1 //===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This 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/InlineCost.h"
37 #include "llvm/Analysis/LoopInfo.h"
38 #include "llvm/Analysis/LoopPass.h"
39 #include "llvm/Analysis/Dominators.h"
40 #include "llvm/Transforms/Utils/Cloning.h"
41 #include "llvm/Transforms/Utils/Local.h"
42 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
43 #include "llvm/ADT/Statistic.h"
44 #include "llvm/ADT/SmallPtrSet.h"
45 #include "llvm/ADT/STLExtras.h"
46 #include "llvm/Support/CommandLine.h"
47 #include "llvm/Support/Debug.h"
48 #include "llvm/Support/raw_ostream.h"
53 STATISTIC(NumBranches, "Number of branches unswitched");
54 STATISTIC(NumSwitches, "Number of switches unswitched");
55 STATISTIC(NumSelects , "Number of selects unswitched");
56 STATISTIC(NumTrivial , "Number of unswitches that are trivial");
57 STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
59 // The specific value of 50 here was chosen based only on intuition and a
60 // few specific examples.
61 static cl::opt<unsigned>
62 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
63 cl::init(50), cl::Hidden);
66 class LoopUnswitch : public LoopPass {
67 LoopInfo *LI; // Loop information
70 // LoopProcessWorklist - Used to check if second loop needs processing
71 // after RewriteLoopBodyWithConditionConstant rewrites first loop.
72 std::vector<Loop*> LoopProcessWorklist;
73 SmallPtrSet<Value *,8> UnswitchedVals;
79 DominanceFrontier *DF;
81 BasicBlock *loopHeader;
82 BasicBlock *loopPreheader;
84 // LoopBlocks contains all of the basic blocks of the loop, including the
85 // preheader of the loop, the body of the loop, and the exit blocks of the
86 // loop, in that order.
87 std::vector<BasicBlock*> LoopBlocks;
88 // NewBlocks contained cloned copy of basic blocks from LoopBlocks.
89 std::vector<BasicBlock*> NewBlocks;
92 static char ID; // Pass ID, replacement for typeid
93 explicit LoopUnswitch(bool Os = false) :
94 LoopPass(&ID), OptimizeForSize(Os), redoLoop(false),
95 currentLoop(NULL), DF(NULL), DT(NULL), loopHeader(NULL),
96 loopPreheader(NULL) {}
98 bool runOnLoop(Loop *L, LPPassManager &LPM);
99 bool processCurrentLoop();
101 /// This transformation requires natural loop information & requires that
102 /// loop preheaders be inserted into the CFG...
104 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
105 AU.addRequiredID(LoopSimplifyID);
106 AU.addPreservedID(LoopSimplifyID);
107 AU.addRequired<LoopInfo>();
108 AU.addPreserved<LoopInfo>();
109 AU.addRequiredID(LCSSAID);
110 AU.addPreservedID(LCSSAID);
111 AU.addPreserved<DominatorTree>();
112 AU.addPreserved<DominanceFrontier>();
117 virtual void releaseMemory() {
118 UnswitchedVals.clear();
121 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist,
123 void RemoveLoopFromWorklist(Loop *L) {
124 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(),
125 LoopProcessWorklist.end(), L);
126 if (I != LoopProcessWorklist.end())
127 LoopProcessWorklist.erase(I);
130 void initLoopData() {
131 loopHeader = currentLoop->getHeader();
132 loopPreheader = currentLoop->getLoopPreheader();
135 /// Split all of the edges from inside the loop to their exit blocks.
136 /// Update the appropriate Phi nodes as we do so.
137 void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks);
139 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val);
140 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
141 BasicBlock *ExitBlock);
142 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
144 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
145 Constant *Val, bool isEqual);
147 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
148 BasicBlock *TrueDest,
149 BasicBlock *FalseDest,
150 Instruction *InsertPt);
152 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
153 void RemoveBlockIfDead(BasicBlock *BB,
154 std::vector<Instruction*> &Worklist, Loop *l);
155 void RemoveLoopFromHierarchy(Loop *L);
156 bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0,
157 BasicBlock **LoopExit = 0);
161 char LoopUnswitch::ID = 0;
162 static RegisterPass<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
164 Pass *llvm::createLoopUnswitchPass(bool Os) {
165 return new LoopUnswitch(Os);
168 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
169 /// invariant in the loop, or has an invariant piece, return the invariant.
