1 //===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // Peephole optimize the CFG.
12 //===----------------------------------------------------------------------===//
14 #include "llvm/Transforms/Utils/Local.h"
15 #include "llvm/Constant.h"
16 #include "llvm/Intrinsics.h"
17 #include "llvm/iPHINode.h"
18 #include "llvm/iTerminators.h"
19 #include "llvm/iOther.h"
20 #include "llvm/Support/CFG.h"
26 // PropagatePredecessors - This gets "Succ" ready to have the predecessors from
27 // "BB". This is a little tricky because "Succ" has PHI nodes, which need to
28 // have extra slots added to them to hold the merge edges from BB's
29 // predecessors, and BB itself might have had PHI nodes in it. This function
30 // returns true (failure) if the Succ BB already has a predecessor that is a
31 // predecessor of BB and incoming PHI arguments would not be discernible.
33 // Assumption: Succ is the single successor for BB.
35 static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
36 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
38 if (!isa<PHINode>(Succ->front()))
39 return false; // We can make the transformation, no problem.
41 // If there is more than one predecessor, and there are PHI nodes in
42 // the successor, then we need to add incoming edges for the PHI nodes
44 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
46 // Check to see if one of the predecessors of BB is already a predecessor of
47 // Succ. If so, we cannot do the transformation if there are any PHI nodes
48 // with incompatible values coming in from the two edges!
50 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI)
51 if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
52 // Loop over all of the PHI nodes checking to see if there are
53 // incompatible values coming in.
54 for (BasicBlock::iterator I = Succ->begin();
55 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
56 // Loop up the entries in the PHI node for BB and for *PI if the values
57 // coming in are non-equal, we cannot merge these two blocks (instead we
58 // should insert a conditional move or something, then merge the
60 int Idx1 = PN->getBasicBlockIndex(BB);
61 int Idx2 = PN->getBasicBlockIndex(*PI);
62 assert(Idx1 != -1 && Idx2 != -1 &&
63 "Didn't have entries for my predecessors??");
64 if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2))
65 return true; // Values are not equal...
69 // Loop over all of the PHI nodes in the successor BB
70 for (BasicBlock::iterator I = Succ->begin();
71 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
72 Value *OldVal = PN->removeIncomingValue(BB, false);
73 assert(OldVal && "No entry in PHI for Pred BB!");
75 // If this incoming value is one of the PHI nodes in BB...
76 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
77 PHINode *OldValPN = cast<PHINode>(OldVal);
78 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
79 End = BBPreds.end(); PredI != End; ++PredI) {
80 PN->addIncoming(OldValPN->getIncomingValueForBlock(*PredI), *PredI);
83 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
84 End = BBPreds.end(); PredI != End; ++PredI) {
85 // Add an incoming value for each of the new incoming values...
86 PN->addIncoming(OldVal, *PredI);
94 // SimplifyCFG - This function is used to do simplification of a CFG. For
95 // example, it adjusts branches to branches to eliminate the extra hop, it
96 // eliminates unreachable basic blocks, and does other "peephole" optimization
97 // of the CFG. It returns true if a modification was made.
99 // WARNING: The entry node of a function may not be simplified.
101 bool SimplifyCFG(BasicBlock *BB) {
102 bool Changed = false;
103 Function *M = BB->getParent();
105 assert(BB && BB->getParent() && "Block not embedded in function!");
106 assert(BB->getTerminator() && "Degenerate basic block encountered!");
107 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
109 // Check to see if the first instruction in this block is just an
110 // 'llvm.unwind'. If so, replace any invoke instructions which use this as an
111 // exception destination with call instructions.
113 if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->getTerminator()))
114 if (BB->begin() == BasicBlock::iterator(UI)) { // Empty block?
115 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
116 while (!Preds.empty()) {
117 BasicBlock *Pred = Preds.back();
118 if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
119 if (II->getExceptionalDest() == BB) {
120 // Insert a new branch instruction before the invoke, because this
121 // is now a fall through...
122 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
123 Pred->getInstList().remove(II); // Take out of symbol table
125 // Insert the call now...
126 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
127 CallInst *CI = new CallInst(II->getCalledValue(), Args,
129 // If the invoke produced a value, the Call now does instead
130 II->replaceAllUsesWith(CI);
139 // Remove basic blocks that have no predecessors... which are unreachable.
140 if (pred_begin(BB) == pred_end(BB) &&
141 !BB->hasConstantReferences()) {
142 //cerr << "Removing BB: \n" << BB;
144 // Loop through all of our successors and make sure they know that one
145 // of their predecessors is going away.
