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/Module.h"
26 #include "llvm/Method.h"
27 #include "llvm/BasicBlock.h"
28 #include "llvm/iTerminators.h"
29 #include "llvm/iOther.h"
30 #include "llvm/Opt/AllOpts.h"
31 #include "llvm/Assembly/Writer.h"
33 #include "llvm/Tools/STLExtras.h"
40 static bool isDCEable(const Value *) { return true; }
43 struct BasicBlockDCE {
45 static bool isDCEable(const Instruction *I) {
46 return !I->hasSideEffects();
51 template<class ValueSubclass, class ItemParentType, class DCEController>
52 static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
53 DCEController DCEControl) {
55 typedef ValueHolder<ValueSubclass, ItemParentType> Container;
57 int Offset = DCEController::EndOffs;
58 for (Container::iterator DI = Vals.begin(); DI != Vals.end()-Offset; ) {
59 // Look for un"used" definitions...
60 if ((*DI)->use_empty() && DCEController::isDCEable(*DI)) {
62 //cerr << "Removing: " << *DI;
63 delete Vals.remove(DI);
72 // RemoveSingularPHIs - This removes PHI nodes from basic blocks that have only
73 // a single predecessor. This means that the PHI node must only have a single
74 // RHS value and can be eliminated.
76 // This routine is very simple because we know that PHI nodes must be the first
77 // things in a basic block, if they are present.
79 static bool RemoveSingularPHIs(BasicBlock *BB) {
80 pred_iterator PI(pred_begin(BB));
81 if (PI == pred_end(BB) || ++PI != pred_end(BB))
82 return false; // More than one predecessor...
84 Instruction *I = BB->front();
85 if (!I->isPHINode()) return false; // No PHI nodes
87 //cerr << "Killing PHIs from " << BB;
88 //cerr << "Pred #0 = " << *pred_begin(BB);
90 //cerr << "Method == " << BB->getParent();
93 PHINode *PN = (PHINode*)I;
94 assert(PN->getOperand(2) == 0 && "PHI node should only have one value!");
95 Value *V = PN->getOperand(0);
97 PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
98 delete BB->getInstList().remove(BB->begin());
101 } while (I->isPHINode());
103 return true; // Yes, we nuked at least one phi node
106 bool DoRemoveUnusedConstants(SymTabValue *S) {
107 bool Changed = false;
108 ConstantPool &CP = S->getConstantPool();
109 for (ConstantPool::plane_iterator PI = CP.begin(); PI != CP.end(); ++PI)
110 Changed |= RemoveUnusedDefs(**PI, ConstPoolDCE());
114 static void ReplaceUsesWithConstant(Instruction *I) {
115 // Get the method level constant pool
116 ConstantPool &CP = I->getParent()->getParent()->getConstantPool();
118 ConstPoolVal *CPV = 0;
119 ConstantPool::PlaneType *P;
120 if (!CP.getPlane(I->getType(), P)) { // Does plane exist?
122 if (!P->empty()) CPV = P->front();
125 if (CPV == 0) { // We don't have an existing constant to reuse. Just add one.
126 CPV = ConstPoolVal::getNullConstant(I->getType()); // Create a new constant
128 // Add the new value to the constant pool...
132 // Make all users of this instruction reference the constant instead
133 I->replaceAllUsesWith(CPV);
136 // PropogatePredecessors - This gets "Succ" ready to have the predecessors from
137 // "BB". This is a little tricky because "Succ" has PHI nodes, which need to
138 // have extra slots added to them to hold the merge edges from BB's
141 // Assumption: BB is the single predecessor of Succ.
143 static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
144 assert(Succ->front()->isPHINode() && "Only works on PHId BBs!");
146 // If there is more than one predecessor, and there are PHI nodes in
147 // the successor, then we need to add incoming edges for the PHI nodes
149 const vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
151 BasicBlock::iterator I = Succ->begin();
152 do { // Loop over all of the PHI nodes in the successor BB
153 PHINode *PN = (PHINode*)*I;
154 Value *OldVal = PN->removeIncomingValue(BB);
155 assert(OldVal && "No entry in PHI for Pred BB!");
157 for (vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
158 End = BBPreds.end(); PredI != End; ++PredI) {
159 // Add an incoming value for each of the new incoming values...