170 /// Otherwise, return null.
171 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
172 // We can never unswitch on vector conditions.
173 if (Cond->getType()->isVectorTy())
176 // Constants should be folded, not unswitched on!
177 if (isa<Constant>(Cond)) return 0;
179 // TODO: Handle: br (VARIANT|INVARIANT).
181 // Hoist simple values out.
182 if (L->makeLoopInvariant(Cond, Changed))
185 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
186 if (BO->getOpcode() == Instruction::And ||
187 BO->getOpcode() == Instruction::Or) {
188 // If either the left or right side is invariant, we can unswitch on this,
189 // which will cause the branch to go away in one loop and the condition to
190 // simplify in the other one.
191 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
193 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
200 bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
201 LI = &getAnalysis<LoopInfo>();
203 DF = getAnalysisIfAvailable<DominanceFrontier>();
204 DT = getAnalysisIfAvailable<DominatorTree>();
206 Function *F = currentLoop->getHeader()->getParent();
207 bool Changed = false;
209 assert(currentLoop->isLCSSAForm(*DT));
211 Changed |= processCurrentLoop();
215 // FIXME: Reconstruct dom info, because it is not preserved properly.
217 DT->runOnFunction(*F);
219 DF->runOnFunction(*F);
224 /// processCurrentLoop - Do actual work and unswitch loop if possible
226 bool LoopUnswitch::processCurrentLoop() {
227 bool Changed = false;
228 LLVMContext &Context = currentLoop->getHeader()->getContext();
230 // Loop over all of the basic blocks in the loop. If we find an interior
231 // block that is branching on a loop-invariant condition, we can unswitch this
233 for (Loop::block_iterator I = currentLoop->block_begin(),
234 E = currentLoop->block_end(); I != E; ++I) {
235 TerminatorInst *TI = (*I)->getTerminator();
236 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
237 // If this isn't branching on an invariant condition, we can't unswitch
239 if (BI->isConditional()) {
240 // See if this, or some part of it, is loop invariant. If so, we can
241 // unswitch on it if we desire.
242 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
243 currentLoop, Changed);
244 if (LoopCond && UnswitchIfProfitable(LoopCond,
245 ConstantInt::getTrue(Context))) {
250 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
251 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
252 currentLoop, Changed);
253 if (LoopCond && SI->getNumCases() > 1) {
254 // Find a value to unswitch on:
255 // FIXME: this should chose the most expensive case!
256 Constant *UnswitchVal = SI->getCaseValue(1);
257 // Do not process same value again and again.
258 if (!UnswitchedVals.insert(UnswitchVal))
261 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) {
268 // Scan the instructions to check for unswitchable values.
269 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
271 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
272 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
273 currentLoop, Changed);
274 if (LoopCond && UnswitchIfProfitable(LoopCond,
275 ConstantInt::getTrue(Context))) {
284 /// isTrivialLoopExitBlock - Check to see if all paths from BB either:
285 /// 1. Exit the loop with no side effects.
286 /// 2. Branch to the latch block with no side-effects.
288 /// If these conditions are true, we return true and set ExitBB to the block we
291 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
293 std::set<BasicBlock*> &Visited) {
294 if (!Visited.insert(BB).second) {
295 // Already visited and Ok, end of recursion.
297 } else if (!L->contains(BB)) {
298 // Otherwise, this is a loop exit, this is fine so long as this is the
300 if (ExitBB != 0) return false;
305 // Otherwise, this is an unvisited intra-loop node. Check all successors.
306 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
307 // Check to see if the successor is a trivial loop exit.
308 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
312 // Okay, everything after this looks good, check to make sure that this block
313 // doesn't include any side effects.
314 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
315 if (I->mayHaveSideEffects())
321 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally
322 /// leads to an exit from the specified loop, and has no side-effects in the
323 /// process. If so, return the block that is exited to, otherwise return null.
324 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
325 std::set<BasicBlock*> Visited;
326 Visited.insert(L->getHeader()); // Branches to header are ok.
327 BasicBlock *ExitBB = 0;
328 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
333 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
334 /// trivial: that is, that the condition controls whether or not the loop does
335 /// anything at all. If this is a trivial condition, unswitching produces no
336 /// code duplications (equivalently, it produces a simpler loop and a new empty
337 /// loop, which gets deleted).
