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");
52 Statistic<> NumSimplify("loop-unswitch",
53 "Number of simplifications of unswitched code");
55 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
56 cl::init(10), cl::Hidden);
58 class LoopUnswitch : public FunctionPass {
59 LoopInfo *LI; // Loop information
61 virtual bool runOnFunction(Function &F);
62 bool visitLoop(Loop *L);
64 /// This transformation requires natural loop information & requires that
65 /// loop preheaders be inserted into the CFG...
67 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
68 AU.addRequiredID(LoopSimplifyID);
69 AU.addPreservedID(LoopSimplifyID);
70 AU.addRequired<LoopInfo>();
71 AU.addPreserved<LoopInfo>();
75 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L);
76 unsigned getLoopUnswitchCost(Loop *L, Value *LIC);
77 void VersionLoop(Value *LIC, Constant *OnVal,
78 Loop *L, Loop *&Out1, Loop *&Out2);
79 BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To);
80 BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt);
81 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,Constant *Val,
83 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
84 bool EntersWhenTrue, BasicBlock *ExitBlock);
86 RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
89 FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }
91 bool LoopUnswitch::runOnFunction(Function &F) {
93 LI = &getAnalysis<LoopInfo>();
95 // Transform all the top-level loops. Copy the loop list so that the child
96 // can update the loop tree if it needs to delete the loop.
97 std::vector<Loop*> SubLoops(LI->begin(), LI->end());
98 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
99 Changed |= visitLoop(SubLoops[i]);
105 /// LoopValuesUsedOutsideLoop - Return true if there are any values defined in
106 /// the loop that are used by instructions outside of it.
107 static bool LoopValuesUsedOutsideLoop(Loop *L) {
108 // We will be doing lots of "loop contains block" queries. Loop::contains is
109 // linear time, use a set to speed this up.
110 std::set<BasicBlock*> LoopBlocks;
112 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
114 LoopBlocks.insert(*BB);
116 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
118 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
119 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
121 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
122 if (!LoopBlocks.count(UserBB))
129 /// isTrivialLoopExitBlock - Check to see if all paths from BB either:
130 /// 1. Exit the loop with no side effects.
131 /// 2. Branch to the latch block with no side-effects.
133 /// If these conditions are true, we return true and set ExitBB to the block we
136 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
138 std::set<BasicBlock*> &Visited) {
139 if (!Visited.insert(BB).second) {
140 // Already visited and Ok, end of recursion.
142 } else if (!L->contains(BB)) {
143 // Otherwise, this is a loop exit, this is fine so long as this is the
145 if (ExitBB != 0) return false;
150 // Otherwise, this is an unvisited intra-loop node. Check all successors.
151 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
152 // Check to see if the successor is a trivial loop exit.
153 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
157 // Okay, everything after this looks good, check to make sure that this block
158 // doesn't include any side effects.
159 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
160 if (I->mayWriteToMemory())
166 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
167 std::set<BasicBlock*> Visited;
168 Visited.insert(L->getHeader()); // Branches to header are ok.
169 Visited.insert(BB); // Don't revisit BB after we do.
170 BasicBlock *ExitBB = 0;
171 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
176 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
177 /// trivial: that is, that the condition controls whether or not the loop does
178 /// anything at all. If this is a trivial condition, unswitching produces no
179 /// code duplications (equivalently, it produces a simpler loop and a new empty
180 /// loop, which gets deleted).
182 /// If this is a trivial condition, return ConstantBool::True if the loop body
183 /// runs when the condition is true, False if the loop body executes when the
184 /// condition is false. Otherwise, return null to indicate a complex condition.
185 static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond,
187 bool *EntersWhenTrue = 0,
188 BasicBlock **LoopExit = 0) {
189 BasicBlock *Header = L->getHeader();
190 TerminatorInst *HeaderTerm = Header->getTerminator();
192 BasicBlock *LoopExitBB = 0;
193 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
194 // If the header block doesn't end with a conditional branch on Cond, we
196 if (!BI->isConditional() || BI->getCondition() != Cond)
199 // Check to see if a successor of the branch is guaranteed to go to the
200 // latch block or exit through a one exit block without having any
201 // side-effects. If so, determine the value of Cond that causes it to do
203 if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(0)))) {
204 if (Val) *Val = ConstantBool::False;
205 } else if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(1)))) {
206 if (Val) *Val = ConstantBool::True;
208 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
209 // If this isn't a switch on Cond, we can't handle it.
