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/InstructionSimplify.h"
28 #include "llvm/Analysis/ProfileInfo.h"
29 #include "llvm/Target/TargetData.h"
30 #include "llvm/Support/CFG.h"
31 #include "llvm/Support/Debug.h"
32 #include "llvm/Support/GetElementPtrTypeIterator.h"
33 #include "llvm/Support/MathExtras.h"
34 #include "llvm/Support/raw_ostream.h"
37 //===----------------------------------------------------------------------===//
41 /// isSafeToLoadUnconditionally - Return true if we know that executing a load
42 /// from this value cannot trap. If it is not obviously safe to load from the
43 /// specified pointer, we do a quick local scan of the basic block containing
44 /// ScanFrom, to determine if the address is already accessed.
45 bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom) {
46 // If it is an alloca it is always safe to load from.
47 if (isa<AllocaInst>(V)) return true;
49 // If it is a global variable it is mostly safe to load from.
50 if (const GlobalValue *GV = dyn_cast<GlobalVariable>(V))
51 // Don't try to evaluate aliases. External weak GV can be null.
52 return !isa<GlobalAlias>(GV) && !GV->hasExternalWeakLinkage();
54 // Otherwise, be a little bit agressive by scanning the local block where we
55 // want to check to see if the pointer is already being loaded or stored
56 // from/to. If so, the previous load or store would have already trapped,
57 // so there is no harm doing an extra load (also, CSE will later eliminate
58 // the load entirely).
59 BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin();
64 // If we see a free or a call which may write to memory (i.e. which might do
65 // a free) the pointer could be marked invalid.
66 if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
67 !isa<DbgInfoIntrinsic>(BBI))
70 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
71 if (LI->getOperand(0) == V) return true;
72 } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
73 if (SI->getOperand(1) == V) return true;
80 //===----------------------------------------------------------------------===//
81 // Local constant propagation.
84 // ConstantFoldTerminator - If a terminator instruction is predicated on a
85 // constant value, convert it into an unconditional branch to the constant
88 bool llvm::ConstantFoldTerminator(BasicBlock *BB) {
89 TerminatorInst *T = BB->getTerminator();
91 // Branch - See if we are conditional jumping on constant
92 if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
93 if (BI->isUnconditional()) return false; // Can't optimize uncond branch
94 BasicBlock *Dest1 = BI->getSuccessor(0);
95 BasicBlock *Dest2 = BI->getSuccessor(1);
97 if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
98 // Are we branching on constant?
99 // YES. Change to unconditional branch...
100 BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
101 BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1;
103 //cerr << "Function: " << T->getParent()->getParent()
104 // << "\nRemoving branch from " << T->getParent()
105 // << "\n\nTo: " << OldDest << endl;
107 // Let the basic block know that we are letting go of it. Based on this,
108 // it will adjust it's PHI nodes.
109 assert(BI->getParent() && "Terminator not inserted in block!");
110 OldDest->removePredecessor(BI->getParent());
112 // Set the unconditional destination, and change the insn to be an
113 // unconditional branch.
114 BI->setUnconditionalDest(Destination);
118 if (Dest2 == Dest1) { // Conditional branch to same location?
119 // This branch matches something like this:
120 // br bool %cond, label %Dest, label %Dest
121 // and changes it into: br label %Dest
123 // Let the basic block know that we are letting go of one copy of it.
124 assert(BI->getParent() && "Terminator not inserted in block!");
125 Dest1->removePredecessor(BI->getParent());
127 // Change a conditional branch to unconditional.
128 BI->setUnconditionalDest(Dest1);
134 if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
135 // If we are switching on a constant, we can convert the switch into a
136 // single branch instruction!
137 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
138 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest
139 BasicBlock *DefaultDest = TheOnlyDest;
140 assert(TheOnlyDest == SI->getDefaultDest() &&
141 "Default destination is not successor #0?");
143 // Figure out which case it goes to.
144 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
145 // Found case matching a constant operand?
146 if (SI->getSuccessorValue(i) == CI) {
147 TheOnlyDest = SI->getSuccessor(i);
151 // Check to see if this branch is going to the same place as the default
152 // dest. If so, eliminate it as an explicit compare.
153 if (SI->getSuccessor(i) == DefaultDest) {
154 // Remove this entry.
155 DefaultDest->removePredecessor(SI->getParent());
157 --i; --e; // Don't skip an entry...
161 // Otherwise, check to see if the switch only branches to one destination.
162 // We do this by reseting "TheOnlyDest" to null when we find two non-equal
164 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0;
167 if (CI && !TheOnlyDest) {
168 // Branching on a constant, but not any of the cases, go to the default
170 TheOnlyDest = SI->getDefaultDest();
173 // If we found a single destination that we can fold the switch into, do so
176 // Insert the new branch.
