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/ADT/Statistic.h"
38 #include "llvm/Support/Debug.h"
39 #include "llvm/Support/CommandLine.h"
46 Statistic<> NumUnswitched("loop-unswitch", "Number of loops unswitched");
48 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
49 cl::init(10), cl::Hidden);
51 class LoopUnswitch : public FunctionPass {
52 LoopInfo *LI; // Loop information
54 virtual bool runOnFunction(Function &F);
55 bool visitLoop(Loop *L);
57 /// This transformation requires natural loop information & requires that
58 /// loop preheaders be inserted into the CFG...
60 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
61 AU.addRequiredID(LoopSimplifyID);
62 AU.addPreservedID(LoopSimplifyID);
63 AU.addRequired<LoopInfo>();
64 AU.addPreserved<LoopInfo>();
68 unsigned getLoopUnswitchCost(Loop *L, Value *LIC);
69 void VersionLoop(Value *LIC, Loop *L, Loop *&Out1, Loop *&Out2);
70 BasicBlock *SplitBlock(BasicBlock *BB, bool SplitAtTop);
71 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, bool Val);
72 void UnswitchTrivialCondition(Loop *L, Value *Cond, bool EntersLoopOnCond,
73 BasicBlock *ExitBlock);
75 RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
78 FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }
80 bool LoopUnswitch::runOnFunction(Function &F) {
82 LI = &getAnalysis<LoopInfo>();
84 // Transform all the top-level loops. Copy the loop list so that the child
85 // can update the loop tree if it needs to delete the loop.
86 std::vector<Loop*> SubLoops(LI->begin(), LI->end());
87 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
88 Changed |= visitLoop(SubLoops[i]);
94 /// LoopValuesUsedOutsideLoop - Return true if there are any values defined in
95 /// the loop that are used by instructions outside of it.
96 static bool LoopValuesUsedOutsideLoop(Loop *L) {
97 // We will be doing lots of "loop contains block" queries. Loop::contains is
98 // linear time, use a set to speed this up.
99 std::set<BasicBlock*> LoopBlocks;
101 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
103 LoopBlocks.insert(*BB);
105 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
107 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
108 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
110 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
111 if (!LoopBlocks.count(UserBB))
118 /// FindTrivialLoopExitBlock - We know that we have a branch from the loop
119 /// header to the specified latch block. See if one of the successors of the
120 /// latch block is an exit, and if so what block it is.
121 static BasicBlock *FindTrivialLoopExitBlock(Loop *L, BasicBlock *Latch) {
122 BasicBlock *Header = L->getHeader();
123 BranchInst *LatchBranch = dyn_cast<BranchInst>(Latch->getTerminator());
124 if (!LatchBranch || !LatchBranch->isConditional()) return 0;
126 // Simple case, the latch block is a conditional branch. The target that
127 // doesn't go to the loop header is our block if it is not in the loop.
128 if (LatchBranch->getSuccessor(0) == Header) {
129 if (L->contains(LatchBranch->getSuccessor(1))) return false;
130 return LatchBranch->getSuccessor(1);
132 assert(LatchBranch->getSuccessor(1) == Header);
133 if (L->contains(LatchBranch->getSuccessor(0))) return false;
134 return LatchBranch->getSuccessor(0);
139 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
140 /// trivial: that is, that the condition controls whether or not the loop does
141 /// anything at all. If this is a trivial condition, unswitching produces no
142 /// code duplications (equivalently, it produces a simpler loop and a new empty
143 /// loop, which gets deleted).
145 /// If this is a trivial condition, return ConstantBool::True if the loop body
146 /// runs when the condition is true, False if the loop body executes when the
147 /// condition is false. Otherwise, return null to indicate a complex condition.
148 static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond,
149 bool *CondEntersLoop = 0,
150 BasicBlock **LoopExit = 0) {
151 BasicBlock *Header = L->getHeader();
152 BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator());
154 // If the header block doesn't end with a conditional branch on Cond, we can't
156 if (!HeaderTerm || !HeaderTerm->isConditional() ||
157 HeaderTerm->getCondition() != Cond)
160 // Check to see if the conditional branch goes to the latch block. If not,
161 // it's not trivial. This also determines the value of Cond that will execute
163 BasicBlock *Latch = L->getLoopLatch();
164 if (HeaderTerm->getSuccessor(1) == Latch) {
165 if (CondEntersLoop) *CondEntersLoop = true;
166 } else if (HeaderTerm->getSuccessor(0) == Latch)
167 if (CondEntersLoop) *CondEntersLoop = false;
169 return false; // Doesn't branch to latch block.