339 /// If this is a trivial condition, return true, otherwise return false. When
340 /// returning true, this sets Cond and Val to the condition that controls the
341 /// trivial condition: when Cond dynamically equals Val, the loop is known to
342 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when
345 bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
346 BasicBlock **LoopExit) {
347 BasicBlock *Header = currentLoop->getHeader();
348 TerminatorInst *HeaderTerm = Header->getTerminator();
349 LLVMContext &Context = Header->getContext();
351 BasicBlock *LoopExitBB = 0;
352 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
353 // If the header block doesn't end with a conditional branch on Cond, we
355 if (!BI->isConditional() || BI->getCondition() != Cond)
358 // Check to see if a successor of the branch is guaranteed to go to the
359 // latch block or exit through a one exit block without having any
360 // side-effects. If so, determine the value of Cond that causes it to do
362 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
363 BI->getSuccessor(0)))) {
364 if (Val) *Val = ConstantInt::getTrue(Context);
365 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
366 BI->getSuccessor(1)))) {
367 if (Val) *Val = ConstantInt::getFalse(Context);
369 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
370 // If this isn't a switch on Cond, we can't handle it.
371 if (SI->getCondition() != Cond) return false;
373 // Check to see if a successor of the switch is guaranteed to go to the
374 // latch block or exit through a one exit block without having any
375 // side-effects. If so, determine the value of Cond that causes it to do
376 // this. Note that we can't trivially unswitch on the default case.
377 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
378 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
379 SI->getSuccessor(i)))) {
380 // Okay, we found a trivial case, remember the value that is trivial.
381 if (Val) *Val = SI->getCaseValue(i);
386 // If we didn't find a single unique LoopExit block, or if the loop exit block
387 // contains phi nodes, this isn't trivial.
388 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
389 return false; // Can't handle this.
391 if (LoopExit) *LoopExit = LoopExitBB;
393 // We already know that nothing uses any scalar values defined inside of this
394 // loop. As such, we just have to check to see if this loop will execute any
395 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
396 // part of the loop that the code *would* execute. We already checked the
397 // tail, check the header now.
398 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
399 if (I->mayHaveSideEffects())
404 /// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
405 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
406 /// unswitch the loop, reprocess the pieces, then return true.
407 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val) {
411 // If LoopSimplify was unable to form a preheader, don't do any unswitching.
415 Function *F = loopHeader->getParent();
417 Constant *CondVal = 0;
418 BasicBlock *ExitBlock = 0;
419 if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
420 // If the condition is trivial, always unswitch. There is no code growth
422 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock);
426 // Check to see if it would be profitable to unswitch current loop.
428 // Do not do non-trivial unswitch while optimizing for size.
429 if (OptimizeForSize || F->hasFnAttr(Attribute::OptimizeForSize))
432 // FIXME: This is overly conservative because it does not take into
433 // consideration code simplification opportunities and code that can
434 // be shared by the resultant unswitched loops.
436 for (Loop::block_iterator I = currentLoop->block_begin(),
437 E = currentLoop->block_end();
439 Metrics.analyzeBasicBlock(*I);
441 // Limit the number of instructions to avoid causing significant code
442 // expansion, and the number of basic blocks, to avoid loops with
443 // large numbers of branches which cause loop unswitching to go crazy.
444 // This is a very ad-hoc heuristic.
445 if (Metrics.NumInsts > Threshold ||
446 Metrics.NumBlocks * 5 > Threshold ||
447 Metrics.NeverInline) {
448 DEBUG(dbgs() << "NOT unswitching loop %"
449 << currentLoop->getHeader()->getName() << ", cost too high: "
450 << currentLoop->getBlocks().size() << "\n");
454 UnswitchNontrivialCondition(LoopCond, Val, currentLoop);
458 // RemapInstruction - Convert the instruction operands from referencing the
459 // current values into those specified by ValueMap.
461 static inline void RemapInstruction(Instruction *I,
462 DenseMap<const Value *, Value*> &ValueMap) {
463 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
464 Value *Op = I->getOperand(op);
465 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op);
466 if (It != ValueMap.end()) Op = It->second;
467 I->setOperand(op, Op);
471 /// CloneLoop - Recursively clone the specified loop and all of its children,
472 /// mapping the blocks with the specified map.
