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
13 // TODO: This should REALLY be worklist driven instead of iterative. Right now,
14 // we scan linearly through values, removing unused ones as we go. The problem
15 // is that this may cause other earlier values to become unused. To make sure
16 // that we get them all, we iterate until things stop changing. Instead, when
17 // removing a value, recheck all of its operands to see if they are now unused.
18 // Piece of cake, and more efficient as well.
20 // Note, this is not trivial, because we have to worry about invalidating
23 //===----------------------------------------------------------------------===//
25 #include "llvm/Optimizations/DCE.h"
26 #include "llvm/Tools/STLExtras.h"
27 #include "llvm/Module.h"
28 #include "llvm/Method.h"
29 #include "llvm/BasicBlock.h"
30 #include "llvm/iTerminators.h"
31 #include "llvm/iOther.h"
32 #include "llvm/Assembly/Writer.h"
40 static bool isDCEable(const ConstPoolVal *CPV) {
41 // TODO: The bytecode writer requires that all used types are in the
42 // constant pool for the current method. This is messy and is really
44 return CPV->getType() != Type::TypeTy; // Don't DCE Type plane constants!
48 struct BasicBlockDCE {
50 static bool isDCEable(const Instruction *I) {
51 return !I->hasSideEffects();
56 template<class Container, class DCEController>
57 static bool RemoveUnusedDefs(Container &Vals, DCEController DCEControl) {
59 int Offset = DCEController::EndOffs;
61 for (typename Container::iterator DI = Vals.begin();
62 DI != Vals.end()-Offset; ) {
63 // Look for un"used" definitions...
64 if ((*DI)->use_empty() && DCEController::isDCEable(*DI)) {
66 //cerr << "Removing: " << *DI;
67 delete Vals.remove(DI);
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 (!I->isPHINode()) 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 = (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 (I->isPHINode());
107 return true; // Yes, we nuked at least one phi node
110 bool opt::DoRemoveUnusedConstants(SymTabValue *S) {
111 bool Changed = false;
112 ConstantPool &CP = S->getConstantPool();
113 for (ConstantPool::plane_iterator PI = CP.begin(); PI != CP.end(); ++PI)
114 Changed |= RemoveUnusedDefs(**PI, ConstPoolDCE());
118 static void ReplaceUsesWithConstant(Instruction *I) {
119 // Get the method level constant pool
120 ConstantPool &CP = I->getParent()->getParent()->getConstantPool();
122 ConstPoolVal *CPV = 0;
123 ConstantPool::PlaneType *P;
124 if (!CP.getPlane(I->getType(), P)) { // Does plane exist?
126 if (!P->empty()) CPV = P->front();
129 if (CPV == 0) { // We don't have an existing constant to reuse. Just add one.
130 CPV = ConstPoolVal::getNullConstant(I->getType()); // Create a new constant
132 // Add the new value to the constant pool...
136 // Make all users of this instruction reference the constant instead
137 I->replaceAllUsesWith(CPV);
140 // PropogatePredecessors - This gets "Succ" ready to have the predecessors from
141 // "BB". This is a little tricky because "Succ" has PHI nodes, which need to
142 // have extra slots added to them to hold the merge edges from BB's
145 // Assumption: BB is the single predecessor of Succ.
147 static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
148 assert(Succ->front()->isPHINode() && "Only works on PHId BBs!");
150 // If there is more than one predecessor, and there are PHI nodes in
151 // the successor, then we need to add incoming edges for the PHI nodes
153 const vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
155 BasicBlock::iterator I = Succ->begin();
156 do { // Loop over all of the PHI nodes in the successor BB
157 PHINode *PN = (PHINode*)*I;
158 Value *OldVal = PN->removeIncomingValue(BB);
159 assert(OldVal && "No entry in PHI for Pred BB!");
161 for (vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
162 End = BBPreds.end(); PredI != End; ++PredI) {
163 // Add an incoming value for each of the new incoming values...
164 PN->addIncoming(OldVal, *PredI);
168 } while ((*I)->isPHINode());
172 // SimplifyCFG - This function is used to do simplification of a CFG. For
173 // example, it adjusts branches to branches to eliminate the extra hop, it
174 // eliminates unreachable basic blocks, and does other "peephole" optimization
175 // of the CFG. It returns true if a modification was made, and returns an
176 // iterator that designates the first element remaining after the block that
179 // WARNING: The entry node of a method may not be simplified.
181 bool opt::SimplifyCFG(Method::iterator &BBIt) {
182 assert(*BBIt && (*BBIt)->getParent() && "Block not embedded in method!");
183 BasicBlock *BB = *BBIt;
184 Method *M = BB->getParent();
185 assert(BB->getTerminator() && "Degenerate basic block encountered!");
186 assert(BB->getParent()->front() != BB && "Can't Simplify entry block!");
188 // Remove basic blocks that have no predecessors... which are unreachable.