146 for_each(succ_begin(BB), succ_end(BB),
147 std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
149 while (!BB->empty()) {
150 Instruction &I = BB->back();
151 // If this instruction is used, replace uses with an arbitrary
152 // constant value. Because control flow can't get here, we don't care
153 // what we replace the value with. Note that since this block is
154 // unreachable, and all values contained within it must dominate their
155 // uses, that all uses will eventually be removed.
157 // Make all users of this instruction reference the constant instead
158 I.replaceAllUsesWith(Constant::getNullValue(I.getType()));
160 // Remove the instruction from the basic block
161 BB->getInstList().pop_back();
163 M->getBasicBlockList().erase(BB);
167 // Check to see if we can constant propagate this terminator instruction
169 Changed |= ConstantFoldTerminator(BB);
171 // Check to see if this block has no non-phi instructions and only a single
172 // successor. If so, replace references to this basic block with references
174 succ_iterator SI(succ_begin(BB));
175 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
177 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
178 while (isa<PHINode>(*BBI)) ++BBI;
180 if (BBI->isTerminator()) { // Terminator is the only non-phi instruction!
181 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
183 if (Succ != BB) { // Arg, don't hurt infinite loops!
184 // If our successor has PHI nodes, then we need to update them to
185 // include entries for BB's predecessors, not for BB itself.
186 // Be careful though, if this transformation fails (returns true) then
187 // we cannot do this transformation!
189 if (!PropagatePredecessorsForPHIs(BB, Succ)) {
190 //cerr << "Killing Trivial BB: \n" << BB;
191 std::string OldName = BB->getName();
193 std::vector<BasicBlock*>
194 OldSuccPreds(pred_begin(Succ), pred_end(Succ));
196 // Move all PHI nodes in BB to Succ if they are alive, otherwise
198 while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
200 BB->getInstList().erase(BB->begin()); // Nuke instruction...
202 // The instruction is alive, so this means that Succ must have
203 // *ONLY* had BB as a predecessor, and the PHI node is still valid
204 // now. Simply move it into Succ, because we know that BB
205 // strictly dominated Succ.
206 BB->getInstList().remove(BB->begin());
207 Succ->getInstList().push_front(PN);
209 // We need to add new entries for the PHI node to account for
210 // predecessors of Succ that the PHI node does not take into
211 // account. At this point, since we know that BB dominated succ,
212 // this means that we should any newly added incoming edges should
213 // use the PHI node as the value for these edges, because they are
216 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
217 if (OldSuccPreds[i] != BB)
218 PN->addIncoming(PN, OldSuccPreds[i]);
221 // Everything that jumped to BB now goes to Succ...
222 BB->replaceAllUsesWith(Succ);
224 // Delete the old basic block...
225 M->getBasicBlockList().erase(BB);
227 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
228 Succ->setName(OldName);
230 //cerr << "Function after removal: \n" << M;
237 // Merge basic blocks into their predecessor if there is only one distinct
238 // pred, and if there is only one distinct successor of the predecessor, and
239 // if there are no PHI nodes.
241 if (!BB->hasConstantReferences()) {
242 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
243 BasicBlock *OnlyPred = *PI++;
244 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
245 if (*PI != OnlyPred) {
246 OnlyPred = 0; // There are multiple different predecessors...
250 BasicBlock *OnlySucc = 0;
251 if (OnlyPred && OnlyPred != BB && // Don't break self loops
252 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
253 // Check to see if there is only one distinct successor...
254 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
256 for (; SI != SE; ++SI)
257 if (*SI != OnlySucc) {
258 OnlySucc = 0; // There are multiple distinct successors!
264 //cerr << "Merging: " << BB << "into: " << OnlyPred;
265 TerminatorInst *Term = OnlyPred->getTerminator();
267 // Resolve any PHI nodes at the start of the block. They are all
268 // guaranteed to have exactly one entry if they exist, unless there are
269 // multiple duplicate (but guaranteed to be equal) entries for the
270 // incoming edges. This occurs when there are multiple edges from
271 // OnlyPred to OnlySucc.
273 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
274 PN->replaceAllUsesWith(PN->getIncomingValue(0));
275 BB->getInstList().pop_front(); // Delete the phi node...
278 // Delete the unconditional branch from the predecessor...
279 OnlyPred->getInstList().pop_back();
281 // Move all definitions in the successor to the predecessor...
282 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
284 // Make all PHI nodes that referred to BB now refer to Pred as their
286 BB->replaceAllUsesWith(OnlyPred);
288 std::string OldName = BB->getName();
290 // Erase basic block from the function...
291 M->getBasicBlockList().erase(BB);
293 // Inherit predecessors name if it exists...
294 if (!OldName.empty() && !OnlyPred->hasName())
295 OnlyPred->setName(OldName);
304 } // End llvm namespace