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/InlineCost.h"
36 #include "llvm/Analysis/InstructionSimplify.h"
37 #include "llvm/Analysis/LoopInfo.h"
38 #include "llvm/Analysis/LoopPass.h"
39 #include "llvm/Analysis/Dominators.h"
40 #include "llvm/Analysis/ScalarEvolution.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),
97 initializeLoopUnswitchPass(*PassRegistry::getPassRegistry());
100 bool runOnLoop(Loop *L, LPPassManager &LPM);
101 bool processCurrentLoop();
103 /// This transformation requires natural loop information & requires that
104 /// loop preheaders be inserted into the CFG.
106 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
107 AU.addRequiredID(LoopSimplifyID);
108 AU.addPreservedID(LoopSimplifyID);
109 AU.addRequired<LoopInfo>();
110 AU.addPreserved<LoopInfo>();
111 AU.addRequiredID(LCSSAID);
112 AU.addPreservedID(LCSSAID);
113 AU.addPreserved<DominatorTree>();
114 AU.addPreserved<ScalarEvolution>();
119 virtual void releaseMemory() {
120 UnswitchedVals.clear();
123 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist,
125 void RemoveLoopFromWorklist(Loop *L) {
126 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(),
127 LoopProcessWorklist.end(), L);
128 if (I != LoopProcessWorklist.end())
129 LoopProcessWorklist.erase(I);
132 void initLoopData() {
133 loopHeader = currentLoop->getHeader();
134 loopPreheader = currentLoop->getLoopPreheader();
137 /// Split all of the edges from inside the loop to their exit blocks.
138 /// Update the appropriate Phi nodes as we do so.
139 void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks);
141 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val);
142 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
143 BasicBlock *ExitBlock);
144 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
146 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
147 Constant *Val, bool isEqual);
149 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
150 BasicBlock *TrueDest,
151 BasicBlock *FalseDest,
152 Instruction *InsertPt);
154 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
155 void RemoveBlockIfDead(BasicBlock *BB,
156 std::vector<Instruction*> &Worklist, Loop *l);
157 void RemoveLoopFromHierarchy(Loop *L);
158 bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0,
159 BasicBlock **LoopExit = 0);
163 char LoopUnswitch::ID = 0;
164 INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops",
166 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
167 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
168 INITIALIZE_PASS_DEPENDENCY(LCSSA)
169 INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops",
172 Pass *llvm::createLoopUnswitchPass(bool Os) {
173 return new LoopUnswitch(Os);
176 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
177 /// invariant in the loop, or has an invariant piece, return the invariant.
178 /// Otherwise, return null.
179 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
180 // We can never unswitch on vector conditions.
181 if (Cond->getType()->isVectorTy())
184 // Constants should be folded, not unswitched on!
185 if (isa<Constant>(Cond)) return 0;
187 // TODO: Handle: br (VARIANT|INVARIANT).
189 // Hoist simple values out.
190 if (L->makeLoopInvariant(Cond, Changed))
193 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
194 if (BO->getOpcode() == Instruction::And ||
195 BO->getOpcode() == Instruction::Or) {
196 // If either the left or right side is invariant, we can unswitch on this,
197 // which will cause the branch to go away in one loop and the condition to
198 // simplify in the other one.
199 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
201 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
208 bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
209 LI = &getAnalysis<LoopInfo>();
211 DT = getAnalysisIfAvailable<DominatorTree>();
213 Function *F = currentLoop->getHeader()->getParent();
214 bool Changed = false;
216 assert(currentLoop->isLCSSAForm(*DT));
218 Changed |= processCurrentLoop();
222 // FIXME: Reconstruct dom info, because it is not preserved properly.
