1 //===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
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 // This pass transforms loops that contain branches on loop-invariant conditions
11 // to have multiple loops. For example, it turns the left into the right code:
20 // This can increase the size of the code exponentially (doubling it every time
21 // a loop is unswitched) so we only unswitch if the resultant code will be
22 // smaller than a threshold.
24 // This pass expects LICM to be run before it to hoist invariant conditions out
25 // of the loop, to make the unswitching opportunity obvious.
27 //===----------------------------------------------------------------------===//
29 #define DEBUG_TYPE "loop-unswitch"
30 #include "llvm/Transforms/Scalar.h"
31 #include "llvm/Constants.h"
32 #include "llvm/DerivedTypes.h"
33 #include "llvm/Function.h"
34 #include "llvm/Instructions.h"
35 #include "llvm/Analysis/ConstantFolding.h"
36 #include "llvm/Analysis/LoopInfo.h"
37 #include "llvm/Analysis/LoopPass.h"
38 #include "llvm/Transforms/Utils/Cloning.h"
39 #include "llvm/Transforms/Utils/Local.h"
40 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
41 #include "llvm/ADT/Statistic.h"
42 #include "llvm/ADT/SmallPtrSet.h"
43 #include "llvm/ADT/PostOrderIterator.h"
44 #include "llvm/Support/CommandLine.h"
45 #include "llvm/Support/Compiler.h"
46 #include "llvm/Support/Debug.h"
51 STATISTIC(NumBranches, "Number of branches unswitched");
52 STATISTIC(NumSwitches, "Number of switches unswitched");
53 STATISTIC(NumSelects , "Number of selects unswitched");
54 STATISTIC(NumTrivial , "Number of unswitches that are trivial");
55 STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
59 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
60 cl::init(10), cl::Hidden);
62 class VISIBILITY_HIDDEN LoopUnswitch : public LoopPass {
63 LoopInfo *LI; // Loop information
66 // LoopProcessWorklist - Used to check if second loop needs processing
67 // after RewriteLoopBodyWithConditionConstant rewrites first loop.
68 std::vector<Loop*> LoopProcessWorklist;
69 SmallPtrSet<Value *,8> UnswitchedVals;
72 static char ID; // Pass ID, replacement for typeid
73 LoopUnswitch() : LoopPass((intptr_t)&ID) {}
75 bool runOnLoop(Loop *L, LPPassManager &LPM);
77 /// This transformation requires natural loop information & requires that
78 /// loop preheaders be inserted into the CFG...
80 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
81 AU.addRequiredID(LoopSimplifyID);
82 AU.addPreservedID(LoopSimplifyID);
83 AU.addRequired<LoopInfo>();
84 AU.addPreserved<LoopInfo>();
85 AU.addRequiredID(LCSSAID);
86 AU.addPreservedID(LCSSAID);
90 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist,
92 void RemoveLoopFromWorklist(Loop *L) {
93 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(),
94 LoopProcessWorklist.end(), L);
95 if (I != LoopProcessWorklist.end())
96 LoopProcessWorklist.erase(I);
99 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L);
100 unsigned getLoopUnswitchCost(Loop *L, Value *LIC);
101 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
102 BasicBlock *ExitBlock);
103 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
104 BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To);
105 BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt);
107 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
108 Constant *Val, bool isEqual);
110 void SimplifyCode(std::vector<Instruction*> &Worklist);
111 void RemoveBlockIfDead(BasicBlock *BB,
112 std::vector<Instruction*> &Worklist);
113 void RemoveLoopFromHierarchy(Loop *L);
115 char LoopUnswitch::ID = 0;
116 RegisterPass<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
119 LoopPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }
121 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
122 /// invariant in the loop, or has an invariant piece, return the invariant.
123 /// Otherwise, return null.
124 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
125 // Constants should be folded, not unswitched on!
126 if (isa<Constant>(Cond)) return false;
128 // TODO: Handle: br (VARIANT|INVARIANT).
129 // TODO: Hoist simple expressions out of loops.
