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"
47 Statistic<> NumBranches("loop-unswitch", "Number of branches unswitched");
48 Statistic<> NumSwitches("loop-unswitch", "Number of switches unswitched");
49 Statistic<> NumSelects ("loop-unswitch", "Number of selects unswitched");
50 Statistic<> NumTrivial ("loop-unswitch",
51 "Number of unswitches that are trivial");
52 Statistic<> NumSimplify("loop-unswitch",
53 "Number of simplifications of unswitched code");
55 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
56 cl::init(10), cl::Hidden);
58 class LoopUnswitch : public FunctionPass {
59 LoopInfo *LI; // Loop information
61 // LoopProcessWorklist - List of loops we need to process.
62 std::vector<Loop*> LoopProcessWorklist;
64 virtual bool runOnFunction(Function &F);
65 bool visitLoop(Loop *L);
67 /// This transformation requires natural loop information & requires that
68 /// loop preheaders be inserted into the CFG...
70 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
71 AU.addRequiredID(LoopSimplifyID);
72 AU.addPreservedID(LoopSimplifyID);
73 AU.addRequired<LoopInfo>();
74 AU.addPreserved<LoopInfo>();
75 AU.addRequiredID(LCSSAID);
76 AU.addPreservedID(LCSSAID);
80 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist,
82 void RemoveLoopFromWorklist(Loop *L) {
83 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(),
84 LoopProcessWorklist.end(), L);
85 if (I != LoopProcessWorklist.end())
86 LoopProcessWorklist.erase(I);
89 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L);
90 unsigned getLoopUnswitchCost(Loop *L, Value *LIC);
91 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
92 BasicBlock *ExitBlock);
93 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
94 BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To);
95 BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt);
97 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
98 Constant *Val, bool isEqual);
100 void SimplifyCode(std::vector<Instruction*> &Worklist);
101 void RemoveBlockIfDead(BasicBlock *BB,
102 std::vector<Instruction*> &Worklist);
103 void RemoveLoopFromHierarchy(Loop *L);
105 RegisterPass<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
108 FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }
110 bool LoopUnswitch::runOnFunction(Function &F) {
111 bool Changed = false;
112 LI = &getAnalysis<LoopInfo>();
114 // Populate the worklist of loops to process in post-order.
115 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
116 for (po_iterator<Loop*> LI = po_begin(*I), E = po_end(*I); LI != E; ++LI)
117 LoopProcessWorklist.push_back(*LI);
119 // Process the loops in worklist order, this is a post-order visitation of
120 // the loops. We use a worklist of loops so that loops can be removed at any
121 // time if they are deleted (e.g. the backedge of a loop is removed).
122 while (!LoopProcessWorklist.empty()) {
123 Loop *L = LoopProcessWorklist.back();
124 LoopProcessWorklist.pop_back();
125 Changed |= visitLoop(L);
131 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
132 /// invariant in the loop, or has an invariant piece, return the invariant.
133 /// Otherwise, return null.
134 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
135 // Constants should be folded, not unswitched on!
136 if (isa<Constant>(Cond)) return false;
138 // TODO: Handle: br (VARIANT|INVARIANT).
139 // TODO: Hoist simple expressions out of loops.
140 if (L->isLoopInvariant(Cond)) return Cond;
142 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
143 if (BO->getOpcode() == Instruction::And ||
144 BO->getOpcode() == Instruction::Or) {
145 // If either the left or right side is invariant, we can unswitch on this,
146 // which will cause the branch to go away in one loop and the condition to
147 // simplify in the other one.
148 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
150 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
157 bool LoopUnswitch::visitLoop(Loop *L) {
158 assert(L->isLCSSAForm());
160 bool Changed = false;
162 // Loop over all of the basic blocks in the loop. If we find an interior
163 // block that is branching on a loop-invariant condition, we can unswitch this
165 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
167 TerminatorInst *TI = (*I)->getTerminator();
168 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
169 // If this isn't branching on an invariant condition, we can't unswitch
171 if (BI->isConditional()) {
172 // See if this, or some part of it, is loop invariant. If so, we can
173 // unswitch on it if we desire.