177 BranchInst::Create(TheOnlyDest, SI);
178 BasicBlock *BB = SI->getParent();
180 // Remove entries from PHI nodes which we no longer branch to...
181 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
182 // Found case matching a constant operand?
183 BasicBlock *Succ = SI->getSuccessor(i);
184 if (Succ == TheOnlyDest)
185 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest
187 Succ->removePredecessor(BB);
190 // Delete the old switch.
191 BB->getInstList().erase(SI);
195 if (SI->getNumSuccessors() == 2) {
196 // Otherwise, we can fold this switch into a conditional branch
197 // instruction if it has only one non-default destination.
198 Value *Cond = new ICmpInst(SI, ICmpInst::ICMP_EQ, SI->getCondition(),
199 SI->getSuccessorValue(1), "cond");
200 // Insert the new branch.
201 BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);
203 // Delete the old switch.
204 SI->eraseFromParent();
210 if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(T)) {
211 // indirectbr blockaddress(@F, @BB) -> br label @BB
212 if (BlockAddress *BA =
213 dyn_cast<BlockAddress>(IBI->getAddress()->stripPointerCasts())) {
214 BasicBlock *TheOnlyDest = BA->getBasicBlock();
215 // Insert the new branch.
216 BranchInst::Create(TheOnlyDest, IBI);
218 for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
219 if (IBI->getDestination(i) == TheOnlyDest)
222 IBI->getDestination(i)->removePredecessor(IBI->getParent());
224 IBI->eraseFromParent();
226 // If we didn't find our destination in the IBI successor list, then we
227 // have undefined behavior. Replace the unconditional branch with an
228 // 'unreachable' instruction.
230 BB->getTerminator()->eraseFromParent();
231 new UnreachableInst(BB->getContext(), BB);
242 //===----------------------------------------------------------------------===//
243 // Local dead code elimination.
246 /// isInstructionTriviallyDead - Return true if the result produced by the
247 /// instruction is not used, and the instruction has no side effects.
249 bool llvm::isInstructionTriviallyDead(Instruction *I) {
250 if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
252 // We don't want debug info removed by anything this general.
253 if (isa<DbgInfoIntrinsic>(I)) return false;
255 if (!I->mayHaveSideEffects()) return true;
257 // Special case intrinsics that "may have side effects" but can be deleted
259 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
260 // Safe to delete llvm.stacksave if dead.
261 if (II->getIntrinsicID() == Intrinsic::stacksave)
266 /// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a
267 /// trivially dead instruction, delete it. If that makes any of its operands
268 /// trivially dead, delete them too, recursively.
269 void llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) {
270 Instruction *I = dyn_cast<Instruction>(V);
271 if (!I || !I->use_empty() || !isInstructionTriviallyDead(I))
274 SmallVector<Instruction*, 16> DeadInsts;
275 DeadInsts.push_back(I);
277 while (!DeadInsts.empty()) {
278 I = DeadInsts.pop_back_val();
280 // Null out all of the instruction's operands to see if any operand becomes
282 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
283 Value *OpV = I->getOperand(i);
286 if (!OpV->use_empty()) continue;
288 // If the operand is an instruction that became dead as we nulled out the
289 // operand, and if it is 'trivially' dead, delete it in a future loop
291 if (Instruction *OpI = dyn_cast<Instruction>(OpV))
292 if (isInstructionTriviallyDead(OpI))
293 DeadInsts.push_back(OpI);
296 I->eraseFromParent();
300 /// RecursivelyDeleteDeadPHINode - If the specified value is an effectively
301 /// dead PHI node, due to being a def-use chain of single-use nodes that
302 /// either forms a cycle or is terminated by a trivially dead instruction,
303 /// delete it. If that makes any of its operands trivially dead, delete them
304 /// too, recursively.
306 llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) {
307 // We can remove a PHI if it is on a cycle in the def-use graph
308 // where each node in the cycle has degree one, i.e. only one use,
309 // and is an instruction with no side effects.
310 if (!PN->hasOneUse())
313 SmallPtrSet<PHINode *, 4> PHIs;
315 for (Instruction *J = cast<Instruction>(*PN->use_begin());
316 J->hasOneUse() && !J->mayHaveSideEffects();
317 J = cast<Instruction>(*J->use_begin()))
318 // If we find a PHI more than once, we're on a cycle that
319 // won't prove fruitful.
320 if (PHINode *JP = dyn_cast<PHINode>(J))
321 if (!PHIs.insert(cast<PHINode>(JP))) {
322 // Break the cycle and delete the PHI and its operands.