224 DT->runOnFunction(*F);
229 /// processCurrentLoop - Do actual work and unswitch loop if possible
231 bool LoopUnswitch::processCurrentLoop() {
232 bool Changed = false;
233 LLVMContext &Context = currentLoop->getHeader()->getContext();
235 // Loop over all of the basic blocks in the loop. If we find an interior
236 // block that is branching on a loop-invariant condition, we can unswitch this
238 for (Loop::block_iterator I = currentLoop->block_begin(),
239 E = currentLoop->block_end(); I != E; ++I) {
240 TerminatorInst *TI = (*I)->getTerminator();
241 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
242 // If this isn't branching on an invariant condition, we can't unswitch
244 if (BI->isConditional()) {
245 // See if this, or some part of it, is loop invariant. If so, we can
246 // unswitch on it if we desire.
247 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
248 currentLoop, Changed);
249 if (LoopCond && UnswitchIfProfitable(LoopCond,
250 ConstantInt::getTrue(Context))) {
255 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
256 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
257 currentLoop, Changed);
258 if (LoopCond && SI->getNumCases() > 1) {
259 // Find a value to unswitch on:
260 // FIXME: this should chose the most expensive case!
261 // FIXME: scan for a case with a non-critical edge?
262 Constant *UnswitchVal = SI->getCaseValue(1);
263 // Do not process same value again and again.
264 if (!UnswitchedVals.insert(UnswitchVal))
267 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) {
274 // Scan the instructions to check for unswitchable values.
275 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
277 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
278 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
279 currentLoop, Changed);
280 if (LoopCond && UnswitchIfProfitable(LoopCond,
281 ConstantInt::getTrue(Context))) {
290 /// isTrivialLoopExitBlock - Check to see if all paths from BB exit the
291 /// loop with no side effects (including infinite loops).
293 /// If true, we return true and set ExitBB to the block we
296 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
298 std::set<BasicBlock*> &Visited) {
299 if (!Visited.insert(BB).second) {
300 // Already visited. Without more analysis, this could indicate an infinte loop.
302 } else if (!L->contains(BB)) {
303 // Otherwise, this is a loop exit, this is fine so long as this is the
305 if (ExitBB != 0) return false;
310 // Otherwise, this is an unvisited intra-loop node. Check all successors.
311 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
312 // Check to see if the successor is a trivial loop exit.
313 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
317 // Okay, everything after this looks good, check to make sure that this block
318 // doesn't include any side effects.
319 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
320 if (I->mayHaveSideEffects())
326 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally
327 /// leads to an exit from the specified loop, and has no side-effects in the
328 /// process. If so, return the block that is exited to, otherwise return null.
329 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
330 std::set<BasicBlock*> Visited;
331 Visited.insert(L->getHeader()); // Branches to header make infinite loops.
332 BasicBlock *ExitBB = 0;
333 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
338 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
339 /// trivial: that is, that the condition controls whether or not the loop does
340 /// anything at all. If this is a trivial condition, unswitching produces no
341 /// code duplications (equivalently, it produces a simpler loop and a new empty
342 /// loop, which gets deleted).
344 /// If this is a trivial condition, return true, otherwise return false. When
345 /// returning true, this sets Cond and Val to the condition that controls the
346 /// trivial condition: when Cond dynamically equals Val, the loop is known to
347 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when
350 bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
351 BasicBlock **LoopExit) {
352 BasicBlock *Header = currentLoop->getHeader();
353 TerminatorInst *HeaderTerm = Header->getTerminator();
354 LLVMContext &Context = Header->getContext();
356 BasicBlock *LoopExitBB = 0;
357 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
358 // If the header block doesn't end with a conditional branch on Cond, we
360 if (!BI->isConditional() || BI->getCondition() != Cond)
363 // Check to see if a successor of the branch is guaranteed to
364 // exit through a unique exit block without having any
365 // side-effects. If so, determine the value of Cond that causes it to do
367 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
368 BI->getSuccessor(0)))) {
369 if (Val) *Val = ConstantInt::getTrue(Context);
370 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
371 BI->getSuccessor(1)))) {
372 if (Val) *Val = ConstantInt::getFalse(Context);
374 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
375 // If this isn't a switch on Cond, we can't handle it.