174 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed);
175 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::getTrue(),
181 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
182 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
183 if (LoopCond && SI->getNumCases() > 1) {
184 // Find a value to unswitch on:
185 // FIXME: this should chose the most expensive case!
186 Constant *UnswitchVal = SI->getCaseValue(1);
187 if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) {
194 // Scan the instructions to check for unswitchable values.
195 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
197 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
198 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
199 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::getTrue(),
207 assert(L->isLCSSAForm());
212 /// isTrivialLoopExitBlock - Check to see if all paths from BB either:
213 /// 1. Exit the loop with no side effects.
214 /// 2. Branch to the latch block with no side-effects.
216 /// If these conditions are true, we return true and set ExitBB to the block we
219 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
221 std::set<BasicBlock*> &Visited) {
222 if (!Visited.insert(BB).second) {
223 // Already visited and Ok, end of recursion.
225 } else if (!L->contains(BB)) {
226 // Otherwise, this is a loop exit, this is fine so long as this is the
228 if (ExitBB != 0) return false;
233 // Otherwise, this is an unvisited intra-loop node. Check all successors.
234 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
235 // Check to see if the successor is a trivial loop exit.
236 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
240 // Okay, everything after this looks good, check to make sure that this block
241 // doesn't include any side effects.
242 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
243 if (I->mayWriteToMemory())
249 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally
250 /// leads to an exit from the specified loop, and has no side-effects in the
251 /// process. If so, return the block that is exited to, otherwise return null.
252 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
253 std::set<BasicBlock*> Visited;
254 Visited.insert(L->getHeader()); // Branches to header are ok.
255 BasicBlock *ExitBB = 0;
256 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
261 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
262 /// trivial: that is, that the condition controls whether or not the loop does
263 /// anything at all. If this is a trivial condition, unswitching produces no
264 /// code duplications (equivalently, it produces a simpler loop and a new empty
265 /// loop, which gets deleted).
267 /// If this is a trivial condition, return true, otherwise return false. When
268 /// returning true, this sets Cond and Val to the condition that controls the
269 /// trivial condition: when Cond dynamically equals Val, the loop is known to
270 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when
273 static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond, Constant **Val = 0,
274 BasicBlock **LoopExit = 0) {
275 BasicBlock *Header = L->getHeader();
276 TerminatorInst *HeaderTerm = Header->getTerminator();
278 BasicBlock *LoopExitBB = 0;
279 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
280 // If the header block doesn't end with a conditional branch on Cond, we
282 if (!BI->isConditional() || BI->getCondition() != Cond)
285 // Check to see if a successor of the branch is guaranteed to go to the
286 // latch block or exit through a one exit block without having any
287 // side-effects. If so, determine the value of Cond that causes it to do
289 if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(0)))) {
290 if (Val) *Val = ConstantBool::getTrue();
291 } else if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(1)))) {
292 if (Val) *Val = ConstantBool::getFalse();
294 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
295 // If this isn't a switch on Cond, we can't handle it.
296 if (SI->getCondition() != Cond) return false;
298 // Check to see if a successor of the switch is guaranteed to go to the
299 // latch block or exit through a one exit block without having any
300 // side-effects. If so, determine the value of Cond that causes it to do
301 // this. Note that we can't trivially unswitch on the default case.
302 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
303 if ((LoopExitBB = isTrivialLoopExitBlock(L, SI->getSuccessor(i)))) {
304 // Okay, we found a trivial case, remember the value that is trivial.
305 if (Val) *Val = SI->getCaseValue(i);
310 // If we didn't find a single unique LoopExit block, or if the loop exit block
311 // contains phi nodes, this isn't trivial.
312 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
313 return false; // Can't handle this.
315 if (LoopExit) *LoopExit = LoopExitBB;
317 // We already know that nothing uses any scalar values defined inside of this
318 // loop. As such, we just have to check to see if this loop will execute any
319 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
320 // part of the loop that the code *would* execute. We already checked the
321 // tail, check the header now.
322 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
323 if (I->mayWriteToMemory())
328 /// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
329 /// we choose to unswitch the specified loop on the specified value.
