1 //===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass transforms loops that contain branches on loop-invariant conditions
11 // to have multiple loops. For example, it turns the left into the right code:
20 // This can increase the size of the code exponentially (doubling it every time
21 // a loop is unswitched) so we only unswitch if the resultant code will be
22 // smaller than a threshold.
24 // This pass expects LICM to be run before it to hoist invariant conditions out
25 // of the loop, to make the unswitching opportunity obvious.
27 //===----------------------------------------------------------------------===//
29 #define DEBUG_TYPE "loop-unswitch"
30 #include "llvm/Transforms/Scalar.h"
31 #include "llvm/Constants.h"
32 #include "llvm/Function.h"
33 #include "llvm/Instructions.h"
34 #include "llvm/Analysis/LoopInfo.h"
35 #include "llvm/Transforms/Utils/Cloning.h"
36 #include "llvm/Transforms/Utils/Local.h"
37 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
38 #include "llvm/ADT/Statistic.h"
39 #include "llvm/Support/Debug.h"
40 #include "llvm/Support/CommandLine.h"
47 Statistic<> NumBranches("loop-unswitch", "Number of branches unswitched");
48 Statistic<> NumSwitches("loop-unswitch", "Number of switches unswitched");
49 Statistic<> NumSelects ("loop-unswitch", "Number of selects unswitched");
50 Statistic<> NumTrivial ("loop-unswitch",
51 "Number of unswitches that are trivial");
53 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
54 cl::init(10), cl::Hidden);
56 class LoopUnswitch : public FunctionPass {
57 LoopInfo *LI; // Loop information
59 virtual bool runOnFunction(Function &F);
60 bool visitLoop(Loop *L);
62 /// This transformation requires natural loop information & requires that
63 /// loop preheaders be inserted into the CFG...
65 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
66 AU.addRequiredID(LoopSimplifyID);
67 AU.addPreservedID(LoopSimplifyID);
68 AU.addRequired<LoopInfo>();
69 AU.addPreserved<LoopInfo>();
73 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L);
74 unsigned getLoopUnswitchCost(Loop *L, Value *LIC);
75 void VersionLoop(Value *LIC, Constant *OnVal,
76 Loop *L, Loop *&Out1, Loop *&Out2);
77 BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To);
78 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,Constant *Val,
80 void UnswitchTrivialCondition(Loop *L, Value *Cond, bool EntersLoopOnCond,
81 BasicBlock *ExitBlock);
83 RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
86 FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }
88 bool LoopUnswitch::runOnFunction(Function &F) {
90 LI = &getAnalysis<LoopInfo>();
92 // Transform all the top-level loops. Copy the loop list so that the child
93 // can update the loop tree if it needs to delete the loop.
94 std::vector<Loop*> SubLoops(LI->begin(), LI->end());
95 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
96 Changed |= visitLoop(SubLoops[i]);
102 /// LoopValuesUsedOutsideLoop - Return true if there are any values defined in
103 /// the loop that are used by instructions outside of it.
104 static bool LoopValuesUsedOutsideLoop(Loop *L) {
105 // We will be doing lots of "loop contains block" queries. Loop::contains is
106 // linear time, use a set to speed this up.
107 std::set<BasicBlock*> LoopBlocks;
109 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
111 LoopBlocks.insert(*BB);
113 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
115 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
116 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
118 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
119 if (!LoopBlocks.count(UserBB))
126 /// FindTrivialLoopExitBlock - We know that we have a branch from the loop
127 /// header to the specified latch block. See if one of the successors of the
128 /// latch block is an exit, and if so what block it is.
129 static BasicBlock *FindTrivialLoopExitBlock(Loop *L, BasicBlock *Latch) {
130 BasicBlock *Header = L->getHeader();
131 BranchInst *LatchBranch = dyn_cast<BranchInst>(Latch->getTerminator());
132 if (!LatchBranch || !LatchBranch->isConditional()) return 0;
134 // Simple case, the latch block is a conditional branch. The target that
135 // doesn't go to the loop header is our block if it is not in the loop.
