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 //===----------------------------------------------------------------------===//
22 #include "llvm/Module.h"
23 #include "llvm/Method.h"
24 #include "llvm/BasicBlock.h"
25 #include "llvm/iTerminators.h"
26 #include "llvm/iOther.h"
27 #include "llvm/Opt/AllOpts.h"
28 #include "llvm/Assembly/Writer.h"
32 static bool isDCEable(const Value *) { return true; }
35 struct BasicBlockDCE {
37 static bool isDCEable(const Instruction *I) {
38 return !I->hasSideEffects();
43 template<class ValueSubclass, class ItemParentType, class DCEController>
44 static bool RemoveUnusedDefs(ValueHolder<ValueSubclass, ItemParentType> &Vals,
45 DCEController DCEControl) {
47 typedef ValueHolder<ValueSubclass, ItemParentType> Container;
49 int Offset = DCEController::EndOffs;
50 for (Container::iterator DI = Vals.begin(); DI != Vals.end()-Offset; ) {
51 // Look for un"used" definitions...
52 if ((*DI)->use_empty() && DCEController::isDCEable(*DI)) {
54 //cerr << "Removing: " << *DI;
55 delete Vals.remove(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->getInstList().front();
77 if (I->getInstType() != Instruction::PHINode) 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 = (PHINode*)I;
86 assert(PN->getOperand(1) == 0 && "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->getInstList().begin());
92 I = BB->getInstList().front();
93 } while (I->getInstType() == Instruction::PHINode);
95 return true; // Yes, we nuked at least one phi node
98 bool DoRemoveUnusedConstants(SymTabValue *S) {
100 ConstantPool &CP = S->getConstantPool();
101 for (ConstantPool::plane_iterator PI = CP.begin(); PI != CP.end(); ++PI)
102 Changed |= RemoveUnusedDefs(**PI, ConstPoolDCE());
106 static void ReplaceUsesWithConstant(Instruction *I) {
107 // Get the method level constant pool
108 ConstantPool &CP = I->getParent()->getParent()->getConstantPool();
110 ConstPoolVal *CPV = 0;
111 ConstantPool::PlaneType *P;
112 if (!CP.getPlane(I->getType(), P)) { // Does plane exist?
114 if (!P->empty()) CPV = P->front();
117 if (CPV == 0) { // We don't have an existing constant to reuse. Just add one.
118 CPV = ConstPoolVal::getNullConstant(I->getType()); // Create a new constant
120 // Add the new value to the constant pool...
124 // Make all users of this instruction reference the constant instead
125 I->replaceAllUsesWith(CPV);
128 // RemovePredecessorFromBlock - This function is called when we are about
129 // to remove a predecessor from a basic block. This function takes care of
130 // removing the predecessor from the PHI nodes in BB so that after the pred
131 // is removed, the number of PHI slots per bb is equal to the number of
134 static void RemovePredecessorFromBlock(BasicBlock *BB, BasicBlock *Pred) {
135 BasicBlock::pred_iterator PI(BB->pred_begin()), EI(BB->pred_end());
136 unsigned pred_idx = 0, max_idx;
138 cerr << "RPFB: " << Pred << "From Block: " << BB;
140 // Find out what index the predecessor is...
141 for (; *PI != BB; ++PI, ++pred_idx) {
142 assert(PI != EI && "Pred is not a predecessor of BB!");
145 // Loop over the rest of the predecssors until we run out, or until we find
146 // out that there are more than 2 predecessors.
147 for (max_idx = pred_idx; PI != EI && max_idx < 2; ++PI, ++max_idx) /*empty*/;
149 // If there are exactly two predecessors, then we want to nuke the PHI nodes
151 bool NukePHIs = max_idx == 1;
153 // Okay, now we know that we need to remove predecessor #pred_idx from all
154 // PHI nodes. Iterate over each PHI node fixing them up
155 BasicBlock::InstListType::iterator II(BB->getInstList().begin());
156 for (; (*II)->getInstType() == Instruction::PHINode; ++II) {
157 PHINode *PN = (PHINode*)*II;
158 PN->removeIncomingValue(pred_idx);
160 if (NukePHIs) { // Destroy the PHI altogether??
161 assert(PN->getOperand(1) == 0 && "PHI node should only have one value!");
162 Value *V = PN->getOperand(0);
164 PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
165 delete BB->getInstList().remove(II);
170 // PropogatePredecessors - This gets "Succ" ready to have the predecessors from
171 // "BB". This is a little tricky because "Succ" has PHI nodes, which need to
172 // have extra slots added to them to hold the merge edges from BB's
175 // Assumption: BB is the single predecessor of Succ.
177 static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
178 assert(BB && Succ && *Succ->pred_begin() == BB && "BB is only pred of Succ" &&
179 ++Succ->pred_begin() == Succ->pred_end());
181 // If there is more than one predecessor, and there are PHI nodes in
182 // the successor, then we need to add incoming edges for the PHI nodes
183 BasicBlock::pred_iterator PI(BB->pred_begin());
184 for (; PI != BB->pred_end(); ++PI) {
189 static bool DoDCEPass(Method *M) {
190 Method::BasicBlocksType &BBs = M->getBasicBlocks();
191 Method::BasicBlocksType::iterator BBIt, BBEnd = BBs.end();
192 if (BBs.begin() == BBEnd) return false; // Nothing to do
193 bool Changed = false;
195 // Loop through now and remove instructions that have no uses...
