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/Function.h"
33 #include "llvm/Instructions.h"
34 #include "llvm/Analysis/LoopInfo.h"
35 #include "llvm/Transforms/Utils/Cloning.h"
36 #include "llvm/Transforms/Utils/Local.h"
37 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
38 #include "llvm/ADT/Statistic.h"
39 #include "llvm/ADT/PostOrderIterator.h"
40 #include "llvm/Support/Debug.h"
41 #include "llvm/Support/CommandLine.h"
46 STATISTIC(NumBranches, "Number of branches unswitched");
47 STATISTIC(NumSwitches, "Number of switches unswitched");
48 STATISTIC(NumSelects , "Number of selects unswitched");
49 STATISTIC(NumTrivial , "Number of unswitches that are trivial");
50 STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
54 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
55 cl::init(10), cl::Hidden);
57 class LoopUnswitch : public FunctionPass {
58 LoopInfo *LI; // Loop information
60 // LoopProcessWorklist - List of loops we need to process.
61 std::vector<Loop*> LoopProcessWorklist;
63 virtual bool runOnFunction(Function &F);
64 bool visitLoop(Loop *L);
66 /// This transformation requires natural loop information & requires that
67 /// loop preheaders be inserted into the CFG...
69 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
70 AU.addRequiredID(LoopSimplifyID);
71 AU.addPreservedID(LoopSimplifyID);
72 AU.addRequired<LoopInfo>();
73 AU.addPreserved<LoopInfo>();
74 AU.addRequiredID(LCSSAID);
75 AU.addPreservedID(LCSSAID);
79 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist,
81 void RemoveLoopFromWorklist(Loop *L) {
82 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(),
83 LoopProcessWorklist.end(), L);
84 if (I != LoopProcessWorklist.end())
85 LoopProcessWorklist.erase(I);
88 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L);
89 unsigned getLoopUnswitchCost(Loop *L, Value *LIC);
90 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
91 BasicBlock *ExitBlock);
92 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
93 BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To);
94 BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt);
96 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
97 Constant *Val, bool isEqual);
99 void SimplifyCode(std::vector<Instruction*> &Worklist);
100 void RemoveBlockIfDead(BasicBlock *BB,
101 std::vector<Instruction*> &Worklist);
102 void RemoveLoopFromHierarchy(Loop *L);
104 RegisterPass<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
107 FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }
109 bool LoopUnswitch::runOnFunction(Function &F) {
110 bool Changed = false;
111 LI = &getAnalysis<LoopInfo>();
113 // Populate the worklist of loops to process in post-order.
114 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
115 for (po_iterator<Loop*> LI = po_begin(*I), E = po_end(*I); LI != E; ++LI)
116 LoopProcessWorklist.push_back(*LI);
118 // Process the loops in worklist order, this is a post-order visitation of
119 // the loops. We use a worklist of loops so that loops can be removed at any
120 // time if they are deleted (e.g. the backedge of a loop is removed).
121 while (!LoopProcessWorklist.empty()) {
122 Loop *L = LoopProcessWorklist.back();
123 LoopProcessWorklist.pop_back();
124 Changed |= visitLoop(L);
130 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
131 /// invariant in the loop, or has an invariant piece, return the invariant.
132 /// Otherwise, return null.
133 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
134 // Constants should be folded, not unswitched on!
135 if (isa<Constant>(Cond)) return false;
137 // TODO: Handle: br (VARIANT|INVARIANT).
138 // TODO: Hoist simple expressions out of loops.
139 if (L->isLoopInvariant(Cond)) return Cond;
141 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
142 if (BO->getOpcode() == Instruction::And ||
143 BO->getOpcode() == Instruction::Or) {
144 // If either the left or right side is invariant, we can unswitch on this,
145 // which will cause the branch to go away in one loop and the condition to
146 // simplify in the other one.
147 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
149 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
156 bool LoopUnswitch::visitLoop(Loop *L) {
157 assert(L->isLCSSAForm());
159 bool Changed = false;
161 // Loop over all of the basic blocks in the loop. If we find an interior
162 // block that is branching on a loop-invariant condition, we can unswitch this
164 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
166 TerminatorInst *TI = (*I)->getTerminator();
167 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
168 // If this isn't branching on an invariant condition, we can't unswitch
170 if (BI->isConditional()) {
171 // See if this, or some part of it, is loop invariant. If so, we can
172 // unswitch on it if we desire.
