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/InstructionSimplify.h"
38 #include "llvm/Analysis/LoopInfo.h"
39 #include "llvm/Analysis/LoopPass.h"
40 #include "llvm/Analysis/Dominators.h"
41 #include "llvm/Transforms/Utils/Cloning.h"
42 #include "llvm/Transforms/Utils/Local.h"
43 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
44 #include "llvm/ADT/Statistic.h"
45 #include "llvm/ADT/SmallPtrSet.h"
46 #include "llvm/ADT/STLExtras.h"
47 #include "llvm/Support/CommandLine.h"
48 #include "llvm/Support/Debug.h"
49 #include "llvm/Support/raw_ostream.h"
54 STATISTIC(NumBranches, "Number of branches unswitched");
55 STATISTIC(NumSwitches, "Number of switches unswitched");
56 STATISTIC(NumSelects , "Number of selects unswitched");
57 STATISTIC(NumTrivial , "Number of unswitches that are trivial");
58 STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
60 // The specific value of 50 here was chosen based only on intuition and a
61 // few specific examples.
62 static cl::opt<unsigned>
63 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
64 cl::init(50), cl::Hidden);
67 class LoopUnswitch : public LoopPass {
68 LoopInfo *LI; // Loop information
71 // LoopProcessWorklist - Used to check if second loop needs processing
72 // after RewriteLoopBodyWithConditionConstant rewrites first loop.
73 std::vector<Loop*> LoopProcessWorklist;
74 SmallPtrSet<Value *,8> UnswitchedVals;
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), 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>();
116 virtual void releaseMemory() {
117 UnswitchedVals.clear();
120 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist,
122 void RemoveLoopFromWorklist(Loop *L) {
123 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(),
124 LoopProcessWorklist.end(), L);
125 if (I != LoopProcessWorklist.end())
126 LoopProcessWorklist.erase(I);
129 void initLoopData() {
130 loopHeader = currentLoop->getHeader();
131 loopPreheader = currentLoop->getLoopPreheader();
134 /// Split all of the edges from inside the loop to their exit blocks.
135 /// Update the appropriate Phi nodes as we do so.
136 void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks);
138 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val);
139 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
140 BasicBlock *ExitBlock);
141 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
143 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
144 Constant *Val, bool isEqual);
146 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
147 BasicBlock *TrueDest,
148 BasicBlock *FalseDest,
149 Instruction *InsertPt);
151 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
152 void RemoveBlockIfDead(BasicBlock *BB,
153 std::vector<Instruction*> &Worklist, Loop *l);
154 void RemoveLoopFromHierarchy(Loop *L);
155 bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0,
156 BasicBlock **LoopExit = 0);
160 char LoopUnswitch::ID = 0;
161 INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops",
163 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
164 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
165 INITIALIZE_PASS_DEPENDENCY(LCSSA)
166 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
167 INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops",
170 Pass *llvm::createLoopUnswitchPass(bool Os) {
171 return new LoopUnswitch(Os);
174 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
175 /// invariant in the loop, or has an invariant piece, return the invariant.
176 /// Otherwise, return null.
177 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
178 // We can never unswitch on vector conditions.
179 if (Cond->getType()->isVectorTy())
182 // Constants should be folded, not unswitched on!
183 if (isa<Constant>(Cond)) return 0;
185 // TODO: Handle: br (VARIANT|INVARIANT).
187 // Hoist simple values out.
188 if (L->makeLoopInvariant(Cond, Changed))
191 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
192 if (BO->getOpcode() == Instruction::And ||
193 BO->getOpcode() == Instruction::Or) {
194 // If either the left or right side is invariant, we can unswitch on this,
195 // which will cause the branch to go away in one loop and the condition to
196 // simplify in the other one.
197 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
199 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
206 bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
207 LI = &getAnalysis<LoopInfo>();
209 DT = getAnalysisIfAvailable<DominatorTree>();
211 Function *F = currentLoop->getHeader()->getParent();
212 bool Changed = false;
214 assert(currentLoop->isLCSSAForm(*DT));
216 Changed |= processCurrentLoop();
220 // FIXME: Reconstruct dom info, because it is not preserved properly.