196 for (BBIt = BBs.begin(); BBIt != BBEnd; BBIt++) {
197 Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE());
198 Changed |= RemoveSingularPHIs(*BBIt);
201 // Loop over all of the basic blocks (except the first one) and remove them
202 // if they are unneeded...
204 for (BBIt = BBs.begin(), ++BBIt; BBIt != BBs.end(); ++BBIt) {
205 BasicBlock *BB = *BBIt;
206 assert(BB->getTerminator() && "Degenerate basic block encountered!");
209 // Remove basic blocks that have no predecessors... which are unreachable.
210 if (BB->pred_begin() == BB->pred_end() &&
211 !BB->hasConstantPoolReferences() && 0) {
212 cerr << "Removing BB: \n" << BB;
214 // Loop through all of our successors and make sure they know that one
215 // of their predecessors is going away.
216 for (BasicBlock::succ_iterator SI = BB->succ_begin(), EI = BB->succ_end();
218 RemovePredecessorFromBlock(*SI, BB);
220 while (!BB->getInstList().empty()) {
221 Instruction *I = BB->getInstList().front();
222 // If this instruction is used, replace uses with an arbitrary
223 // constant value. Because control flow can't get here, we don't care
224 // what we replace the value with.
225 if (!I->use_empty()) ReplaceUsesWithConstant(I);
227 // Remove the instruction from the basic block
228 delete BB->getInstList().remove(BB->getInstList().begin());
230 delete BBs.remove(BBIt);
231 --BBIt; // remove puts use on the next block, we want the previous one
236 // Check to see if this block has no instructions and only a single
237 // successor. If so, replace block references with successor.
238 BasicBlock::succ_iterator SI(BB->succ_begin());
239 if (SI != BB->succ_end() && ++SI == BB->succ_end()) { // One succ?
240 Instruction *I = BB->getInstList().front();
241 if (I->isTerminator()) { // Terminator is the only instruction!
243 if (Succ->getInstList().front()->getInstType() == Instruction::PHINode){
244 // Add entries to the PHI nodes so that the PHI nodes have the right
245 // number of entries...
246 PropogatePredecessorsForPHIs(BB, Succ);
249 BasicBlock *Succ = *BB->succ_begin(); // There is exactly one successor
250 BB->replaceAllUsesWith(Succ);
251 cerr << "Killing Trivial BB: \n" << BB;
253 BB = BBs.remove(BBIt);
254 --BBIt; // remove puts use on the next block, we want the previous one
256 if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
257 Succ->setName(BB->getName());
258 delete BB; // Delete basic block
260 cerr << "Method after removal: \n" << M;
267 // Merge basic blocks into their predecessor if there is only one pred,
268 // and if there is only one successor of the predecessor.
269 BasicBlock::pred_iterator PI(BB->pred_begin());
270 if (PI != BB->pred_end() && *PI != BB && // Not empty? Not same BB?
271 ++PI == BB->pred_end() && !BB->hasConstantPoolReferences()) {
272 BasicBlock *Pred = *BB->pred_begin();
273 TerminatorInst *Term = Pred->getTerminator();
274 assert(Term != 0 && "malformed basic block without terminator!");
276 // Does the predecessor block only have a single successor?
277 BasicBlock::succ_iterator SI(Pred->succ_begin());
278 if (++SI == Pred->succ_end()) {
279 //cerr << "Merging: " << BB << "into: " << Pred;
281 // Delete the unconditianal branch from the predecessor...
282 BasicBlock::InstListType::iterator DI = Pred->getInstList().end();
283 assert(Pred->getTerminator() &&
284 "Degenerate basic block encountered!"); // Empty bb???
285 delete Pred->getInstList().remove(--DI);
287 // Move all definitions in the succecessor to the predecessor...
288 while (!BB->getInstList().empty()) {
289 DI = BB->getInstList().begin();
290 Instruction *Def = BB->getInstList().remove(DI); // Remove from front
291 Pred->getInstList().push_back(Def); // Add to end...
294 // Remove basic block from the method... and advance iterator to the
295 // next valid block...
296 BB = BBs.remove(BBIt);
297 --BBIt; // remove puts us on the NEXT bb. We want the prev BB
300 // Inherit predecessors name if it exists...
301 if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
303 // You ARE the weakest link... goodbye
309 // Remove unused constants
310 Changed |= DoRemoveUnusedConstants(M);
315 // It is possible that we may require multiple passes over the code to fully
316 // eliminate dead code. Iterate until we are done.
318 bool DoDeadCodeElimination(Method *M) {
319 bool Changed = false;
320 while (DoDCEPass(M)) Changed = true;
324 bool DoDeadCodeElimination(Module *C) {
325 bool Val = ApplyOptToAllMethods(C, DoDeadCodeElimination);
326 while (DoRemoveUnusedConstants(C)) Val = true;