171 // The latch block must end with a conditional branch where one edge goes to
172 // the header (this much we know) and one edge goes OUT of the loop.
173 BasicBlock *LoopExitBlock = FindTrivialLoopExitBlock(L, Latch);
174 if (!LoopExitBlock) return 0;
175 if (LoopExit) *LoopExit = LoopExitBlock;
177 // We already know that nothing uses any scalar values defined inside of this
178 // loop. As such, we just have to check to see if this loop will execute any
179 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
180 // part of the loop that the code *would* execute.
181 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
182 if (I->mayWriteToMemory())
184 for (BasicBlock::iterator I = Latch->begin(), E = Latch->end(); I != E; ++I)
185 if (I->mayWriteToMemory())
190 /// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
191 /// we choose to unswitch the specified loop on the specified value.
193 unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
194 // If the condition is trivial, always unswitch. There is no code growth for
196 if (IsTrivialUnswitchCondition(L, LIC))
200 // FIXME: this is brain dead. It should take into consideration code
202 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
205 // Do not include empty blocks in the cost calculation. This happen due to
206 // loop canonicalization and will be removed.
207 if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
210 // Count basic blocks.
217 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
218 /// invariant in the loop, or has an invariant piece, return the invariant.
219 /// Otherwise, return null.
220 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
221 // Constants should be folded, not unswitched on!
222 if (isa<Constant>(Cond)) return false;
224 // TODO: Handle: br (VARIANT|INVARIANT).
225 // TODO: Hoist simple expressions out of loops.
226 if (L->isLoopInvariant(Cond)) return Cond;
228 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
229 if (BO->getOpcode() == Instruction::And ||
230 BO->getOpcode() == Instruction::Or) {
231 // If either the left or right side is invariant, we can unswitch on this,
232 // which will cause the branch to go away in one loop and the condition to
233 // simplify in the other one.
234 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
236 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
243 bool LoopUnswitch::visitLoop(Loop *L) {
244 bool Changed = false;
246 // Recurse through all subloops before we process this loop. Copy the loop
247 // list so that the child can update the loop tree if it needs to delete the
249 std::vector<Loop*> SubLoops(L->begin(), L->end());
250 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
251 Changed |= visitLoop(SubLoops[i]);
253 // Loop over all of the basic blocks in the loop. If we find an interior
254 // block that is branching on a loop-invariant condition, we can unswitch this
256 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
258 TerminatorInst *TI = (*I)->getTerminator();
259 // FIXME: Handle invariant select instructions.
261 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
262 if (!isa<Constant>(SI) && L->isLoopInvariant(SI->getCondition()))
263 DEBUG(std::cerr << "TODO: Implement unswitching 'switch' loop %"
264 << L->getHeader()->getName() << ", cost = "
265 << L->getBlocks().size() << "\n" << **I);
269 BranchInst *BI = dyn_cast<BranchInst>(TI);
272 // If this isn't branching on an invariant condition, we can't unswitch it.
273 if (!BI->isConditional())
276 // See if this, or some part of it, is loop invariant. If so, we can
277 // unswitch on it if we desire.
278 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed);
279 if (LoopCond == 0) continue;
281 // Check to see if it would be profitable to unswitch this loop.
282 if (getLoopUnswitchCost(L, LoopCond) > Threshold) {
283 // FIXME: this should estimate growth by the amount of code shared by the
284 // resultant unswitched loops. This should have no code growth:
285 // for () { if (iv) {...} }
286 // as one copy of the loop will be empty.
288 DEBUG(std::cerr << "NOT unswitching loop %"
289 << L->getHeader()->getName() << ", cost too high: "
290 << L->getBlocks().size() << "\n");
294 // If this loop has live-out values, we can't unswitch it. We need something
295 // like loop-closed SSA form in order to know how to insert PHI nodes for
297 if (LoopValuesUsedOutsideLoop(L)) {
298 DEBUG(std::cerr << "NOT unswitching loop %"
299 << L->getHeader()->getName()
300 << ", a loop value is used outside loop!\n");
304 //std::cerr << "BEFORE:\n"; LI->dump();
305 Loop *NewLoop1 = 0, *NewLoop2 = 0;
307 // If this is a trivial condition to unswitch (which results in no code
308 // duplication), do it now.