210 if (SI->getCondition() != Cond) return false;
212 // Check to see if a successor of the switch is guaranteed to go to the
213 // latch block or exit through a one exit block without having any
214 // side-effects. If so, determine the value of Cond that causes it to do
215 // this. Note that we can't trivially unswitch on the default case.
216 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
217 if ((LoopExitBB = isTrivialLoopExitBlock(L, SI->getSuccessor(i)))) {
218 // Okay, we found a trivial case, remember the value that is trivial.
219 if (Val) *Val = SI->getCaseValue(i);
220 if (EntersWhenTrue) *EntersWhenTrue = false;
226 return false; // Can't handle this.
228 if (LoopExit) *LoopExit = LoopExitBB;
230 // We already know that nothing uses any scalar values defined inside of this
231 // loop. As such, we just have to check to see if this loop will execute any
232 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
233 // part of the loop that the code *would* execute. We already checked the
234 // tail, check the header now.
235 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
236 if (I->mayWriteToMemory())
241 /// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
242 /// we choose to unswitch the specified loop on the specified value.
244 unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
245 // If the condition is trivial, always unswitch. There is no code growth for
247 if (IsTrivialUnswitchCondition(L, LIC))
251 // FIXME: this is brain dead. It should take into consideration code
253 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
256 // Do not include empty blocks in the cost calculation. This happen due to
257 // loop canonicalization and will be removed.
258 if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
261 // Count basic blocks.
268 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
269 /// invariant in the loop, or has an invariant piece, return the invariant.
270 /// Otherwise, return null.
271 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
272 // Constants should be folded, not unswitched on!
273 if (isa<Constant>(Cond)) return false;
275 // TODO: Handle: br (VARIANT|INVARIANT).
276 // TODO: Hoist simple expressions out of loops.
277 if (L->isLoopInvariant(Cond)) return Cond;
279 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
280 if (BO->getOpcode() == Instruction::And ||
281 BO->getOpcode() == Instruction::Or) {
282 // If either the left or right side is invariant, we can unswitch on this,
283 // which will cause the branch to go away in one loop and the condition to
284 // simplify in the other one.
285 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
287 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
294 bool LoopUnswitch::visitLoop(Loop *L) {
295 bool Changed = false;
297 // Recurse through all subloops before we process this loop. Copy the loop
298 // list so that the child can update the loop tree if it needs to delete the
300 std::vector<Loop*> SubLoops(L->begin(), L->end());
301 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
302 Changed |= visitLoop(SubLoops[i]);
304 // Loop over all of the basic blocks in the loop. If we find an interior
305 // block that is branching on a loop-invariant condition, we can unswitch this
307 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
309 TerminatorInst *TI = (*I)->getTerminator();
310 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
311 // If this isn't branching on an invariant condition, we can't unswitch
313 if (BI->isConditional()) {
314 // See if this, or some part of it, is loop invariant. If so, we can
315 // unswitch on it if we desire.
316 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed);
317 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) {
322 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
323 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
324 if (LoopCond && SI->getNumCases() > 1) {
325 // Find a value to unswitch on:
326 // FIXME: this should chose the most expensive case!
327 Constant *UnswitchVal = SI->getCaseValue(1);
328 if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) {
335 // Scan the instructions to check for unswitchable values.
336 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
338 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
339 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
340 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) {
350 /// UnswitchIfProfitable - We have found that we can unswitch L when
351 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
352 /// unswitch the loop, reprocess the pieces, then return true.
353 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){
354 // Check to see if it would be profitable to unswitch this loop.
355 if (getLoopUnswitchCost(L, LoopCond) > Threshold) {
356 // FIXME: this should estimate growth by the amount of code shared by the
357 // resultant unswitched loops.