222 DT->runOnFunction(*F);
227 /// processCurrentLoop - Do actual work and unswitch loop if possible
229 bool LoopUnswitch::processCurrentLoop() {
230 bool Changed = false;
231 LLVMContext &Context = currentLoop->getHeader()->getContext();
233 // Loop over all of the basic blocks in the loop. If we find an interior
234 // block that is branching on a loop-invariant condition, we can unswitch this
236 for (Loop::block_iterator I = currentLoop->block_begin(),
237 E = currentLoop->block_end(); I != E; ++I) {
238 TerminatorInst *TI = (*I)->getTerminator();
239 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
240 // If this isn't branching on an invariant condition, we can't unswitch
242 if (BI->isConditional()) {
243 // See if this, or some part of it, is loop invariant. If so, we can
244 // unswitch on it if we desire.
245 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
246 currentLoop, Changed);
247 if (LoopCond && UnswitchIfProfitable(LoopCond,
248 ConstantInt::getTrue(Context))) {
253 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
254 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
255 currentLoop, Changed);
256 if (LoopCond && SI->getNumCases() > 1) {
257 // Find a value to unswitch on:
258 // FIXME: this should chose the most expensive case!
259 Constant *UnswitchVal = SI->getCaseValue(1);
260 // Do not process same value again and again.
261 if (!UnswitchedVals.insert(UnswitchVal))
264 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) {
271 // Scan the instructions to check for unswitchable values.
272 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
274 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
275 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
276 currentLoop, Changed);
277 if (LoopCond && UnswitchIfProfitable(LoopCond,
278 ConstantInt::getTrue(Context))) {
287 /// isTrivialLoopExitBlock - Check to see if all paths from BB exit the
288 /// loop with no side effects (including infinite loops).
290 /// If true, we return true and set ExitBB to the block we
293 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
295 std::set<BasicBlock*> &Visited) {
296 if (!Visited.insert(BB).second) {
297 // Already visited. Without more analysis, this could indicate an infinte loop.
299 } else if (!L->contains(BB)) {
300 // Otherwise, this is a loop exit, this is fine so long as this is the
302 if (ExitBB != 0) return false;
307 // Otherwise, this is an unvisited intra-loop node. Check all successors.
308 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
309 // Check to see if the successor is a trivial loop exit.
310 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
314 // Okay, everything after this looks good, check to make sure that this block
315 // doesn't include any side effects.
316 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
317 if (I->mayHaveSideEffects())
323 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally
324 /// leads to an exit from the specified loop, and has no side-effects in the
325 /// process. If so, return the block that is exited to, otherwise return null.
326 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
327 std::set<BasicBlock*> Visited;
328 Visited.insert(L->getHeader()); // Branches to header make infinite loops.
329 BasicBlock *ExitBB = 0;
330 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
335 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
336 /// trivial: that is, that the condition controls whether or not the loop does
337 /// anything at all. If this is a trivial condition, unswitching produces no
338 /// code duplications (equivalently, it produces a simpler loop and a new empty
339 /// loop, which gets deleted).
341 /// If this is a trivial condition, return true, otherwise return false. When
342 /// returning true, this sets Cond and Val to the condition that controls the
343 /// trivial condition: when Cond dynamically equals Val, the loop is known to
344 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when
347 bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
348 BasicBlock **LoopExit) {
349 BasicBlock *Header = currentLoop->getHeader();
350 TerminatorInst *HeaderTerm = Header->getTerminator();
351 LLVMContext &Context = Header->getContext();
353 BasicBlock *LoopExitBB = 0;
354 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
355 // If the header block doesn't end with a conditional branch on Cond, we
357 if (!BI->isConditional() || BI->getCondition() != Cond)
360 // Check to see if a successor of the branch is guaranteed to
361 // exit through a unique exit block without having any
362 // side-effects. If so, determine the value of Cond that causes it to do
364 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
365 BI->getSuccessor(0)))) {
366 if (Val) *Val = ConstantInt::getTrue(Context);
367 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
368 BI->getSuccessor(1)))) {
369 if (Val) *Val = ConstantInt::getFalse(Context);
371 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
372 // If this isn't a switch on Cond, we can't handle it.
