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 recursive instead of iterative. Right now, we
14 // scan linearly through values, removing unused ones as we go. The problem is
15 // that this may cause other earlier values to become unused. To make sure that
16 // 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"
38 static bool isDCEable(const Value *) { return true; }
41 struct BasicBlockDCE {
43 static bool isDCEable(const Instruction *I) {
44 return !I->hasSideEffects();
49 template<class ValueSubclass, class ItemParentType, class DCEController>
50 static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
51 DCEController DCEControl) {
53 typedef ValueHolder<ValueSubclass, ItemParentType> Container;
55 int Offset = DCEController::EndOffs;
56 for (Container::iterator DI = Vals.begin(); DI != Vals.end()-Offset; ) {
57 // Look for un"used" definitions...
58 if ((*DI)->use_empty() && DCEController::isDCEable(*DI)) {
60 //cerr << "Removing: " << *DI;
61 delete Vals.remove(DI);
70 // RemoveSingularPHIs - This removes PHI nodes from basic blocks that have only
71 // a single predecessor. This means that the PHI node must only have a single
72 // RHS value and can be eliminated.
74 // This routine is very simple because we know that PHI nodes must be the first
75 // things in a basic block, if they are present.
77 static bool RemoveSingularPHIs(BasicBlock *BB) {
78 pred_iterator PI(pred_begin(BB));
79 if (PI == pred_end(BB) || ++PI != pred_end(BB))
80 return false; // More than one predecessor...
82 Instruction *I = BB->getInstList().front();
83 if (I->getInstType() != Instruction::PHINode) return false; // No PHI nodes
85 //cerr << "Killing PHIs from " << BB;
86 //cerr << "Pred #0 = " << *pred_begin(BB);
88 //cerr << "Method == " << BB->getParent();
91 PHINode *PN = (PHINode*)I;
92 assert(PN->getOperand(2) == 0 && "PHI node should only have one value!");
93 Value *V = PN->getOperand(0);
95 PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
96 delete BB->getInstList().remove(BB->getInstList().begin());
98 I = BB->getInstList().front();
99 } while (I->getInstType() == Instruction::PHINode);
101 return true; // Yes, we nuked at least one phi node
104 bool DoRemoveUnusedConstants(SymTabValue *S) {
105 bool Changed = false;
106 ConstantPool &CP = S->getConstantPool();
107 for (ConstantPool::plane_iterator PI = CP.begin(); PI != CP.end(); ++PI)
108 Changed |= RemoveUnusedDefs(**PI, ConstPoolDCE());
112 static void ReplaceUsesWithConstant(Instruction *I) {
113 // Get the method level constant pool
114 ConstantPool &CP = I->getParent()->getParent()->getConstantPool();
116 ConstPoolVal *CPV = 0;
117 ConstantPool::PlaneType *P;
118 if (!CP.getPlane(I->getType(), P)) { // Does plane exist?
120 if (!P->empty()) CPV = P->front();
123 if (CPV == 0) { // We don't have an existing constant to reuse. Just add one.
124 CPV = ConstPoolVal::getNullConstant(I->getType()); // Create a new constant
126 // Add the new value to the constant pool...
130 // Make all users of this instruction reference the constant instead
131 I->replaceAllUsesWith(CPV);
134 // RemovePredecessorFromBlock - This function is called when we are about
135 // to remove a predecessor from a basic block. This function takes care of
136 // removing the predecessor from the PHI nodes in BB so that after the pred
137 // is removed, the number of PHI slots per bb is equal to the number of
140 static void RemovePredecessorFromBlock(BasicBlock *BB, BasicBlock *Pred) {
141 pred_iterator PI(pred_begin(BB)), EI(pred_end(BB));
144 //cerr << "RPFB: " << Pred << "From Block: " << BB;
146 // Loop over the rest of the predecssors until we run out, or until we find
147 // out that there are more than 2 predecessors.
148 for (max_idx = 0; PI != EI && max_idx < 3; ++PI, ++max_idx) /*empty*/;
150 // If there are exactly two predecessors, then we want to nuke the PHI nodes
152 bool NukePHIs = max_idx == 2;
153 assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!");
155 // Okay, now we know that we need to remove predecessor #pred_idx from all
156 // PHI nodes. Iterate over each PHI node fixing them up
157 BasicBlock::InstListType::iterator II(BB->getInstList().begin());
158 for (; (*II)->getInstType() == Instruction::PHINode; ++II) {
159 PHINode *PN = (PHINode*)*II;
160 PN->removeIncomingValue(BB);
162 if (NukePHIs) { // Destroy the PHI altogether??
163 assert(PN->getOperand(1) == 0 && "PHI node should only have one value!");
164 Value *V = PN->getOperand(0);
166 PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
167 delete BB->getInstList().remove(II);
172 // PropogatePredecessors - This gets "Succ" ready to have the predecessors from
173 // "BB". This is a little tricky because "Succ" has PHI nodes, which need to
174 // have extra slots added to them to hold the merge edges from BB's
177 // Assumption: BB is the single predecessor of Succ.
