1 //===-- Local.cpp - Functions to perform local transformations ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This family of functions perform various local transformations to the
13 //===----------------------------------------------------------------------===//
15 #include "llvm/Transforms/Utils/Local.h"
16 #include "llvm/Constants.h"
17 #include "llvm/GlobalAlias.h"
18 #include "llvm/GlobalVariable.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Instructions.h"
21 #include "llvm/Intrinsics.h"
22 #include "llvm/IntrinsicInst.h"
23 #include "llvm/LLVMContext.h"
24 #include "llvm/ADT/SmallPtrSet.h"
25 #include "llvm/Analysis/ConstantFolding.h"
26 #include "llvm/Analysis/DebugInfo.h"
27 #include "llvm/Analysis/MallocHelper.h"
28 #include "llvm/Analysis/ProfileInfo.h"
29 #include "llvm/Target/TargetData.h"
30 #include "llvm/Support/GetElementPtrTypeIterator.h"
31 #include "llvm/Support/MathExtras.h"
34 //===----------------------------------------------------------------------===//
38 /// isSafeToLoadUnconditionally - Return true if we know that executing a load
39 /// from this value cannot trap. If it is not obviously safe to load from the
40 /// specified pointer, we do a quick local scan of the basic block containing
41 /// ScanFrom, to determine if the address is already accessed.
42 bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom) {
43 // If it is an alloca it is always safe to load from.
44 if (isa<AllocaInst>(V)) return true;
46 // If it is a global variable it is mostly safe to load from.
47 if (const GlobalValue *GV = dyn_cast<GlobalVariable>(V))
48 // Don't try to evaluate aliases. External weak GV can be null.
49 return !isa<GlobalAlias>(GV) && !GV->hasExternalWeakLinkage();
51 // Otherwise, be a little bit agressive by scanning the local block where we
52 // want to check to see if the pointer is already being loaded or stored
53 // from/to. If so, the previous load or store would have already trapped,
54 // so there is no harm doing an extra load (also, CSE will later eliminate
55 // the load entirely).
56 BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin();
61 // If we see a free or a call which may write to memory (i.e. which might do
62 // a free) the pointer could be marked invalid.
63 if (isFreeCall(BBI) || (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
64 !isa<DbgInfoIntrinsic>(BBI)))
67 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
68 if (LI->getOperand(0) == V) return true;
69 } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
70 if (SI->getOperand(1) == V) return true;
77 //===----------------------------------------------------------------------===//
78 // Local constant propagation.
81 // ConstantFoldTerminator - If a terminator instruction is predicated on a
82 // constant value, convert it into an unconditional branch to the constant
85 bool llvm::ConstantFoldTerminator(BasicBlock *BB) {
86 TerminatorInst *T = BB->getTerminator();
88 // Branch - See if we are conditional jumping on constant
89 if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
90 if (BI->isUnconditional()) return false; // Can't optimize uncond branch
91 BasicBlock *Dest1 = BI->getSuccessor(0);
92 BasicBlock *Dest2 = BI->getSuccessor(1);
94 if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
95 // Are we branching on constant?
96 // YES. Change to unconditional branch...
97 BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
98 BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1;
100 //cerr << "Function: " << T->getParent()->getParent()
101 // << "\nRemoving branch from " << T->getParent()
102 // << "\n\nTo: " << OldDest << endl;
104 // Let the basic block know that we are letting go of it. Based on this,
105 // it will adjust it's PHI nodes.
106 assert(BI->getParent() && "Terminator not inserted in block!");
107 OldDest->removePredecessor(BI->getParent());
109 // Set the unconditional destination, and change the insn to be an
110 // unconditional branch.
111 BI->setUnconditionalDest(Destination);
113 } else if (Dest2 == Dest1) { // Conditional branch to same location?
114 // This branch matches something like this:
115 // br bool %cond, label %Dest, label %Dest
116 // and changes it into: br label %Dest
118 // Let the basic block know that we are letting go of one copy of it.
119 assert(BI->getParent() && "Terminator not inserted in block!");
120 Dest1->removePredecessor(BI->getParent());
122 // Change a conditional branch to unconditional.
123 BI->setUnconditionalDest(Dest1);
126 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
127 // If we are switching on a constant, we can convert the switch into a
128 // single branch instruction!
129 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
130 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest
131 BasicBlock *DefaultDest = TheOnlyDest;
132 assert(TheOnlyDest == SI->getDefaultDest() &&
133 "Default destination is not successor #0?");
135 // Figure out which case it goes to...
136 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
137 // Found case matching a constant operand?
138 if (SI->getSuccessorValue(i) == CI) {
139 TheOnlyDest = SI->getSuccessor(i);
143 // Check to see if this branch is going to the same place as the default
144 // dest. If so, eliminate it as an explicit compare.
145 if (SI->getSuccessor(i) == DefaultDest) {
146 // Remove this entry...
147 DefaultDest->removePredecessor(SI->getParent());
149 --i; --e; // Don't skip an entry...
153 // Otherwise, check to see if the switch only branches to one destination.
154 // We do this by reseting "TheOnlyDest" to null when we find two non-equal
156 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0;
159 if (CI && !TheOnlyDest) {
160 // Branching on a constant, but not any of the cases, go to the default
162 TheOnlyDest = SI->getDefaultDest();
165 // If we found a single destination that we can fold the switch into, do so
168 // Insert the new branch..
169 BranchInst::Create(TheOnlyDest, SI);
170 BasicBlock *BB = SI->getParent();
172 // Remove entries from PHI nodes which we no longer branch to...