173 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed);
174 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::getTrue(),
180 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
181 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
182 if (LoopCond && SI->getNumCases() > 1) {
183 // Find a value to unswitch on:
184 // FIXME: this should chose the most expensive case!
185 Constant *UnswitchVal = SI->getCaseValue(1);
186 if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) {
193 // Scan the instructions to check for unswitchable values.
194 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
196 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
197 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
198 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::getTrue(),
206 assert(L->isLCSSAForm());
211 /// isTrivialLoopExitBlock - Check to see if all paths from BB either:
212 /// 1. Exit the loop with no side effects.
213 /// 2. Branch to the latch block with no side-effects.
215 /// If these conditions are true, we return true and set ExitBB to the block we
218 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
220 std::set<BasicBlock*> &Visited) {
221 if (!Visited.insert(BB).second) {
222 // Already visited and Ok, end of recursion.
224 } else if (!L->contains(BB)) {
225 // Otherwise, this is a loop exit, this is fine so long as this is the
227 if (ExitBB != 0) return false;
232 // Otherwise, this is an unvisited intra-loop node. Check all successors.
233 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
234 // Check to see if the successor is a trivial loop exit.
235 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
239 // Okay, everything after this looks good, check to make sure that this block
240 // doesn't include any side effects.
241 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
242 if (I->mayWriteToMemory())
248 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally
249 /// leads to an exit from the specified loop, and has no side-effects in the
250 /// process. If so, return the block that is exited to, otherwise return null.
251 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
252 std::set<BasicBlock*> Visited;
253 Visited.insert(L->getHeader()); // Branches to header are ok.
254 BasicBlock *ExitBB = 0;
255 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
260 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
261 /// trivial: that is, that the condition controls whether or not the loop does
262 /// anything at all. If this is a trivial condition, unswitching produces no
263 /// code duplications (equivalently, it produces a simpler loop and a new empty
264 /// loop, which gets deleted).
266 /// If this is a trivial condition, return true, otherwise return false. When
267 /// returning true, this sets Cond and Val to the condition that controls the
268 /// trivial condition: when Cond dynamically equals Val, the loop is known to
269 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when
272 static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond, Constant **Val = 0,
273 BasicBlock **LoopExit = 0) {
274 BasicBlock *Header = L->getHeader();
275 TerminatorInst *HeaderTerm = Header->getTerminator();
277 BasicBlock *LoopExitBB = 0;
278 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
279 // If the header block doesn't end with a conditional branch on Cond, we
281 if (!BI->isConditional() || BI->getCondition() != Cond)
284 // Check to see if a successor of the branch is guaranteed to go to the
285 // latch block or exit through a one exit block without having any
286 // side-effects. If so, determine the value of Cond that causes it to do
288 if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(0)))) {
289 if (Val) *Val = ConstantBool::getTrue();
290 } else if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(1)))) {
291 if (Val) *Val = ConstantBool::getFalse();
293 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
294 // If this isn't a switch on Cond, we can't handle it.
295 if (SI->getCondition() != Cond) return false;
297 // Check to see if a successor of the switch is guaranteed to go to the
298 // latch block or exit through a one exit block without having any
299 // side-effects. If so, determine the value of Cond that causes it to do
300 // this. Note that we can't trivially unswitch on the default case.
301 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
302 if ((LoopExitBB = isTrivialLoopExitBlock(L, SI->getSuccessor(i)))) {
303 // Okay, we found a trivial case, remember the value that is trivial.
304 if (Val) *Val = SI->getCaseValue(i);
309 // If we didn't find a single unique LoopExit block, or if the loop exit block
310 // contains phi nodes, this isn't trivial.
311 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
312 return false; // Can't handle this.
314 if (LoopExit) *LoopExit = LoopExitBB;
316 // We already know that nothing uses any scalar values defined inside of this
317 // loop. As such, we just have to check to see if this loop will execute any
318 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
319 // part of the loop that the code *would* execute. We already checked the
320 // tail, check the header now.