331 unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
332 // If the condition is trivial, always unswitch. There is no code growth for
334 if (IsTrivialUnswitchCondition(L, LIC))
337 // FIXME: This is really overly conservative. However, more liberal
338 // estimations have thus far resulted in excessive unswitching, which is bad
339 // both in compile time and in code size. This should be replaced once
340 // someone figures out how a good estimation.
341 return L->getBlocks().size();
344 // FIXME: this is brain dead. It should take into consideration code
346 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
349 // Do not include empty blocks in the cost calculation. This happen due to
350 // loop canonicalization and will be removed.
351 if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
354 // Count basic blocks.
361 /// UnswitchIfProfitable - We have found that we can unswitch L when
362 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
363 /// unswitch the loop, reprocess the pieces, then return true.
364 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){
365 // Check to see if it would be profitable to unswitch this loop.
366 unsigned Cost = getLoopUnswitchCost(L, LoopCond);
367 if (Cost > Threshold) {
368 // FIXME: this should estimate growth by the amount of code shared by the
369 // resultant unswitched loops.
371 DOUT << "NOT unswitching loop %"
372 << L->getHeader()->getName() << ", cost too high: "
373 << L->getBlocks().size() << "\n";
377 // If this is a trivial condition to unswitch (which results in no code
378 // duplication), do it now.
380 BasicBlock *ExitBlock;
381 if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal, &ExitBlock)) {
382 UnswitchTrivialCondition(L, LoopCond, CondVal, ExitBlock);
384 UnswitchNontrivialCondition(LoopCond, Val, L);
390 /// SplitBlock - Split the specified block at the specified instruction - every
391 /// thing before SplitPt stays in Old and everything starting with SplitPt moves
392 /// to a new block. The two blocks are joined by an unconditional branch and
393 /// the loop info is updated.
395 BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *Old, Instruction *SplitPt) {
396 BasicBlock::iterator SplitIt = SplitPt;
397 while (isa<PHINode>(SplitIt))
399 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
401 // The new block lives in whichever loop the old one did.
402 if (Loop *L = LI->getLoopFor(Old))
403 L->addBasicBlockToLoop(New, *LI);
409 BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) {
410 TerminatorInst *LatchTerm = BB->getTerminator();
411 unsigned SuccNum = 0;
412 for (unsigned i = 0, e = LatchTerm->getNumSuccessors(); ; ++i) {
413 assert(i != e && "Didn't find edge?");
414 if (LatchTerm->getSuccessor(i) == Succ) {
420 // If this is a critical edge, let SplitCriticalEdge do it.
421 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, this))
422 return LatchTerm->getSuccessor(SuccNum);
424 // If the edge isn't critical, then BB has a single successor or Succ has a
425 // single pred. Split the block.
426 BasicBlock::iterator SplitPoint;
427 if (BasicBlock *SP = Succ->getSinglePredecessor()) {
428 // If the successor only has a single pred, split the top of the successor
430 assert(SP == BB && "CFG broken");
431 return SplitBlock(Succ, Succ->begin());
433 // Otherwise, if BB has a single successor, split it at the bottom of the
435 assert(BB->getTerminator()->getNumSuccessors() == 1 &&
436 "Should have a single succ!");
437 return SplitBlock(BB, BB->getTerminator());
443 // RemapInstruction - Convert the instruction operands from referencing the
444 // current values into those specified by ValueMap.
446 static inline void RemapInstruction(Instruction *I,
447 std::map<const Value *, Value*> &ValueMap) {
448 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
449 Value *Op = I->getOperand(op);
450 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
451 if (It != ValueMap.end()) Op = It->second;
452 I->setOperand(op, Op);
456 /// CloneLoop - Recursively clone the specified loop and all of its children,
457 /// mapping the blocks with the specified map.
458 static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
460 Loop *New = new Loop();
463 PL->addChildLoop(New);
465 LI->addTopLevelLoop(New);
467 // Add all of the blocks in L to the new loop.
468 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
470 if (LI->getLoopFor(*I) == L)
471 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
473 // Add all of the subloops to the new loop.
474 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
475 CloneLoop(*I, New, VM, LI);
480 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
481 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
482 /// code immediately before InsertPt.