136 if (LatchBranch->getSuccessor(0) == Header) {
137 if (L->contains(LatchBranch->getSuccessor(1))) return false;
138 return LatchBranch->getSuccessor(1);
140 assert(LatchBranch->getSuccessor(1) == Header);
141 if (L->contains(LatchBranch->getSuccessor(0))) return false;
142 return LatchBranch->getSuccessor(0);
147 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
148 /// trivial: that is, that the condition controls whether or not the loop does
149 /// anything at all. If this is a trivial condition, unswitching produces no
150 /// code duplications (equivalently, it produces a simpler loop and a new empty
151 /// loop, which gets deleted).
153 /// If this is a trivial condition, return ConstantBool::True if the loop body
154 /// runs when the condition is true, False if the loop body executes when the
155 /// condition is false. Otherwise, return null to indicate a complex condition.
156 static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond,
157 bool *CondEntersLoop = 0,
158 BasicBlock **LoopExit = 0) {
159 BasicBlock *Header = L->getHeader();
160 BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator());
162 // If the header block doesn't end with a conditional branch on Cond, we can't
164 if (!HeaderTerm || !HeaderTerm->isConditional() ||
165 HeaderTerm->getCondition() != Cond)
168 // Check to see if the conditional branch goes to the latch block. If not,
169 // it's not trivial. This also determines the value of Cond that will execute
171 BasicBlock *Latch = L->getLoopLatch();
172 if (HeaderTerm->getSuccessor(1) == Latch) {
173 if (CondEntersLoop) *CondEntersLoop = true;
174 } else if (HeaderTerm->getSuccessor(0) == Latch)
175 if (CondEntersLoop) *CondEntersLoop = false;
177 return false; // Doesn't branch to latch block.
179 // The latch block must end with a conditional branch where one edge goes to
180 // the header (this much we know) and one edge goes OUT of the loop.
181 BasicBlock *LoopExitBlock = FindTrivialLoopExitBlock(L, Latch);
182 if (!LoopExitBlock) return 0;
183 if (LoopExit) *LoopExit = LoopExitBlock;
185 // We already know that nothing uses any scalar values defined inside of this
186 // loop. As such, we just have to check to see if this loop will execute any
187 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
188 // part of the loop that the code *would* execute.
189 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
190 if (I->mayWriteToMemory())
192 for (BasicBlock::iterator I = Latch->begin(), E = Latch->end(); I != E; ++I)
193 if (I->mayWriteToMemory())
198 /// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
199 /// we choose to unswitch the specified loop on the specified value.
201 unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
202 // If the condition is trivial, always unswitch. There is no code growth for
204 if (IsTrivialUnswitchCondition(L, LIC))
208 // FIXME: this is brain dead. It should take into consideration code
210 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
213 // Do not include empty blocks in the cost calculation. This happen due to
214 // loop canonicalization and will be removed.
215 if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
218 // Count basic blocks.
225 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
226 /// invariant in the loop, or has an invariant piece, return the invariant.
227 /// Otherwise, return null.
228 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
229 // Constants should be folded, not unswitched on!
230 if (isa<Constant>(Cond)) return false;
232 // TODO: Handle: br (VARIANT|INVARIANT).
233 // TODO: Hoist simple expressions out of loops.
234 if (L->isLoopInvariant(Cond)) return Cond;
236 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
237 if (BO->getOpcode() == Instruction::And ||
238 BO->getOpcode() == Instruction::Or) {
239 // If either the left or right side is invariant, we can unswitch on this,
240 // which will cause the branch to go away in one loop and the condition to
241 // simplify in the other one.
242 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
244 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
251 bool LoopUnswitch::visitLoop(Loop *L) {
252 bool Changed = false;
254 // Recurse through all subloops before we process this loop. Copy the loop
255 // list so that the child can update the loop tree if it needs to delete the
257 std::vector<Loop*> SubLoops(L->begin(), L->end());
258 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
259 Changed |= visitLoop(SubLoops[i]);
261 // Loop over all of the basic blocks in the loop. If we find an interior
262 // block that is branching on a loop-invariant condition, we can unswitch this
264 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
266 TerminatorInst *TI = (*I)->getTerminator();
267 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
268 // If this isn't branching on an invariant condition, we can't unswitch
270 if (BI->isConditional()) {
271 // See if this, or some part of it, is loop invariant. If so, we can
272 // unswitch on it if we desire.