323 JP->replaceAllUsesWith(UndefValue::get(JP->getType()));
324 RecursivelyDeleteTriviallyDeadInstructions(JP);
329 //===----------------------------------------------------------------------===//
330 // Control Flow Graph Restructuring.
334 /// RemovePredecessorAndSimplify - Like BasicBlock::removePredecessor, this
335 /// method is called when we're about to delete Pred as a predecessor of BB. If
336 /// BB contains any PHI nodes, this drops the entries in the PHI nodes for Pred.
338 /// Unlike the removePredecessor method, this attempts to simplify uses of PHI
339 /// nodes that collapse into identity values. For example, if we have:
340 /// x = phi(1, 0, 0, 0)
343 /// .. and delete the predecessor corresponding to the '1', this will attempt to
344 /// recursively fold the and to 0.
345 void llvm::RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred,
347 // This only adjusts blocks with PHI nodes.
348 if (!isa<PHINode>(BB->begin()))
351 // Remove the entries for Pred from the PHI nodes in BB, but do not simplify
352 // them down. This will leave us with single entry phi nodes and other phis
353 // that can be removed.
354 BB->removePredecessor(Pred, true);
356 WeakVH PhiIt = &BB->front();
357 while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) {
358 PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt));
360 Value *PNV = PN->hasConstantValue();
361 if (PNV == 0) continue;
363 // If we're able to simplify the phi to a single value, substitute the new
364 // value into all of its uses.
365 assert(PNV != PN && "hasConstantValue broken");
367 ReplaceAndSimplifyAllUses(PN, PNV, TD);
369 // If recursive simplification ended up deleting the next PHI node we would
370 // iterate to, then our iterator is invalid, restart scanning from the top
372 if (PhiIt == 0) PhiIt = &BB->front();
377 /// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
378 /// predecessor is known to have one successor (DestBB!). Eliminate the edge
379 /// between them, moving the instructions in the predecessor into DestBB and
380 /// deleting the predecessor block.
382 void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) {
383 // If BB has single-entry PHI nodes, fold them.
384 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
385 Value *NewVal = PN->getIncomingValue(0);
386 // Replace self referencing PHI with undef, it must be dead.
387 if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
388 PN->replaceAllUsesWith(NewVal);
389 PN->eraseFromParent();
392 BasicBlock *PredBB = DestBB->getSinglePredecessor();
393 assert(PredBB && "Block doesn't have a single predecessor!");
395 // Splice all the instructions from PredBB to DestBB.
396 PredBB->getTerminator()->eraseFromParent();
397 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
399 // Anything that branched to PredBB now branches to DestBB.
400 PredBB->replaceAllUsesWith(DestBB);
403 ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>();
405 PI->replaceAllUses(PredBB, DestBB);
406 PI->removeEdge(ProfileInfo::getEdge(PredBB, DestBB));
410 PredBB->eraseFromParent();
413 /// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
414 /// almost-empty BB ending in an unconditional branch to Succ, into succ.
416 /// Assumption: Succ is the single successor for BB.
418 static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
419 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
421 DEBUG(errs() << "Looking to fold " << BB->getName() << " into "
422 << Succ->getName() << "\n");
423 // Shortcut, if there is only a single predecessor it must be BB and merging
425 if (Succ->getSinglePredecessor()) return true;
427 // Make a list of the predecessors of BB
428 typedef SmallPtrSet<BasicBlock*, 16> BlockSet;
429 BlockSet BBPreds(pred_begin(BB), pred_end(BB));
431 // Use that list to make another list of common predecessors of BB and Succ
432 BlockSet CommonPreds;
433 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);
435 if (BBPreds.count(*PI))
436 CommonPreds.insert(*PI);
438 // Shortcut, if there are no common predecessors, merging is always safe
439 if (CommonPreds.empty())
442 // Look at all the phi nodes in Succ, to see if they present a conflict when
443 // merging these blocks
444 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
445 PHINode *PN = cast<PHINode>(I);
447 // If the incoming value from BB is again a PHINode in
448 // BB which has the same incoming value for *PI as PN does, we can
449 // merge the phi nodes and then the blocks can still be merged
450 PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB));
451 if (BBPN && BBPN->getParent() == BB) {
452 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end();
454 if (BBPN->getIncomingValueForBlock(*PI)
455 != PN->getIncomingValueForBlock(*PI)) {
456 DEBUG(errs() << "Can't fold, phi node " << PN->getName() << " in "
457 << Succ->getName() << " is conflicting with "
458 << BBPN->getName() << " with regard to common predecessor "
459 << (*PI)->getName() << "\n");
464 Value* Val = PN->getIncomingValueForBlock(BB);
465 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end();
467 // See if the incoming value for the common predecessor is equal to the
468 // one for BB, in which case this phi node will not prevent the merging
470 if (Val != PN->getIncomingValueForBlock(*PI)) {
471 DEBUG(errs() << "Can't fold, phi node " << PN->getName() << " in "
472 << Succ->getName() << " is conflicting with regard to common "
473 << "predecessor " << (*PI)->getName() << "\n");
483 /// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an
484 /// unconditional branch, and contains no instructions other than PHI nodes,
485 /// potential debug intrinsics and the branch. If possible, eliminate BB by
486 /// rewriting all the predecessors to branch to the successor block and return
487 /// true. If we can't transform, return false.