359 DEBUG(std::cerr << "NOT unswitching loop %"
360 << L->getHeader()->getName() << ", cost too high: "
361 << L->getBlocks().size() << "\n");
365 // If this loop has live-out values, we can't unswitch it. We need something
366 // like loop-closed SSA form in order to know how to insert PHI nodes for
368 if (LoopValuesUsedOutsideLoop(L)) {
369 DEBUG(std::cerr << "NOT unswitching loop %" << L->getHeader()->getName()
370 << ", a loop value is used outside loop!\n");
374 //std::cerr << "BEFORE:\n"; LI->dump();
375 Loop *NewLoop1 = 0, *NewLoop2 = 0;
377 // If this is a trivial condition to unswitch (which results in no code
378 // duplication), do it now.
380 bool EntersWhenTrue = true;
381 BasicBlock *ExitBlock;
382 if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal,
383 &EntersWhenTrue, &ExitBlock)) {
384 UnswitchTrivialCondition(L, LoopCond, CondVal, EntersWhenTrue, ExitBlock);
387 VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2);
390 //std::cerr << "AFTER:\n"; LI->dump();
392 // Try to unswitch each of our new loops now!
393 if (NewLoop1) visitLoop(NewLoop1);
394 if (NewLoop2) visitLoop(NewLoop2);
398 /// SplitBlock - Split the specified block at the specified instruction - every
399 /// thing before SplitPt stays in Old and everything starting with SplitPt moves
400 /// to a new block. The two blocks are joined by an unconditional branch and
401 /// the loop info is updated.
403 BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *Old, Instruction *SplitPt) {
404 BasicBlock::iterator SplitIt = SplitPt;
405 while (isa<PHINode>(SplitIt))
407 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
409 // The new block lives in whichever loop the old one did.
410 if (Loop *L = LI->getLoopFor(Old))
411 L->addBasicBlockToLoop(New, *LI);
417 BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) {
418 TerminatorInst *LatchTerm = BB->getTerminator();
419 unsigned SuccNum = 0;
420 for (unsigned i = 0, e = LatchTerm->getNumSuccessors(); ; ++i) {
421 assert(i != e && "Didn't find edge?");
422 if (LatchTerm->getSuccessor(i) == Succ) {
428 // If this is a critical edge, let SplitCriticalEdge do it.
429 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, this))
430 return LatchTerm->getSuccessor(SuccNum);
432 // If the edge isn't critical, then BB has a single successor or Succ has a
433 // single pred. Split the block.
434 BasicBlock::iterator SplitPoint;
435 if (BasicBlock *SP = Succ->getSinglePredecessor()) {
436 // If the successor only has a single pred, split the top of the successor
438 assert(SP == BB && "CFG broken");
439 return SplitBlock(Succ, Succ->begin());
441 // Otherwise, if BB has a single successor, split it at the bottom of the
443 assert(BB->getTerminator()->getNumSuccessors() == 1 &&
444 "Should have a single succ!");
445 return SplitBlock(BB, BB->getTerminator());
451 // RemapInstruction - Convert the instruction operands from referencing the
452 // current values into those specified by ValueMap.
454 static inline void RemapInstruction(Instruction *I,
455 std::map<const Value *, Value*> &ValueMap) {
456 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
457 Value *Op = I->getOperand(op);
458 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
459 if (It != ValueMap.end()) Op = It->second;
460 I->setOperand(op, Op);
464 /// CloneLoop - Recursively clone the specified loop and all of its children,
465 /// mapping the blocks with the specified map.
466 static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
468 Loop *New = new Loop();
471 PL->addChildLoop(New);
473 LI->addTopLevelLoop(New);
475 // Add all of the blocks in L to the new loop.
476 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
478 if (LI->getLoopFor(*I) == L)
479 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
481 // Add all of the subloops to the new loop.
482 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
483 CloneLoop(*I, New, VM, LI);
488 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
489 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
490 /// code immediately before InsertPt.
491 static void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
492 BasicBlock *TrueDest,
493 BasicBlock *FalseDest,
494 Instruction *InsertPt) {
495 // Insert a conditional branch on LIC to the two preheaders. The original
496 // code is the true version and the new code is the false version.
