1 //===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
3 // Peephole optimize the CFG.
5 //===----------------------------------------------------------------------===//
7 #include "llvm/Transforms/Utils/Local.h"
8 #include "llvm/Constant.h"
9 #include "llvm/Intrinsics.h"
10 #include "llvm/iPHINode.h"
11 #include "llvm/iTerminators.h"
12 #include "llvm/iOther.h"
13 #include "llvm/Support/CFG.h"
17 // PropagatePredecessors - This gets "Succ" ready to have the predecessors from
18 // "BB". This is a little tricky because "Succ" has PHI nodes, which need to
19 // have extra slots added to them to hold the merge edges from BB's
20 // predecessors, and BB itself might have had PHI nodes in it. This function
21 // returns true (failure) if the Succ BB already has a predecessor that is a
22 // predecessor of BB and incoming PHI arguments would not be discernable.
24 // Assumption: Succ is the single successor for BB.
26 static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
27 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
29 if (!isa<PHINode>(Succ->front()))
30 return false; // We can make the transformation, no problem.
32 // If there is more than one predecessor, and there are PHI nodes in
33 // the successor, then we need to add incoming edges for the PHI nodes
35 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
37 // Check to see if one of the predecessors of BB is already a predecessor of
38 // Succ. If so, we cannot do the transformation if there are any PHI nodes
39 // with incompatible values coming in from the two edges!
41 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI)
42 if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
43 // Loop over all of the PHI nodes checking to see if there are
44 // incompatible values coming in.
45 for (BasicBlock::iterator I = Succ->begin();
46 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
47 // Loop up the entries in the PHI node for BB and for *PI if the values
48 // coming in are non-equal, we cannot merge these two blocks (instead we
49 // should insert a conditional move or something, then merge the
51 int Idx1 = PN->getBasicBlockIndex(BB);
52 int Idx2 = PN->getBasicBlockIndex(*PI);
53 assert(Idx1 != -1 && Idx2 != -1 &&
54 "Didn't have entries for my predecessors??");
55 if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2))
56 return true; // Values are not equal...
60 // Loop over all of the PHI nodes in the successor BB
61 for (BasicBlock::iterator I = Succ->begin();
62 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
63 Value *OldVal = PN->removeIncomingValue(BB, false);
64 assert(OldVal && "No entry in PHI for Pred BB!");
66 // If this incoming value is one of the PHI nodes in BB...
67 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
68 PHINode *OldValPN = cast<PHINode>(OldVal);
69 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
70 End = BBPreds.end(); PredI != End; ++PredI) {
71 PN->addIncoming(OldValPN->getIncomingValueForBlock(*PredI), *PredI);
74 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
75 End = BBPreds.end(); PredI != End; ++PredI) {
76 // Add an incoming value for each of the new incoming values...
77 PN->addIncoming(OldVal, *PredI);
85 // SimplifyCFG - This function is used to do simplification of a CFG. For
86 // example, it adjusts branches to branches to eliminate the extra hop, it
87 // eliminates unreachable basic blocks, and does other "peephole" optimization
88 // of the CFG. It returns true if a modification was made.
90 // WARNING: The entry node of a function may not be simplified.
92 bool SimplifyCFG(BasicBlock *BB) {
94 Function *M = BB->getParent();
96 assert(BB && BB->getParent() && "Block not embedded in function!");
97 assert(BB->getTerminator() && "Degenerate basic block encountered!");
98 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
100 // Check to see if the first instruction in this block is just an
101 // 'llvm.unwind'. If so, replace any invoke instructions which use this as an
102 // exception destination with call instructions.
104 if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->getTerminator()))
105 if (BB->begin() == BasicBlock::iterator(UI)) { // Empty block?
106 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
107 while (!Preds.empty()) {
108 BasicBlock *Pred = Preds.back();
109 if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
110 if (II->getExceptionalDest() == BB) {
111 // Insert a new branch instruction before the invoke, because this
112 // is now a fall through...
113 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
114 Pred->getInstList().remove(II); // Take out of symbol table
116 // Insert the call now...
117 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
118 CallInst *CI = new CallInst(II->getCalledValue(), Args,
120 // If the invoke produced a value, the Call now does instead
121 II->replaceAllUsesWith(CI);
130 // Remove basic blocks that have no predecessors... which are unreachable.
131 if (pred_begin(BB) == pred_end(BB) &&
132 !BB->hasConstantReferences()) {
133 //cerr << "Removing BB: \n" << BB;
135 // Loop through all of our successors and make sure they know that one
136 // of their predecessors is going away.