321 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
322 if (I->mayWriteToMemory())
327 /// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
328 /// we choose to unswitch the specified loop on the specified value.
330 unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
331 // If the condition is trivial, always unswitch. There is no code growth for
333 if (IsTrivialUnswitchCondition(L, LIC))
336 // FIXME: This is really overly conservative. However, more liberal
337 // estimations have thus far resulted in excessive unswitching, which is bad
338 // both in compile time and in code size. This should be replaced once
339 // someone figures out how a good estimation.
340 return L->getBlocks().size();
343 // FIXME: this is brain dead. It should take into consideration code
345 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
348 // Do not include empty blocks in the cost calculation. This happen due to
349 // loop canonicalization and will be removed.
350 if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
353 // Count basic blocks.
360 /// UnswitchIfProfitable - We have found that we can unswitch L when
361 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
362 /// unswitch the loop, reprocess the pieces, then return true.
363 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){
364 // Check to see if it would be profitable to unswitch this loop.
365 unsigned Cost = getLoopUnswitchCost(L, LoopCond);
366 if (Cost > Threshold) {
367 // FIXME: this should estimate growth by the amount of code shared by the
368 // resultant unswitched loops.
370 DOUT << "NOT unswitching loop %"
371 << L->getHeader()->getName() << ", cost too high: "
372 << L->getBlocks().size() << "\n";
376 // If this is a trivial condition to unswitch (which results in no code
377 // duplication), do it now.
379 BasicBlock *ExitBlock;
380 if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal, &ExitBlock)) {
381 UnswitchTrivialCondition(L, LoopCond, CondVal, ExitBlock);
383 UnswitchNontrivialCondition(LoopCond, Val, L);
389 /// SplitBlock - Split the specified block at the specified instruction - every
390 /// thing before SplitPt stays in Old and everything starting with SplitPt moves
391 /// to a new block. The two blocks are joined by an unconditional branch and
392 /// the loop info is updated.
394 BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *Old, Instruction *SplitPt) {
395 BasicBlock::iterator SplitIt = SplitPt;
396 while (isa<PHINode>(SplitIt))
398 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
400 // The new block lives in whichever loop the old one did.
401 if (Loop *L = LI->getLoopFor(Old))
402 L->addBasicBlockToLoop(New, *LI);
408 BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) {
409 TerminatorInst *LatchTerm = BB->getTerminator();
410 unsigned SuccNum = 0;
411 for (unsigned i = 0, e = LatchTerm->getNumSuccessors(); ; ++i) {
412 assert(i != e && "Didn't find edge?");
413 if (LatchTerm->getSuccessor(i) == Succ) {
419 // If this is a critical edge, let SplitCriticalEdge do it.
420 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, this))
421 return LatchTerm->getSuccessor(SuccNum);
423 // If the edge isn't critical, then BB has a single successor or Succ has a
424 // single pred. Split the block.
425 BasicBlock::iterator SplitPoint;
426 if (BasicBlock *SP = Succ->getSinglePredecessor()) {
427 // If the successor only has a single pred, split the top of the successor
429 assert(SP == BB && "CFG broken");
430 return SplitBlock(Succ, Succ->begin());
432 // Otherwise, if BB has a single successor, split it at the bottom of the
434 assert(BB->getTerminator()->getNumSuccessors() == 1 &&
435 "Should have a single succ!");
436 return SplitBlock(BB, BB->getTerminator());
442 // RemapInstruction - Convert the instruction operands from referencing the
443 // current values into those specified by ValueMap.
445 static inline void RemapInstruction(Instruction *I,
446 std::map<const Value *, Value*> &ValueMap) {
447 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
448 Value *Op = I->getOperand(op);
449 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
450 if (It != ValueMap.end()) Op = It->second;
451 I->setOperand(op, Op);
455 /// CloneLoop - Recursively clone the specified loop and all of its children,
456 /// mapping the blocks with the specified map.
457 static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
459 Loop *New = new Loop();
462 PL->addChildLoop(New);
464 LI->addTopLevelLoop(New);
466 // Add all of the blocks in L to the new loop.
467 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
469 if (LI->getLoopFor(*I) == L)
470 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
472 // Add all of the subloops to the new loop.