130 if (L->isLoopInvariant(Cond)) return Cond;
132 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
133 if (BO->getOpcode() == Instruction::And ||
134 BO->getOpcode() == Instruction::Or) {
135 // If either the left or right side is invariant, we can unswitch on this,
136 // which will cause the branch to go away in one loop and the condition to
137 // simplify in the other one.
138 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
140 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
147 bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
148 assert(L->isLCSSAForm());
149 LI = &getAnalysis<LoopInfo>();
151 bool Changed = false;
153 // Loop over all of the basic blocks in the loop. If we find an interior
154 // block that is branching on a loop-invariant condition, we can unswitch this
156 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
158 TerminatorInst *TI = (*I)->getTerminator();
159 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
160 // If this isn't branching on an invariant condition, we can't unswitch
162 if (BI->isConditional()) {
163 // See if this, or some part of it, is loop invariant. If so, we can
164 // unswitch on it if we desire.
165 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed);
166 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(),
172 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
173 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
174 if (LoopCond && SI->getNumCases() > 1) {
175 // Find a value to unswitch on:
176 // FIXME: this should chose the most expensive case!
177 Constant *UnswitchVal = SI->getCaseValue(1);
178 // Do not process same value again and again.
179 if (!UnswitchedVals.insert(UnswitchVal))
182 if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) {
189 // Scan the instructions to check for unswitchable values.
190 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
192 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
193 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
194 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(),
202 assert(L->isLCSSAForm());
207 /// isTrivialLoopExitBlock - Check to see if all paths from BB either:
208 /// 1. Exit the loop with no side effects.
209 /// 2. Branch to the latch block with no side-effects.
211 /// If these conditions are true, we return true and set ExitBB to the block we
214 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
216 std::set<BasicBlock*> &Visited) {
217 if (!Visited.insert(BB).second) {
218 // Already visited and Ok, end of recursion.
220 } else if (!L->contains(BB)) {
221 // Otherwise, this is a loop exit, this is fine so long as this is the
223 if (ExitBB != 0) return false;
228 // Otherwise, this is an unvisited intra-loop node. Check all successors.
229 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
230 // Check to see if the successor is a trivial loop exit.
231 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
235 // Okay, everything after this looks good, check to make sure that this block
236 // doesn't include any side effects.
237 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
238 if (I->mayWriteToMemory())
244 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally
245 /// leads to an exit from the specified loop, and has no side-effects in the
246 /// process. If so, return the block that is exited to, otherwise return null.
247 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
248 std::set<BasicBlock*> Visited;
249 Visited.insert(L->getHeader()); // Branches to header are ok.
250 BasicBlock *ExitBB = 0;
251 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
256 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
257 /// trivial: that is, that the condition controls whether or not the loop does
258 /// anything at all. If this is a trivial condition, unswitching produces no
259 /// code duplications (equivalently, it produces a simpler loop and a new empty
260 /// loop, which gets deleted).
262 /// If this is a trivial condition, return true, otherwise return false. When
263 /// returning true, this sets Cond and Val to the condition that controls the
264 /// trivial condition: when Cond dynamically equals Val, the loop is known to
265 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when
268 static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond, Constant **Val = 0,
269 BasicBlock **LoopExit = 0) {
270 BasicBlock *Header = L->getHeader();
271 TerminatorInst *HeaderTerm = Header->getTerminator();
273 BasicBlock *LoopExitBB = 0;
274 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
275 // If the header block doesn't end with a conditional branch on Cond, we
277 if (!BI->isConditional() || BI->getCondition() != Cond)
280 // Check to see if a successor of the branch is guaranteed to go to the
281 // latch block or exit through a one exit block without having any
282 // side-effects. If so, determine the value of Cond that causes it to do
284 if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(0)))) {
285 if (Val) *Val = ConstantInt::getTrue();
286 } else if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(1)))) {
287 if (Val) *Val = ConstantInt::getFalse();
289 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
290 // If this isn't a switch on Cond, we can't handle it.
291 if (SI->getCondition() != Cond) return false;
293 // Check to see if a successor of the switch is guaranteed to go to the
294 // latch block or exit through a one exit block without having any
295 // side-effects. If so, determine the value of Cond that causes it to do
296 // this. Note that we can't trivially unswitch on the default case.