376 if (SI->getCondition() != Cond) return false;
378 // Check to see if a successor of the switch is guaranteed to go to the
379 // latch block or exit through a one exit block without having any
380 // side-effects. If so, determine the value of Cond that causes it to do
381 // this. Note that we can't trivially unswitch on the default case.
382 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
383 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
384 SI->getSuccessor(i)))) {
385 // Okay, we found a trivial case, remember the value that is trivial.
386 if (Val) *Val = SI->getCaseValue(i);
391 // If we didn't find a single unique LoopExit block, or if the loop exit block
392 // contains phi nodes, this isn't trivial.
393 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
394 return false; // Can't handle this.
396 if (LoopExit) *LoopExit = LoopExitBB;
398 // We already know that nothing uses any scalar values defined inside of this
399 // loop. As such, we just have to check to see if this loop will execute any
400 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
401 // part of the loop that the code *would* execute. We already checked the
402 // tail, check the header now.
403 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
404 if (I->mayHaveSideEffects())
409 /// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
410 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
411 /// unswitch the loop, reprocess the pieces, then return true.
412 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val) {
416 // If LoopSimplify was unable to form a preheader, don't do any unswitching.
420 Function *F = loopHeader->getParent();
422 Constant *CondVal = 0;
423 BasicBlock *ExitBlock = 0;
424 if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
425 // If the condition is trivial, always unswitch. There is no code growth
427 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock);
431 // Check to see if it would be profitable to unswitch current loop.
433 // Do not do non-trivial unswitch while optimizing for size.
434 if (OptimizeForSize || F->hasFnAttr(Attribute::OptimizeForSize))
437 // FIXME: This is overly conservative because it does not take into
438 // consideration code simplification opportunities and code that can
439 // be shared by the resultant unswitched loops.
441 for (Loop::block_iterator I = currentLoop->block_begin(),
442 E = currentLoop->block_end();
444 Metrics.analyzeBasicBlock(*I);
446 // Limit the number of instructions to avoid causing significant code
447 // expansion, and the number of basic blocks, to avoid loops with
448 // large numbers of branches which cause loop unswitching to go crazy.
449 // This is a very ad-hoc heuristic.
450 if (Metrics.NumInsts > Threshold ||
451 Metrics.NumBlocks * 5 > Threshold ||
452 Metrics.containsIndirectBr || Metrics.isRecursive) {
453 DEBUG(dbgs() << "NOT unswitching loop %"
454 << currentLoop->getHeader()->getName() << ", cost too high: "
455 << currentLoop->getBlocks().size() << "\n");
459 UnswitchNontrivialCondition(LoopCond, Val, currentLoop);
463 /// CloneLoop - Recursively clone the specified loop and all of its children,
464 /// mapping the blocks with the specified map.
465 static Loop *CloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM,
466 LoopInfo *LI, LPPassManager *LPM) {
467 Loop *New = new Loop();
468 LPM->insertLoop(New, PL);
470 // Add all of the blocks in L to the new loop.
471 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
473 if (LI->getLoopFor(*I) == L)
474 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), LI->getBase());
476 // Add all of the subloops to the new loop.
477 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
478 CloneLoop(*I, New, VM, LI, LPM);
483 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
484 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
485 /// code immediately before InsertPt.
486 void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
487 BasicBlock *TrueDest,
488 BasicBlock *FalseDest,
489 Instruction *InsertPt) {
490 // Insert a conditional branch on LIC to the two preheaders. The original
491 // code is the true version and the new code is the false version.
492 Value *BranchVal = LIC;
493 if (!isa<ConstantInt>(Val) ||
494 Val->getType() != Type::getInt1Ty(LIC->getContext()))
495 BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val);
496 else if (Val != ConstantInt::getTrue(Val->getContext()))
497 // We want to enter the new loop when the condition is true.