137 for_each(succ_begin(BB), succ_end(BB),
138 std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
140 while (!BB->empty()) {
141 Instruction &I = BB->back();
142 // If this instruction is used, replace uses with an arbitrary
143 // constant value. Because control flow can't get here, we don't care
144 // what we replace the value with. Note that since this block is
145 // unreachable, and all values contained within it must dominate their
146 // uses, that all uses will eventually be removed.
148 // Make all users of this instruction reference the constant instead
149 I.replaceAllUsesWith(Constant::getNullValue(I.getType()));
151 // Remove the instruction from the basic block
152 BB->getInstList().pop_back();
154 M->getBasicBlockList().erase(BB);
158 // Check to see if we can constant propagate this terminator instruction
160 Changed |= ConstantFoldTerminator(BB);
162 // Check to see if this block has no non-phi instructions and only a single
163 // successor. If so, replace references to this basic block with references
165 succ_iterator SI(succ_begin(BB));
166 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
168 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
169 while (isa<PHINode>(*BBI)) ++BBI;
171 if (BBI->isTerminator()) { // Terminator is the only non-phi instruction!
172 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
174 if (Succ != BB) { // Arg, don't hurt infinite loops!
175 // If our successor has PHI nodes, then we need to update them to
176 // include entries for BB's predecessors, not for BB itself.
177 // Be careful though, if this transformation fails (returns true) then
178 // we cannot do this transformation!
180 if (!PropagatePredecessorsForPHIs(BB, Succ)) {
181 //cerr << "Killing Trivial BB: \n" << BB;
182 std::string OldName = BB->getName();
184 std::vector<BasicBlock*>
185 OldSuccPreds(pred_begin(Succ), pred_end(Succ));
187 // Move all PHI nodes in BB to Succ if they are alive, otherwise
189 while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
191 BB->getInstList().erase(BB->begin()); // Nuke instruction...
193 // The instruction is alive, so this means that Succ must have
194 // *ONLY* had BB as a predecessor, and the PHI node is still valid
195 // now. Simply move it into Succ, because we know that BB
196 // strictly dominated Succ.
197 BB->getInstList().remove(BB->begin());
198 Succ->getInstList().push_front(PN);
200 // We need to add new entries for the PHI node to account for
201 // predecessors of Succ that the PHI node does not take into
202 // account. At this point, since we know that BB dominated succ,
203 // this means that we should any newly added incoming edges should
204 // use the PHI node as the value for these edges, because they are
207 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
208 if (OldSuccPreds[i] != BB)
209 PN->addIncoming(PN, OldSuccPreds[i]);
212 // Everything that jumped to BB now goes to Succ...
213 BB->replaceAllUsesWith(Succ);
215 // Delete the old basic block...
216 M->getBasicBlockList().erase(BB);
218 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
219 Succ->setName(OldName);
221 //cerr << "Function after removal: \n" << M;
228 // Merge basic blocks into their predecessor if there is only one distinct
229 // pred, and if there is only one distinct successor of the predecessor, and
230 // if there are no PHI nodes.
232 if (!BB->hasConstantReferences()) {
233 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
234 BasicBlock *OnlyPred = *PI++;
235 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
236 if (*PI != OnlyPred) {
237 OnlyPred = 0; // There are multiple different predecessors...
241 BasicBlock *OnlySucc = 0;
242 if (OnlyPred && OnlyPred != BB && // Don't break self loops
243 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
244 // Check to see if there is only one distinct successor...
245 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
247 for (; SI != SE; ++SI)
248 if (*SI != OnlySucc) {
249 OnlySucc = 0; // There are multiple distinct successors!
255 //cerr << "Merging: " << BB << "into: " << OnlyPred;
256 TerminatorInst *Term = OnlyPred->getTerminator();
258 // Resolve any PHI nodes at the start of the block. They are all
259 // guaranteed to have exactly one entry if they exist, unless there are
260 // multiple duplicate (but guaranteed to be equal) entries for the
261 // incoming edges. This occurs when there are multiple edges from
262 // OnlyPred to OnlySucc.
264 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
265 PN->replaceAllUsesWith(PN->getIncomingValue(0));
266 BB->getInstList().pop_front(); // Delete the phi node...
269 // Delete the unconditional branch from the predecessor...
270 OnlyPred->getInstList().pop_back();
272 // Move all definitions in the succecessor to the predecessor...
273 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
275 // Make all PHI nodes that refered to BB now refer to Pred as their
277 BB->replaceAllUsesWith(OnlyPred);
279 std::string OldName = BB->getName();
281 // Erase basic block from the function...
282 M->getBasicBlockList().erase(BB);
284 // Inherit predecessors name if it exists...
285 if (!OldName.empty() && !OnlyPred->hasName())
286 OnlyPred->setName(OldName);