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 // Constants should be folded, not unswitched on!
173 if (isa<Constant>(Cond)) return 0;
175 // TODO: Handle: br (VARIANT|INVARIANT).
177 // Hoist simple values out.
178 if (L->makeLoopInvariant(Cond, Changed))
181 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
182 if (BO->getOpcode() == Instruction::And ||
183 BO->getOpcode() == Instruction::Or) {
184 // If either the left or right side is invariant, we can unswitch on this,
185 // which will cause the branch to go away in one loop and the condition to
186 // simplify in the other one.
187 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
189 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
196 bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
197 LI = &getAnalysis<LoopInfo>();
199 DF = getAnalysisIfAvailable<DominanceFrontier>();
200 DT = getAnalysisIfAvailable<DominatorTree>();
202 Function *F = currentLoop->getHeader()->getParent();
203 bool Changed = false;
205 assert(currentLoop->isLCSSAForm());
207 Changed |= processCurrentLoop();
211 // FIXME: Reconstruct dom info, because it is not preserved properly.
213 DT->runOnFunction(*F);
215 DF->runOnFunction(*F);
220 /// processCurrentLoop - Do actual work and unswitch loop if possible
222 bool LoopUnswitch::processCurrentLoop() {
223 bool Changed = false;
224 LLVMContext &Context = currentLoop->getHeader()->getContext();
226 // Loop over all of the basic blocks in the loop. If we find an interior
227 // block that is branching on a loop-invariant condition, we can unswitch this
229 for (Loop::block_iterator I = currentLoop->block_begin(),
230 E = currentLoop->block_end();
232 TerminatorInst *TI = (*I)->getTerminator();
233 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
234 // If this isn't branching on an invariant condition, we can't unswitch
236 if (BI->isConditional()) {
237 // See if this, or some part of it, is loop invariant. If so, we can
238 // unswitch on it if we desire.
239 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
240 currentLoop, Changed);
241 if (LoopCond && UnswitchIfProfitable(LoopCond,
242 ConstantInt::getTrue(Context))) {
247 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
248 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
249 currentLoop, Changed);
250 if (LoopCond && SI->getNumCases() > 1) {
251 // Find a value to unswitch on:
252 // FIXME: this should chose the most expensive case!
253 Constant *UnswitchVal = SI->getCaseValue(1);
254 // Do not process same value again and again.
255 if (!UnswitchedVals.insert(UnswitchVal))
258 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) {
265 // Scan the instructions to check for unswitchable values.
266 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
268 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
269 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
270 currentLoop, Changed);
271 if (LoopCond && UnswitchIfProfitable(LoopCond,
272 ConstantInt::getTrue(Context))) {
281 /// isTrivialLoopExitBlock - Check to see if all paths from BB either:
282 /// 1. Exit the loop with no side effects.
283 /// 2. Branch to the latch block with no side-effects.
285 /// If these conditions are true, we return true and set ExitBB to the block we
288 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
290 std::set<BasicBlock*> &Visited) {
291 if (!Visited.insert(BB).second) {
292 // Already visited and Ok, end of recursion.
294 } else if (!L->contains(BB)) {
295 // Otherwise, this is a loop exit, this is fine so long as this is the
297 if (ExitBB != 0) return false;
302 // Otherwise, this is an unvisited intra-loop node. Check all successors.
303 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
304 // Check to see if the successor is a trivial loop exit.
305 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
309 // Okay, everything after this looks good, check to make sure that this block
310 // doesn't include any side effects.
311 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
312 if (I->mayHaveSideEffects())
318 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally
319 /// leads to an exit from the specified loop, and has no side-effects in the
320 /// process. If so, return the block that is exited to, otherwise return null.
321 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
322 std::set<BasicBlock*> Visited;
323 Visited.insert(L->getHeader()); // Branches to header are ok.
324 BasicBlock *ExitBB = 0;
325 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
330 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
331 /// trivial: that is, that the condition controls whether or not the loop does
332 /// anything at all. If this is a trivial condition, unswitching produces no
333 /// code duplications (equivalently, it produces a simpler loop and a new empty
334 /// loop, which gets deleted).