497 Value *BranchVal = LIC;
498 if (!isa<ConstantBool>(Val)) {
499 BranchVal = BinaryOperator::createSetEQ(LIC, Val, "tmp", InsertPt);
500 } else if (Val != ConstantBool::True) {
501 // We want to enter the new loop when the condition is true.
502 std::swap(TrueDest, FalseDest);
505 // Insert the new branch.
506 new BranchInst(TrueDest, FalseDest, BranchVal, InsertPt);
510 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
511 /// condition in it (a cond branch from its header block to its latch block,
512 /// where the path through the loop that doesn't execute its body has no
513 /// side-effects), unswitch it. This doesn't involve any code duplication, just
514 /// moving the conditional branch outside of the loop and updating loop info.
515 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
516 Constant *Val, bool EntersWhenTrue,
517 BasicBlock *ExitBlock) {
518 DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %"
519 << L->getHeader()->getName() << " [" << L->getBlocks().size()
520 << " blocks] in Function " << L->getHeader()->getParent()->getName()
521 << " on cond: " << *Val << (EntersWhenTrue ? " == " : " != ") <<
524 // First step, split the preheader, so that we know that there is a safe place
525 // to insert the conditional branch. We will change 'OrigPH' to have a
526 // conditional branch on Cond.
527 BasicBlock *OrigPH = L->getLoopPreheader();
528 BasicBlock *NewPH = SplitEdge(OrigPH, L->getHeader());
530 // Now that we have a place to insert the conditional branch, create a place
531 // to branch to: this is the exit block out of the loop that we should
534 // Split this block now, so that the loop maintains its exit block, and so
535 // that the jump from the preheader can execute the contents of the exit block
536 // without actually branching to it (the exit block should be dominated by the
537 // loop header, not the preheader).
538 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
539 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin());
541 // Okay, now we have a position to branch from and a position to branch to,
542 // insert the new conditional branch.
544 BasicBlock *TrueDest = NewPH, *FalseDest = NewExit;
545 if (!EntersWhenTrue) std::swap(TrueDest, FalseDest);
546 EmitPreheaderBranchOnCondition(Cond, Val, TrueDest, FalseDest,
547 OrigPH->getTerminator());
549 OrigPH->getTerminator()->eraseFromParent();
551 // Now that we know that the loop is never entered when this condition is a
552 // particular value, rewrite the loop with this info. We know that this will
553 // at least eliminate the old branch.
554 RewriteLoopBodyWithConditionConstant(L, Cond, Val, EntersWhenTrue);
559 /// VersionLoop - We determined that the loop is profitable to unswitch when LIC
560 /// equal Val. Split it into loop versions and test the condition outside of
561 /// either loop. Return the loops created as Out1/Out2.
562 void LoopUnswitch::VersionLoop(Value *LIC, Constant *Val, Loop *L,
563 Loop *&Out1, Loop *&Out2) {
564 Function *F = L->getHeader()->getParent();
566 DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
567 << L->getHeader()->getName() << " [" << L->getBlocks().size()
568 << " blocks] in Function " << F->getName()
569 << " when '" << *Val << "' == " << *LIC << "\n");
571 // LoopBlocks contains all of the basic blocks of the loop, including the
572 // preheader of the loop, the body of the loop, and the exit blocks of the
573 // loop, in that order.
574 std::vector<BasicBlock*> LoopBlocks;
576 // First step, split the preheader and exit blocks, and add these blocks to
577 // the LoopBlocks list.
578 BasicBlock *OrigPreheader = L->getLoopPreheader();
579 LoopBlocks.push_back(SplitEdge(OrigPreheader, L->getHeader()));
581 // We want the loop to come after the preheader, but before the exit blocks.
582 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
584 std::vector<BasicBlock*> ExitBlocks;
585 L->getExitBlocks(ExitBlocks);
586 std::sort(ExitBlocks.begin(), ExitBlocks.end());
587 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
590 // Split all of the edges from inside the loop to their exit blocks. This
591 // unswitching trivial: no phi nodes to update.