309 bool EntersLoopOnCond;
310 BasicBlock *ExitBlock;
311 if (IsTrivialUnswitchCondition(L, LoopCond, &EntersLoopOnCond, &ExitBlock)){
312 UnswitchTrivialCondition(L, LoopCond, EntersLoopOnCond, ExitBlock);
315 VersionLoop(LoopCond, L, NewLoop1, NewLoop2);
318 //std::cerr << "AFTER:\n"; LI->dump();
320 // Try to unswitch each of our new loops now!
321 if (NewLoop1) visitLoop(NewLoop1);
322 if (NewLoop2) visitLoop(NewLoop2);
329 /// SplitBlock - Split the specified basic block into two pieces. If SplitAtTop
330 /// is false, this splits the block so the second half only has an unconditional
331 /// branch. If SplitAtTop is true, it makes it so the first half of the block
332 /// only has an unconditional branch in it.
334 /// This method updates the LoopInfo for this function to correctly reflect the
335 /// CFG changes made.
337 /// This routine returns the new basic block that was inserted, which is always
338 /// the later part of the block.
339 BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *BB, bool SplitAtTop) {
340 BasicBlock::iterator SplitPoint;
342 SplitPoint = BB->getTerminator();
344 SplitPoint = BB->begin();
345 while (isa<PHINode>(SplitPoint)) ++SplitPoint;
348 BasicBlock *New = BB->splitBasicBlock(SplitPoint, BB->getName()+".tail");
349 // New now lives in whichever loop that BB used to.
350 if (Loop *L = LI->getLoopFor(BB))
351 L->addBasicBlockToLoop(New, *LI);
356 // RemapInstruction - Convert the instruction operands from referencing the
357 // current values into those specified by ValueMap.
359 static inline void RemapInstruction(Instruction *I,
360 std::map<const Value *, Value*> &ValueMap) {
361 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
362 Value *Op = I->getOperand(op);
363 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
364 if (It != ValueMap.end()) Op = It->second;
365 I->setOperand(op, Op);
369 /// CloneLoop - Recursively clone the specified loop and all of its children,
370 /// mapping the blocks with the specified map.
371 static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
373 Loop *New = new Loop();
376 PL->addChildLoop(New);
378 LI->addTopLevelLoop(New);
380 // Add all of the blocks in L to the new loop.
381 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
383 if (LI->getLoopFor(*I) == L)
384 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
386 // Add all of the subloops to the new loop.
387 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
388 CloneLoop(*I, New, VM, LI);
393 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
394 /// condition in it (a cond branch from its header block to its latch block,
395 /// where the path through the loop that doesn't execute its body has no
396 /// side-effects), unswitch it. This doesn't involve any code duplication, just
397 /// moving the conditional branch outside of the loop and updating loop info.
398 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
400 BasicBlock *ExitBlock) {
401 DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %"
402 << L->getHeader()->getName() << " [" << L->getBlocks().size()
403 << " blocks] in Function " << L->getHeader()->getParent()->getName()
404 << " on cond:" << *Cond << "\n");
406 // First step, split the preahder, so that we know that there is a safe place
407 // to insert the conditional branch. We will change 'OrigPH' to have a
408 // conditional branch on Cond.
409 BasicBlock *OrigPH = L->getLoopPreheader();
410 BasicBlock *NewPH = SplitBlock(OrigPH, false);
412 // Now that we have a place to insert the conditional branch, create a place
413 // to branch to: this is the exit block out of the loop that we should
416 // Split this block now, so that the loop maintains its exit block.
417 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
418 BasicBlock *NewExit = SplitBlock(ExitBlock, true);
420 // Okay, now we have a position to branch from and a position to branch to,
421 // insert the new conditional branch.
422 new BranchInst(EnterOnCond ? NewPH : NewExit, EnterOnCond ? NewExit : NewPH,
423 Cond, OrigPH->getTerminator());
424 OrigPH->getTerminator()->eraseFromParent();
426 // Now that we know that the loop is never entered when this condition is a
427 // particular value, rewrite the loop with this info. We know that this will
428 // at least eliminate the old branch.
