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
12 // * Eliminates method prototypes that are not referenced
14 // TODO: This should REALLY be worklist driven instead of iterative. Right now,
15 // we scan linearly through values, removing unused ones as we go. The problem
16 // is that this may cause other earlier values to become unused. To make sure
17 // that we get them all, we iterate until things stop changing. Instead, when
18 // removing a value, recheck all of its operands to see if they are now unused.
19 // Piece of cake, and more efficient as well.
21 // Note, this is not trivial, because we have to worry about invalidating
24 //===----------------------------------------------------------------------===//
26 #include "llvm/Optimizations/DCE.h"
27 #include "llvm/Support/STLExtras.h"
28 #include "llvm/Module.h"
29 #include "llvm/Method.h"
30 #include "llvm/BasicBlock.h"
31 #include "llvm/iTerminators.h"
32 #include "llvm/iOther.h"
33 #include "llvm/Assembly/Writer.h"
41 static bool isDCEable(const ConstPoolVal *CPV) {
42 // TODO: The bytecode writer requires that all used types are in the
43 // constant pool for the current method. This is messy and is really
45 return CPV->getType() != Type::TypeTy; // Don't DCE Type plane constants!
49 struct BasicBlockDCE {
51 static bool isDCEable(const Instruction *I) {
52 return !I->hasSideEffects();
57 template<class Container, class DCEController>
58 static bool RemoveUnusedDefs(Container &Vals, DCEController DCEControl) {
60 int Offset = DCEController::EndOffs;
62 for (typename Container::iterator DI = Vals.begin();
63 DI != Vals.end()-Offset; ) {
64 // Look for un"used" definitions...
65 if ((*DI)->use_empty() && DCEController::isDCEable(*DI)) {
67 //cerr << "Removing: " << *DI;
68 delete Vals.remove(DI);
77 // RemoveSingularPHIs - This removes PHI nodes from basic blocks that have only
78 // a single predecessor. This means that the PHI node must only have a single
79 // RHS value and can be eliminated.
81 // This routine is very simple because we know that PHI nodes must be the first
82 // things in a basic block, if they are present.
84 static bool RemoveSingularPHIs(BasicBlock *BB) {
85 pred_iterator PI(pred_begin(BB));
86 if (PI == pred_end(BB) || ++PI != pred_end(BB))
87 return false; // More than one predecessor...
89 Instruction *I = BB->front();
90 if (!I->isPHINode()) return false; // No PHI nodes
92 //cerr << "Killing PHIs from " << BB;
93 //cerr << "Pred #0 = " << *pred_begin(BB);
95 //cerr << "Method == " << BB->getParent();
98 PHINode *PN = (PHINode*)I;
99 assert(PN->getNumOperands() == 2 && "PHI node should only have one value!");
100 Value *V = PN->getOperand(0);
102 PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
103 delete BB->getInstList().remove(BB->begin());
106 } while (I->isPHINode());
108 return true; // Yes, we nuked at least one phi node
111 bool opt::DoRemoveUnusedConstants(SymTabValue *S) {
112 bool Changed = false;
113 ConstantPool &CP = S->getConstantPool();
114 for (ConstantPool::plane_iterator PI = CP.begin(); PI != CP.end(); ++PI)
115 Changed |= RemoveUnusedDefs(**PI, ConstPoolDCE());
119 static void ReplaceUsesWithConstant(Instruction *I) {
120 // Get the method level constant pool
121 ConstantPool &CP = I->getParent()->getParent()->getConstantPool();
123 ConstPoolVal *CPV = 0;
124 ConstantPool::PlaneType *P;
125 if (!CP.getPlane(I->getType(), P)) { // Does plane exist?
127 if (!P->empty()) CPV = P->front();
130 if (CPV == 0) { // We don't have an existing constant to reuse. Just add one.
131 CPV = ConstPoolVal::getNullConstant(I->getType()); // Create a new constant
133 // Add the new value to the constant pool...
137 // Make all users of this instruction reference the constant instead
138 I->replaceAllUsesWith(CPV);
141 // PropogatePredecessors - This gets "Succ" ready to have the predecessors from
142 // "BB". This is a little tricky because "Succ" has PHI nodes, which need to
143 // have extra slots added to them to hold the merge edges from BB's
146 // Assumption: BB is the single predecessor of Succ.
148 static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
149 assert(Succ->front()->isPHINode() && "Only works on PHId BBs!");
151 // If there is more than one predecessor, and there are PHI nodes in
152 // the successor, then we need to add incoming edges for the PHI nodes
154 const vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
156 BasicBlock::iterator I = Succ->begin();
157 do { // Loop over all of the PHI nodes in the successor BB
158 PHINode *PN = (PHINode*)*I;
159 Value *OldVal = PN->removeIncomingValue(BB);
160 assert(OldVal && "No entry in PHI for Pred BB!");
162 for (vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
163 End = BBPreds.end(); PredI != End; ++PredI) {
164 // Add an incoming value for each of the new incoming values...
165 PN->addIncoming(OldVal, *PredI);
169 } while ((*I)->isPHINode());
173 // SimplifyCFG - This function is used to do simplification of a CFG. For
174 // example, it adjusts branches to branches to eliminate the extra hop, it
175 // eliminates unreachable basic blocks, and does other "peephole" optimization
176 // of the CFG. It returns true if a modification was made, and returns an
177 // iterator that designates the first element remaining after the block that
180 // WARNING: The entry node of a method may not be simplified.