297 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
298 if ((LoopExitBB = isTrivialLoopExitBlock(L, SI->getSuccessor(i)))) {
299 // Okay, we found a trivial case, remember the value that is trivial.
300 if (Val) *Val = SI->getCaseValue(i);
305 // If we didn't find a single unique LoopExit block, or if the loop exit block
306 // contains phi nodes, this isn't trivial.
307 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
308 return false; // Can't handle this.
310 if (LoopExit) *LoopExit = LoopExitBB;
312 // We already know that nothing uses any scalar values defined inside of this
313 // loop. As such, we just have to check to see if this loop will execute any
314 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
315 // part of the loop that the code *would* execute. We already checked the
316 // tail, check the header now.
317 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
318 if (I->mayWriteToMemory())
323 /// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
324 /// we choose to unswitch the specified loop on the specified value.
326 unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
327 // If the condition is trivial, always unswitch. There is no code growth for
329 if (IsTrivialUnswitchCondition(L, LIC))
332 // FIXME: This is really overly conservative. However, more liberal
333 // estimations have thus far resulted in excessive unswitching, which is bad
334 // both in compile time and in code size. This should be replaced once
335 // someone figures out how a good estimation.
336 return L->getBlocks().size();
339 // FIXME: this is brain dead. It should take into consideration code
341 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
344 // Do not include empty blocks in the cost calculation. This happen due to
345 // loop canonicalization and will be removed.
346 if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
349 // Count basic blocks.
356 /// UnswitchIfProfitable - We have found that we can unswitch L when
357 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
358 /// unswitch the loop, reprocess the pieces, then return true.
359 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){
360 // Check to see if it would be profitable to unswitch this loop.
361 unsigned Cost = getLoopUnswitchCost(L, LoopCond);
362 if (Cost > Threshold) {
363 // FIXME: this should estimate growth by the amount of code shared by the
364 // resultant unswitched loops.
366 DOUT << "NOT unswitching loop %"
367 << L->getHeader()->getName() << ", cost too high: "
368 << L->getBlocks().size() << "\n";
372 // If this is a trivial condition to unswitch (which results in no code
373 // duplication), do it now.
375 BasicBlock *ExitBlock;
376 if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal, &ExitBlock)) {
377 UnswitchTrivialCondition(L, LoopCond, CondVal, ExitBlock);
379 UnswitchNontrivialCondition(LoopCond, Val, L);
385 /// SplitBlock - Split the specified block at the specified instruction - every
386 /// thing before SplitPt stays in Old and everything starting with SplitPt moves
387 /// to a new block. The two blocks are joined by an unconditional branch and
388 /// the loop info is updated.
390 BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *Old, Instruction *SplitPt) {
391 BasicBlock::iterator SplitIt = SplitPt;
392 while (isa<PHINode>(SplitIt))
394 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
396 // The new block lives in whichever loop the old one did.
397 if (Loop *L = LI->getLoopFor(Old))
398 L->addBasicBlockToLoop(New, *LI);
404 BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) {
405 TerminatorInst *LatchTerm = BB->getTerminator();
406 unsigned SuccNum = 0;
407 for (unsigned i = 0, e = LatchTerm->getNumSuccessors(); ; ++i) {
408 assert(i != e && "Didn't find edge?");
409 if (LatchTerm->getSuccessor(i) == Succ) {
415 // If this is a critical edge, let SplitCriticalEdge do it.
416 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, this))
417 return LatchTerm->getSuccessor(SuccNum);
419 // If the edge isn't critical, then BB has a single successor or Succ has a
420 // single pred. Split the block.
421 BasicBlock::iterator SplitPoint;
422 if (BasicBlock *SP = Succ->getSinglePredecessor()) {
423 // If the successor only has a single pred, split the top of the successor
425 assert(SP == BB && "CFG broken");
426 return SplitBlock(Succ, Succ->begin());
428 // Otherwise, if BB has a single successor, split it at the bottom of the
430 assert(BB->getTerminator()->getNumSuccessors() == 1 &&
431 "Should have a single succ!");
432 return SplitBlock(BB, BB->getTerminator());
438 // RemapInstruction - Convert the instruction operands from referencing the
439 // current values into those specified by ValueMap.