473 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
474 CloneLoop(*I, New, VM, LI);
479 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
480 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
481 /// code immediately before InsertPt.
482 static void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
483 BasicBlock *TrueDest,
484 BasicBlock *FalseDest,
485 Instruction *InsertPt) {
486 // Insert a conditional branch on LIC to the two preheaders. The original
487 // code is the true version and the new code is the false version.
488 Value *BranchVal = LIC;
489 if (!isa<ConstantBool>(Val)) {
490 BranchVal = BinaryOperator::createSetEQ(LIC, Val, "tmp", InsertPt);
491 } else if (Val != ConstantBool::getTrue()) {
492 // We want to enter the new loop when the condition is true.
493 std::swap(TrueDest, FalseDest);
496 // Insert the new branch.
497 new BranchInst(TrueDest, FalseDest, BranchVal, InsertPt);
501 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
502 /// condition in it (a cond branch from its header block to its latch block,
503 /// where the path through the loop that doesn't execute its body has no
504 /// side-effects), unswitch it. This doesn't involve any code duplication, just
505 /// moving the conditional branch outside of the loop and updating loop info.
506 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
508 BasicBlock *ExitBlock) {
509 DOUT << "loop-unswitch: Trivial-Unswitch loop %"
510 << L->getHeader()->getName() << " [" << L->getBlocks().size()
511 << " blocks] in Function " << L->getHeader()->getParent()->getName()
512 << " on cond: " << *Val << " == " << *Cond << "\n";
514 // First step, split the preheader, so that we know that there is a safe place
515 // to insert the conditional branch. We will change 'OrigPH' to have a
516 // conditional branch on Cond.
517 BasicBlock *OrigPH = L->getLoopPreheader();
518 BasicBlock *NewPH = SplitEdge(OrigPH, L->getHeader());
520 // Now that we have a place to insert the conditional branch, create a place
521 // to branch to: this is the exit block out of the loop that we should
524 // Split this block now, so that the loop maintains its exit block, and so
525 // that the jump from the preheader can execute the contents of the exit block
526 // without actually branching to it (the exit block should be dominated by the
527 // loop header, not the preheader).
528 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
529 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin());
531 // Okay, now we have a position to branch from and a position to branch to,
532 // insert the new conditional branch.
533 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
534 OrigPH->getTerminator());
535 OrigPH->getTerminator()->eraseFromParent();
537 // We need to reprocess this loop, it could be unswitched again.
538 LoopProcessWorklist.push_back(L);
540 // Now that we know that the loop is never entered when this condition is a
541 // particular value, rewrite the loop with this info. We know that this will
542 // at least eliminate the old branch.
543 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
548 /// VersionLoop - We determined that the loop is profitable to unswitch when LIC
549 /// equal Val. Split it into loop versions and test the condition outside of
550 /// either loop. Return the loops created as Out1/Out2.
551 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
553 Function *F = L->getHeader()->getParent();
554 DOUT << "loop-unswitch: Unswitching loop %"
555 << L->getHeader()->getName() << " [" << L->getBlocks().size()
556 << " blocks] in Function " << F->getName()
557 << " when '" << *Val << "' == " << *LIC << "\n";
559 // LoopBlocks contains all of the basic blocks of the loop, including the
560 // preheader of the loop, the body of the loop, and the exit blocks of the
561 // loop, in that order.
562 std::vector<BasicBlock*> LoopBlocks;
564 // First step, split the preheader and exit blocks, and add these blocks to
565 // the LoopBlocks list.
566 BasicBlock *OrigPreheader = L->getLoopPreheader();
567 LoopBlocks.push_back(SplitEdge(OrigPreheader, L->getHeader()));
569 // We want the loop to come after the preheader, but before the exit blocks.
570 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
572 std::vector<BasicBlock*> ExitBlocks;
573 L->getUniqueExitBlocks(ExitBlocks);
575 // Split all of the edges from inside the loop to their exit blocks. Update
576 // the appropriate Phi nodes as we do so.