373 if (SI->getCondition() != Cond) return false;
375 // Check to see if a successor of the switch is guaranteed to go to the
376 // latch block or exit through a one exit block without having any
377 // side-effects. If so, determine the value of Cond that causes it to do
378 // this. Note that we can't trivially unswitch on the default case.
379 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
380 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
381 SI->getSuccessor(i)))) {
382 // Okay, we found a trivial case, remember the value that is trivial.
383 if (Val) *Val = SI->getCaseValue(i);
388 // If we didn't find a single unique LoopExit block, or if the loop exit block
389 // contains phi nodes, this isn't trivial.
390 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
391 return false; // Can't handle this.
393 if (LoopExit) *LoopExit = LoopExitBB;
395 // We already know that nothing uses any scalar values defined inside of this
396 // loop. As such, we just have to check to see if this loop will execute any
397 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
398 // part of the loop that the code *would* execute. We already checked the
399 // tail, check the header now.
400 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
401 if (I->mayHaveSideEffects())
406 /// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
407 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
408 /// unswitch the loop, reprocess the pieces, then return true.
409 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val) {
413 // If LoopSimplify was unable to form a preheader, don't do any unswitching.
417 Function *F = loopHeader->getParent();
419 Constant *CondVal = 0;
420 BasicBlock *ExitBlock = 0;
421 if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
422 // If the condition is trivial, always unswitch. There is no code growth
424 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock);
428 // Check to see if it would be profitable to unswitch current loop.
430 // Do not do non-trivial unswitch while optimizing for size.
431 if (OptimizeForSize || F->hasFnAttr(Attribute::OptimizeForSize))
434 // FIXME: This is overly conservative because it does not take into
435 // consideration code simplification opportunities and code that can
436 // be shared by the resultant unswitched loops.
438 for (Loop::block_iterator I = currentLoop->block_begin(),
439 E = currentLoop->block_end();
441 Metrics.analyzeBasicBlock(*I);
443 // Limit the number of instructions to avoid causing significant code
444 // expansion, and the number of basic blocks, to avoid loops with
445 // large numbers of branches which cause loop unswitching to go crazy.
446 // This is a very ad-hoc heuristic.
447 if (Metrics.NumInsts > Threshold ||
448 Metrics.NumBlocks * 5 > Threshold ||
449 Metrics.containsIndirectBr || Metrics.isRecursive) {
450 DEBUG(dbgs() << "NOT unswitching loop %"
451 << currentLoop->getHeader()->getName() << ", cost too high: "
452 << currentLoop->getBlocks().size() << "\n");
456 UnswitchNontrivialCondition(LoopCond, Val, currentLoop);
460 // RemapInstruction - Convert the instruction operands from referencing the
461 // current values into those specified by VMap.
463 static inline void RemapInstruction(Instruction *I,
464 ValueMap<const Value *, Value*> &VMap) {
465 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
466 Value *Op = I->getOperand(op);
467 ValueMap<const Value *, Value*>::iterator It = VMap.find(Op);
468 if (It != VMap.end()) Op = It->second;
469 I->setOperand(op, Op);
473 /// CloneLoop - Recursively clone the specified loop and all of its children,
474 /// mapping the blocks with the specified map.
475 static Loop *CloneLoop(Loop *L, Loop *PL, ValueMap<const Value*, Value*> &VM,
476 LoopInfo *LI, LPPassManager *LPM) {
477 Loop *New = new Loop();
478 LPM->insertLoop(New, PL);
480 // Add all of the blocks in L to the new loop.
481 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
483 if (LI->getLoopFor(*I) == L)
484 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), LI->getBase());
486 // Add all of the subloops to the new loop.
487 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
488 CloneLoop(*I, New, VM, LI, LPM);
493 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
494 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
495 /// code immediately before InsertPt.
496 void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
497 BasicBlock *TrueDest,
498 BasicBlock *FalseDest,
499 Instruction *InsertPt) {
500 // Insert a conditional branch on LIC to the two preheaders. The original
501 // code is the true version and the new code is the false version.