441 static inline void RemapInstruction(Instruction *I,
442 DenseMap<const Value *, Value*> &ValueMap) {
443 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
444 Value *Op = I->getOperand(op);
445 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op);
446 if (It != ValueMap.end()) Op = It->second;
447 I->setOperand(op, Op);
451 /// CloneLoop - Recursively clone the specified loop and all of its children,
452 /// mapping the blocks with the specified map.
453 static Loop *CloneLoop(Loop *L, Loop *PL, DenseMap<const Value*, Value*> &VM,
454 LoopInfo *LI, LPPassManager *LPM) {
455 Loop *New = new Loop();
457 LPM->insertLoop(New, PL);
459 // Add all of the blocks in L to the new loop.
460 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
462 if (LI->getLoopFor(*I) == L)
463 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
465 // Add all of the subloops to the new loop.
466 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
467 CloneLoop(*I, New, VM, LI, LPM);
472 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
473 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
474 /// code immediately before InsertPt.
475 static void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
476 BasicBlock *TrueDest,
477 BasicBlock *FalseDest,
478 Instruction *InsertPt) {
479 // Insert a conditional branch on LIC to the two preheaders. The original
480 // code is the true version and the new code is the false version.
481 Value *BranchVal = LIC;
482 if (!isa<ConstantInt>(Val) || Val->getType() != Type::Int1Ty)
483 BranchVal = new ICmpInst(ICmpInst::ICMP_EQ, LIC, Val, "tmp", InsertPt);
484 else if (Val != ConstantInt::getTrue())
485 // We want to enter the new loop when the condition is true.
486 std::swap(TrueDest, FalseDest);
488 // Insert the new branch.
489 new BranchInst(TrueDest, FalseDest, BranchVal, InsertPt);
493 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
494 /// condition in it (a cond branch from its header block to its latch block,
495 /// where the path through the loop that doesn't execute its body has no
496 /// side-effects), unswitch it. This doesn't involve any code duplication, just
497 /// moving the conditional branch outside of the loop and updating loop info.
498 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
500 BasicBlock *ExitBlock) {
501 DOUT << "loop-unswitch: Trivial-Unswitch loop %"
502 << L->getHeader()->getName() << " [" << L->getBlocks().size()
503 << " blocks] in Function " << L->getHeader()->getParent()->getName()
504 << " on cond: " << *Val << " == " << *Cond << "\n";
506 // First step, split the preheader, so that we know that there is a safe place
507 // to insert the conditional branch. We will change 'OrigPH' to have a
508 // conditional branch on Cond.
509 BasicBlock *OrigPH = L->getLoopPreheader();
510 BasicBlock *NewPH = SplitEdge(OrigPH, L->getHeader());
512 // Now that we have a place to insert the conditional branch, create a place
513 // to branch to: this is the exit block out of the loop that we should
516 // Split this block now, so that the loop maintains its exit block, and so
517 // that the jump from the preheader can execute the contents of the exit block
518 // without actually branching to it (the exit block should be dominated by the
519 // loop header, not the preheader).
520 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
521 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin());
523 // Okay, now we have a position to branch from and a position to branch to,
524 // insert the new conditional branch.
525 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
526 OrigPH->getTerminator());
527 OrigPH->getTerminator()->eraseFromParent();
529 // We need to reprocess this loop, it could be unswitched again.
532 // Now that we know that the loop is never entered when this condition is a
533 // particular value, rewrite the loop with this info. We know that this will
534 // at least eliminate the old branch.
535 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
540 /// VersionLoop - We determined that the loop is profitable to unswitch when LIC
541 /// equal Val. Split it into loop versions and test the condition outside of
542 /// either loop. Return the loops created as Out1/Out2.
543 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
545 Function *F = L->getHeader()->getParent();
546 DOUT << "loop-unswitch: Unswitching loop %"
547 << L->getHeader()->getName() << " [" << L->getBlocks().size()
548 << " blocks] in Function " << F->getName()
549 << " when '" << *Val << "' == " << *LIC << "\n";
551 // LoopBlocks contains all of the basic blocks of the loop, including the
552 // preheader of the loop, the body of the loop, and the exit blocks of the
553 // loop, in that order.