498 std::swap(TrueDest, FalseDest);
500 // Insert the new branch.
501 BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt);
503 // If either edge is critical, split it. This helps preserve LoopSimplify
504 // form for enclosing loops.
505 SplitCriticalEdge(BI, 0, this);
506 SplitCriticalEdge(BI, 1, this);
509 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
510 /// condition in it (a cond branch from its header block to its latch block,
511 /// where the path through the loop that doesn't execute its body has no
512 /// side-effects), unswitch it. This doesn't involve any code duplication, just
513 /// moving the conditional branch outside of the loop and updating loop info.
514 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
516 BasicBlock *ExitBlock) {
517 DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
518 << loopHeader->getName() << " [" << L->getBlocks().size()
519 << " blocks] in Function " << L->getHeader()->getParent()->getName()
520 << " on cond: " << *Val << " == " << *Cond << "\n");
522 // First step, split the preheader, so that we know that there is a safe place
523 // to insert the conditional branch. We will change loopPreheader to have a
524 // conditional branch on Cond.
525 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this);
527 // Now that we have a place to insert the conditional branch, create a place
528 // to branch to: this is the exit block out of the loop that we should
531 // Split this block now, so that the loop maintains its exit block, and so
532 // that the jump from the preheader can execute the contents of the exit block
533 // without actually branching to it (the exit block should be dominated by the
534 // loop header, not the preheader).
535 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
536 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this);
538 // Okay, now we have a position to branch from and a position to branch to,
539 // insert the new conditional branch.
540 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
541 loopPreheader->getTerminator());
542 LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L);
543 loopPreheader->getTerminator()->eraseFromParent();
545 // We need to reprocess this loop, it could be unswitched again.
548 // Now that we know that the loop is never entered when this condition is a
549 // particular value, rewrite the loop with this info. We know that this will
550 // at least eliminate the old branch.
551 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
555 /// SplitExitEdges - Split all of the edges from inside the loop to their exit
556 /// blocks. Update the appropriate Phi nodes as we do so.
557 void LoopUnswitch::SplitExitEdges(Loop *L,
558 const SmallVector<BasicBlock *, 8> &ExitBlocks){
560 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
561 BasicBlock *ExitBlock = ExitBlocks[i];
562 SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
563 pred_end(ExitBlock));
565 // Although SplitBlockPredecessors doesn't preserve loop-simplify in
566 // general, if we call it on all predecessors of all exits then it does.
567 if (!ExitBlock->isLandingPad()) {
568 SplitBlockPredecessors(ExitBlock, Preds.data(), Preds.size(),
571 SmallVector<BasicBlock*, 2> NewBBs;
572 SplitLandingPadPredecessors(ExitBlock, Preds, ".us-lcssa", ".us-lcssa",
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(dbgs() << "loop-unswitch: Unswitching loop %"
585 << loopHeader->getName() << " [" << L->getBlocks().size()
586 << " blocks] in Function " << F->getName()
587 << " when '" << *Val << "' == " << *LIC << "\n");
589 if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>())
595 // First step, split the preheader and exit blocks, and add these blocks to
596 // the LoopBlocks list.
597 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this);
598 LoopBlocks.push_back(NewPreheader);
600 // We want the loop to come after the preheader, but before the exit blocks.
601 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
603 SmallVector<BasicBlock*, 8> ExitBlocks;
604 L->getUniqueExitBlocks(ExitBlocks);
606 // Split all of the edges from inside the loop to their exit blocks. Update
607 // the appropriate Phi nodes as we do so.
608 SplitExitEdges(L, ExitBlocks);
610 // The exit blocks may have been changed due to edge splitting, recompute.
612 L->getUniqueExitBlocks(ExitBlocks);
614 // Add exit blocks to the loop blocks.
615 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
617 // Next step, clone all of the basic blocks that make up the loop (including
618 // the loop preheader and exit blocks), keeping track of the mapping between
619 // the instructions and blocks.