502 Value *BranchVal = LIC;
503 if (!isa<ConstantInt>(Val) ||
504 Val->getType() != Type::getInt1Ty(LIC->getContext()))
505 BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val, "tmp");
506 else if (Val != ConstantInt::getTrue(Val->getContext()))
507 // We want to enter the new loop when the condition is true.
508 std::swap(TrueDest, FalseDest);
510 // Insert the new branch.
511 BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt);
513 // If either edge is critical, split it. This helps preserve LoopSimplify
514 // form for enclosing loops.
515 SplitCriticalEdge(BI, 0, this);
516 SplitCriticalEdge(BI, 1, this);
519 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
520 /// condition in it (a cond branch from its header block to its latch block,
521 /// where the path through the loop that doesn't execute its body has no
522 /// side-effects), unswitch it. This doesn't involve any code duplication, just
523 /// moving the conditional branch outside of the loop and updating loop info.
524 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
526 BasicBlock *ExitBlock) {
527 DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
528 << loopHeader->getName() << " [" << L->getBlocks().size()
529 << " blocks] in Function " << L->getHeader()->getParent()->getName()
530 << " on cond: " << *Val << " == " << *Cond << "\n");
532 // First step, split the preheader, so that we know that there is a safe place
533 // to insert the conditional branch. We will change loopPreheader to have a
534 // conditional branch on Cond.
535 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this);
537 // Now that we have a place to insert the conditional branch, create a place
538 // to branch to: this is the exit block out of the loop that we should
541 // Split this block now, so that the loop maintains its exit block, and so
542 // that the jump from the preheader can execute the contents of the exit block
543 // without actually branching to it (the exit block should be dominated by the
544 // loop header, not the preheader).
545 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
546 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this);
548 // Okay, now we have a position to branch from and a position to branch to,
549 // insert the new conditional branch.
550 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
551 loopPreheader->getTerminator());
552 LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L);
553 loopPreheader->getTerminator()->eraseFromParent();
555 // We need to reprocess this loop, it could be unswitched again.
558 // Now that we know that the loop is never entered when this condition is a
559 // particular value, rewrite the loop with this info. We know that this will
560 // at least eliminate the old branch.
561 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
565 /// SplitExitEdges - Split all of the edges from inside the loop to their exit
566 /// blocks. Update the appropriate Phi nodes as we do so.
567 void LoopUnswitch::SplitExitEdges(Loop *L,
568 const SmallVector<BasicBlock *, 8> &ExitBlocks){
570 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
571 BasicBlock *ExitBlock = ExitBlocks[i];
572 SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
573 pred_end(ExitBlock));
574 SplitBlockPredecessors(ExitBlock, Preds.data(), Preds.size(),
579 /// UnswitchNontrivialCondition - We determined that the loop is profitable
580 /// to unswitch when LIC equal Val. Split it into loop versions and test the
581 /// condition outside of either loop. Return the loops created as Out1/Out2.
582 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
584 Function *F = loopHeader->getParent();
585 DEBUG(dbgs() << "loop-unswitch: Unswitching loop %"
586 << loopHeader->getName() << " [" << L->getBlocks().size()
587 << " blocks] in Function " << F->getName()
588 << " when '" << *Val << "' == " << *LIC << "\n");
593 // First step, split the preheader and exit blocks, and add these blocks to
594 // the LoopBlocks list.
595 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this);
596 LoopBlocks.push_back(NewPreheader);
598 // We want the loop to come after the preheader, but before the exit blocks.
599 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
601 SmallVector<BasicBlock*, 8> ExitBlocks;
602 L->getUniqueExitBlocks(ExitBlocks);
604 // Split all of the edges from inside the loop to their exit blocks. Update
605 // the appropriate Phi nodes as we do so.
606 SplitExitEdges(L, ExitBlocks);
608 // The exit blocks may have been changed due to edge splitting, recompute.
610 L->getUniqueExitBlocks(ExitBlocks);
612 // Add exit blocks to the loop blocks.
613 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
615 // Next step, clone all of the basic blocks that make up the loop (including
616 // the loop preheader and exit blocks), keeping track of the mapping between
617 // the instructions and blocks.