273 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed);
274 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) {
279 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
280 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
281 if (LoopCond && SI->getNumCases() > 1) {
282 // Find a value to unswitch on:
283 // FIXME: this should chose the most expensive case!
284 Constant *UnswitchVal = SI->getCaseValue(1);
285 if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) {
292 // Scan the instructions to check for unswitchable values.
293 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
295 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
296 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
297 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) {
307 /// UnswitchIfProfitable - We have found that we can unswitch L when
308 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
309 /// unswitch the loop, reprocess the pieces, then return true.
310 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){
311 // Check to see if it would be profitable to unswitch this loop.
312 if (getLoopUnswitchCost(L, LoopCond) > Threshold) {
313 // FIXME: this should estimate growth by the amount of code shared by the
314 // resultant unswitched loops.
316 DEBUG(std::cerr << "NOT unswitching loop %"
317 << L->getHeader()->getName() << ", cost too high: "
318 << L->getBlocks().size() << "\n");
322 // If this loop has live-out values, we can't unswitch it. We need something
323 // like loop-closed SSA form in order to know how to insert PHI nodes for
325 if (LoopValuesUsedOutsideLoop(L)) {
326 DEBUG(std::cerr << "NOT unswitching loop %" << L->getHeader()->getName()
327 << ", a loop value is used outside loop!\n");
331 //std::cerr << "BEFORE:\n"; LI->dump();
332 Loop *NewLoop1 = 0, *NewLoop2 = 0;
334 // If this is a trivial condition to unswitch (which results in no code
335 // duplication), do it now.
336 bool EntersLoopOnCond;
337 BasicBlock *ExitBlock;
338 if (IsTrivialUnswitchCondition(L, LoopCond, &EntersLoopOnCond, &ExitBlock)){
339 UnswitchTrivialCondition(L, LoopCond, EntersLoopOnCond, ExitBlock);
342 VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2);
345 //std::cerr << "AFTER:\n"; LI->dump();
347 // Try to unswitch each of our new loops now!
348 if (NewLoop1) visitLoop(NewLoop1);
349 if (NewLoop2) visitLoop(NewLoop2);
353 BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) {
354 TerminatorInst *LatchTerm = BB->getTerminator();
355 unsigned SuccNum = 0;
356 for (unsigned i = 0, e = LatchTerm->getNumSuccessors(); ; ++i) {
357 assert(i != e && "Didn't find edge?");
358 if (LatchTerm->getSuccessor(i) == Succ) {
364 // If this is a critical edge, let SplitCriticalEdge do it.
365 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, this))
366 return LatchTerm->getSuccessor(SuccNum);
368 // If the edge isn't critical, then BB has a single successor or Succ has a
369 // single pred. Split the block.
370 BasicBlock *BlockToSplit;
371 BasicBlock::iterator SplitPoint;
372 if (BasicBlock *SP = Succ->getSinglePredecessor()) {
373 // If the successor only has a single pred, split the top of the successor
375 assert(SP == BB && "CFG broken");
377 SplitPoint = Succ->begin();
379 // Otherwise, if BB has a single successor, split it at the bottom of the
381 assert(BB->getTerminator()->getNumSuccessors() == 1 &&
382 "Should have a single succ!");
384 SplitPoint = BB->getTerminator();
388 BlockToSplit->splitBasicBlock(SplitPoint,
389 BlockToSplit->getName()+".tail");
390 // New now lives in whichever loop that BB used to.
391 if (Loop *L = LI->getLoopFor(BlockToSplit))
392 L->addBasicBlockToLoop(New, *LI);
398 // RemapInstruction - Convert the instruction operands from referencing the
399 // current values into those specified by ValueMap.
401 static inline void RemapInstruction(Instruction *I,
402 std::map<const Value *, Value*> &ValueMap) {
403 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
404 Value *Op = I->getOperand(op);
405 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
406 if (It != ValueMap.end()) Op = It->second;
407 I->setOperand(op, Op);
411 /// CloneLoop - Recursively clone the specified loop and all of its children,
412 /// mapping the blocks with the specified map.