336 /// If this is a trivial condition, return true, otherwise return false. When
337 /// returning true, this sets Cond and Val to the condition that controls the
338 /// trivial condition: when Cond dynamically equals Val, the loop is known to
339 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when
342 bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
343 BasicBlock **LoopExit) {
344 BasicBlock *Header = currentLoop->getHeader();
345 TerminatorInst *HeaderTerm = Header->getTerminator();
346 LLVMContext &Context = Header->getContext();
348 BasicBlock *LoopExitBB = 0;
349 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
350 // If the header block doesn't end with a conditional branch on Cond, we
352 if (!BI->isConditional() || BI->getCondition() != Cond)
355 // Check to see if a successor of the branch is guaranteed to go to the
356 // latch block or exit through a one exit block without having any
357 // side-effects. If so, determine the value of Cond that causes it to do
359 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
360 BI->getSuccessor(0)))) {
361 if (Val) *Val = ConstantInt::getTrue(Context);
362 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
363 BI->getSuccessor(1)))) {
364 if (Val) *Val = ConstantInt::getFalse(Context);
366 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
367 // If this isn't a switch on Cond, we can't handle it.
368 if (SI->getCondition() != Cond) return false;
370 // Check to see if a successor of the switch is guaranteed to go to the
371 // latch block or exit through a one exit block without having any
372 // side-effects. If so, determine the value of Cond that causes it to do
373 // this. Note that we can't trivially unswitch on the default case.
374 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
375 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
376 SI->getSuccessor(i)))) {
377 // Okay, we found a trivial case, remember the value that is trivial.
378 if (Val) *Val = SI->getCaseValue(i);
383 // If we didn't find a single unique LoopExit block, or if the loop exit block
384 // contains phi nodes, this isn't trivial.
385 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
386 return false; // Can't handle this.
388 if (LoopExit) *LoopExit = LoopExitBB;
390 // We already know that nothing uses any scalar values defined inside of this
391 // loop. As such, we just have to check to see if this loop will execute any
392 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
393 // part of the loop that the code *would* execute. We already checked the
394 // tail, check the header now.
395 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
396 if (I->mayHaveSideEffects())
401 /// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
402 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
403 /// unswitch the loop, reprocess the pieces, then return true.
404 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val){
408 // If LoopSimplify was unable to form a preheader, don't do any unswitching.
412 Function *F = loopHeader->getParent();
414 // If the condition is trivial, always unswitch. There is no code growth for
416 if (!IsTrivialUnswitchCondition(LoopCond)) {
417 // Check to see if it would be profitable to unswitch current loop.
419 // Do not do non-trivial unswitch while optimizing for size.
420 if (OptimizeForSize || F->hasFnAttr(Attribute::OptimizeForSize))
423 // FIXME: This is overly conservative because it does not take into
424 // consideration code simplification opportunities and code that can
425 // be shared by the resultant unswitched loops.
427 for (Loop::block_iterator I = currentLoop->block_begin(),
428 E = currentLoop->block_end();
430 Metrics.analyzeBasicBlock(*I);
432 // Limit the number of instructions to avoid causing significant code
433 // expansion, and the number of basic blocks, to avoid loops with
434 // large numbers of branches which cause loop unswitching to go crazy.
435 // This is a very ad-hoc heuristic.
436 if (Metrics.NumInsts > Threshold ||
437 Metrics.NumBlocks * 5 > Threshold ||
438 Metrics.NeverInline) {
439 DEBUG(errs() << "NOT unswitching loop %"
440 << currentLoop->getHeader()->getName() << ", cost too high: "
441 << currentLoop->getBlocks().size() << "\n");
447 BasicBlock *ExitBlock;
448 if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
449 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock);
451 UnswitchNontrivialCondition(LoopCond, Val, currentLoop);
457 // RemapInstruction - Convert the instruction operands from referencing the
458 // current values into those specified by ValueMap.
460 static inline void RemapInstruction(Instruction *I,
461 DenseMap<const Value *, Value*> &ValueMap) {
462 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
463 Value *Op = I->getOperand(op);
464 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op);
465 if (It != ValueMap.end()) Op = It->second;
466 I->setOperand(op, Op);
470 /// CloneLoop - Recursively clone the specified loop and all of its children,
471 /// mapping the blocks with the specified map.