618 NewBlocks.reserve(LoopBlocks.size());
619 ValueMap<const Value*, Value*> VMap;
620 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
621 BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], VMap, ".us", F);
622 NewBlocks.push_back(NewBB);
623 VMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping.
624 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L);
627 // Splice the newly inserted blocks into the function right before the
628 // original preheader.
629 F->getBasicBlockList().splice(NewPreheader, F->getBasicBlockList(),
630 NewBlocks[0], F->end());
632 // Now we create the new Loop object for the versioned loop.
633 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), VMap, LI, LPM);
634 Loop *ParentLoop = L->getParentLoop();
636 // Make sure to add the cloned preheader and exit blocks to the parent loop
638 ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase());
641 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
642 BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[i]]);
643 // The new exit block should be in the same loop as the old one.
644 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
645 ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase());
647 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
648 "Exit block should have been split to have one successor!");
649 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
651 // If the successor of the exit block had PHI nodes, add an entry for
654 for (BasicBlock::iterator I = ExitSucc->begin(); isa<PHINode>(I); ++I) {
655 PN = cast<PHINode>(I);
656 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
657 ValueMap<const Value *, Value*>::iterator It = VMap.find(V);
658 if (It != VMap.end()) V = It->second;
659 PN->addIncoming(V, NewExit);
663 // Rewrite the code to refer to itself.
664 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
665 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
666 E = NewBlocks[i]->end(); I != E; ++I)
667 RemapInstruction(I, VMap);
669 // Rewrite the original preheader to select between versions of the loop.
670 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
671 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
672 "Preheader splitting did not work correctly!");
674 // Emit the new branch that selects between the two versions of this loop.
675 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
676 LPM->deleteSimpleAnalysisValue(OldBR, L);
677 OldBR->eraseFromParent();
679 LoopProcessWorklist.push_back(NewLoop);
682 // Keep a WeakVH holding onto LIC. If the first call to RewriteLoopBody
683 // deletes the instruction (for example by simplifying a PHI that feeds into
684 // the condition that we're unswitching on), we don't rewrite the second
686 WeakVH LICHandle(LIC);
688 // Now we rewrite the original code to know that the condition is true and the
689 // new code to know that the condition is false.
690 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
692 // It's possible that simplifying one loop could cause the other to be
693 // changed to another value or a constant. If its a constant, don't simplify
695 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
696 LICHandle && !isa<Constant>(LICHandle))
697 RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true);
700 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
702 static void RemoveFromWorklist(Instruction *I,
703 std::vector<Instruction*> &Worklist) {
704 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
706 while (WI != Worklist.end()) {
707 unsigned Offset = WI-Worklist.begin();
709 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
713 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
714 /// program, replacing all uses with V and update the worklist.
715 static void ReplaceUsesOfWith(Instruction *I, Value *V,
716 std::vector<Instruction*> &Worklist,
717 Loop *L, LPPassManager *LPM) {
718 DEBUG(dbgs() << "Replace with '" << *V << "': " << *I);
720 // Add uses to the worklist, which may be dead now.
721 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
722 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
723 Worklist.push_back(Use);
725 // Add users to the worklist which may be simplified now.
726 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
728 Worklist.push_back(cast<Instruction>(*UI));
729 LPM->deleteSimpleAnalysisValue(I, L);
730 RemoveFromWorklist(I, Worklist);
731 I->replaceAllUsesWith(V);
732 I->eraseFromParent();
736 /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
737 /// information, and remove any dead successors it has.
739 void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
740 std::vector<Instruction*> &Worklist,
742 if (pred_begin(BB) != pred_end(BB)) {
743 // This block isn't dead, since an edge to BB was just removed, see if there
744 // are any easy simplifications we can do now.
745 if (BasicBlock *Pred = BB->getSinglePredecessor()) {
746 // If it has one pred, fold phi nodes in BB.
747 while (isa<PHINode>(BB->begin()))
748 ReplaceUsesOfWith(BB->begin(),
749 cast<PHINode>(BB->begin())->getIncomingValue(0),
752 // If this is the header of a loop and the only pred is the latch, we now
753 // have an unreachable loop.