488 bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) {
489 // We can't eliminate infinite loops.
490 BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0);
491 if (BB == Succ) return false;
493 // Check to see if merging these blocks would cause conflicts for any of the
494 // phi nodes in BB or Succ. If not, we can safely merge.
495 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
497 // Check for cases where Succ has multiple predecessors and a PHI node in BB
498 // has uses which will not disappear when the PHI nodes are merged. It is
499 // possible to handle such cases, but difficult: it requires checking whether
500 // BB dominates Succ, which is non-trivial to calculate in the case where
501 // Succ has multiple predecessors. Also, it requires checking whether
502 // constructing the necessary self-referential PHI node doesn't intoduce any
503 // conflicts; this isn't too difficult, but the previous code for doing this
506 // Note that if this check finds a live use, BB dominates Succ, so BB is
507 // something like a loop pre-header (or rarely, a part of an irreducible CFG);
508 // folding the branch isn't profitable in that case anyway.
509 if (!Succ->getSinglePredecessor()) {
510 BasicBlock::iterator BBI = BB->begin();
511 while (isa<PHINode>(*BBI)) {
512 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end();
514 if (PHINode* PN = dyn_cast<PHINode>(*UI)) {
515 if (PN->getIncomingBlock(UI) != BB)
525 DEBUG(errs() << "Killing Trivial BB: \n" << *BB);
527 if (isa<PHINode>(Succ->begin())) {
528 // If there is more than one pred of succ, and there are PHI nodes in
529 // the successor, then we need to add incoming edges for the PHI nodes
531 const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
533 // Loop over all of the PHI nodes in the successor of BB.
534 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
535 PHINode *PN = cast<PHINode>(I);
536 Value *OldVal = PN->removeIncomingValue(BB, false);
537 assert(OldVal && "No entry in PHI for Pred BB!");
539 // If this incoming value is one of the PHI nodes in BB, the new entries
540 // in the PHI node are the entries from the old PHI.
541 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
542 PHINode *OldValPN = cast<PHINode>(OldVal);
543 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
544 // Note that, since we are merging phi nodes and BB and Succ might
545 // have common predecessors, we could end up with a phi node with
546 // identical incoming branches. This will be cleaned up later (and
547 // will trigger asserts if we try to clean it up now, without also
548 // simplifying the corresponding conditional branch).
549 PN->addIncoming(OldValPN->getIncomingValue(i),
550 OldValPN->getIncomingBlock(i));
552 // Add an incoming value for each of the new incoming values.
553 for (unsigned i = 0, e = BBPreds.size(); i != e; ++i)
554 PN->addIncoming(OldVal, BBPreds[i]);
559 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
560 if (Succ->getSinglePredecessor()) {
561 // BB is the only predecessor of Succ, so Succ will end up with exactly
562 // the same predecessors BB had.
563 Succ->getInstList().splice(Succ->begin(),
564 BB->getInstList(), BB->begin());
566 // We explicitly check for such uses in CanPropagatePredecessorsForPHIs.
567 assert(PN->use_empty() && "There shouldn't be any uses here!");
568 PN->eraseFromParent();
572 // Everything that jumped to BB now goes to Succ.
573 BB->replaceAllUsesWith(Succ);
574 if (!Succ->hasName()) Succ->takeName(BB);
575 BB->eraseFromParent(); // Delete the old basic block.
581 /// OnlyUsedByDbgIntrinsics - Return true if the instruction I is only used
582 /// by DbgIntrinsics. If DbgInUses is specified then the vector is filled
583 /// with the DbgInfoIntrinsic that use the instruction I.
584 bool llvm::OnlyUsedByDbgInfoIntrinsics(Instruction *I,
585 SmallVectorImpl<DbgInfoIntrinsic *> *DbgInUses) {
589 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); UI != UE;
591 if (DbgInfoIntrinsic *DI = dyn_cast<DbgInfoIntrinsic>(*UI)) {
593 DbgInUses->push_back(DI);