472 static Loop *CloneLoop(Loop *L, Loop *PL, DenseMap<const Value*, Value*> &VM,
473 LoopInfo *LI, LPPassManager *LPM) {
474 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(errs() << "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)
569 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
570 BasicBlock *ExitBlock = ExitBlocks[i];
571 SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
572 pred_end(ExitBlock));
573 SplitBlockPredecessors(ExitBlock, Preds.data(), Preds.size(),
578 /// UnswitchNontrivialCondition - We determined that the loop is profitable
579 /// to unswitch when LIC equal Val. Split it into loop versions and test the
580 /// condition outside of either loop. Return the loops created as Out1/Out2.
581 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
583 Function *F = loopHeader->getParent();
584 DEBUG(errs() << "loop-unswitch: Unswitching loop %"
585 << loopHeader->getName() << " [" << L->getBlocks().size()
586 << " blocks] in Function " << F->getName()
587 << " when '" << *Val << "' == " << *LIC << "\n");
592 // First step, split the preheader and exit blocks, and add these blocks to
593 // the LoopBlocks list.
594 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this);
595 LoopBlocks.push_back(NewPreheader);
597 // We want the loop to come after the preheader, but before the exit blocks.
598 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
600 SmallVector<BasicBlock*, 8> ExitBlocks;
601 L->getUniqueExitBlocks(ExitBlocks);
603 // Split all of the edges from inside the loop to their exit blocks. Update
604 // the appropriate Phi nodes as we do so.
605 SplitExitEdges(L, ExitBlocks);
607 // The exit blocks may have been changed due to edge splitting, recompute.
609 L->getUniqueExitBlocks(ExitBlocks);
611 // Add exit blocks to the loop blocks.
612 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
614 // Next step, clone all of the basic blocks that make up the loop (including
615 // the loop preheader and exit blocks), keeping track of the mapping between
616 // the instructions and blocks.
617 NewBlocks.reserve(LoopBlocks.size());
618 DenseMap<const Value*, Value*> ValueMap;
619 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
620 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F);
621 NewBlocks.push_back(New);
622 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping.
623 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], New, L);
626 // Splice the newly inserted blocks into the function right before the
627 // original preheader.
628 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
629 NewBlocks[0], F->end());
631 // Now we create the new Loop object for the versioned loop.
632 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI, LPM);
633 Loop *ParentLoop = L->getParentLoop();
635 // Make sure to add the cloned preheader and exit blocks to the parent loop
637 ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase());
640 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
641 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]);
642 // The new exit block should be in the same loop as the old one.
643 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
644 ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase());
646 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
647 "Exit block should have been split to have one successor!");
648 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
650 // If the successor of the exit block had PHI nodes, add an entry for
653 for (BasicBlock::iterator I = ExitSucc->begin();
654 (PN = dyn_cast<PHINode>(I)); ++I) {
655 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
656 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(V);
657 if (It != ValueMap.end()) V = It->second;
658 PN->addIncoming(V, NewExit);
662 // Rewrite the code to refer to itself.
663 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
664 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
665 E = NewBlocks[i]->end(); I != E; ++I)
666 RemapInstruction(I, ValueMap);
668 // Rewrite the original preheader to select between versions of the loop.
669 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
670 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
671 "Preheader splitting did not work correctly!");
673 // Emit the new branch that selects between the two versions of this loop.
674 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
675 LPM->deleteSimpleAnalysisValue(OldBR, L);
676 OldBR->eraseFromParent();
678 LoopProcessWorklist.push_back(NewLoop);
681 // Now we rewrite the original code to know that the condition is true and the
682 // new code to know that the condition is false.
683 RewriteLoopBodyWithConditionConstant(L , LIC, Val, false);
685 // It's possible that simplifying one loop could cause the other to be
686 // deleted. If so, don't simplify it.
687 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop)
688 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
692 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
694 static void RemoveFromWorklist(Instruction *I,
695 std::vector<Instruction*> &Worklist) {
696 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
698 while (WI != Worklist.end()) {
699 unsigned Offset = WI-Worklist.begin();
701 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
705 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
706 /// program, replacing all uses with V and update the worklist.
707 static void ReplaceUsesOfWith(Instruction *I, Value *V,
708 std::vector<Instruction*> &Worklist,
709 Loop *L, LPPassManager *LPM) {
710 DEBUG(errs() << "Replace with '" << *V << "': " << *I);
712 // Add uses to the worklist, which may be dead now.
713 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
714 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
715 Worklist.push_back(Use);
717 // Add users to the worklist which may be simplified now.