554 std::vector<BasicBlock*> LoopBlocks;
556 // First step, split the preheader and exit blocks, and add these blocks to
557 // the LoopBlocks list.
558 BasicBlock *OrigPreheader = L->getLoopPreheader();
559 LoopBlocks.push_back(SplitEdge(OrigPreheader, L->getHeader()));
561 // We want the loop to come after the preheader, but before the exit blocks.
562 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
564 std::vector<BasicBlock*> ExitBlocks;
565 L->getUniqueExitBlocks(ExitBlocks);
567 // Split all of the edges from inside the loop to their exit blocks. Update
568 // the appropriate Phi nodes as we do so.
569 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
570 BasicBlock *ExitBlock = ExitBlocks[i];
571 std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock));
573 for (unsigned j = 0, e = Preds.size(); j != e; ++j) {
574 assert(L->contains(Preds[j]) &&
575 "All preds of loop exit blocks must be the same loop!");
576 BasicBlock* MiddleBlock = SplitEdge(Preds[j], ExitBlock);
577 BasicBlock* StartBlock = Preds[j];
578 BasicBlock* EndBlock;
579 if (MiddleBlock->getSinglePredecessor() == ExitBlock) {
580 EndBlock = MiddleBlock;
581 MiddleBlock = EndBlock->getSinglePredecessor();;
583 EndBlock = ExitBlock;
586 std::set<PHINode*> InsertedPHIs;
587 PHINode* OldLCSSA = 0;
588 for (BasicBlock::iterator I = EndBlock->begin();
589 (OldLCSSA = dyn_cast<PHINode>(I)); ++I) {
590 Value* OldValue = OldLCSSA->getIncomingValueForBlock(MiddleBlock);
591 PHINode* NewLCSSA = new PHINode(OldLCSSA->getType(),
592 OldLCSSA->getName() + ".us-lcssa",
593 MiddleBlock->getTerminator());
594 NewLCSSA->addIncoming(OldValue, StartBlock);
595 OldLCSSA->setIncomingValue(OldLCSSA->getBasicBlockIndex(MiddleBlock),
597 InsertedPHIs.insert(NewLCSSA);
600 BasicBlock::iterator InsertPt = EndBlock->begin();
601 while (dyn_cast<PHINode>(InsertPt)) ++InsertPt;
602 for (BasicBlock::iterator I = MiddleBlock->begin();
603 (OldLCSSA = dyn_cast<PHINode>(I)) && InsertedPHIs.count(OldLCSSA) == 0;
605 PHINode *NewLCSSA = new PHINode(OldLCSSA->getType(),
606 OldLCSSA->getName() + ".us-lcssa",
608 OldLCSSA->replaceAllUsesWith(NewLCSSA);
609 NewLCSSA->addIncoming(OldLCSSA, MiddleBlock);
614 // The exit blocks may have been changed due to edge splitting, recompute.
616 L->getUniqueExitBlocks(ExitBlocks);
618 // Add exit blocks to the loop blocks.
619 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
621 // Next step, clone all of the basic blocks that make up the loop (including
622 // the loop preheader and exit blocks), keeping track of the mapping between
623 // the instructions and blocks.
624 std::vector<BasicBlock*> NewBlocks;
625 NewBlocks.reserve(LoopBlocks.size());
626 DenseMap<const Value*, Value*> ValueMap;
627 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
628 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F);
629 NewBlocks.push_back(New);
630 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping.
633 // Splice the newly inserted blocks into the function right before the
634 // original preheader.
635 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
636 NewBlocks[0], F->end());
638 // Now we create the new Loop object for the versioned loop.
639 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI, LPM);
640 Loop *ParentLoop = L->getParentLoop();
642 // Make sure to add the cloned preheader and exit blocks to the parent loop
644 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI);
647 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
648 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]);
649 // The new exit block should be in the same loop as the old one.