182 bool opt::SimplifyCFG(Method::iterator &BBIt) {
183 assert(*BBIt && (*BBIt)->getParent() && "Block not embedded in method!");
184 BasicBlock *BB = *BBIt;
185 Method *M = BB->getParent();
186 assert(BB->getTerminator() && "Degenerate basic block encountered!");
187 assert(BB->getParent()->front() != BB && "Can't Simplify entry block!");
189 // Remove basic blocks that have no predecessors... which are unreachable.
190 if (pred_begin(BB) == pred_end(BB) &&
191 !BB->hasConstantPoolReferences()) {
192 //cerr << "Removing BB: \n" << BB;
194 // Loop through all of our successors and make sure they know that one
195 // of their predecessors is going away.
196 for_each(succ_begin(BB), succ_end(BB),
197 std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
199 while (!BB->empty()) {
200 Instruction *I = BB->back();
201 // If this instruction is used, replace uses with an arbitrary
202 // constant value. Because control flow can't get here, we don't care
203 // what we replace the value with. Note that since this block is
204 // unreachable, and all values contained within it must dominate their
205 // uses, that all uses will eventually be removed.
206 if (!I->use_empty()) ReplaceUsesWithConstant(I);
208 // Remove the instruction from the basic block
209 delete BB->getInstList().pop_back();
211 delete M->getBasicBlocks().remove(BBIt);
215 // Check to see if this block has no instructions and only a single
216 // successor. If so, replace block references with successor.
217 succ_iterator SI(succ_begin(BB));
218 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
219 Instruction *I = BB->front();
220 if (I->isTerminator()) { // Terminator is the only instruction!
221 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
222 //cerr << "Killing Trivial BB: \n" << BB;
224 if (Succ != BB) { // Arg, don't hurt infinite loops!
225 if (Succ->front()->isPHINode()) {
226 // If our successor has PHI nodes, then we need to update them to
227 // include entries for BB's predecessors, not for BB itself.
229 PropogatePredecessorsForPHIs(BB, Succ);
232 BB->replaceAllUsesWith(Succ);
233 BB = M->getBasicBlocks().remove(BBIt);
235 if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
236 Succ->setName(BB->getName());
237 delete BB; // Delete basic block
239 //cerr << "Method after removal: \n" << M;
245 // Merge basic blocks into their predecessor if there is only one pred,
246 // and if there is only one successor of the predecessor.
247 pred_iterator PI(pred_begin(BB));
248 if (PI != pred_end(BB) && *PI != BB && // Not empty? Not same BB?
249 ++PI == pred_end(BB) && !BB->hasConstantPoolReferences()) {
250 BasicBlock *Pred = *pred_begin(BB);
251 TerminatorInst *Term = Pred->getTerminator();
252 assert(Term != 0 && "malformed basic block without terminator!");
254 // Does the predecessor block only have a single successor?
255 succ_iterator SI(succ_begin(Pred));
256 if (++SI == succ_end(Pred)) {
257 //cerr << "Merging: " << BB << "into: " << Pred;
259 // Delete the unconditianal branch from the predecessor...
260 BasicBlock::iterator DI = Pred->end();
261 assert(Pred->getTerminator() &&
262 "Degenerate basic block encountered!"); // Empty bb???
263 delete Pred->getInstList().remove(--DI); // Destroy uncond branch
265 // Move all definitions in the succecessor to the predecessor...
266 while (!BB->empty()) {
268 Instruction *Def = BB->getInstList().remove(DI); // Remove from front
269 Pred->getInstList().push_back(Def); // Add to end...
272 // Remove basic block from the method... and advance iterator to the
273 // next valid block...
274 BB = M->getBasicBlocks().remove(BBIt);
276 // Make all PHI nodes that refered to BB now refer to Pred as their
278 BB->replaceAllUsesWith(Pred);
280 // Inherit predecessors name if it exists...
281 if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName());
283 delete BB; // You ARE the weakest link... goodbye
291 static bool DoDCEPass(Method *M) {
292 Method::iterator BBIt, BBEnd = M->end();
293 if (M->begin() == BBEnd) return false; // Nothing to do
294 bool Changed = false;
296 // Loop through now and remove instructions that have no uses...
297 for (BBIt = M->begin(); BBIt != BBEnd; ++BBIt) {
298 Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE());
299 Changed |= RemoveSingularPHIs(*BBIt);
302 // Loop over all of the basic blocks (except the first one) and remove them
303 // if they are unneeded...
305 for (BBIt = M->begin(), ++BBIt; BBIt != M->end(); ) {
306 if (opt::SimplifyCFG(BBIt)) {
313 // Remove unused constants
314 return Changed | opt::DoRemoveUnusedConstants(M);
318 // It is possible that we may require multiple passes over the code to fully
319 // eliminate dead code. Iterate until we are done.
321 bool opt::DoDeadCodeElimination(Method *M) {
322 bool Changed = false;
323 while (DoDCEPass(M)) Changed = true;
327 bool opt::DoDeadCodeElimination(Module *Mod) {
328 bool Changed = false;
330 for (Module::iterator MI = Mod->begin(); MI != Mod->end(); ) {
332 if (!Meth->isExternal()) { // DCE normal methods
333 Changed |= DoDeadCodeElimination(Meth);
334 ++MI; // Next method please
335 } else if (Meth->use_size() == 0) { // No references to prototype?
336 //cerr << "Removing method proto: " << Meth->getName() << endl;
337 delete Mod->getMethodList().remove(MI); // Remove prototype
338 // Remove moves iterator to point to the next one automatically
340 ++MI; // Skip prototype in use.
344 while (DoRemoveUnusedConstants(Mod)) Changed = true;