754 if (Loop *L = LI->getLoopFor(BB))
755 if (loopHeader == BB && L->contains(Pred)) {
756 // Remove the branch from the latch to the header block, this makes
757 // the header dead, which will make the latch dead (because the header
758 // dominates the latch).
759 LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L);
760 Pred->getTerminator()->eraseFromParent();
761 new UnreachableInst(BB->getContext(), Pred);
763 // The loop is now broken, remove it from LI.
764 RemoveLoopFromHierarchy(L);
766 // Reprocess the header, which now IS dead.
767 RemoveBlockIfDead(BB, Worklist, L);
771 // If pred ends in a uncond branch, add uncond branch to worklist so that
772 // the two blocks will get merged.
773 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
774 if (BI->isUnconditional())
775 Worklist.push_back(BI);
780 DEBUG(dbgs() << "Nuking dead block: " << *BB);
782 // Remove the instructions in the basic block from the worklist.
783 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
784 RemoveFromWorklist(I, Worklist);
786 // Anything that uses the instructions in this basic block should have their
787 // uses replaced with undefs.
788 // If I is not void type then replaceAllUsesWith undef.
789 // This allows ValueHandlers and custom metadata to adjust itself.
790 if (!I->getType()->isVoidTy())
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 SmallVector<BasicBlock*, 4> 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 array_pod_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 LPM->deleteSimpleAnalysisValue(BB, L);
820 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, L);
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 LPM->deleteLoopFromQueue(L);
850 RemoveLoopFromWorklist(L);
853 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
854 // the value specified by Val in the specified loop, or we know it does NOT have
855 // that value. Rewrite any uses of LIC or of properties correlated to it.
856 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
859 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
861 // FIXME: Support correlated properties, like:
868 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
869 // selects, switches.
870 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
871 std::vector<Instruction*> Worklist;
872 LLVMContext &Context = Val->getContext();
875 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
876 // in the loop with the appropriate one directly.
877 if (IsEqual || (isa<ConstantInt>(Val) &&
878 Val->getType()->isIntegerTy(1))) {
883 Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
884 !cast<ConstantInt>(Val)->getZExtValue());
886 for (unsigned i = 0, e = Users.size(); i != e; ++i)
887 if (Instruction *U = cast<Instruction>(Users[i])) {
890 U->replaceUsesOfWith(LIC, Replacement);
891 Worklist.push_back(U);
893 SimplifyCode(Worklist, L);
897 // Otherwise, we don't know the precise value of LIC, but we do know that it
898 // is certainly NOT "Val". As such, simplify any uses in the loop that we
899 // can. This case occurs when we unswitch switch statements.
900 for (unsigned i = 0, e = Users.size(); i != e; ++i) {
901 Instruction *U = cast<Instruction>(Users[i]);
905 Worklist.push_back(U);
907 // TODO: We could do other simplifications, for example, turning
908 // 'icmp eq LIC, Val' -> false.
910 // If we know that LIC is not Val, use this info to simplify code.
911 SwitchInst *SI = dyn_cast<SwitchInst>(U);
912 if (SI == 0 || !isa<ConstantInt>(Val)) continue;
914 unsigned DeadCase = SI->findCaseValue(cast<ConstantInt>(Val));
915 if (DeadCase == 0) continue; // Default case is live for multiple values.
917 // Found a dead case value. Don't remove PHI nodes in the
918 // successor if they become single-entry, those PHI nodes may
919 // be in the Users list.
921 // FIXME: This is a hack. We need to keep the successor around
922 // and hooked up so as to preserve the loop structure, because
923 // trying to update it is complicated. So instead we preserve the
924 // loop structure and put the block on a dead code path.
925 BasicBlock *Switch = SI->getParent();
926 SplitEdge(Switch, SI->getSuccessor(DeadCase), this);
927 // Compute the successors instead of relying on the return value
928 // of SplitEdge, since it may have split the switch successor
930 BasicBlock *NewSISucc = SI->getSuccessor(DeadCase);
931 BasicBlock *OldSISucc = *succ_begin(NewSISucc);
932 // Create an "unreachable" destination.