173 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
174 // Found case matching a constant operand?
175 BasicBlock *Succ = SI->getSuccessor(i);
176 if (Succ == TheOnlyDest)
177 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest
179 Succ->removePredecessor(BB);
182 // Delete the old switch...
183 BB->getInstList().erase(SI);
185 } else if (SI->getNumSuccessors() == 2) {
186 // Otherwise, we can fold this switch into a conditional branch
187 // instruction if it has only one non-default destination.
188 Value *Cond = new ICmpInst(SI, ICmpInst::ICMP_EQ, SI->getCondition(),
189 SI->getSuccessorValue(1), "cond");
190 // Insert the new branch...
191 BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);
193 // Delete the old switch...
194 SI->eraseFromParent();
202 //===----------------------------------------------------------------------===//
203 // Local dead code elimination...
206 /// isInstructionTriviallyDead - Return true if the result produced by the
207 /// instruction is not used, and the instruction has no side effects.
209 bool llvm::isInstructionTriviallyDead(Instruction *I) {
210 if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
212 // We don't want debug info removed by anything this general.
213 if (isa<DbgInfoIntrinsic>(I)) return false;
215 if (!I->mayHaveSideEffects()) return true;
217 // Special case intrinsics that "may have side effects" but can be deleted
219 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
220 // Safe to delete llvm.stacksave if dead.
221 if (II->getIntrinsicID() == Intrinsic::stacksave)
226 /// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a
227 /// trivially dead instruction, delete it. If that makes any of its operands
228 /// trivially dead, delete them too, recursively.
229 void llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) {
230 Instruction *I = dyn_cast<Instruction>(V);
231 if (!I || !I->use_empty() || !isInstructionTriviallyDead(I))
234 SmallVector<Instruction*, 16> DeadInsts;
235 DeadInsts.push_back(I);
237 while (!DeadInsts.empty()) {
238 I = DeadInsts.pop_back_val();
240 // Null out all of the instruction's operands to see if any operand becomes
242 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
243 Value *OpV = I->getOperand(i);
246 if (!OpV->use_empty()) continue;
248 // If the operand is an instruction that became dead as we nulled out the
249 // operand, and if it is 'trivially' dead, delete it in a future loop
251 if (Instruction *OpI = dyn_cast<Instruction>(OpV))
252 if (isInstructionTriviallyDead(OpI))
253 DeadInsts.push_back(OpI);
256 I->eraseFromParent();
260 /// RecursivelyDeleteDeadPHINode - If the specified value is an effectively
261 /// dead PHI node, due to being a def-use chain of single-use nodes that
262 /// either forms a cycle or is terminated by a trivially dead instruction,
263 /// delete it. If that makes any of its operands trivially dead, delete them
264 /// too, recursively.
266 llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) {
267 // We can remove a PHI if it is on a cycle in the def-use graph
268 // where each node in the cycle has degree one, i.e. only one use,
269 // and is an instruction with no side effects.
270 if (!PN->hasOneUse())
273 SmallPtrSet<PHINode *, 4> PHIs;
275 for (Instruction *J = cast<Instruction>(*PN->use_begin());
276 J->hasOneUse() && !J->mayHaveSideEffects();
277 J = cast<Instruction>(*J->use_begin()))
278 // If we find a PHI more than once, we're on a cycle that
279 // won't prove fruitful.
280 if (PHINode *JP = dyn_cast<PHINode>(J))
281 if (!PHIs.insert(cast<PHINode>(JP))) {
282 // Break the cycle and delete the PHI and its operands.
283 JP->replaceAllUsesWith(UndefValue::get(JP->getType()));
284 RecursivelyDeleteTriviallyDeadInstructions(JP);
289 //===----------------------------------------------------------------------===//
290 // Control Flow Graph Restructuring...
293 /// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
294 /// predecessor is known to have one successor (DestBB!). Eliminate the edge
295 /// between them, moving the instructions in the predecessor into DestBB and
296 /// deleting the predecessor block.
298 void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) {
299 // If BB has single-entry PHI nodes, fold them.
300 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
301 Value *NewVal = PN->getIncomingValue(0);
302 // Replace self referencing PHI with undef, it must be dead.
303 if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
304 PN->replaceAllUsesWith(NewVal);
305 PN->eraseFromParent();
308 BasicBlock *PredBB = DestBB->getSinglePredecessor();
309 assert(PredBB && "Block doesn't have a single predecessor!");
311 // Splice all the instructions from PredBB to DestBB.
312 PredBB->getTerminator()->eraseFromParent();
313 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
315 // Anything that branched to PredBB now branches to DestBB.
316 PredBB->replaceAllUsesWith(DestBB);
319 ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>();
321 PI->replaceAllUses(PredBB, DestBB);
322 PI->removeEdge(ProfileInfo::getEdge(PredBB, DestBB));
326 PredBB->eraseFromParent();
329 /// OnlyUsedByDbgIntrinsics - Return true if the instruction I is only used
330 /// by DbgIntrinsics. If DbgInUses is specified then the vector is filled
331 /// with the DbgInfoIntrinsic that use the instruction I.
332 bool llvm::OnlyUsedByDbgInfoIntrinsics(Instruction *I,
333 SmallVectorImpl<DbgInfoIntrinsic *> *DbgInUses) {
337 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); UI != UE;
339 if (DbgInfoIntrinsic *DI = dyn_cast<DbgInfoIntrinsic>(*UI)) {
341 DbgInUses->push_back(DI);