577 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
578 BasicBlock *ExitBlock = ExitBlocks[i];
579 std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock));
581 for (unsigned j = 0, e = Preds.size(); j != e; ++j) {
582 assert(L->contains(Preds[j]) &&
583 "All preds of loop exit blocks must be the same loop!");
584 BasicBlock* MiddleBlock = SplitEdge(Preds[j], ExitBlock);
585 BasicBlock* StartBlock = Preds[j];
586 BasicBlock* EndBlock;
587 if (MiddleBlock->getSinglePredecessor() == ExitBlock) {
588 EndBlock = MiddleBlock;
589 MiddleBlock = EndBlock->getSinglePredecessor();;
591 EndBlock = ExitBlock;
594 std::set<PHINode*> InsertedPHIs;
595 PHINode* OldLCSSA = 0;
596 for (BasicBlock::iterator I = EndBlock->begin();
597 (OldLCSSA = dyn_cast<PHINode>(I)); ++I) {
598 Value* OldValue = OldLCSSA->getIncomingValueForBlock(MiddleBlock);
599 PHINode* NewLCSSA = new PHINode(OldLCSSA->getType(),
600 OldLCSSA->getName() + ".us-lcssa",
601 MiddleBlock->getTerminator());
602 NewLCSSA->addIncoming(OldValue, StartBlock);
603 OldLCSSA->setIncomingValue(OldLCSSA->getBasicBlockIndex(MiddleBlock),
605 InsertedPHIs.insert(NewLCSSA);
608 BasicBlock::iterator InsertPt = EndBlock->begin();
609 while (dyn_cast<PHINode>(InsertPt)) ++InsertPt;
610 for (BasicBlock::iterator I = MiddleBlock->begin();
611 (OldLCSSA = dyn_cast<PHINode>(I)) && InsertedPHIs.count(OldLCSSA) == 0;
613 PHINode *NewLCSSA = new PHINode(OldLCSSA->getType(),
614 OldLCSSA->getName() + ".us-lcssa",
616 OldLCSSA->replaceAllUsesWith(NewLCSSA);
617 NewLCSSA->addIncoming(OldLCSSA, MiddleBlock);
622 // The exit blocks may have been changed due to edge splitting, recompute.
624 L->getUniqueExitBlocks(ExitBlocks);
626 // Add exit blocks to the loop blocks.
627 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
629 // Next step, clone all of the basic blocks that make up the loop (including
630 // the loop preheader and exit blocks), keeping track of the mapping between
631 // the instructions and blocks.
632 std::vector<BasicBlock*> NewBlocks;
633 NewBlocks.reserve(LoopBlocks.size());
634 std::map<const Value*, Value*> ValueMap;
635 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
636 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F);
637 NewBlocks.push_back(New);
638 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping.
641 // Splice the newly inserted blocks into the function right before the
642 // original preheader.
643 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
644 NewBlocks[0], F->end());
646 // Now we create the new Loop object for the versioned loop.
647 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
648 Loop *ParentLoop = L->getParentLoop();
650 // Make sure to add the cloned preheader and exit blocks to the parent loop
652 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI);
655 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
656 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]);
657 // The new exit block should be in the same loop as the old one.
658 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
659 ExitBBLoop->addBasicBlockToLoop(NewExit, *LI);
661 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
662 "Exit block should have been split to have one successor!");
663 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
665 // If the successor of the exit block had PHI nodes, add an entry for
668 for (BasicBlock::iterator I = ExitSucc->begin();
669 (PN = dyn_cast<PHINode>(I)); ++I) {
670 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
671 std::map<const Value *, Value*>::iterator It = ValueMap.find(V);
672 if (It != ValueMap.end()) V = It->second;
673 PN->addIncoming(V, NewExit);
677 // Rewrite the code to refer to itself.
678 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
679 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
680 E = NewBlocks[i]->end(); I != E; ++I)
681 RemapInstruction(I, ValueMap);
683 // Rewrite the original preheader to select between versions of the loop.
684 BranchInst *OldBR = cast<BranchInst>(OrigPreheader->getTerminator());
685 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
686 "Preheader splitting did not work correctly!");
688 // Emit the new branch that selects between the two versions of this loop.
689 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
690 OldBR->eraseFromParent();
692 LoopProcessWorklist.push_back(L);
693 LoopProcessWorklist.push_back(NewLoop);
695 // Now we rewrite the original code to know that the condition is true and the
696 // new code to know that the condition is false.