650 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
651 ExitBBLoop->addBasicBlockToLoop(NewExit, *LI);
653 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
654 "Exit block should have been split to have one successor!");
655 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
657 // If the successor of the exit block had PHI nodes, add an entry for
660 for (BasicBlock::iterator I = ExitSucc->begin();
661 (PN = dyn_cast<PHINode>(I)); ++I) {
662 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
663 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(V);
664 if (It != ValueMap.end()) V = It->second;
665 PN->addIncoming(V, NewExit);
669 // Rewrite the code to refer to itself.
670 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
671 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
672 E = NewBlocks[i]->end(); I != E; ++I)
673 RemapInstruction(I, ValueMap);
675 // Rewrite the original preheader to select between versions of the loop.
676 BranchInst *OldBR = cast<BranchInst>(OrigPreheader->getTerminator());
677 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
678 "Preheader splitting did not work correctly!");
680 // Emit the new branch that selects between the two versions of this loop.
681 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
682 OldBR->eraseFromParent();
684 LoopProcessWorklist.push_back(NewLoop);
687 // Now we rewrite the original code to know that the condition is true and the
688 // new code to know that the condition is false.
689 RewriteLoopBodyWithConditionConstant(L , LIC, Val, false);
691 // It's possible that simplifying one loop could cause the other to be
692 // deleted. If so, don't simplify it.
693 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop)
694 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
697 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
699 static void RemoveFromWorklist(Instruction *I,
700 std::vector<Instruction*> &Worklist) {
701 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
703 while (WI != Worklist.end()) {
704 unsigned Offset = WI-Worklist.begin();
706 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
710 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
711 /// program, replacing all uses with V and update the worklist.
712 static void ReplaceUsesOfWith(Instruction *I, Value *V,
713 std::vector<Instruction*> &Worklist) {
714 DOUT << "Replace with '" << *V << "': " << *I;
716 // Add uses to the worklist, which may be dead now.
717 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
718 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
719 Worklist.push_back(Use);
721 // Add users to the worklist which may be simplified now.
722 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
724 Worklist.push_back(cast<Instruction>(*UI));
725 I->replaceAllUsesWith(V);
726 I->eraseFromParent();
727 RemoveFromWorklist(I, Worklist);
731 /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
732 /// information, and remove any dead successors it has.
734 void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
735 std::vector<Instruction*> &Worklist) {
736 if (pred_begin(BB) != pred_end(BB)) {
737 // This block isn't dead, since an edge to BB was just removed, see if there
738 // are any easy simplifications we can do now.
739 if (BasicBlock *Pred = BB->getSinglePredecessor()) {
740 // If it has one pred, fold phi nodes in BB.
741 while (isa<PHINode>(BB->begin()))
742 ReplaceUsesOfWith(BB->begin(),
743 cast<PHINode>(BB->begin())->getIncomingValue(0),
746 // If this is the header of a loop and the only pred is the latch, we now
747 // have an unreachable loop.
748 if (Loop *L = LI->getLoopFor(BB))
749 if (L->getHeader() == BB && L->contains(Pred)) {
750 // Remove the branch from the latch to the header block, this makes
751 // the header dead, which will make the latch dead (because the header
752 // dominates the latch).
753 Pred->getTerminator()->eraseFromParent();
754 new UnreachableInst(Pred);
756 // The loop is now broken, remove it from LI.
757 RemoveLoopFromHierarchy(L);
759 // Reprocess the header, which now IS dead.
760 RemoveBlockIfDead(BB, Worklist);
764 // If pred ends in a uncond branch, add uncond branch to worklist so that
765 // the two blocks will get merged.
766 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
767 if (BI->isUnconditional())
768 Worklist.push_back(BI);
773 DOUT << "Nuking dead block: " << *BB;
775 // Remove the instructions in the basic block from the worklist.
776 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
777 RemoveFromWorklist(I, Worklist);
779 // Anything that uses the instructions in this basic block should have their
780 // uses replaced with undefs.
782 I->replaceAllUsesWith(UndefValue::get(I->getType()));
785 // If this is the edge to the header block for a loop, remove the loop and
786 // promote all subloops.
787 if (Loop *BBLoop = LI->getLoopFor(BB)) {
788 if (BBLoop->getLoopLatch() == BB)
789 RemoveLoopFromHierarchy(BBLoop);
792 // Remove the block from the loop info, which removes it from any loops it
797 // Remove phi node entries in successors for this block.