483 static void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
484 BasicBlock *TrueDest,
485 BasicBlock *FalseDest,
486 Instruction *InsertPt) {
487 // Insert a conditional branch on LIC to the two preheaders. The original
488 // code is the true version and the new code is the false version.
489 Value *BranchVal = LIC;
490 if (!isa<ConstantBool>(Val)) {
491 BranchVal = BinaryOperator::createSetEQ(LIC, Val, "tmp", InsertPt);
492 } else if (Val != ConstantBool::getTrue()) {
493 // We want to enter the new loop when the condition is true.
494 std::swap(TrueDest, FalseDest);
497 // Insert the new branch.
498 new BranchInst(TrueDest, FalseDest, BranchVal, InsertPt);
502 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
503 /// condition in it (a cond branch from its header block to its latch block,
504 /// where the path through the loop that doesn't execute its body has no
505 /// side-effects), unswitch it. This doesn't involve any code duplication, just
506 /// moving the conditional branch outside of the loop and updating loop info.
507 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
509 BasicBlock *ExitBlock) {
510 DOUT << "loop-unswitch: Trivial-Unswitch loop %"
511 << L->getHeader()->getName() << " [" << L->getBlocks().size()
512 << " blocks] in Function " << L->getHeader()->getParent()->getName()
513 << " on cond: " << *Val << " == " << *Cond << "\n";
515 // First step, split the preheader, so that we know that there is a safe place
516 // to insert the conditional branch. We will change 'OrigPH' to have a
517 // conditional branch on Cond.
518 BasicBlock *OrigPH = L->getLoopPreheader();
519 BasicBlock *NewPH = SplitEdge(OrigPH, L->getHeader());
521 // Now that we have a place to insert the conditional branch, create a place
522 // to branch to: this is the exit block out of the loop that we should
525 // Split this block now, so that the loop maintains its exit block, and so
526 // that the jump from the preheader can execute the contents of the exit block
527 // without actually branching to it (the exit block should be dominated by the
528 // loop header, not the preheader).
529 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
530 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin());
532 // Okay, now we have a position to branch from and a position to branch to,
533 // insert the new conditional branch.
534 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
535 OrigPH->getTerminator());
536 OrigPH->getTerminator()->eraseFromParent();
538 // We need to reprocess this loop, it could be unswitched again.
539 LoopProcessWorklist.push_back(L);
541 // Now that we know that the loop is never entered when this condition is a
542 // particular value, rewrite the loop with this info. We know that this will
543 // at least eliminate the old branch.
544 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
549 /// VersionLoop - We determined that the loop is profitable to unswitch when LIC
550 /// equal Val. Split it into loop versions and test the condition outside of
551 /// either loop. Return the loops created as Out1/Out2.
552 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
554 Function *F = L->getHeader()->getParent();
555 DOUT << "loop-unswitch: Unswitching loop %"
556 << L->getHeader()->getName() << " [" << L->getBlocks().size()
557 << " blocks] in Function " << F->getName()
558 << " when '" << *Val << "' == " << *LIC << "\n";
560 // LoopBlocks contains all of the basic blocks of the loop, including the
561 // preheader of the loop, the body of the loop, and the exit blocks of the
562 // loop, in that order.
563 std::vector<BasicBlock*> LoopBlocks;
565 // First step, split the preheader and exit blocks, and add these blocks to
566 // the LoopBlocks list.
567 BasicBlock *OrigPreheader = L->getLoopPreheader();
568 LoopBlocks.push_back(SplitEdge(OrigPreheader, L->getHeader()));
570 // We want the loop to come after the preheader, but before the exit blocks.
571 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
573 std::vector<BasicBlock*> ExitBlocks;
574 L->getUniqueExitBlocks(ExitBlocks);
576 // Split all of the edges from inside the loop to their exit blocks. Update
577 // the appropriate Phi nodes as we do so.