697 RewriteLoopBodyWithConditionConstant(L , LIC, Val, false);
699 // It's possible that simplifying one loop could cause the other to be
700 // deleted. If so, don't simplify it.
701 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop)
702 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
705 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
707 static void RemoveFromWorklist(Instruction *I,
708 std::vector<Instruction*> &Worklist) {
709 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
711 while (WI != Worklist.end()) {
712 unsigned Offset = WI-Worklist.begin();
714 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
718 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
719 /// program, replacing all uses with V and update the worklist.
720 static void ReplaceUsesOfWith(Instruction *I, Value *V,
721 std::vector<Instruction*> &Worklist) {
722 DOUT << "Replace with '" << *V << "': " << *I;
724 // Add uses to the worklist, which may be dead now.
725 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
726 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
727 Worklist.push_back(Use);
729 // Add users to the worklist which may be simplified now.
730 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
732 Worklist.push_back(cast<Instruction>(*UI));
733 I->replaceAllUsesWith(V);
734 I->eraseFromParent();
735 RemoveFromWorklist(I, Worklist);
739 /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
740 /// information, and remove any dead successors it has.
742 void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
743 std::vector<Instruction*> &Worklist) {
744 if (pred_begin(BB) != pred_end(BB)) {
745 // This block isn't dead, since an edge to BB was just removed, see if there
746 // are any easy simplifications we can do now.
747 if (BasicBlock *Pred = BB->getSinglePredecessor()) {
748 // If it has one pred, fold phi nodes in BB.
749 while (isa<PHINode>(BB->begin()))
750 ReplaceUsesOfWith(BB->begin(),
751 cast<PHINode>(BB->begin())->getIncomingValue(0),
754 // If this is the header of a loop and the only pred is the latch, we now
755 // have an unreachable loop.
756 if (Loop *L = LI->getLoopFor(BB))
757 if (L->getHeader() == BB && L->contains(Pred)) {
758 // Remove the branch from the latch to the header block, this makes
759 // the header dead, which will make the latch dead (because the header
760 // dominates the latch).
761 Pred->getTerminator()->eraseFromParent();
762 new UnreachableInst(Pred);
764 // The loop is now broken, remove it from LI.
765 RemoveLoopFromHierarchy(L);
767 // Reprocess the header, which now IS dead.
768 RemoveBlockIfDead(BB, Worklist);
772 // If pred ends in a uncond branch, add uncond branch to worklist so that
773 // the two blocks will get merged.
774 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
775 if (BI->isUnconditional())
776 Worklist.push_back(BI);
781 DOUT << "Nuking dead block: " << *BB;
783 // Remove the instructions in the basic block from the worklist.
784 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
785 RemoveFromWorklist(I, Worklist);
787 // Anything that uses the instructions in this basic block should have their
788 // uses replaced with undefs.
790 I->replaceAllUsesWith(UndefValue::get(I->getType()));
793 // If this is the edge to the header block for a loop, remove the loop and
794 // promote all subloops.
795 if (Loop *BBLoop = LI->getLoopFor(BB)) {
796 if (BBLoop->getLoopLatch() == BB)
797 RemoveLoopFromHierarchy(BBLoop);
800 // Remove the block from the loop info, which removes it from any loops it
805 // Remove phi node entries in successors for this block.
806 TerminatorInst *TI = BB->getTerminator();
807 std::vector<BasicBlock*> Succs;
808 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
809 Succs.push_back(TI->getSuccessor(i));
810 TI->getSuccessor(i)->removePredecessor(BB);
813 // Unique the successors, remove anything with multiple uses.
814 std::sort(Succs.begin(), Succs.end());
815 Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end());
817 // Remove the basic block, including all of the instructions contained in it.
818 BB->eraseFromParent();
820 // Remove successor blocks here that are not dead, so that we know we only
821 // have dead blocks in this list. Nondead blocks have a way of becoming dead,
822 // then getting removed before we revisit them, which is badness.
824 for (unsigned i = 0; i != Succs.size(); ++i)
825 if (pred_begin(Succs[i]) != pred_end(Succs[i])) {
826 // One exception is loop headers. If this block was the preheader for a
827 // loop, then we DO want to visit the loop so the loop gets deleted.