620 NewBlocks.reserve(LoopBlocks.size());
621 ValueToValueMapTy VMap;
622 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
623 BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], VMap, ".us", F);
624 NewBlocks.push_back(NewBB);
625 VMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping.
626 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L);
629 // Splice the newly inserted blocks into the function right before the
630 // original preheader.
631 F->getBasicBlockList().splice(NewPreheader, F->getBasicBlockList(),
632 NewBlocks[0], F->end());
634 // Now we create the new Loop object for the versioned loop.
635 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), VMap, LI, LPM);
636 Loop *ParentLoop = L->getParentLoop();
638 // Make sure to add the cloned preheader and exit blocks to the parent loop
640 ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase());
643 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
644 BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[i]]);
645 // The new exit block should be in the same loop as the old one.
646 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
647 ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase());
649 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
650 "Exit block should have been split to have one successor!");
651 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
653 // If the successor of the exit block had PHI nodes, add an entry for
656 for (BasicBlock::iterator I = ExitSucc->begin(); isa<PHINode>(I); ++I) {
657 PN = cast<PHINode>(I);
658 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
659 ValueToValueMapTy::iterator It = VMap.find(V);
660 if (It != VMap.end()) V = It->second;
661 PN->addIncoming(V, NewExit);
664 if (LandingPadInst *LPad = NewExit->getLandingPadInst()) {
665 PN = PHINode::Create(LPad->getType(), 0, "",
666 ExitSucc->getFirstInsertionPt());
668 for (pred_iterator I = pred_begin(ExitSucc), E = pred_end(ExitSucc);
671 LandingPadInst *LPI = BB->getLandingPadInst();
672 LPI->replaceAllUsesWith(PN);
673 PN->addIncoming(LPI, BB);
678 // Rewrite the code to refer to itself.
679 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
680 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
681 E = NewBlocks[i]->end(); I != E; ++I)
682 RemapInstruction(I, VMap,RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
684 // Rewrite the original preheader to select between versions of the loop.
685 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
686 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
687 "Preheader splitting did not work correctly!");
689 // Emit the new branch that selects between the two versions of this loop.
690 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
691 LPM->deleteSimpleAnalysisValue(OldBR, L);
692 OldBR->eraseFromParent();
694 LoopProcessWorklist.push_back(NewLoop);
697 // Keep a WeakVH holding onto LIC. If the first call to RewriteLoopBody
698 // deletes the instruction (for example by simplifying a PHI that feeds into
699 // the condition that we're unswitching on), we don't rewrite the second
701 WeakVH LICHandle(LIC);
703 // Now we rewrite the original code to know that the condition is true and the
704 // new code to know that the condition is false.
705 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
707 // It's possible that simplifying one loop could cause the other to be
708 // changed to another value or a constant. If its a constant, don't simplify
710 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
711 LICHandle && !isa<Constant>(LICHandle))
712 RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true);
715 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
717 static void RemoveFromWorklist(Instruction *I,
718 std::vector<Instruction*> &Worklist) {
719 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
721 while (WI != Worklist.end()) {
722 unsigned Offset = WI-Worklist.begin();
724 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
728 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
729 /// program, replacing all uses with V and update the worklist.
730 static void ReplaceUsesOfWith(Instruction *I, Value *V,
731 std::vector<Instruction*> &Worklist,
732 Loop *L, LPPassManager *LPM) {
733 DEBUG(dbgs() << "Replace with '" << *V << "': " << *I);
735 // Add uses to the worklist, which may be dead now.
736 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
737 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
738 Worklist.push_back(Use);
740 // Add users to the worklist which may be simplified now.
741 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
743 Worklist.push_back(cast<Instruction>(*UI));
744 LPM->deleteSimpleAnalysisValue(I, L);
745 RemoveFromWorklist(I, Worklist);
746 I->replaceAllUsesWith(V);
747 I->eraseFromParent();
751 /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
752 /// information, and remove any dead successors it has.
