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->front();
83 if (!I->isPHINode()) 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->begin());
99 } while (I->isPHINode());
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::iterator II(BB->begin());
158 for (; (*II)->isPHINode(); ++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(Succ->front()->isPHINode() && "Only works on PHId BBs!");
182 // If there is more than one predecessor, and there are PHI nodes in
183 // the successor, then we need to add incoming edges for the PHI nodes
185 const vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
187 BasicBlock::iterator I = Succ->begin();
188 do { // Loop over all of the PHI nodes in the successor BB
189 PHINode *PN = (PHINode*)*I;
190 Value *OldVal = PN->removeIncomingValue(BB);
191 assert(OldVal && "No entry in PHI for Pred BB!");
193 for (vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
194 End = BBPreds.end(); PredI != End; ++PredI) {
195 // Add an incoming value for each of the new incoming values...
196 PN->addIncoming(OldVal, *PredI);
200 } while ((*I)->isPHINode());
203 static bool DoDCEPass(Method *M) {
204 Method::iterator BBIt, BBEnd = M->end();
205 if (M->begin() == BBEnd) return false; // Nothing to do
206 bool Changed = false;
208 // Loop through now and remove instructions that have no uses...
209 for (BBIt = M->begin(); BBIt != BBEnd; ++BBIt) {
210 Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE());
211 Changed |= RemoveSingularPHIs(*BBIt);
214 // Loop over all of the basic blocks (except the first one) and remove them
215 // if they are unneeded...
217 for (BBIt = M->begin(), ++BBIt; BBIt != M->end(); ++BBIt) {
218 BasicBlock *BB = *BBIt;
219 assert(BB->getTerminator() && "Degenerate basic block encountered!");
221 #if 0 // This is know to basically work?
222 // Remove basic blocks that have no predecessors... which are unreachable.
223 if (pred_begin(BB) == pred_end(BB) &&
224 !BB->hasConstantPoolReferences() && 0) {
225 cerr << "Removing BB: \n" << BB;
227 // Loop through all of our successors and make sure they know that one
228 // of their predecessors is going away.
229 for_each(succ_begin(BB), succ_end(BB),
230 bind_2nd(RemovePredecessorFromBlock, BB));
232 while (!BB->empty()) {
233 Instruction *I = BB->front();
234 // If this instruction is used, replace uses with an arbitrary
235 // constant value. Because control flow can't get here, we don't care
236 // what we replace the value with.
237 if (!I->use_empty()) ReplaceUsesWithConstant(I);
239 // Remove the instruction from the basic block
240 delete BB->getInstList().remove(BB->begin());
242 delete M->getBasicBlocks().remove(BBIt);
243 --BBIt; // remove puts use on the next block, we want the previous one
249 #if 0 // This has problems
250 // Check to see if this block has no instructions and only a single
251 // successor. If so, replace block references with successor.
252 succ_iterator SI(succ_begin(BB));
253 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
254 Instruction *I = BB->front();
255 if (I->isTerminator()) { // Terminator is the only instruction!
256 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
257 cerr << "Killing Trivial BB: \n" << BB;
259 if (Succ->front()->isPHINode()) {
260 // If our successor has PHI nodes, then we need to update them to
261 // include entries for BB's predecessors, not for BB itself.
263 PropogatePredecessorsForPHIs(BB, Succ);
266 BB->replaceAllUsesWith(Succ);
268 BB = M->getBasicBlocks().remove(BBIt);
269 --BBIt; // remove puts use on the next block, we want the previous one
271 if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
272 Succ->setName(BB->getName());
273 delete BB; // Delete basic block
275 cerr << "Method after removal: \n" << M;
282 // Merge basic blocks into their predecessor if there is only one pred,
283 // and if there is only one successor of the predecessor.
284 pred_iterator PI(pred_begin(BB));
285 if (PI != pred_end(BB) && *PI != BB && // Not empty? Not same BB?
286 ++PI == pred_end(BB) && !BB->hasConstantPoolReferences()) {
287 BasicBlock *Pred = *pred_begin(BB);
288 TerminatorInst *Term = Pred->getTerminator();
289 assert(Term != 0 && "malformed basic block without terminator!");
291 // Does the predecessor block only have a single successor?
292 succ_iterator SI(succ_begin(Pred));
293 if (++SI == succ_end(Pred)) {
294 //cerr << "Merging: " << BB << "into: " << Pred;
296 // Delete the unconditianal branch from the predecessor...
297 BasicBlock::iterator DI = Pred->end();
298 assert(Pred->getTerminator() &&
299 "Degenerate basic block encountered!"); // Empty bb???
300 delete Pred->getInstList().remove(--DI); // Destroy uncond branch
302 // Move all definitions in the succecessor to the predecessor...
303 while (!BB->empty()) {
305 Instruction *Def = BB->getInstList().remove(DI); // Remove from front
306 Pred->getInstList().push_back(Def); // Add to end...
309 // Remove basic block from the method... and advance iterator to the
310 // next valid block...
311 BB = M->getBasicBlocks().remove(BBIt);
312 --BBIt; // remove puts us on the NEXT bb. We want the prev BB
315 // Make all PHI nodes that refered to BB now refer to Pred as their
317 BB->replaceAllUsesWith(Pred);
319 // Inherit predecessors name if it exists...
320 if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
322 // You ARE the weakest link... goodbye
325 //WriteToVCG(M, "MergedInto");
330 // Remove unused constants
331 Changed |= DoRemoveUnusedConstants(M);
336 // It is possible that we may require multiple passes over the code to fully
337 // eliminate dead code. Iterate until we are done.
339 bool DoDeadCodeElimination(Method *M) {
340 bool Changed = false;
341 while (DoDCEPass(M)) Changed = true;
345 bool DoDeadCodeElimination(Module *C) {
346 bool Val = ApplyOptToAllMethods(C, DoDeadCodeElimination);
347 while (DoRemoveUnusedConstants(C)) Val = true;