798 TerminatorInst *TI = BB->getTerminator();
799 std::vector<BasicBlock*> Succs;
800 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
801 Succs.push_back(TI->getSuccessor(i));
802 TI->getSuccessor(i)->removePredecessor(BB);
805 // Unique the successors, remove anything with multiple uses.
806 std::sort(Succs.begin(), Succs.end());
807 Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end());
809 // Remove the basic block, including all of the instructions contained in it.
810 BB->eraseFromParent();
812 // Remove successor blocks here that are not dead, so that we know we only
813 // have dead blocks in this list. Nondead blocks have a way of becoming dead,
814 // then getting removed before we revisit them, which is badness.
816 for (unsigned i = 0; i != Succs.size(); ++i)
817 if (pred_begin(Succs[i]) != pred_end(Succs[i])) {
818 // One exception is loop headers. If this block was the preheader for a
819 // loop, then we DO want to visit the loop so the loop gets deleted.
820 // We know that if the successor is a loop header, that this loop had to
821 // be the preheader: the case where this was the latch block was handled
822 // above and headers can only have two predecessors.
823 if (!LI->isLoopHeader(Succs[i])) {
824 Succs.erase(Succs.begin()+i);
829 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
830 RemoveBlockIfDead(Succs[i], Worklist);
833 /// RemoveLoopFromHierarchy - We have discovered that the specified loop has
834 /// become unwrapped, either because the backedge was deleted, or because the
835 /// edge into the header was removed. If the edge into the header from the
836 /// latch block was removed, the loop is unwrapped but subloops are still alive,
837 /// so they just reparent loops. If the loops are actually dead, they will be
839 void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) {
840 LPM->deleteLoopFromQueue(L);
841 RemoveLoopFromWorklist(L);
846 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
847 // the value specified by Val in the specified loop, or we know it does NOT have
848 // that value. Rewrite any uses of LIC or of properties correlated to it.
849 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
852 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
854 // FIXME: Support correlated properties, like:
861 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
862 // selects, switches.
863 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
864 std::vector<Instruction*> Worklist;
866 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
867 // in the loop with the appropriate one directly.
868 if (IsEqual || (isa<ConstantInt>(Val) && Val->getType() == Type::Int1Ty)) {
873 Replacement = ConstantInt::get(Type::Int1Ty,
874 !cast<ConstantInt>(Val)->getZExtValue());
876 for (unsigned i = 0, e = Users.size(); i != e; ++i)
877 if (Instruction *U = cast<Instruction>(Users[i])) {
878 if (!L->contains(U->getParent()))
880 U->replaceUsesOfWith(LIC, Replacement);
881 Worklist.push_back(U);
884 // Otherwise, we don't know the precise value of LIC, but we do know that it
885 // is certainly NOT "Val". As such, simplify any uses in the loop that we
886 // can. This case occurs when we unswitch switch statements.
887 for (unsigned i = 0, e = Users.size(); i != e; ++i)
888 if (Instruction *U = cast<Instruction>(Users[i])) {
889 if (!L->contains(U->getParent()))
892 Worklist.push_back(U);
894 // If we know that LIC is not Val, use this info to simplify code.
895 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
896 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
897 if (SI->getCaseValue(i) == Val) {
898 // Found a dead case value. Don't remove PHI nodes in the
899 // successor if they become single-entry, those PHI nodes may
900 // be in the Users list.
902 // FIXME: This is a hack. We need to keep the successor around
903 // and hooked up so as to preserve the loop structure, because
904 // trying to update it is complicated. So instead we preserve the
905 // loop structure and put the block on an dead code path.
907 BasicBlock* Old = SI->getParent();
908 BasicBlock* Split = SplitBlock(Old, SI);
910 Instruction* OldTerm = Old->getTerminator();
911 new BranchInst(Split, SI->getSuccessor(i),
912 ConstantInt::getTrue(), OldTerm);
914 Old->getTerminator()->eraseFromParent();
918 for (BasicBlock::iterator II = SI->getSuccessor(i)->begin();
919 (PN = dyn_cast<PHINode>(II)); ++II) {
920 Value *InVal = PN->removeIncomingValue(Split, false);
921 PN->addIncoming(InVal, Old);
930 // TODO: We could do other simplifications, for example, turning
931 // LIC == Val -> false.