189 if (pred_begin(BB) == pred_end(BB) &&
190 !BB->hasConstantPoolReferences()) {
191 //cerr << "Removing BB: \n" << BB;
193 // Loop through all of our successors and make sure they know that one
194 // of their predecessors is going away.
195 for_each(succ_begin(BB), succ_end(BB),
196 std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
198 while (!BB->empty()) {
199 Instruction *I = BB->back();
200 // If this instruction is used, replace uses with an arbitrary
201 // constant value. Because control flow can't get here, we don't care
202 // what we replace the value with. Note that since this block is
203 // unreachable, and all values contained within it must dominate their
204 // uses, that all uses will eventually be removed.
205 if (!I->use_empty()) ReplaceUsesWithConstant(I);
207 // Remove the instruction from the basic block
208 delete BB->getInstList().pop_back();
210 delete M->getBasicBlocks().remove(BBIt);
214 // Check to see if this block has no instructions and only a single
215 // successor. If so, replace block references with successor.
216 succ_iterator SI(succ_begin(BB));
217 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
218 Instruction *I = BB->front();
219 if (I->isTerminator()) { // Terminator is the only instruction!
220 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
221 //cerr << "Killing Trivial BB: \n" << BB;
223 if (Succ != BB) { // Arg, don't hurt infinite loops!
224 if (Succ->front()->isPHINode()) {
225 // If our successor has PHI nodes, then we need to update them to
226 // include entries for BB's predecessors, not for BB itself.
228 PropogatePredecessorsForPHIs(BB, Succ);
231 BB->replaceAllUsesWith(Succ);
232 BB = M->getBasicBlocks().remove(BBIt);
234 if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
235 Succ->setName(BB->getName());
236 delete BB; // Delete basic block
238 //cerr << "Method after removal: \n" << M;
244 // Merge basic blocks into their predecessor if there is only one pred,
245 // and if there is only one successor of the predecessor.
246 pred_iterator PI(pred_begin(BB));
247 if (PI != pred_end(BB) && *PI != BB && // Not empty? Not same BB?
248 ++PI == pred_end(BB) && !BB->hasConstantPoolReferences()) {
249 BasicBlock *Pred = *pred_begin(BB);
250 TerminatorInst *Term = Pred->getTerminator();
251 assert(Term != 0 && "malformed basic block without terminator!");
253 // Does the predecessor block only have a single successor?
254 succ_iterator SI(succ_begin(Pred));
255 if (++SI == succ_end(Pred)) {
256 //cerr << "Merging: " << BB << "into: " << Pred;
258 // Delete the unconditianal branch from the predecessor...
259 BasicBlock::iterator DI = Pred->end();
260 assert(Pred->getTerminator() &&
261 "Degenerate basic block encountered!"); // Empty bb???
262 delete Pred->getInstList().remove(--DI); // Destroy uncond branch
264 // Move all definitions in the succecessor to the predecessor...
265 while (!BB->empty()) {
267 Instruction *Def = BB->getInstList().remove(DI); // Remove from front
268 Pred->getInstList().push_back(Def); // Add to end...
271 // Remove basic block from the method... and advance iterator to the
272 // next valid block...
273 BB = M->getBasicBlocks().remove(BBIt);
275 // Make all PHI nodes that refered to BB now refer to Pred as their
277 BB->replaceAllUsesWith(Pred);
279 // Inherit predecessors name if it exists...
280 if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
282 delete BB; // You ARE the weakest link... goodbye
290 static bool DoDCEPass(Method *M) {
291 Method::iterator BBIt, BBEnd = M->end();
292 if (M->begin() == BBEnd) return false; // Nothing to do
293 bool Changed = false;
295 // Loop through now and remove instructions that have no uses...
296 for (BBIt = M->begin(); BBIt != BBEnd; ++BBIt) {
297 Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE());
298 Changed |= RemoveSingularPHIs(*BBIt);
301 // Loop over all of the basic blocks (except the first one) and remove them
302 // if they are unneeded...
304 for (BBIt = M->begin(), ++BBIt; BBIt != M->end(); ) {
305 if (opt::SimplifyCFG(BBIt)) {
312 // Remove unused constants
313 return Changed | opt::DoRemoveUnusedConstants(M);
317 // It is possible that we may require multiple passes over the code to fully
318 // eliminate dead code. Iterate until we are done.
320 bool opt::DoDeadCodeElimination(Method *M) {
321 bool Changed = false;
322 while (DoDCEPass(M)) Changed = true;
326 bool opt::DoDeadCodeElimination(Module *C) {
327 bool Val = C->reduceApply(DoDeadCodeElimination);
329 while (DoRemoveUnusedConstants(C)) Val = true;