429 RewriteLoopBodyWithConditionConstant(L, Cond, EnterOnCond);
435 /// VersionLoop - We determined that the loop is profitable to unswitch and
436 /// contains a branch on a loop invariant condition. Split it into loop
437 /// versions and test the condition outside of either loop. Return the loops
438 /// created as Out1/Out2.
439 void LoopUnswitch::VersionLoop(Value *LIC, Loop *L, Loop *&Out1, Loop *&Out2) {
440 Function *F = L->getHeader()->getParent();
442 DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
443 << L->getHeader()->getName() << " [" << L->getBlocks().size()
444 << " blocks] in Function " << F->getName()
445 << " on cond:" << *LIC << "\n");
447 std::vector<BasicBlock*> LoopBlocks;
449 // First step, split the preheader and exit blocks, and add these blocks to
450 // the LoopBlocks list.
451 BasicBlock *OrigPreheader = L->getLoopPreheader();
452 LoopBlocks.push_back(SplitBlock(OrigPreheader, false));
454 // We want the loop to come after the preheader, but before the exit blocks.
455 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
457 std::vector<BasicBlock*> ExitBlocks;
458 L->getExitBlocks(ExitBlocks);
459 std::sort(ExitBlocks.begin(), ExitBlocks.end());
460 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
462 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
463 SplitBlock(ExitBlocks[i], true);
464 LoopBlocks.push_back(ExitBlocks[i]);
467 // Next step, clone all of the basic blocks that make up the loop (including
468 // the loop preheader and exit blocks), keeping track of the mapping between
469 // the instructions and blocks.
470 std::vector<BasicBlock*> NewBlocks;
471 NewBlocks.reserve(LoopBlocks.size());
472 std::map<const Value*, Value*> ValueMap;
473 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
474 NewBlocks.push_back(CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F));
475 ValueMap[LoopBlocks[i]] = NewBlocks.back(); // Keep the BB mapping.
478 // Splice the newly inserted blocks into the function right before the
479 // original preheader.
480 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
481 NewBlocks[0], F->end());
483 // Now we create the new Loop object for the versioned loop.
484 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
485 if (Loop *Parent = L->getParentLoop()) {
486 // Make sure to add the cloned preheader and exit blocks to the parent loop
488 Parent->addBasicBlockToLoop(NewBlocks[0], *LI);
489 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
490 Parent->addBasicBlockToLoop(cast<BasicBlock>(ValueMap[ExitBlocks[i]]),
494 // Rewrite the code to refer to itself.
495 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
496 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
497 E = NewBlocks[i]->end(); I != E; ++I)
498 RemapInstruction(I, ValueMap);
500 // Rewrite the original preheader to select between versions of the loop.
501 assert(isa<BranchInst>(OrigPreheader->getTerminator()) &&
502 cast<BranchInst>(OrigPreheader->getTerminator())->isUnconditional() &&
503 OrigPreheader->getTerminator()->getSuccessor(0) == LoopBlocks[0] &&
504 "Preheader splitting did not work correctly!");
505 // Remove the unconditional branch to LoopBlocks[0].
506 OrigPreheader->getInstList().pop_back();
508 // Insert a conditional branch on LIC to the two preheaders. The original
509 // code is the true version and the new code is the false version.
510 new BranchInst(LoopBlocks[0], NewBlocks[0], LIC, OrigPreheader);
512 // Now we rewrite the original code to know that the condition is true and the
513 // new code to know that the condition is false.
514 RewriteLoopBodyWithConditionConstant(L, LIC, true);
515 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, false);
521 // RewriteLoopBodyWithConditionConstant - We know that the boolean value LIC has
522 // the value specified by Val in the specified loop. Rewrite any uses of LIC or
523 // of properties correlated to it.
524 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
526 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
527 // FIXME: Support correlated properties, like:
533 ConstantBool *BoolVal = ConstantBool::get(Val);
535 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
536 // selects, switches.
537 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
538 for (unsigned i = 0, e = Users.size(); i != e; ++i)
539 if (Instruction *U = cast<Instruction>(Users[i]))
540 if (L->contains(U->getParent()))
541 U->replaceUsesOfWith(LIC, BoolVal);