473 static Loop *CloneLoop(Loop *L, Loop *PL, DenseMap<const Value*, Value*> &VM,
474 LoopInfo *LI, LPPassManager *LPM) {
475 Loop *New = new Loop();
476 LPM->insertLoop(New, PL);
478 // Add all of the blocks in L to the new loop.
479 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
481 if (LI->getLoopFor(*I) == L)
482 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), LI->getBase());
484 // Add all of the subloops to the new loop.
485 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
486 CloneLoop(*I, New, VM, LI, LPM);
491 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
492 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
493 /// code immediately before InsertPt.
494 void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
495 BasicBlock *TrueDest,
496 BasicBlock *FalseDest,
497 Instruction *InsertPt) {
498 // Insert a conditional branch on LIC to the two preheaders. The original
499 // code is the true version and the new code is the false version.
500 Value *BranchVal = LIC;
501 if (!isa<ConstantInt>(Val) ||
502 Val->getType() != Type::getInt1Ty(LIC->getContext()))
503 BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val, "tmp");
504 else if (Val != ConstantInt::getTrue(Val->getContext()))
505 // We want to enter the new loop when the condition is true.
506 std::swap(TrueDest, FalseDest);
508 // Insert the new branch.
509 BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt);
511 // If either edge is critical, split it. This helps preserve LoopSimplify
512 // form for enclosing loops.
513 SplitCriticalEdge(BI, 0, this);
514 SplitCriticalEdge(BI, 1, this);
517 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
518 /// condition in it (a cond branch from its header block to its latch block,
519 /// where the path through the loop that doesn't execute its body has no
520 /// side-effects), unswitch it. This doesn't involve any code duplication, just
521 /// moving the conditional branch outside of the loop and updating loop info.
522 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
524 BasicBlock *ExitBlock) {
525 DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
526 << loopHeader->getName() << " [" << L->getBlocks().size()
527 << " blocks] in Function " << L->getHeader()->getParent()->getName()
528 << " on cond: " << *Val << " == " << *Cond << "\n");
530 // First step, split the preheader, so that we know that there is a safe place
531 // to insert the conditional branch. We will change loopPreheader to have a
532 // conditional branch on Cond.
533 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this);
535 // Now that we have a place to insert the conditional branch, create a place
536 // to branch to: this is the exit block out of the loop that we should
539 // Split this block now, so that the loop maintains its exit block, and so
540 // that the jump from the preheader can execute the contents of the exit block
541 // without actually branching to it (the exit block should be dominated by the
542 // loop header, not the preheader).
543 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
544 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this);
546 // Okay, now we have a position to branch from and a position to branch to,
547 // insert the new conditional branch.
548 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
549 loopPreheader->getTerminator());
550 LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L);
551 loopPreheader->getTerminator()->eraseFromParent();
553 // We need to reprocess this loop, it could be unswitched again.
556 // Now that we know that the loop is never entered when this condition is a
557 // particular value, rewrite the loop with this info. We know that this will
558 // at least eliminate the old branch.
559 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
563 /// SplitExitEdges - Split all of the edges from inside the loop to their exit
564 /// blocks. Update the appropriate Phi nodes as we do so.
565 void LoopUnswitch::SplitExitEdges(Loop *L,
566 const SmallVector<BasicBlock *, 8> &ExitBlocks){
568 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
569 BasicBlock *ExitBlock = ExitBlocks[i];
570 SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
571 pred_end(ExitBlock));
572 SplitBlockPredecessors(ExitBlock, Preds.data(), Preds.size(),
577 /// UnswitchNontrivialCondition - We determined that the loop is profitable
578 /// to unswitch when LIC equal Val. Split it into loop versions and test the
579 /// condition outside of either loop. Return the loops created as Out1/Out2.
580 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
582 Function *F = loopHeader->getParent();
583 DEBUG(dbgs() << "loop-unswitch: Unswitching loop %"
584 << loopHeader->getName() << " [" << L->getBlocks().size()
585 << " blocks] in Function " << F->getName()
586 << " when '" << *Val << "' == " << *LIC << "\n");
591 // First step, split the preheader and exit blocks, and add these blocks to
592 // the LoopBlocks list.
593 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this);
594 LoopBlocks.push_back(NewPreheader);
596 // We want the loop to come after the preheader, but before the exit blocks.
597 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
599 SmallVector<BasicBlock*, 8> ExitBlocks;
600 L->getUniqueExitBlocks(ExitBlocks);
602 // Split all of the edges from inside the loop to their exit blocks. Update
603 // the appropriate Phi nodes as we do so.