754 void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
755 std::vector<Instruction*> &Worklist,
757 if (pred_begin(BB) != pred_end(BB)) {
758 // This block isn't dead, since an edge to BB was just removed, see if there
759 // are any easy simplifications we can do now.
760 if (BasicBlock *Pred = BB->getSinglePredecessor()) {
761 // If it has one pred, fold phi nodes in BB.
762 while (isa<PHINode>(BB->begin()))
763 ReplaceUsesOfWith(BB->begin(),
764 cast<PHINode>(BB->begin())->getIncomingValue(0),
767 // If this is the header of a loop and the only pred is the latch, we now
768 // have an unreachable loop.
769 if (Loop *L = LI->getLoopFor(BB))
770 if (loopHeader == BB && L->contains(Pred)) {
771 // Remove the branch from the latch to the header block, this makes
772 // the header dead, which will make the latch dead (because the header
773 // dominates the latch).
774 LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L);
775 Pred->getTerminator()->eraseFromParent();
776 new UnreachableInst(BB->getContext(), Pred);
778 // The loop is now broken, remove it from LI.
779 RemoveLoopFromHierarchy(L);
781 // Reprocess the header, which now IS dead.
782 RemoveBlockIfDead(BB, Worklist, L);
786 // If pred ends in a uncond branch, add uncond branch to worklist so that
787 // the two blocks will get merged.
788 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
789 if (BI->isUnconditional())
790 Worklist.push_back(BI);
795 DEBUG(dbgs() << "Nuking dead block: " << *BB);
797 // Remove the instructions in the basic block from the worklist.
798 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
799 RemoveFromWorklist(I, Worklist);
801 // Anything that uses the instructions in this basic block should have their
802 // uses replaced with undefs.
803 // If I is not void type then replaceAllUsesWith undef.
804 // This allows ValueHandlers and custom metadata to adjust itself.
805 if (!I->getType()->isVoidTy())
806 I->replaceAllUsesWith(UndefValue::get(I->getType()));
809 // If this is the edge to the header block for a loop, remove the loop and
810 // promote all subloops.
811 if (Loop *BBLoop = LI->getLoopFor(BB)) {
812 if (BBLoop->getLoopLatch() == BB) {
813 RemoveLoopFromHierarchy(BBLoop);
814 if (currentLoop == BBLoop) {
821 // Remove the block from the loop info, which removes it from any loops it
826 // Remove phi node entries in successors for this block.
827 TerminatorInst *TI = BB->getTerminator();
828 SmallVector<BasicBlock*, 4> Succs;
829 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
830 Succs.push_back(TI->getSuccessor(i));
831 TI->getSuccessor(i)->removePredecessor(BB);
834 // Unique the successors, remove anything with multiple uses.
835 array_pod_sort(Succs.begin(), Succs.end());
836 Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end());
838 // Remove the basic block, including all of the instructions contained in it.
839 LPM->deleteSimpleAnalysisValue(BB, L);
840 BB->eraseFromParent();
841 // Remove successor blocks here that are not dead, so that we know we only
842 // have dead blocks in this list. Nondead blocks have a way of becoming dead,
843 // then getting removed before we revisit them, which is badness.
845 for (unsigned i = 0; i != Succs.size(); ++i)
846 if (pred_begin(Succs[i]) != pred_end(Succs[i])) {
847 // One exception is loop headers. If this block was the preheader for a
848 // loop, then we DO want to visit the loop so the loop gets deleted.
849 // We know that if the successor is a loop header, that this loop had to
850 // be the preheader: the case where this was the latch block was handled
851 // above and headers can only have two predecessors.