413 static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
415 Loop *New = new Loop();
418 PL->addChildLoop(New);
420 LI->addTopLevelLoop(New);
422 // Add all of the blocks in L to the new loop.
423 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
425 if (LI->getLoopFor(*I) == L)
426 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
428 // Add all of the subloops to the new loop.
429 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
430 CloneLoop(*I, New, VM, LI);
435 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
436 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
437 /// code immediately before InsertPt.
438 static void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
439 BasicBlock *TrueDest,
440 BasicBlock *FalseDest,
441 Instruction *InsertPt) {
442 // Insert a conditional branch on LIC to the two preheaders. The original
443 // code is the true version and the new code is the false version.
444 Value *BranchVal = LIC;
445 if (!isa<ConstantBool>(BranchVal)) {
446 BranchVal = BinaryOperator::createSetEQ(LIC, Val, "tmp", InsertPt);
447 } else if (Val != ConstantBool::True) {
448 // We want to enter the new loop when the condition is true.
449 std::swap(TrueDest, FalseDest);
452 // Insert the new branch.
453 new BranchInst(TrueDest, FalseDest, BranchVal, InsertPt);
457 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
458 /// condition in it (a cond branch from its header block to its latch block,
459 /// where the path through the loop that doesn't execute its body has no
460 /// side-effects), unswitch it. This doesn't involve any code duplication, just
461 /// moving the conditional branch outside of the loop and updating loop info.
462 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
464 BasicBlock *ExitBlock) {
465 DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %"
466 << L->getHeader()->getName() << " [" << L->getBlocks().size()
467 << " blocks] in Function " << L->getHeader()->getParent()->getName()
468 << " on cond:" << *Cond << "\n");
470 // First step, split the preheader, so that we know that there is a safe place
471 // to insert the conditional branch. We will change 'OrigPH' to have a
472 // conditional branch on Cond.
473 BasicBlock *OrigPH = L->getLoopPreheader();
474 BasicBlock *NewPH = SplitEdge(OrigPH, L->getHeader());
476 // Now that we have a place to insert the conditional branch, create a place
477 // to branch to: this is the exit block out of the loop that we should
480 // Split this edge now, so that the loop maintains its exit block.
481 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
482 BasicBlock *NewExit = SplitEdge(L->getLoopLatch(), ExitBlock);
483 assert(NewExit != ExitBlock && "Edge not split!");
485 // Okay, now we have a position to branch from and a position to branch to,
486 // insert the new conditional branch.
487 new BranchInst(EnterOnCond ? NewPH : NewExit, EnterOnCond ? NewExit : NewPH,
488 Cond, OrigPH->getTerminator());
489 OrigPH->getTerminator()->eraseFromParent();
491 // Now that we know that the loop is never entered when this condition is a
492 // particular value, rewrite the loop with this info. We know that this will
493 // at least eliminate the old branch.
494 RewriteLoopBodyWithConditionConstant(L, Cond, ConstantBool::get(EnterOnCond),
500 /// VersionLoop - We determined that the loop is profitable to unswitch when LIC
501 /// equal Val. Split it into loop versions and test the condition outside of
502 /// either loop. Return the loops created as Out1/Out2.
503 void LoopUnswitch::VersionLoop(Value *LIC, Constant *Val, Loop *L,
504 Loop *&Out1, Loop *&Out2) {
505 Function *F = L->getHeader()->getParent();
507 DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
508 << L->getHeader()->getName() << " [" << L->getBlocks().size()
509 << " blocks] in Function " << F->getName()
510 << " when '" << *Val << "' == " << *LIC << "\n");
512 // LoopBlocks contains all of the basic blocks of the loop, including the
513 // preheader of the loop, the body of the loop, and the exit blocks of the
514 // loop, in that order.
515 std::vector<BasicBlock*> LoopBlocks;
517 // First step, split the preheader and exit blocks, and add these blocks to
518 // the LoopBlocks list.
519 BasicBlock *OrigPreheader = L->getLoopPreheader();
520 LoopBlocks.push_back(SplitEdge(OrigPreheader, L->getHeader()));
522 // We want the loop to come after the preheader, but before the exit blocks.