578 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
579 BasicBlock *ExitBlock = ExitBlocks[i];
580 std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock));
582 for (unsigned j = 0, e = Preds.size(); j != e; ++j) {
583 assert(L->contains(Preds[j]) &&
584 "All preds of loop exit blocks must be the same loop!");
585 BasicBlock* MiddleBlock = SplitEdge(Preds[j], ExitBlock);
586 BasicBlock* StartBlock = Preds[j];
587 BasicBlock* EndBlock;
588 if (MiddleBlock->getSinglePredecessor() == ExitBlock) {
589 EndBlock = MiddleBlock;
590 MiddleBlock = EndBlock->getSinglePredecessor();;
592 EndBlock = ExitBlock;
595 std::set<PHINode*> InsertedPHIs;
596 PHINode* OldLCSSA = 0;
597 for (BasicBlock::iterator I = EndBlock->begin();
598 (OldLCSSA = dyn_cast<PHINode>(I)); ++I) {
599 Value* OldValue = OldLCSSA->getIncomingValueForBlock(MiddleBlock);
600 PHINode* NewLCSSA = new PHINode(OldLCSSA->getType(),
601 OldLCSSA->getName() + ".us-lcssa",
602 MiddleBlock->getTerminator());
603 NewLCSSA->addIncoming(OldValue, StartBlock);
604 OldLCSSA->setIncomingValue(OldLCSSA->getBasicBlockIndex(MiddleBlock),
606 InsertedPHIs.insert(NewLCSSA);
609 BasicBlock::iterator InsertPt = EndBlock->begin();
610 while (dyn_cast<PHINode>(InsertPt)) ++InsertPt;
611 for (BasicBlock::iterator I = MiddleBlock->begin();
612 (OldLCSSA = dyn_cast<PHINode>(I)) && InsertedPHIs.count(OldLCSSA) == 0;
614 PHINode *NewLCSSA = new PHINode(OldLCSSA->getType(),
615 OldLCSSA->getName() + ".us-lcssa",
617 OldLCSSA->replaceAllUsesWith(NewLCSSA);
618 NewLCSSA->addIncoming(OldLCSSA, MiddleBlock);
623 // The exit blocks may have been changed due to edge splitting, recompute.
625 L->getUniqueExitBlocks(ExitBlocks);
627 // Add exit blocks to the loop blocks.
628 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
630 // Next step, clone all of the basic blocks that make up the loop (including
631 // the loop preheader and exit blocks), keeping track of the mapping between
632 // the instructions and blocks.
633 std::vector<BasicBlock*> NewBlocks;
634 NewBlocks.reserve(LoopBlocks.size());
635 std::map<const Value*, Value*> ValueMap;
636 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
637 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F);
638 NewBlocks.push_back(New);
639 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping.
642 // Splice the newly inserted blocks into the function right before the
643 // original preheader.
644 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
645 NewBlocks[0], F->end());
647 // Now we create the new Loop object for the versioned loop.
648 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
649 Loop *ParentLoop = L->getParentLoop();
651 // Make sure to add the cloned preheader and exit blocks to the parent loop
653 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI);
656 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
657 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]);
658 // The new exit block should be in the same loop as the old one.
659 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
660 ExitBBLoop->addBasicBlockToLoop(NewExit, *LI);
662 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
663 "Exit block should have been split to have one successor!");
664 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
666 // If the successor of the exit block had PHI nodes, add an entry for
669 for (BasicBlock::iterator I = ExitSucc->begin();
670 (PN = dyn_cast<PHINode>(I)); ++I) {
671 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
672 std::map<const Value *, Value*>::iterator It = ValueMap.find(V);
673 if (It != ValueMap.end()) V = It->second;
674 PN->addIncoming(V, NewExit);
678 // Rewrite the code to refer to itself.
679 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
680 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
681 E = NewBlocks[i]->end(); I != E; ++I)
682 RemapInstruction(I, ValueMap);
684 // Rewrite the original preheader to select between versions of the loop.
685 BranchInst *OldBR = cast<BranchInst>(OrigPreheader->getTerminator());
686 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
687 "Preheader splitting did not work correctly!");
689 // Emit the new branch that selects between the two versions of this loop.
690 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
691 OldBR->eraseFromParent();
693 LoopProcessWorklist.push_back(L);
694 LoopProcessWorklist.push_back(NewLoop);
696 // Now we rewrite the original code to know that the condition is true and the
697 // new code to know that the condition is false.
698 RewriteLoopBodyWithConditionConstant(L , LIC, Val, false);
700 // It's possible that simplifying one loop could cause the other to be
701 // deleted. If so, don't simplify it.