718 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
720 Worklist.push_back(cast<Instruction>(*UI));
721 LPM->deleteSimpleAnalysisValue(I, L);
722 RemoveFromWorklist(I, Worklist);
723 I->replaceAllUsesWith(V);
724 I->eraseFromParent();
728 /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
729 /// information, and remove any dead successors it has.
731 void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
732 std::vector<Instruction*> &Worklist,
734 if (pred_begin(BB) != pred_end(BB)) {
735 // This block isn't dead, since an edge to BB was just removed, see if there
736 // are any easy simplifications we can do now.
737 if (BasicBlock *Pred = BB->getSinglePredecessor()) {
738 // If it has one pred, fold phi nodes in BB.
739 while (isa<PHINode>(BB->begin()))
740 ReplaceUsesOfWith(BB->begin(),
741 cast<PHINode>(BB->begin())->getIncomingValue(0),
744 // If this is the header of a loop and the only pred is the latch, we now
745 // have an unreachable loop.
746 if (Loop *L = LI->getLoopFor(BB))
747 if (loopHeader == BB && L->contains(Pred)) {
748 // Remove the branch from the latch to the header block, this makes
749 // the header dead, which will make the latch dead (because the header
750 // dominates the latch).
751 LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L);
752 Pred->getTerminator()->eraseFromParent();
753 new UnreachableInst(BB->getContext(), Pred);
755 // The loop is now broken, remove it from LI.
756 RemoveLoopFromHierarchy(L);
758 // Reprocess the header, which now IS dead.
759 RemoveBlockIfDead(BB, Worklist, L);
763 // If pred ends in a uncond branch, add uncond branch to worklist so that
764 // the two blocks will get merged.
765 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
766 if (BI->isUnconditional())
767 Worklist.push_back(BI);
772 DEBUG(errs() << "Nuking dead block: " << *BB);
774 // Remove the instructions in the basic block from the worklist.
775 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
776 RemoveFromWorklist(I, Worklist);
778 // Anything that uses the instructions in this basic block should have their
779 // uses replaced with undefs.
780 // If I is not void type then replaceAllUsesWith undef.
781 // This allows ValueHandlers and custom metadata to adjust itself.
782 if (!I->getType()->isVoidTy())
783 I->replaceAllUsesWith(UndefValue::get(I->getType()));
786 // If this is the edge to the header block for a loop, remove the loop and
787 // promote all subloops.
788 if (Loop *BBLoop = LI->getLoopFor(BB)) {
789 if (BBLoop->getLoopLatch() == BB)
790 RemoveLoopFromHierarchy(BBLoop);
793 // Remove the block from the loop info, which removes it from any loops it
798 // Remove phi node entries in successors for this block.
799 TerminatorInst *TI = BB->getTerminator();
800 SmallVector<BasicBlock*, 4> Succs;
801 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
802 Succs.push_back(TI->getSuccessor(i));
803 TI->getSuccessor(i)->removePredecessor(BB);
806 // Unique the successors, remove anything with multiple uses.
807 array_pod_sort(Succs.begin(), Succs.end());
808 Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end());
810 // Remove the basic block, including all of the instructions contained in it.
811 LPM->deleteSimpleAnalysisValue(BB, L);
812 BB->eraseFromParent();
813 // Remove successor blocks here that are not dead, so that we know we only
814 // have dead blocks in this list. Nondead blocks have a way of becoming dead,
815 // then getting removed before we revisit them, which is badness.
817 for (unsigned i = 0; i != Succs.size(); ++i)
818 if (pred_begin(Succs[i]) != pred_end(Succs[i])) {
819 // One exception is loop headers. If this block was the preheader for a
820 // loop, then we DO want to visit the loop so the loop gets deleted.
821 // We know that if the successor is a loop header, that this loop had to
822 // be the preheader: the case where this was the latch block was handled
823 // above and headers can only have two predecessors.
824 if (!LI->isLoopHeader(Succs[i])) {
825 Succs.erase(Succs.begin()+i);
830 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
831 RemoveBlockIfDead(Succs[i], Worklist, L);
834 /// RemoveLoopFromHierarchy - We have discovered that the specified loop has
835 /// become unwrapped, either because the backedge was deleted, or because the
836 /// edge into the header was removed. If the edge into the header from the
837 /// latch block was removed, the loop is unwrapped but subloops are still alive,
838 /// so they just reparent loops. If the loops are actually dead, they will be
840 void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) {
841 LPM->deleteLoopFromQueue(L);
842 RemoveLoopFromWorklist(L);
845 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
846 // the value specified by Val in the specified loop, or we know it does NOT have
847 // that value. Rewrite any uses of LIC or of properties correlated to it.