179 static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
180 assert(BB && Succ && *pred_begin(Succ) == BB && "BB is only pred of Succ" &&
181 ++pred_begin(Succ) == pred_end(Succ));
183 // If there is more than one predecessor, and there are PHI nodes in
184 // the successor, then we need to add incoming edges for the PHI nodes
185 pred_iterator PI(pred_begin(BB));
186 for (; PI != pred_end(BB); ++PI) {
191 static bool DoDCEPass(Method *M) {
192 Method::BasicBlocksType &BBs = M->getBasicBlocks();
193 Method::BasicBlocksType::iterator BBIt, BBEnd = BBs.end();
194 if (BBs.begin() == BBEnd) return false; // Nothing to do
195 bool Changed = false;
197 // Loop through now and remove instructions that have no uses...
198 for (BBIt = BBs.begin(); BBIt != BBEnd; BBIt++) {
199 Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE());
200 Changed |= RemoveSingularPHIs(*BBIt);
203 // Loop over all of the basic blocks (except the first one) and remove them
204 // if they are unneeded...
206 for (BBIt = BBs.begin(), ++BBIt; BBIt != BBs.end(); ++BBIt) {
207 BasicBlock *BB = *BBIt;
208 assert(BB->getTerminator() && "Degenerate basic block encountered!");
211 // Remove basic blocks that have no predecessors... which are unreachable.
212 if (pred_begin(BB) == pred_end(BB) &&
213 !BB->hasConstantPoolReferences() && 0) {
214 cerr << "Removing BB: \n" << BB;
216 // Loop through all of our successors and make sure they know that one
217 // of their predecessors is going away.
218 for (succ_iterator SI = succ_begin(BB), EI = succ_end(BB); SI != EI; ++SI)
219 RemovePredecessorFromBlock(*SI, BB);
221 while (!BB->getInstList().empty()) {
222 Instruction *I = BB->getInstList().front();
223 // If this instruction is used, replace uses with an arbitrary
224 // constant value. Because control flow can't get here, we don't care
225 // what we replace the value with.
226 if (!I->use_empty()) ReplaceUsesWithConstant(I);
228 // Remove the instruction from the basic block
229 delete BB->getInstList().remove(BB->getInstList().begin());
231 delete BBs.remove(BBIt);
232 --BBIt; // remove puts use on the next block, we want the previous one
237 // Check to see if this block has no instructions and only a single
238 // successor. If so, replace block references with successor.
239 succ_iterator SI(succ_begin(BB));
240 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
241 Instruction *I = BB->getInstList().front();
242 if (I->isTerminator()) { // Terminator is the only instruction!
244 if (Succ->getInstList().front()->getInstType() == Instruction::PHINode){
245 // Add entries to the PHI nodes so that the PHI nodes have the right
246 // number of entries...
247 PropogatePredecessorsForPHIs(BB, Succ);
250 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
251 BB->replaceAllUsesWith(Succ);
252 cerr << "Killing Trivial BB: \n" << BB;
254 BB = BBs.remove(BBIt);
255 --BBIt; // remove puts use on the next block, we want the previous one
257 if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
258 Succ->setName(BB->getName());
259 delete BB; // Delete basic block
261 cerr << "Method after removal: \n" << M;
268 // Merge basic blocks into their predecessor if there is only one pred,
269 // and if there is only one successor of the predecessor.
270 pred_iterator PI(pred_begin(BB));
271 if (PI != pred_end(BB) && *PI != BB && // Not empty? Not same BB?
272 ++PI == pred_end(BB) && !BB->hasConstantPoolReferences()) {
273 BasicBlock *Pred = *pred_begin(BB);
274 TerminatorInst *Term = Pred->getTerminator();
275 assert(Term != 0 && "malformed basic block without terminator!");
277 // Does the predecessor block only have a single successor?
278 succ_iterator SI(succ_begin(Pred));
279 if (++SI == succ_end(Pred)) {
280 //cerr << "Merging: " << BB << "into: " << Pred;
282 // Delete the unconditianal branch from the predecessor...
283 BasicBlock::InstListType::iterator DI = Pred->getInstList().end();
284 assert(Pred->getTerminator() &&
285 "Degenerate basic block encountered!"); // Empty bb???
286 delete Pred->getInstList().remove(--DI); // Destroy uncond branch
288 // Move all definitions in the succecessor to the predecessor...
289 while (!BB->getInstList().empty()) {
290 DI = BB->getInstList().begin();
291 Instruction *Def = BB->getInstList().remove(DI); // Remove from front
292 Pred->getInstList().push_back(Def); // Add to end...
295 // Remove basic block from the method... and advance iterator to the
296 // next valid block...
297 BB = BBs.remove(BBIt);
298 --BBIt; // remove puts us on the NEXT bb. We want the prev BB
301 // Make all PHI nodes that refered to BB now refer to Pred as their
303 BB->replaceAllUsesWith(Pred);
305 // Inherit predecessors name if it exists...
306 if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
308 // You ARE the weakest link... goodbye
311 WriteToVCG(M, "MergedInto");
316 // Remove unused constants
317 Changed |= DoRemoveUnusedConstants(M);
322 // It is possible that we may require multiple passes over the code to fully
323 // eliminate dead code. Iterate until we are done.
325 bool DoDeadCodeElimination(Method *M) {
326 bool Changed = false;
327 while (DoDCEPass(M)) Changed = true;
331 bool DoDeadCodeElimination(Module *C) {
332 bool Val = ApplyOptToAllMethods(C, DoDeadCodeElimination);
333 while (DoRemoveUnusedConstants(C)) Val = true;