828 // We know that if the successor is a loop header, that this loop had to
829 // be the preheader: the case where this was the latch block was handled
830 // above and headers can only have two predecessors.
831 if (!LI->isLoopHeader(Succs[i])) {
832 Succs.erase(Succs.begin()+i);
837 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
838 RemoveBlockIfDead(Succs[i], Worklist);
841 /// RemoveLoopFromHierarchy - We have discovered that the specified loop has
842 /// become unwrapped, either because the backedge was deleted, or because the
843 /// edge into the header was removed. If the edge into the header from the
844 /// latch block was removed, the loop is unwrapped but subloops are still alive,
845 /// so they just reparent loops. If the loops are actually dead, they will be
847 void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) {
848 if (Loop *ParentLoop = L->getParentLoop()) { // Not a top-level loop.
849 // Reparent all of the blocks in this loop. Since BBLoop had a parent,
850 // they are now all in it.
851 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
853 if (LI->getLoopFor(*I) == L) // Don't change blocks in subloops.
854 LI->changeLoopFor(*I, ParentLoop);
856 // Remove the loop from its parent loop.
857 for (Loop::iterator I = ParentLoop->begin(), E = ParentLoop->end();;
859 assert(I != E && "Couldn't find loop");
861 ParentLoop->removeChildLoop(I);
866 // Move all subloops into the parent loop.
867 while (L->begin() != L->end())
868 ParentLoop->addChildLoop(L->removeChildLoop(L->end()-1));
870 // Reparent all of the blocks in this loop. Since BBLoop had no parent,
871 // they no longer in a loop at all.
873 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
874 // Don't change blocks in subloops.
875 if (LI->getLoopFor(L->getBlocks()[i]) == L) {
876 LI->removeBlock(L->getBlocks()[i]);
881 // Remove the loop from the top-level LoopInfo object.
882 for (LoopInfo::iterator I = LI->begin(), E = LI->end();; ++I) {
883 assert(I != E && "Couldn't find loop");
890 // Move all of the subloops to the top-level.
891 while (L->begin() != L->end())
892 LI->addTopLevelLoop(L->removeChildLoop(L->end()-1));
896 RemoveLoopFromWorklist(L);
901 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
902 // the value specified by Val in the specified loop, or we know it does NOT have
903 // that value. Rewrite any uses of LIC or of properties correlated to it.
904 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
907 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
909 // FIXME: Support correlated properties, like:
916 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
917 // selects, switches.
918 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
919 std::vector<Instruction*> Worklist;
921 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
922 // in the loop with the appropriate one directly.
923 if (IsEqual || isa<ConstantBool>(Val)) {
928 Replacement = ConstantBool::get(!cast<ConstantBool>(Val)->getValue());
930 for (unsigned i = 0, e = Users.size(); i != e; ++i)
931 if (Instruction *U = cast<Instruction>(Users[i])) {
932 if (!L->contains(U->getParent()))
934 U->replaceUsesOfWith(LIC, Replacement);
935 Worklist.push_back(U);
938 // Otherwise, we don't know the precise value of LIC, but we do know that it
939 // is certainly NOT "Val". As such, simplify any uses in the loop that we
940 // can. This case occurs when we unswitch switch statements.
941 for (unsigned i = 0, e = Users.size(); i != e; ++i)
942 if (Instruction *U = cast<Instruction>(Users[i])) {
943 if (!L->contains(U->getParent()))
946 Worklist.push_back(U);
948 // If we know that LIC is not Val, use this info to simplify code.
949 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
950 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
951 if (SI->getCaseValue(i) == Val) {
952 // Found a dead case value. Don't remove PHI nodes in the
953 // successor if they become single-entry, those PHI nodes may
954 // be in the Users list.
956 // FIXME: This is a hack. We need to keep the successor around
957 // and hooked up so as to preserve the loop structure, because
958 // trying to update it is complicated. So instead we preserve the
959 // loop structure and put the block on an dead code path.