592 unsigned NumBlocks = L->getBlocks().size();
594 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
595 BasicBlock *ExitBlock = ExitBlocks[i];
596 std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock));
598 for (unsigned j = 0, e = Preds.size(); j != e; ++j) {
599 assert(L->contains(Preds[j]) &&
600 "All preds of loop exit blocks must be the same loop!");
601 SplitEdge(Preds[j], ExitBlock);
605 // The exit blocks may have been changed due to edge splitting, recompute.
607 L->getExitBlocks(ExitBlocks);
608 std::sort(ExitBlocks.begin(), ExitBlocks.end());
609 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
612 // Add exit blocks to the loop blocks.
613 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
615 // Next step, clone all of the basic blocks that make up the loop (including
616 // the loop preheader and exit blocks), keeping track of the mapping between
617 // the instructions and blocks.
618 std::vector<BasicBlock*> NewBlocks;
619 NewBlocks.reserve(LoopBlocks.size());
620 std::map<const Value*, Value*> ValueMap;
621 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
622 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F);
623 NewBlocks.push_back(New);
624 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping.
627 // Splice the newly inserted blocks into the function right before the
628 // original preheader.
629 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
630 NewBlocks[0], F->end());
632 // Now we create the new Loop object for the versioned loop.
633 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
634 Loop *ParentLoop = L->getParentLoop();
636 // Make sure to add the cloned preheader and exit blocks to the parent loop
638 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI);
641 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
642 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]);
643 // The new exit block should be in the same loop as the old one.
644 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
645 ExitBBLoop->addBasicBlockToLoop(NewExit, *LI);
647 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
648 "Exit block should have been split to have one successor!");
649 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
651 // If the successor of the exit block had PHI nodes, add an entry for
654 for (BasicBlock::iterator I = ExitSucc->begin();
655 (PN = dyn_cast<PHINode>(I)); ++I) {
656 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
657 std::map<const Value *, Value*>::iterator It = ValueMap.find(V);
658 if (It != ValueMap.end()) V = It->second;
659 PN->addIncoming(V, NewExit);
663 // Rewrite the code to refer to itself.
664 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
665 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
666 E = NewBlocks[i]->end(); I != E; ++I)
667 RemapInstruction(I, ValueMap);
669 // Rewrite the original preheader to select between versions of the loop.
670 BranchInst *OldBR = cast<BranchInst>(OrigPreheader->getTerminator());
671 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
672 "Preheader splitting did not work correctly!");
674 // Emit the new branch that selects between the two versions of this loop.
675 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
676 OldBR->eraseFromParent();
678 // Now we rewrite the original code to know that the condition is true and the
679 // new code to know that the condition is false.
680 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
681 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
686 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
688 static void RemoveFromWorklist(Instruction *I,
689 std::vector<Instruction*> &Worklist) {
690 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
692 while (WI != Worklist.end()) {
693 unsigned Offset = WI-Worklist.begin();
695 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
699 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
700 /// program, replacing all uses with V and update the worklist.
701 static void ReplaceUsesOfWith(Instruction *I, Value *V,
702 std::vector<Instruction*> &Worklist) {
703 DEBUG(std::cerr << "Replace with '" << *V << "': " << *I);
705 // Add uses to the worklist, which may be dead now.
706 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
707 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
708 Worklist.push_back(Use);
710 // Add users to the worklist which may be simplified now.
711 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
713 Worklist.push_back(cast<Instruction>(*UI));
714 I->replaceAllUsesWith(V);
715 I->eraseFromParent();
716 RemoveFromWorklist(I, Worklist);
722 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
723 // the value specified by Val in the specified loop, or we know it does NOT have
724 // that value. Rewrite any uses of LIC or of properties correlated to it.
725 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
728 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
730 // FIXME: Support correlated properties, like:
737 // NotVal - If Val is a bool, this contains its inverse.
738 Constant *NotVal = 0;
739 if (ConstantBool *CB = dyn_cast<ConstantBool>(Val))
740 NotVal = ConstantBool::get(!CB->getValue());
742 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
743 // selects, switches.
744 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
746 std::vector<Instruction*> Worklist;
748 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
749 // in the loop with the appropriate one directly.