523 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
525 std::vector<BasicBlock*> ExitBlocks;
526 L->getExitBlocks(ExitBlocks);
527 std::sort(ExitBlocks.begin(), ExitBlocks.end());
528 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
531 // Split all of the edges from inside the loop to their exit blocks. This
532 // unswitching trivial: no phi nodes to update.
533 unsigned NumBlocks = L->getBlocks().size();
534 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
535 BasicBlock *ExitBlock = ExitBlocks[i];
536 std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock));
538 for (unsigned j = 0, e = Preds.size(); j != e; ++j) {
539 assert(L->contains(Preds[j]) &&
540 "All preds of loop exit blocks must be the same loop!");
541 SplitEdge(Preds[j], ExitBlock);
545 // The exit blocks may have been changed due to edge splitting, recompute.
547 L->getExitBlocks(ExitBlocks);
548 std::sort(ExitBlocks.begin(), ExitBlocks.end());
549 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
552 // Add exit blocks to the loop blocks.
553 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
555 // Next step, clone all of the basic blocks that make up the loop (including
556 // the loop preheader and exit blocks), keeping track of the mapping between
557 // the instructions and blocks.
558 std::vector<BasicBlock*> NewBlocks;
559 NewBlocks.reserve(LoopBlocks.size());
560 std::map<const Value*, Value*> ValueMap;
561 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
562 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F);
563 NewBlocks.push_back(New);
564 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping.
567 // Splice the newly inserted blocks into the function right before the
568 // original preheader.
569 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
570 NewBlocks[0], F->end());
572 // Now we create the new Loop object for the versioned loop.
573 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
574 Loop *ParentLoop = L->getParentLoop();
576 // Make sure to add the cloned preheader and exit blocks to the parent loop
578 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI);
581 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
582 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]);
584 ParentLoop->addBasicBlockToLoop(cast<BasicBlock>(NewExit), *LI);
586 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
587 "Exit block should have been split to have one successor!");
588 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
590 // If the successor of the exit block had PHI nodes, add an entry for
593 for (BasicBlock::iterator I = ExitSucc->begin();
594 (PN = dyn_cast<PHINode>(I)); ++I) {
595 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
596 std::map<const Value *, Value*>::iterator It = ValueMap.find(V);
597 if (It != ValueMap.end()) V = It->second;
598 PN->addIncoming(V, NewExit);
602 // Rewrite the code to refer to itself.
603 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
604 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
605 E = NewBlocks[i]->end(); I != E; ++I)
606 RemapInstruction(I, ValueMap);
608 // Rewrite the original preheader to select between versions of the loop.
609 BranchInst *OldBR = cast<BranchInst>(OrigPreheader->getTerminator());
610 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
611 "Preheader splitting did not work correctly!");
613 // Emit the new branch that selects between the two versions of this loop.
614 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
615 OldBR->eraseFromParent();
617 // Now we rewrite the original code to know that the condition is true and the
618 // new code to know that the condition is false.
619 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
620 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
625 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
626 // the value specified by Val in the specified loop, or we know it does NOT have
627 // that value. Rewrite any uses of LIC or of properties correlated to it.
628 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
631 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
633 // FIXME: Support correlated properties, like:
640 // NotVal - If Val is a bool, this contains its inverse.
641 Constant *NotVal = 0;
642 if (ConstantBool *CB = dyn_cast<ConstantBool>(Val))
643 NotVal = ConstantBool::get(!CB->getValue());
645 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
646 // selects, switches.
647 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
649 // Haha, this loop could be unswitched. Get it? The unswitch pass could
650 // unswitch itself. Amazing.
651 for (unsigned i = 0, e = Users.size(); i != e; ++i)
652 if (Instruction *U = cast<Instruction>(Users[i]))
653 if (L->contains(U->getParent()))
655 U->replaceUsesOfWith(LIC, Val);
657 U->replaceUsesOfWith(LIC, NotVal);
659 // If we know that LIC is not Val, use this info to simplify code.
660 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
661 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
662 if (SI->getCaseValue(i) == Val) {
663 // Found a dead case value. Don't remove PHI nodes in the
664 // successor if they become single-entry, those PHI nodes may
665 // be in the Users list.
666 SI->getSuccessor(i)->removePredecessor(SI->getParent(), true);
673 // TODO: We could simplify stuff like X == C.