935 SimplifyCode(Worklist);
938 /// SimplifyCode - Okay, now that we have simplified some instructions in the
939 /// loop, walk over it and constant prop, dce, and fold control flow where
940 /// possible. Note that this is effectively a very simple loop-structure-aware
941 /// optimizer. During processing of this loop, L could very well be deleted, so
942 /// it must not be used.
944 /// FIXME: When the loop optimizer is more mature, separate this out to a new
947 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist) {
948 while (!Worklist.empty()) {
949 Instruction *I = Worklist.back();
952 // Simple constant folding.
953 if (Constant *C = ConstantFoldInstruction(I)) {
954 ReplaceUsesOfWith(I, C, Worklist);
959 if (isInstructionTriviallyDead(I)) {
960 DOUT << "Remove dead instruction '" << *I;
962 // Add uses to the worklist, which may be dead now.
963 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
964 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
965 Worklist.push_back(Use);
966 I->eraseFromParent();
967 RemoveFromWorklist(I, Worklist);
972 // Special case hacks that appear commonly in unswitched code.
973 switch (I->getOpcode()) {
974 case Instruction::Select:
975 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(0))) {
976 ReplaceUsesOfWith(I, I->getOperand(!CB->getZExtValue()+1), Worklist);
980 case Instruction::And:
981 if (isa<ConstantInt>(I->getOperand(0)) &&
982 I->getOperand(0)->getType() == Type::Int1Ty) // constant -> RHS
983 cast<BinaryOperator>(I)->swapOperands();
984 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1)))
985 if (CB->getType() == Type::Int1Ty) {
986 if (CB->isOne()) // X & 1 -> X
987 ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
989 ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
993 case Instruction::Or:
994 if (isa<ConstantInt>(I->getOperand(0)) &&
995 I->getOperand(0)->getType() == Type::Int1Ty) // constant -> RHS
996 cast<BinaryOperator>(I)->swapOperands();
997 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1)))
998 if (CB->getType() == Type::Int1Ty) {
999 if (CB->isOne()) // X | 1 -> 1
1000 ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
1002 ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
1006 case Instruction::Br: {
1007 BranchInst *BI = cast<BranchInst>(I);
1008 if (BI->isUnconditional()) {
1009 // If BI's parent is the only pred of the successor, fold the two blocks
1011 BasicBlock *Pred = BI->getParent();
1012 BasicBlock *Succ = BI->getSuccessor(0);
1013 BasicBlock *SinglePred = Succ->getSinglePredecessor();
1014 if (!SinglePred) continue; // Nothing to do.
1015 assert(SinglePred == Pred && "CFG broken");
1017 DOUT << "Merging blocks: " << Pred->getName() << " <- "
1018 << Succ->getName() << "\n";
1020 // Resolve any single entry PHI nodes in Succ.
1021 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1022 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist);
1024 // Move all of the successor contents from Succ to Pred.
1025 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1027 BI->eraseFromParent();
1028 RemoveFromWorklist(BI, Worklist);
1030 // If Succ has any successors with PHI nodes, update them to have
1031 // entries coming from Pred instead of Succ.
1032 Succ->replaceAllUsesWith(Pred);
1034 // Remove Succ from the loop tree.
1035 LI->removeBlock(Succ);
1036 Succ->eraseFromParent();
1038 } else if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){
1039 // Conditional branch. Turn it into an unconditional branch, then
1040 // remove dead blocks.
1041 break; // FIXME: Enable.
1043 DOUT << "Folded branch: " << *BI;
1044 BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue());
1045 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue());
1046 DeadSucc->removePredecessor(BI->getParent(), true);
1047 Worklist.push_back(new BranchInst(LiveSucc, BI));
1048 BI->eraseFromParent();
1049 RemoveFromWorklist(BI, Worklist);
1052 RemoveBlockIfDead(DeadSucc, Worklist);