702 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop)
703 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
706 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
708 static void RemoveFromWorklist(Instruction *I,
709 std::vector<Instruction*> &Worklist) {
710 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
712 while (WI != Worklist.end()) {
713 unsigned Offset = WI-Worklist.begin();
715 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
719 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
720 /// program, replacing all uses with V and update the worklist.
721 static void ReplaceUsesOfWith(Instruction *I, Value *V,
722 std::vector<Instruction*> &Worklist) {
723 DOUT << "Replace with '" << *V << "': " << *I;
725 // Add uses to the worklist, which may be dead now.
726 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
727 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
728 Worklist.push_back(Use);
730 // Add users to the worklist which may be simplified now.
731 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
733 Worklist.push_back(cast<Instruction>(*UI));
734 I->replaceAllUsesWith(V);
735 I->eraseFromParent();
736 RemoveFromWorklist(I, Worklist);
740 /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
741 /// information, and remove any dead successors it has.
743 void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
744 std::vector<Instruction*> &Worklist) {
745 if (pred_begin(BB) != pred_end(BB)) {
746 // This block isn't dead, since an edge to BB was just removed, see if there
747 // are any easy simplifications we can do now.
748 if (BasicBlock *Pred = BB->getSinglePredecessor()) {
749 // If it has one pred, fold phi nodes in BB.
750 while (isa<PHINode>(BB->begin()))
751 ReplaceUsesOfWith(BB->begin(),
752 cast<PHINode>(BB->begin())->getIncomingValue(0),
755 // If this is the header of a loop and the only pred is the latch, we now
756 // have an unreachable loop.
757 if (Loop *L = LI->getLoopFor(BB))
758 if (L->getHeader() == BB && L->contains(Pred)) {
759 // Remove the branch from the latch to the header block, this makes
760 // the header dead, which will make the latch dead (because the header
761 // dominates the latch).
762 Pred->getTerminator()->eraseFromParent();
763 new UnreachableInst(Pred);
765 // The loop is now broken, remove it from LI.
766 RemoveLoopFromHierarchy(L);
768 // Reprocess the header, which now IS dead.
769 RemoveBlockIfDead(BB, Worklist);
773 // If pred ends in a uncond branch, add uncond branch to worklist so that
774 // the two blocks will get merged.
775 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
776 if (BI->isUnconditional())
777 Worklist.push_back(BI);
782 DOUT << "Nuking dead block: " << *BB;
784 // Remove the instructions in the basic block from the worklist.
785 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
786 RemoveFromWorklist(I, Worklist);
788 // Anything that uses the instructions in this basic block should have their
789 // uses replaced with undefs.
791 I->replaceAllUsesWith(UndefValue::get(I->getType()));
794 // If this is the edge to the header block for a loop, remove the loop and
795 // promote all subloops.
796 if (Loop *BBLoop = LI->getLoopFor(BB)) {
797 if (BBLoop->getLoopLatch() == BB)
798 RemoveLoopFromHierarchy(BBLoop);
801 // Remove the block from the loop info, which removes it from any loops it
806 // Remove phi node entries in successors for this block.
807 TerminatorInst *TI = BB->getTerminator();
808 std::vector<BasicBlock*> Succs;
809 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
810 Succs.push_back(TI->getSuccessor(i));
811 TI->getSuccessor(i)->removePredecessor(BB);
814 // Unique the successors, remove anything with multiple uses.
815 std::sort(Succs.begin(), Succs.end());
816 Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end());
818 // Remove the basic block, including all of the instructions contained in it.
819 BB->eraseFromParent();
821 // Remove successor blocks here that are not dead, so that we know we only
822 // have dead blocks in this list. Nondead blocks have a way of becoming dead,
823 // then getting removed before we revisit them, which is badness.
825 for (unsigned i = 0; i != Succs.size(); ++i)
826 if (pred_begin(Succs[i]) != pred_end(Succs[i])) {
827 // One exception is loop headers. If this block was the preheader for a
828 // loop, then we DO want to visit the loop so the loop gets deleted.
829 // We know that if the successor is a loop header, that this loop had to
830 // be the preheader: the case where this was the latch block was handled
831 // above and headers can only have two predecessors.
