entries for BB's predecessors, not for BB itself.
195 PropogatePredecessorsForPHIs(BB, Succ);
198 BB->replaceAllUsesWith(Succ);
199 BB = M->getBasicBlocks().remove(BBIt);
201 if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
202 Succ->setName(BB->getName());
203 delete BB; // Delete basic block
205 //cerr << "Method after removal: \n" << M;
211 // Merge basic blocks into their predecessor if there is only one pred,
212 // and if there is only one successor of the predecessor.
213 BasicBlock::pred_iterator PI(BB->pred_begin());
214 if (PI != BB->pred_end() && *PI != BB && // Not empty? Not same BB?
215 ++PI == BB->pred_end() && !BB->hasConstantPoolReferences()) {
216 BasicBlock *Pred = *BB->pred_begin();
217 TerminatorInst *Term = Pred->getTerminator();
218 assert(Term != 0 && "malformed basic block without terminator!");
220 // Does the predecessor block only have a single successor?
221 BasicBlock::succ_iterator SI(Pred->succ_begin());
222 if (++SI == Pred->succ_end()) {
223 //cerr << "Merging: " << BB << "into: " << Pred;
225 // Delete the unconditianal branch from the predecessor...
226 BasicBlock::iterator DI = Pred->end();
227 assert(Pred->getTerminator() &&
228 "Degenerate basic block encountered!"); // Empty bb???
229 delete Pred->getInstList().remove(--DI); // Destroy uncond branch
231 // Move all definitions in the succecessor to the predecessor...
232 while (!BB->empty()) {
234 Instruction *Def = BB->getInstList().remove(DI); // Remove from front
235 Pred->getInstList().push_back(Def); // Add to end...
238 // Remove basic block from the method... and advance iterator to the
239 // next valid block...
240 BB = M->getBasicBlocks().remove(BBIt);
242 // Make all PHI nodes that refered to BB now refer to Pred as their
244 BB->replaceAllUsesWith(Pred);
246 // Inherit predecessors name if it exists...
247 if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
249 delete BB; // You ARE the weakest link... goodbye
257 static bool DoDCEPass(Method *M) {
258 Method::iterator BBIt, BBEnd = M->end();
259 if (M->begin() == BBEnd) return false; // Nothing to do
260 bool Changed = false;
262 // Loop through now and remove instructions that have no uses...
263 for (BBIt = M->begin(); BBIt != BBEnd; ++BBIt) {
264 Changed |= RemoveUnusedDefs((*BBIt)->getInstList());
265 Changed |= RemoveSingularPHIs(*BBIt);
268 // Loop over all of the basic blocks (except the first one) and remove them
269 // if they are unneeded...
271 for (BBIt = M->begin(), ++BBIt; BBIt != M->end(); ) {
272 if (opt::SimplifyCFG(BBIt)) {
283 // It is possible that we may require multiple passes over the code to fully
284 // eliminate dead code. Iterate until we are done.
286 bool opt::DeadCodeElimination::doDCE(Method *M) {
287 bool Changed = false;
288 while (DoDCEPass(M)) Changed = true;
292 bool opt::DeadCodeElimination::RemoveUnusedGlobalValues(Module *Mod) {
293 bool Changed = false;
295 for (Module::iterator MI = Mod->begin(); MI != Mod->end(); ) {
297 if (Meth->isExternal() && Meth->use_size() == 0) {
298 // No references to prototype?
299 //cerr << "Removing method proto: " << Meth->getName() << endl;
300 delete Mod->getMethodList().remove(MI); // Remove prototype
301 // Remove moves iterator to point to the next one automatically
304 ++MI; // Skip prototype in use.
308 for (Module::giterator GI = Mod->gbegin(); GI != Mod->gend(); ) {
309 GlobalVariable *GV = *GI;
310 if (!GV->hasInitializer() && GV->use_size() == 0) {
311 // No references to uninitialized global variable?
312 //cerr << "Removing global var: " << GV->getName() << endl;
313 delete Mod->getGlobalList().remove(GI);
314 // Remove moves iterator to point to the next one automatically