604 SplitExitEdges(L, ExitBlocks);
606 // The exit blocks may have been changed due to edge splitting, recompute.
608 L->getUniqueExitBlocks(ExitBlocks);
610 // Add exit blocks to the loop blocks.
611 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
613 // Next step, clone all of the basic blocks that make up the loop (including
614 // the loop preheader and exit blocks), keeping track of the mapping between
615 // the instructions and blocks.
616 NewBlocks.reserve(LoopBlocks.size());
617 DenseMap<const Value*, Value*> ValueMap;
618 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
619 BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F);
620 NewBlocks.push_back(NewBB);
621 ValueMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping.
622 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L);
625 // Splice the newly inserted blocks into the function right before the
626 // original preheader.
627 F->getBasicBlockList().splice(NewPreheader, F->getBasicBlockList(),
628 NewBlocks[0], F->end());
630 // Now we create the new Loop object for the versioned loop.
631 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI, LPM);
632 Loop *ParentLoop = L->getParentLoop();
634 // Make sure to add the cloned preheader and exit blocks to the parent loop
636 ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase());
639 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
640 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]);
641 // The new exit block should be in the same loop as the old one.
642 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
643 ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase());
645 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
646 "Exit block should have been split to have one successor!");
647 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
649 // If the successor of the exit block had PHI nodes, add an entry for
652 for (BasicBlock::iterator I = ExitSucc->begin(); isa<PHINode>(I); ++I) {
653 PN = cast<PHINode>(I);
654 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
655 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(V);
656 if (It != ValueMap.end()) V = It->second;
657 PN->addIncoming(V, NewExit);
661 // Rewrite the code to refer to itself.
662 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
663 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
664 E = NewBlocks[i]->end(); I != E; ++I)
665 RemapInstruction(I, ValueMap);
667 // Rewrite the original preheader to select between versions of the loop.
668 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
669 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
670 "Preheader splitting did not work correctly!");
672 // Emit the new branch that selects between the two versions of this loop.
673 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
674 LPM->deleteSimpleAnalysisValue(OldBR, L);
675 OldBR->eraseFromParent();
677 LoopProcessWorklist.push_back(NewLoop);
680 // Now we rewrite the original code to know that the condition is true and the
681 // new code to know that the condition is false.
682 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
684 // It's possible that simplifying one loop could cause the other to be
685 // deleted. If so, don't simplify it.
686 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop)
687 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
691 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
693 static void RemoveFromWorklist(Instruction *I,
694 std::vector<Instruction*> &Worklist) {
695 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
697 while (WI != Worklist.end()) {
698 unsigned Offset = WI-Worklist.begin();
700 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
704 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
705 /// program, replacing all uses with V and update the worklist.
706 static void ReplaceUsesOfWith(Instruction *I, Value *V,
707 std::vector<Instruction*> &Worklist,
708 Loop *L, LPPassManager *LPM) {
709 DEBUG(dbgs() << "Replace with '" << *V << "': " << *I);
711 // Add uses to the worklist, which may be dead now.
712 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
713 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
714 Worklist.push_back(Use);
716 // Add users to the worklist which may be simplified now.
717 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
719 Worklist.push_back(cast<Instruction>(*UI));
720 LPM->deleteSimpleAnalysisValue(I, L);
721 RemoveFromWorklist(I, Worklist);
722 I->replaceAllUsesWith(V);
723 I->eraseFromParent();
727 /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
728 /// information, and remove any dead successors it has.
730 void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
731 std::vector<Instruction*> &Worklist,
733 if (pred_begin(BB) != pred_end(BB)) {
734 // This block isn't dead, since an edge to BB was just removed, see if there
735 // are any easy simplifications we can do now.
736 if (BasicBlock *Pred = BB->getSinglePredecessor()) {
737 // If it has one pred, fold phi nodes in BB.
738 while (isa<PHINode>(BB->begin()))
739 ReplaceUsesOfWith(BB->begin(),
740 cast<PHINode>(BB->begin())->getIncomingValue(0),
743 // If this is the header of a loop and the only pred is the latch, we now
744 // have an unreachable loop.
745 if (Loop *L = LI->getLoopFor(BB))
746 if (loopHeader == BB && L->contains(Pred)) {
747 // Remove the branch from the latch to the header block, this makes
748 // the header dead, which will make the latch dead (because the header
749 // dominates the latch).