852 if (!LI->isLoopHeader(Succs[i])) {
853 Succs.erase(Succs.begin()+i);
858 for (unsigned i = 0, e = Succs.size(); i != e; ++i)
859 RemoveBlockIfDead(Succs[i], Worklist, L);
862 /// RemoveLoopFromHierarchy - We have discovered that the specified loop has
863 /// become unwrapped, either because the backedge was deleted, or because the
864 /// edge into the header was removed. If the edge into the header from the
865 /// latch block was removed, the loop is unwrapped but subloops are still alive,
866 /// so they just reparent loops. If the loops are actually dead, they will be
868 void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) {
869 LPM->deleteLoopFromQueue(L);
870 RemoveLoopFromWorklist(L);
873 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
874 // the value specified by Val in the specified loop, or we know it does NOT have
875 // that value. Rewrite any uses of LIC or of properties correlated to it.
876 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
879 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
881 // FIXME: Support correlated properties, like:
888 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
889 // selects, switches.
890 std::vector<Instruction*> Worklist;
891 LLVMContext &Context = Val->getContext();
894 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
895 // in the loop with the appropriate one directly.
896 if (IsEqual || (isa<ConstantInt>(Val) &&
897 Val->getType()->isIntegerTy(1))) {
902 Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
903 !cast<ConstantInt>(Val)->getZExtValue());
905 for (Value::use_iterator UI = LIC->use_begin(), E = LIC->use_end();
907 Instruction *U = dyn_cast<Instruction>(*UI);
908 if (!U || !L->contains(U))
910 Worklist.push_back(U);
913 for (std::vector<Instruction*>::iterator UI = Worklist.begin();
914 UI != Worklist.end(); ++UI)
915 (*UI)->replaceUsesOfWith(LIC, Replacement);
917 SimplifyCode(Worklist, L);
921 // Otherwise, we don't know the precise value of LIC, but we do know that it
922 // is certainly NOT "Val". As such, simplify any uses in the loop that we
923 // can. This case occurs when we unswitch switch statements.
924 for (Value::use_iterator UI = LIC->use_begin(), E = LIC->use_end();
926 Instruction *U = dyn_cast<Instruction>(*UI);
927 if (!U || !L->contains(U))
930 Worklist.push_back(U);
932 // TODO: We could do other simplifications, for example, turning
933 // 'icmp eq LIC, Val' -> false.
935 // If we know that LIC is not Val, use this info to simplify code.
936 SwitchInst *SI = dyn_cast<SwitchInst>(U);
937 if (SI == 0 || !isa<ConstantInt>(Val)) continue;
939 unsigned DeadCase = SI->findCaseValue(cast<ConstantInt>(Val));
940 if (DeadCase == 0) continue; // Default case is live for multiple values.
942 // Found a dead case value. Don't remove PHI nodes in the
943 // successor if they become single-entry, those PHI nodes may
944 // be in the Users list.
946 BasicBlock *Switch = SI->getParent();
947 BasicBlock *SISucc = SI->getSuccessor(DeadCase);
948 BasicBlock *Latch = L->getLoopLatch();
949 if (!SI->findCaseDest(SISucc)) continue; // Edge is critical.
950 // If the DeadCase successor dominates the loop latch, then the
951 // transformation isn't safe since it will delete the sole predecessor edge
953 if (Latch && DT->dominates(SISucc, Latch))
956 // FIXME: This is a hack. We need to keep the successor around
957 // and hooked up so as to preserve the loop structure, because
958 // trying to update it is complicated. So instead we preserve the
959 // loop structure and put the block on a dead code path.
960 SplitEdge(Switch, SISucc, this);
961 // Compute the successors instead of relying on the return value
962 // of SplitEdge, since it may have split the switch successor
964 BasicBlock *NewSISucc = SI->getSuccessor(DeadCase);
965 BasicBlock *OldSISucc = *succ_begin(NewSISucc);
966 // Create an "unreachable" destination.
967 BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
970 new UnreachableInst(Context, Abort);
971 // Force the new case destination to branch to the "unreachable"
972 // block while maintaining a (dead) CFG edge to the old block.