160 PN->addIncoming(OldVal, *PredI);
164 } while ((*I)->isPHINode());
167 static bool DoDCEPass(Method *M) {
168 Method::iterator BBIt, BBEnd = M->end();
169 if (M->begin() == BBEnd) return false; // Nothing to do
170 bool Changed = false;
172 // Loop through now and remove instructions that have no uses...
173 for (BBIt = M->begin(); BBIt != BBEnd; ++BBIt) {
174 Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE());
175 Changed |= RemoveSingularPHIs(*BBIt);
178 // Loop over all of the basic blocks (except the first one) and remove them
179 // if they are unneeded...
181 for (BBIt = M->begin(), ++BBIt; BBIt != M->end(); ++BBIt) {
182 BasicBlock *BB = *BBIt;
183 assert(BB->getTerminator() && "Degenerate basic block encountered!");
185 // Remove basic blocks that have no predecessors... which are unreachable.
186 if (pred_begin(BB) == pred_end(BB) &&
187 !BB->hasConstantPoolReferences() && 0) {
188 //cerr << "Removing BB: \n" << BB;
190 // Loop through all of our successors and make sure they know that one
191 // of their predecessors is going away.
192 for_each(succ_begin(BB), succ_end(BB),
193 bind_obj(BB, &BasicBlock::removePredecessor));
195 while (!BB->empty()) {
196 Instruction *I = BB->front();
197 // If this instruction is used, replace uses with an arbitrary
198 // constant value. Because control flow can't get here, we don't care
199 // what we replace the value with.
200 if (!I->use_empty()) ReplaceUsesWithConstant(I);
202 // Remove the instruction from the basic block
203 delete BB->getInstList().remove(BB->begin());
205 delete M->getBasicBlocks().remove(BBIt);
206 --BBIt; // remove puts use on the next block, we want the previous one
211 // Check to see if this block has no instructions and only a single
212 // successor. If so, replace block references with successor.
213 succ_iterator SI(succ_begin(BB));
214 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
215 Instruction *I = BB->front();
216 if (I->isTerminator()) { // Terminator is the only instruction!
217 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
218 //cerr << "Killing Trivial BB: \n" << BB;
220 if (Succ != BB) { // Arg, don't hurt infinite loops!
221 if (Succ->front()->isPHINode()) {
222 // If our successor has PHI nodes, then we need to update them to
223 // include entries for BB's predecessors, not for BB itself.
225 PropogatePredecessorsForPHIs(BB, Succ);
228 BB->replaceAllUsesWith(Succ);
230 BB = M->getBasicBlocks().remove(BBIt);
231 --BBIt; // remove puts use on the next block, we want the previous one
233 if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
234 Succ->setName(BB->getName());
235 delete BB; // Delete basic block
237 //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);
274 --BBIt; // remove puts us on the NEXT bb. We want the prev BB
277 // Make all PHI nodes that refered to BB now refer to Pred as their
279 BB->replaceAllUsesWith(Pred);
281 // Inherit predecessors name if it exists...
282 if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
284 // You ARE the weakest link... goodbye
287 //WriteToVCG(M, "MergedInto");
292 // Remove unused constants
293 Changed |= DoRemoveUnusedConstants(M);
298 // It is possible that we may require multiple passes over the code to fully
299 // eliminate dead code. Iterate until we are done.
301 bool DoDeadCodeElimination(Method *M) {
302 bool Changed = false;
303 while (DoDCEPass(M)) Changed = true;
307 bool DoDeadCodeElimination(Module *C) {
308 bool Val = ApplyOptToAllMethods(C, DoDeadCodeElimination);
309 while (DoRemoveUnusedConstants(C)) Val = true;