750 LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L);
751 Pred->getTerminator()->eraseFromParent();
752 new UnreachableInst(BB->getContext(), Pred);
754 // The loop is now broken, remove it from LI.
755 RemoveLoopFromHierarchy(L);
757 // Reprocess the header, which now IS dead.
758 RemoveBlockIfDead(BB, Worklist, L);
762 // If pred ends in a uncond branch, add uncond branch to worklist so that
763 // the two blocks will get merged.
764 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
765 if (BI->isUnconditional())
766 Worklist.push_back(BI);
771 DEBUG(dbgs() << "Nuking dead block: " << *BB);
773 // Remove the instructions in the basic block from the worklist.
774 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
775 RemoveFromWorklist(I, Worklist);
777 // Anything that uses the instructions in this basic block should have their
778 // uses replaced with undefs.
779 // If I is not void type then replaceAllUsesWith undef.
780 // This allows ValueHandlers and custom metadata to adjust itself.
781 if (!I->getType()->isVoidTy())
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 SmallVector<BasicBlock*, 4> 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 array_pod_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 LPM->deleteSimpleAnalysisValue(BB, L);
811 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, L);
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);
844 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
845 // the value specified by Val in the specified loop, or we know it does NOT have
846 // that value. Rewrite any uses of LIC or of properties correlated to it.
847 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
850 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
852 // FIXME: Support correlated properties, like:
859 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
860 // selects, switches.
861 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
862 std::vector<Instruction*> Worklist;
863 LLVMContext &Context = Val->getContext();
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) &&
869 Val->getType()->isIntegerTy(1))) {
874 Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
875 !cast<ConstantInt>(Val)->getZExtValue());
877 for (unsigned i = 0, e = Users.size(); i != e; ++i)
878 if (Instruction *U = cast<Instruction>(Users[i])) {
881 U->replaceUsesOfWith(LIC, Replacement);
882 Worklist.push_back(U);
884 SimplifyCode(Worklist, L);
888 // Otherwise, we don't know the precise value of LIC, but we do know that it
889 // is certainly NOT "Val". As such, simplify any uses in the loop that we
890 // can. This case occurs when we unswitch switch statements.
891 for (unsigned i = 0, e = Users.size(); i != e; ++i) {
892 Instruction *U = cast<Instruction>(Users[i]);
896 Worklist.push_back(U);
898 // TODO: We could do other simplifications, for example, turning
899 // 'icmp eq LIC, Val' -> false.
901 // If we know that LIC is not Val, use this info to simplify code.
902 SwitchInst *SI = dyn_cast<SwitchInst>(U);
903 if (SI == 0 || !isa<ConstantInt>(Val)) continue;
905 unsigned DeadCase = SI->findCaseValue(cast<ConstantInt>(Val));
906 if (DeadCase == 0) continue; // Default case is live for multiple values.
908 // Found a dead case value. Don't remove PHI nodes in the
909 // successor if they become single-entry, those PHI nodes may
910 // be in the Users list.
912 // FIXME: This is a hack. We need to keep the successor around
913 // and hooked up so as to preserve the loop structure, because
914 // trying to update it is complicated. So instead we preserve the
915 // loop structure and put the block on a dead code path.
916 BasicBlock *Switch = SI->getParent();
917 SplitEdge(Switch, SI->getSuccessor(DeadCase), this);
918 // Compute the successors instead of relying on the return value
919 // of SplitEdge, since it may have split the switch successor
921 BasicBlock *NewSISucc = SI->getSuccessor(DeadCase);
922 BasicBlock *OldSISucc = *succ_begin(NewSISucc);
923 // Create an "unreachable" destination.
924 BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
927 new UnreachableInst(Context, Abort);
928 // Force the new case destination to branch to the "unreachable"
929 // block while maintaining a (dead) CFG edge to the old block.
930 NewSISucc->getTerminator()->eraseFromParent();
931 BranchInst::Create(Abort, OldSISucc,
932 ConstantInt::getTrue(Context), NewSISucc);
933 // Release the PHI operands for this edge.