973 NewSISucc->getTerminator()->eraseFromParent();
974 BranchInst::Create(Abort, OldSISucc,
975 ConstantInt::getTrue(Context), NewSISucc);
976 // Release the PHI operands for this edge.
977 for (BasicBlock::iterator II = NewSISucc->begin();
978 PHINode *PN = dyn_cast<PHINode>(II); ++II)
979 PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
980 UndefValue::get(PN->getType()));
981 // Tell the domtree about the new block. We don't fully update the
982 // domtree here -- instead we force it to do a full recomputation
983 // after the pass is complete -- but we do need to inform it of
986 DT->addNewBlock(Abort, NewSISucc);
989 SimplifyCode(Worklist, L);
992 /// SimplifyCode - Okay, now that we have simplified some instructions in the
993 /// loop, walk over it and constant prop, dce, and fold control flow where
994 /// possible. Note that this is effectively a very simple loop-structure-aware
995 /// optimizer. During processing of this loop, L could very well be deleted, so
996 /// it must not be used.
998 /// FIXME: When the loop optimizer is more mature, separate this out to a new
1001 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
1002 while (!Worklist.empty()) {
1003 Instruction *I = Worklist.back();
1004 Worklist.pop_back();
1007 if (isInstructionTriviallyDead(I)) {
1008 DEBUG(dbgs() << "Remove dead instruction '" << *I);
1010 // Add uses to the worklist, which may be dead now.
1011 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
1012 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
1013 Worklist.push_back(Use);
1014 LPM->deleteSimpleAnalysisValue(I, L);
1015 RemoveFromWorklist(I, Worklist);
1016 I->eraseFromParent();
1021 // See if instruction simplification can hack this up. This is common for
1022 // things like "select false, X, Y" after unswitching made the condition be
1024 if (Value *V = SimplifyInstruction(I, 0, DT))
1025 if (LI->replacementPreservesLCSSAForm(I, V)) {
1026 ReplaceUsesOfWith(I, V, Worklist, L, LPM);
1030 // Special case hacks that appear commonly in unswitched code.
1031 if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
1032 if (BI->isUnconditional()) {
1033 // If BI's parent is the only pred of the successor, fold the two blocks
1035 BasicBlock *Pred = BI->getParent();
1036 BasicBlock *Succ = BI->getSuccessor(0);
1037 BasicBlock *SinglePred = Succ->getSinglePredecessor();
1038 if (!SinglePred) continue; // Nothing to do.
1039 assert(SinglePred == Pred && "CFG broken");
1041 DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- "
1042 << Succ->getName() << "\n");
1044 // Resolve any single entry PHI nodes in Succ.
1045 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1046 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM);
1048 // If Succ has any successors with PHI nodes, update them to have
1049 // entries coming from Pred instead of Succ.
1050 Succ->replaceAllUsesWith(Pred);
1052 // Move all of the successor contents from Succ to Pred.
1053 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1055 LPM->deleteSimpleAnalysisValue(BI, L);
1056 BI->eraseFromParent();
1057 RemoveFromWorklist(BI, Worklist);
1059 // Remove Succ from the loop tree.
1060 LI->removeBlock(Succ);
1061 LPM->deleteSimpleAnalysisValue(Succ, L);
1062 Succ->eraseFromParent();
1067 if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){
1068 // Conditional branch. Turn it into an unconditional branch, then
1069 // remove dead blocks.
1070 continue; // FIXME: Enable.
1072 DEBUG(dbgs() << "Folded branch: " << *BI);
1073 BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue());
1074 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue());
1075 DeadSucc->removePredecessor(BI->getParent(), true);
1076 Worklist.push_back(BranchInst::Create(LiveSucc, BI));
1077 LPM->deleteSimpleAnalysisValue(BI, L);
1078 BI->eraseFromParent();
1079 RemoveFromWorklist(BI, Worklist);
1082 RemoveBlockIfDead(DeadSucc, Worklist, L);