933 BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
936 new UnreachableInst(Context, Abort);
937 // Force the new case destination to branch to the "unreachable"
938 // block while maintaining a (dead) CFG edge to the old block.
939 NewSISucc->getTerminator()->eraseFromParent();
940 BranchInst::Create(Abort, OldSISucc,
941 ConstantInt::getTrue(Context), NewSISucc);
942 // Release the PHI operands for this edge.
943 for (BasicBlock::iterator II = NewSISucc->begin();
944 PHINode *PN = dyn_cast<PHINode>(II); ++II)
945 PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
946 UndefValue::get(PN->getType()));
947 // Tell the domtree about the new block. We don't fully update the
948 // domtree here -- instead we force it to do a full recomputation
949 // after the pass is complete -- but we do need to inform it of
952 DT->addNewBlock(Abort, NewSISucc);
955 SimplifyCode(Worklist, L);
958 /// SimplifyCode - Okay, now that we have simplified some instructions in the
959 /// loop, walk over it and constant prop, dce, and fold control flow where
960 /// possible. Note that this is effectively a very simple loop-structure-aware
961 /// optimizer. During processing of this loop, L could very well be deleted, so
962 /// it must not be used.
964 /// FIXME: When the loop optimizer is more mature, separate this out to a new
967 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
968 while (!Worklist.empty()) {
969 Instruction *I = Worklist.back();
972 // Simple constant folding.
973 if (Constant *C = ConstantFoldInstruction(I)) {
974 ReplaceUsesOfWith(I, C, Worklist, L, LPM);
979 if (isInstructionTriviallyDead(I)) {
980 DEBUG(dbgs() << "Remove dead instruction '" << *I);
982 // Add uses to the worklist, which may be dead now.
983 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
984 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
985 Worklist.push_back(Use);
986 LPM->deleteSimpleAnalysisValue(I, L);
987 RemoveFromWorklist(I, Worklist);
988 I->eraseFromParent();
993 // See if instruction simplification can hack this up. This is common for
994 // things like "select false, X, Y" after unswitching made the condition be
996 if (Value *V = SimplifyInstruction(I)) {
997 ReplaceUsesOfWith(I, V, Worklist, L, LPM);
1001 // Special case hacks that appear commonly in unswitched code.
1002 if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
1003 if (BI->isUnconditional()) {
1004 // If BI's parent is the only pred of the successor, fold the two blocks
1006 BasicBlock *Pred = BI->getParent();
1007 BasicBlock *Succ = BI->getSuccessor(0);
1008 BasicBlock *SinglePred = Succ->getSinglePredecessor();
1009 if (!SinglePred) continue; // Nothing to do.
1010 assert(SinglePred == Pred && "CFG broken");
1012 DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- "
1013 << Succ->getName() << "\n");
1015 // Resolve any single entry PHI nodes in Succ.
1016 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1017 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM);
1019 // Move all of the successor contents from Succ to Pred.
1020 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1022 LPM->deleteSimpleAnalysisValue(BI, L);
1023 BI->eraseFromParent();
1024 RemoveFromWorklist(BI, Worklist);
1026 // If Succ has any successors with PHI nodes, update them to have
1027 // entries coming from Pred instead of Succ.
1028 Succ->replaceAllUsesWith(Pred);
1030 // Remove Succ from the loop tree.
1031 LI->removeBlock(Succ);
1032 LPM->deleteSimpleAnalysisValue(Succ, L);
1033 Succ->eraseFromParent();
1038 if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){
1039 // Conditional branch. Turn it into an unconditional branch, then
1040 // remove dead blocks.
1041 continue; // FIXME: Enable.
1043 DEBUG(dbgs() << "Folded branch: " << *BI);
1044 BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue());
1045 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue());
1046 DeadSucc->removePredecessor(BI->getParent(), true);
1047 Worklist.push_back(BranchInst::Create(LiveSucc, BI));
1048 LPM->deleteSimpleAnalysisValue(BI, L);
1049 BI->eraseFromParent();
1050 RemoveFromWorklist(BI, Worklist);
1053 RemoveBlockIfDead(DeadSucc, Worklist, L);