961 BasicBlock* Old = SI->getParent();
962 BasicBlock* Split = SplitBlock(Old, SI);
964 Instruction* OldTerm = Old->getTerminator();
965 new BranchInst(Split, SI->getSuccessor(i),
966 ConstantBool::getTrue(), OldTerm);
968 Old->getTerminator()->eraseFromParent();
972 for (BasicBlock::iterator II = SI->getSuccessor(i)->begin();
973 (PN = dyn_cast<PHINode>(II)); ++II) {
974 Value *InVal = PN->removeIncomingValue(Split, false);
975 PN->addIncoming(InVal, Old);
984 // TODO: We could do other simplifications, for example, turning
985 // LIC == Val -> false.
989 SimplifyCode(Worklist);
992 /// SimplifyCode - Okay, now that we have simplified some instructions in the
993 /// loop, walk over it and constant prop, dce, and fold control flow where
994 /// possible. Note that this is effectively a very simple loop-structure-aware
995 /// optimizer. During processing of this loop, L could very well be deleted, so
996 /// it must not be used.
998 /// FIXME: When the loop optimizer is more mature, separate this out to a new
1001 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist) {
1002 while (!Worklist.empty()) {
1003 Instruction *I = Worklist.back();
1004 Worklist.pop_back();
1006 // Simple constant folding.
1007 if (Constant *C = ConstantFoldInstruction(I)) {
1008 ReplaceUsesOfWith(I, C, Worklist);
1013 if (isInstructionTriviallyDead(I)) {
1014 DOUT << "Remove dead instruction '" << *I;
1016 // Add uses to the worklist, which may be dead now.
1017 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
1018 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
1019 Worklist.push_back(Use);
1020 I->eraseFromParent();
1021 RemoveFromWorklist(I, Worklist);
1026 // Special case hacks that appear commonly in unswitched code.
1027 switch (I->getOpcode()) {
1028 case Instruction::Select:
1029 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(0))) {
1030 ReplaceUsesOfWith(I, I->getOperand(!CB->getValue()+1), Worklist);
1034 case Instruction::And:
1035 if (isa<ConstantBool>(I->getOperand(0))) // constant -> RHS
1036 cast<BinaryOperator>(I)->swapOperands();
1037 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) {
1038 if (CB->getValue()) // X & 1 -> X
1039 ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
1041 ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
1045 case Instruction::Or:
1046 if (isa<ConstantBool>(I->getOperand(0))) // constant -> RHS
1047 cast<BinaryOperator>(I)->swapOperands();
1048 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) {
1049 if (CB->getValue()) // X | 1 -> 1
1050 ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
1052 ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
1056 case Instruction::Br: {
1057 BranchInst *BI = cast<BranchInst>(I);
1058 if (BI->isUnconditional()) {
1059 // If BI's parent is the only pred of the successor, fold the two blocks
1061 BasicBlock *Pred = BI->getParent();
1062 BasicBlock *Succ = BI->getSuccessor(0);
1063 BasicBlock *SinglePred = Succ->getSinglePredecessor();
1064 if (!SinglePred) continue; // Nothing to do.
1065 assert(SinglePred == Pred && "CFG broken");
1067 DOUT << "Merging blocks: " << Pred->getName() << " <- "
1068 << Succ->getName() << "\n";
1070 // Resolve any single entry PHI nodes in Succ.
1071 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1072 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist);
1074 // Move all of the successor contents from Succ to Pred.
1075 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1077 BI->eraseFromParent();
1078 RemoveFromWorklist(BI, Worklist);
1080 // If Succ has any successors with PHI nodes, update them to have
1081 // entries coming from Pred instead of Succ.
1082 Succ->replaceAllUsesWith(Pred);
1084 // Remove Succ from the loop tree.
1085 LI->removeBlock(Succ);
1086 Succ->eraseFromParent();
1088 } else if (ConstantBool *CB = dyn_cast<ConstantBool>(BI->getCondition())){
1089 // Conditional branch. Turn it into an unconditional branch, then
1090 // remove dead blocks.
1091 break; // FIXME: Enable.
1093 DOUT << "Folded branch: " << *BI;
1094 BasicBlock *DeadSucc = BI->getSuccessor(CB->getValue());
1095 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getValue());
1096 DeadSucc->removePredecessor(BI->getParent(), true);
1097 Worklist.push_back(new BranchInst(LiveSucc, BI));
1098 BI->eraseFromParent();
1099 RemoveFromWorklist(BI, Worklist);
1102 RemoveBlockIfDead(DeadSucc, Worklist);