848 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
851 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
853 // FIXME: Support correlated properties, like:
860 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
861 // selects, switches.
862 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
863 std::vector<Instruction*> Worklist;
864 LLVMContext &Context = Val->getContext();
867 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
868 // in the loop with the appropriate one directly.
869 if (IsEqual || (isa<ConstantInt>(Val) &&
870 Val->getType() == Type::getInt1Ty(Val->getContext()))) {
875 Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
876 !cast<ConstantInt>(Val)->getZExtValue());
878 for (unsigned i = 0, e = Users.size(); i != e; ++i)
879 if (Instruction *U = cast<Instruction>(Users[i])) {
882 U->replaceUsesOfWith(LIC, Replacement);
883 Worklist.push_back(U);
886 // Otherwise, we don't know the precise value of LIC, but we do know that it
887 // is certainly NOT "Val". As such, simplify any uses in the loop that we
888 // can. This case occurs when we unswitch switch statements.
889 for (unsigned i = 0, e = Users.size(); i != e; ++i)
890 if (Instruction *U = cast<Instruction>(Users[i])) {
894 Worklist.push_back(U);
896 // If we know that LIC is not Val, use this info to simplify code.
897 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
898 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
899 if (SI->getCaseValue(i) == Val) {
900 // Found a dead case value. Don't remove PHI nodes in the
901 // successor if they become single-entry, those PHI nodes may
902 // be in the Users list.
904 // FIXME: This is a hack. We need to keep the successor around
905 // and hooked up so as to preserve the loop structure, because
906 // trying to update it is complicated. So instead we preserve the
907 // loop structure and put the block on a dead code path.
908 BasicBlock *Switch = SI->getParent();
909 SplitEdge(Switch, SI->getSuccessor(i), this);
910 // Compute the successors instead of relying on the return value
911 // of SplitEdge, since it may have split the switch successor
913 BasicBlock *NewSISucc = SI->getSuccessor(i);
914 BasicBlock *OldSISucc = *succ_begin(NewSISucc);
915 // Create an "unreachable" destination.
916 BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
919 new UnreachableInst(Context, Abort);
920 // Force the new case destination to branch to the "unreachable"
921 // block while maintaining a (dead) CFG edge to the old block.
922 NewSISucc->getTerminator()->eraseFromParent();
923 BranchInst::Create(Abort, OldSISucc,
924 ConstantInt::getTrue(Context), NewSISucc);
925 // Release the PHI operands for this edge.
926 for (BasicBlock::iterator II = NewSISucc->begin();
927 PHINode *PN = dyn_cast<PHINode>(II); ++II)
928 PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
929 UndefValue::get(PN->getType()));
930 // Tell the domtree about the new block. We don't fully update the
931 // domtree here -- instead we force it to do a full recomputation
932 // after the pass is complete -- but we do need to inform it of
935 DT->addNewBlock(Abort, NewSISucc);
941 // TODO: We could do other simplifications, for example, turning
942 // LIC == Val -> false.
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(errs() << "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() == Type::getInt1Ty(I->getContext()))
997 cast<BinaryOperator>(I)->swapOperands();
998 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1)))
999 if (CB->getType() == Type::getInt1Ty(I->getContext())) {
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() == Type::getInt1Ty(I->getContext()))
1011 cast<BinaryOperator>(I)->swapOperands();
1012 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1)))
1013 if (CB->getType() == Type::getInt1Ty(I->getContext())) {
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(errs() << "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();
1055 } else if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){
1056 // Conditional branch. Turn it into an unconditional branch, then
1057 // remove dead blocks.
1058 break; // FIXME: Enable.
1060 DEBUG(errs() << "Folded branch: " << *BI);
1061 BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue());
1062 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue());
1063 DeadSucc->removePredecessor(BI->getParent(), true);
1064 Worklist.push_back(BranchInst::Create(LiveSucc, BI));
1065 LPM->deleteSimpleAnalysisValue(BI, L);
1066 BI->eraseFromParent();
1067 RemoveFromWorklist(BI, Worklist);
1070 RemoveBlockIfDead(DeadSucc, Worklist, L);