934 for (BasicBlock::iterator II = NewSISucc->begin();
935 PHINode *PN = dyn_cast<PHINode>(II); ++II)
936 PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
937 UndefValue::get(PN->getType()));
938 // Tell the domtree about the new block. We don't fully update the
939 // domtree here -- instead we force it to do a full recomputation
940 // after the pass is complete -- but we do need to inform it of
943 DT->addNewBlock(Abort, NewSISucc);
946 SimplifyCode(Worklist, L);
949 /// SimplifyCode - Okay, now that we have simplified some instructions in the
950 /// loop, walk over it and constant prop, dce, and fold control flow where
951 /// possible. Note that this is effectively a very simple loop-structure-aware
952 /// optimizer. During processing of this loop, L could very well be deleted, so
953 /// it must not be used.
955 /// FIXME: When the loop optimizer is more mature, separate this out to a new
958 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
959 while (!Worklist.empty()) {
960 Instruction *I = Worklist.back();
963 // Simple constant folding.
964 if (Constant *C = ConstantFoldInstruction(I)) {
965 ReplaceUsesOfWith(I, C, Worklist, L, LPM);
970 if (isInstructionTriviallyDead(I)) {
971 DEBUG(dbgs() << "Remove dead instruction '" << *I);
973 // Add uses to the worklist, which may be dead now.
974 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
975 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
976 Worklist.push_back(Use);
977 LPM->deleteSimpleAnalysisValue(I, L);
978 RemoveFromWorklist(I, Worklist);
979 I->eraseFromParent();
984 // Special case hacks that appear commonly in unswitched code.
985 switch (I->getOpcode()) {
986 case Instruction::Select:
987 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(0))) {
988 ReplaceUsesOfWith(I, I->getOperand(!CB->getZExtValue()+1), Worklist, L,
993 case Instruction::And:
994 if (isa<ConstantInt>(I->getOperand(0)) &&
996 I->getOperand(0)->getType()->isIntegerTy(1))
997 cast<BinaryOperator>(I)->swapOperands();
998 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1)))
999 if (CB->getType()->isIntegerTy(1)) {
1000 if (CB->isOne()) // X & 1 -> X
1001 ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM);
1003 ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM);
1007 case Instruction::Or:
1008 if (isa<ConstantInt>(I->getOperand(0)) &&
1010 I->getOperand(0)->getType()->isIntegerTy(1))
1011 cast<BinaryOperator>(I)->swapOperands();
1012 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1)))
1013 if (CB->getType()->isIntegerTy(1)) {
1014 if (CB->isOne()) // X | 1 -> 1
1015 ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM);
1017 ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM);
1021 case Instruction::Br: {
1022 BranchInst *BI = cast<BranchInst>(I);
1023 if (BI->isUnconditional()) {
1024 // If BI's parent is the only pred of the successor, fold the two blocks
1026 BasicBlock *Pred = BI->getParent();
1027 BasicBlock *Succ = BI->getSuccessor(0);
1028 BasicBlock *SinglePred = Succ->getSinglePredecessor();
1029 if (!SinglePred) continue; // Nothing to do.
1030 assert(SinglePred == Pred && "CFG broken");
1032 DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- "
1033 << Succ->getName() << "\n");
1035 // Resolve any single entry PHI nodes in Succ.
1036 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1037 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM);
1039 // Move all of the successor contents from Succ to Pred.
1040 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1042 LPM->deleteSimpleAnalysisValue(BI, L);
1043 BI->eraseFromParent();
1044 RemoveFromWorklist(BI, Worklist);
1046 // If Succ has any successors with PHI nodes, update them to have
1047 // entries coming from Pred instead of Succ.
1048 Succ->replaceAllUsesWith(Pred);
1050 // Remove Succ from the loop tree.
1051 LI->removeBlock(Succ);
1052 LPM->deleteSimpleAnalysisValue(Succ, L);
1053 Succ->eraseFromParent();
1058 if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){
1059 // Conditional branch. Turn it into an unconditional branch, then
1060 // remove dead blocks.
1061 break; // FIXME: Enable.
1063 DEBUG(dbgs() << "Folded branch: " << *BI);
1064 BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue());
1065 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue());
1066 DeadSucc->removePredecessor(BI->getParent(), true);
1067 Worklist.push_back(BranchInst::Create(LiveSucc, BI));
1068 LPM->deleteSimpleAnalysisValue(BI, L);
1069 BI->eraseFromParent();
1070 RemoveFromWorklist(BI, Worklist);
1073 RemoveBlockIfDead(DeadSucc, Worklist, L);