832 if (!LI->isLoopHeader(Succs[i])) {
833 Succs.erase(Succs.begin()+i);
838 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
839 RemoveBlockIfDead(Succs[i], Worklist);
842 /// RemoveLoopFromHierarchy - We have discovered that the specified loop has
843 /// become unwrapped, either because the backedge was deleted, or because the
844 /// edge into the header was removed. If the edge into the header from the
845 /// latch block was removed, the loop is unwrapped but subloops are still alive,
846 /// so they just reparent loops. If the loops are actually dead, they will be
848 void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) {
849 if (Loop *ParentLoop = L->getParentLoop()) { // Not a top-level loop.
850 // Reparent all of the blocks in this loop. Since BBLoop had a parent,
851 // they are now all in it.
852 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
854 if (LI->getLoopFor(*I) == L) // Don't change blocks in subloops.
855 LI->changeLoopFor(*I, ParentLoop);
857 // Remove the loop from its parent loop.
858 for (Loop::iterator I = ParentLoop->begin(), E = ParentLoop->end();;
860 assert(I != E && "Couldn't find loop");
862 ParentLoop->removeChildLoop(I);
867 // Move all subloops into the parent loop.
868 while (L->begin() != L->end())
869 ParentLoop->addChildLoop(L->removeChildLoop(L->end()-1));
871 // Reparent all of the blocks in this loop. Since BBLoop had no parent,
872 // they no longer in a loop at all.
874 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
875 // Don't change blocks in subloops.
876 if (LI->getLoopFor(L->getBlocks()[i]) == L) {
877 LI->removeBlock(L->getBlocks()[i]);
882 // Remove the loop from the top-level LoopInfo object.
883 for (LoopInfo::iterator I = LI->begin(), E = LI->end();; ++I) {
884 assert(I != E && "Couldn't find loop");
891 // Move all of the subloops to the top-level.
892 while (L->begin() != L->end())
893 LI->addTopLevelLoop(L->removeChildLoop(L->end()-1));
897 RemoveLoopFromWorklist(L);
902 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
903 // the value specified by Val in the specified loop, or we know it does NOT have
904 // that value. Rewrite any uses of LIC or of properties correlated to it.
905 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
908 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
910 // FIXME: Support correlated properties, like:
917 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
918 // selects, switches.
919 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
920 std::vector<Instruction*> Worklist;
922 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
923 // in the loop with the appropriate one directly.
924 if (IsEqual || isa<ConstantBool>(Val)) {
929 Replacement = ConstantBool::get(!cast<ConstantBool>(Val)->getValue());
931 for (unsigned i = 0, e = Users.size(); i != e; ++i)
932 if (Instruction *U = cast<Instruction>(Users[i])) {
933 if (!L->contains(U->getParent()))
935 U->replaceUsesOfWith(LIC, Replacement);
936 Worklist.push_back(U);
939 // Otherwise, we don't know the precise value of LIC, but we do know that it
940 // is certainly NOT "Val". As such, simplify any uses in the loop that we
941 // can. This case occurs when we unswitch switch statements.
942 for (unsigned i = 0, e = Users.size(); i != e; ++i)
943 if (Instruction *U = cast<Instruction>(Users[i])) {
944 if (!L->contains(U->getParent()))
947 Worklist.push_back(U);
949 // If we know that LIC is not Val, use this info to simplify code.
950 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
951 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
952 if (SI->getCaseValue(i) == Val) {
953 // Found a dead case value. Don't remove PHI nodes in the
954 // successor if they become single-entry, those PHI nodes may
955 // be in the Users list.
957 // FIXME: This is a hack. We need to keep the successor around
958 // and hooked up so as to preserve the loop structure, because
959 // trying to update it is complicated. So instead we preserve the
960 // loop structure and put the block on an dead code path.
962 BasicBlock* Old = SI->getParent();
963 BasicBlock* Split = SplitBlock(Old, SI);
965 Instruction* OldTerm = Old->getTerminator();
966 new BranchInst(Split, SI->getSuccessor(i),
967 ConstantBool::getTrue(), OldTerm);
969 Old->getTerminator()->eraseFromParent();
973 for (BasicBlock::iterator II = SI->getSuccessor(i)->begin();
974 (PN = dyn_cast<PHINode>(II)); ++II) {
975 Value *InVal = PN->removeIncomingValue(Split, false);
976 PN->addIncoming(InVal, Old);
985 // TODO: We could do other simplifications, for example, turning
986 // LIC == Val -> false.