750 if (IsEqual || NotVal) {
751 Value *Replacement = NotVal ? NotVal : Val;
753 for (unsigned i = 0, e = Users.size(); i != e; ++i)
754 if (Instruction *U = cast<Instruction>(Users[i])) {
755 if (!L->contains(U->getParent()))
757 U->replaceUsesOfWith(LIC, Replacement);
758 Worklist.push_back(U);
761 // Otherwise, we don't know the precise value of LIC, but we do know that it
762 // is certainly NOT "Val". As such, simplify any uses in the loop that we
763 // can. This case occurs when we unswitch switch statements.
764 for (unsigned i = 0, e = Users.size(); i != e; ++i)
765 if (Instruction *U = cast<Instruction>(Users[i])) {
766 if (!L->contains(U->getParent()))
769 Worklist.push_back(U);
771 // If we know that LIC is not Val, use this info to simplify code.
772 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
773 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
774 if (SI->getCaseValue(i) == Val) {
775 // Found a dead case value. Don't remove PHI nodes in the
776 // successor if they become single-entry, those PHI nodes may
777 // be in the Users list.
778 SI->getSuccessor(i)->removePredecessor(SI->getParent(), true);
785 // TODO: We could do other simplifications, for example, turning
786 // LIC == Val -> false.
790 // Okay, now that we have simplified some instructions in the loop, walk over
791 // it and constant prop, dce, and fold control flow where possible. Note that
792 // this is effectively a very simple loop-structure-aware optimizer.
793 while (!Worklist.empty()) {
794 Instruction *I = Worklist.back();
797 // Simple constant folding.
798 if (Constant *C = ConstantFoldInstruction(I)) {
799 ReplaceUsesOfWith(I, C, Worklist);
804 if (isInstructionTriviallyDead(I)) {
805 DEBUG(std::cerr << "Remove dead instruction '" << *I);
807 // Add uses to the worklist, which may be dead now.
808 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
809 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
810 Worklist.push_back(Use);
811 I->eraseFromParent();
812 RemoveFromWorklist(I, Worklist);
817 // Special case hacks that appear commonly in unswitched code.
818 switch (I->getOpcode()) {
819 case Instruction::Select:
820 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(0))) {
821 ReplaceUsesOfWith(I, I->getOperand(!CB->getValue()+1), Worklist);
825 case Instruction::And:
826 if (isa<ConstantBool>(I->getOperand(0))) // constant -> RHS
827 cast<BinaryOperator>(I)->swapOperands();
828 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) {
829 if (CB->getValue()) // X & 1 -> X
830 ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
832 ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
836 case Instruction::Or:
837 if (isa<ConstantBool>(I->getOperand(0))) // constant -> RHS
838 cast<BinaryOperator>(I)->swapOperands();
839 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) {
840 if (CB->getValue()) // X | 1 -> 1
841 ReplaceUsesOfWith(I, I->getOperand(1), Worklist);
843 ReplaceUsesOfWith(I, I->getOperand(0), Worklist);
847 case Instruction::Br: {
848 BranchInst *BI = cast<BranchInst>(I);
849 if (BI->isUnconditional()) {
850 // If BI's parent is the only pred of the successor, fold the two blocks
852 BasicBlock *Pred = BI->getParent();
853 BasicBlock *Succ = BI->getSuccessor(0);
854 BasicBlock *SinglePred = Succ->getSinglePredecessor();
855 if (!SinglePred) continue; // Nothing to do.
856 assert(SinglePred == Pred && "CFG broken");
858 DEBUG(std::cerr << "Merging blocks: " << Pred->getName() << " <- "
859 << Succ->getName() << "\n");
861 // Resolve any single entry PHI nodes in Succ.
862 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
863 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist);
865 // Move all of the successor contents from Succ to Pred.
866 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
868 BI->eraseFromParent();
869 RemoveFromWorklist(BI, Worklist);
871 // If Succ has any successors with PHI nodes, update them to have
872 // entries coming from Pred instead of Succ.
873 Succ->replaceAllUsesWith(Pred);
875 // Remove Succ from the loop tree.
876 LI->removeBlock(Succ);
877 Succ->eraseFromParent();