990 SimplifyCode(Worklist);
993 /// SimplifyCode - Okay, now that we have simplified some instructions in the
994 /// loop, walk over it and constant prop, dce, and fold control flow where
995 /// possible. Note that this is effectively a very simple loop-structure-aware
996 /// optimizer. During processing of this loop, L could very well be deleted, so
997 /// it must not be used.
999 /// FIXME: When the loop optimizer is more mature, separate this out to a new
1002 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist) {
1003 while (!Worklist.empty()) {
1004 Instruction *I = Worklist.back();
1005 Worklist.pop_back();
1007 // Simple constant folding.
1008 if (Constant *C = ConstantFoldInstruction(I)) {
1009 ReplaceUsesOfWith(I, C, Worklist);
1014 if (isInstructionTriviallyDead(I)) {
1015 DOUT << "Remove dead instruction '" << *I;
1017 // Add uses to the worklist, which may be dead now.
1018 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
1019 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
1020 Worklist.push_back(Use);
1021 I->eraseFromParent();
1022 RemoveFromWorklist(I, Worklist);
1027 // Special case hacks that appear commonly in unswitched code.
1028 switch (I->getOpcode()) {
1029 case Instruction::Select:
1030 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(0))) {
1031 ReplaceUsesOfWith(I, I->getOperand(!CB->getValue()+1), Worklist);
1035 case Instruction::And:
1036 if (isa<ConstantBool>(I->getOperand(0))) // constant -> RHS
1037 cast<BinaryOperator>(I)->swapOperands();
1038 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) {
1039 if (CB->getValue()) // X & 1 -> X
1040 ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
1042 ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
1046 case Instruction::Or:
1047 if (isa<ConstantBool>(I->getOperand(0))) // constant -> RHS
1048 cast<BinaryOperator>(I)->swapOperands();
1049 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) {
1050 if (CB->getValue()) // X | 1 -> 1
1051 ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
1053 ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
1057 case Instruction::Br: {
1058 BranchInst *BI = cast<BranchInst>(I);
1059 if (BI->isUnconditional()) {
1060 // If BI's parent is the only pred of the successor, fold the two blocks
1062 BasicBlock *Pred = BI->getParent();
1063 BasicBlock *Succ = BI->getSuccessor(0);
1064 BasicBlock *SinglePred = Succ->getSinglePredecessor();
1065 if (!SinglePred) continue; // Nothing to do.
1066 assert(SinglePred == Pred && "CFG broken");
1068 DOUT << "Merging blocks: " << Pred->getName() << " <- "
1069 << Succ->getName() << "\n";
1071 // Resolve any single entry PHI nodes in Succ.
1072 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1073 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist);
1075 // Move all of the successor contents from Succ to Pred.
1076 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1078 BI->eraseFromParent();
1079 RemoveFromWorklist(BI, Worklist);
1081 // If Succ has any successors with PHI nodes, update them to have
1082 // entries coming from Pred instead of Succ.
1083 Succ->replaceAllUsesWith(Pred);
1085 // Remove Succ from the loop tree.
1086 LI->removeBlock(Succ);
1087 Succ->eraseFromParent();
1089 } else if (ConstantBool *CB = dyn_cast<ConstantBool>(BI->getCondition())){
1090 // Conditional branch. Turn it into an unconditional branch, then
1091 // remove dead blocks.
1092 break; // FIXME: Enable.
1094 DOUT << "Folded branch: " << *BI;
1095 BasicBlock *DeadSucc = BI->getSuccessor(CB->getValue());
1096 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getValue());
1097 DeadSucc->removePredecessor(BI->getParent(), true);
1098 Worklist.push_back(new BranchInst(LiveSucc, BI));
1099 BI->eraseFromParent();
1100 RemoveFromWorklist(BI, Worklist);
1103 RemoveBlockIfDead(DeadSucc, Worklist);