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, ConstantBool *LoopCond);
74 RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
77 FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }
79 bool LoopUnswitch::runOnFunction(Function &F) {
81 LI = &getAnalysis<LoopInfo>();
83 // Transform all the top-level loops. Copy the loop list so that the child
84 // can update the loop tree if it needs to delete the loop.
85 std::vector<Loop*> SubLoops(LI->begin(), LI->end());
86 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
87 Changed |= visitLoop(SubLoops[i]);
93 /// LoopValuesUsedOutsideLoop - Return true if there are any values defined in
94 /// the loop that are used by instructions outside of it.
95 static bool LoopValuesUsedOutsideLoop(Loop *L) {
96 // We will be doing lots of "loop contains block" queries. Loop::contains is
97 // linear time, use a set to speed this up.
98 std::set<BasicBlock*> LoopBlocks;
100 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
102 LoopBlocks.insert(*BB);
104 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
106 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
107 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
109 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
110 if (!LoopBlocks.count(UserBB))
117 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
118 /// trivial: that is, that the condition controls whether or not the loop does
119 /// anything at all. If this is a trivial condition, unswitching produces no
120 /// code duplications (equivalently, it produces a simpler loop and a new empty
121 /// loop, which gets deleted).
123 /// If this is a trivial condition, return ConstantBool::True if the loop body
124 /// runs when the condition is true, False if the loop body executes when the
125 /// condition is false. Otherwise, return null to indicate a complex condition.
126 static ConstantBool *IsTrivialUnswitchCondition(Loop *L, Value *Cond) {
127 BasicBlock *Header = L->getHeader();
128 BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator());
129 ConstantBool *RetVal = 0;
131 // If the header block doesn't end with a conditional branch on Cond, we can't
133 if (!HeaderTerm || !HeaderTerm->isConditional() ||
134 HeaderTerm->getCondition() != Cond)
137 // Check to see if the conditional branch goes to the latch block. If not,
138 // it's not trivial. This also determines the value of Cond that will execute
140 BasicBlock *Latch = L->getLoopLatch();
141 if (HeaderTerm->getSuccessor(1) == Latch)
142 RetVal = ConstantBool::True;
143 else if (HeaderTerm->getSuccessor(0) == Latch)
144 RetVal = ConstantBool::False;
146 return 0; // Doesn't branch to latch block.
148 // The latch block must end with a conditional branch where one edge goes to
149 // the header (this much we know) and one edge goes OUT of the loop.
150 BranchInst *LatchBranch = dyn_cast<BranchInst>(Latch->getTerminator());
151 if (!LatchBranch || !LatchBranch->isConditional()) return 0;
153 if (LatchBranch->getSuccessor(0) == Header) {
154 if (L->contains(LatchBranch->getSuccessor(1))) return 0;
156 assert(LatchBranch->getSuccessor(1) == Header);
157 if (L->contains(LatchBranch->getSuccessor(0))) return 0;
160 // We already know that nothing uses any scalar values defined inside of this
161 // loop. As such, we just have to check to see if this loop will execute any
162 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
163 // part of the loop that the code *would* execute.
164 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
165 if (I->mayWriteToMemory())
167 for (BasicBlock::iterator I = Latch->begin(), E = Latch->end(); I != E; ++I)
168 if (I->mayWriteToMemory())
173 /// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
174 /// we choose to unswitch the specified loop on the specified value.
176 unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
177 // If the condition is trivial, always unswitch. There is no code growth for
179 if (IsTrivialUnswitchCondition(L, LIC))
183 // FIXME: this is brain dead. It should take into consideration code
185 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
188 // Do not include empty blocks in the cost calculation. This happen due to
189 // loop canonicalization and will be removed.
190 if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
193 // Count basic blocks.
200 bool LoopUnswitch::visitLoop(Loop *L) {
201 bool Changed = false;
203 // Recurse through all subloops before we process this loop. Copy the loop
204 // list so that the child can update the loop tree if it needs to delete the
206 std::vector<Loop*> SubLoops(L->begin(), L->end());
207 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
208 Changed |= visitLoop(SubLoops[i]);
210 // Loop over all of the basic blocks in the loop. If we find an interior
211 // block that is branching on a loop-invariant condition, we can unswitch this
213 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
215 TerminatorInst *TI = (*I)->getTerminator();
216 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
217 if (!isa<Constant>(SI) && L->isLoopInvariant(SI->getCondition()))
218 DEBUG(std::cerr << "TODO: Implement unswitching 'switch' loop %"
219 << L->getHeader()->getName() << ", cost = "
220 << L->getBlocks().size() << "\n" << **I);
224 BranchInst *BI = dyn_cast<BranchInst>(TI);
227 // If this isn't branching on an invariant condition, we can't unswitch it.
228 if (!BI->isConditional() || isa<Constant>(BI->getCondition()) ||
229 !L->isLoopInvariant(BI->getCondition()))
232 // Check to see if it would be profitable to unswitch this loop.
233 if (getLoopUnswitchCost(L, BI->getCondition()) > Threshold) {
234 // FIXME: this should estimate growth by the amount of code shared by the
235 // resultant unswitched loops. This should have no code growth:
236 // for () { if (iv) {...} }
237 // as one copy of the loop will be empty.
239 DEBUG(std::cerr << "NOT unswitching loop %"
240 << L->getHeader()->getName() << ", cost too high: "
241 << L->getBlocks().size() << "\n");
245 // If this loop has live-out values, we can't unswitch it. We need something
246 // like loop-closed SSA form in order to know how to insert PHI nodes for
248 if (LoopValuesUsedOutsideLoop(L)) {
249 DEBUG(std::cerr << "NOT unswitching loop %"
250 << L->getHeader()->getName()
251 << ", a loop value is used outside loop!\n");
255 //std::cerr << "BEFORE:\n"; LI->dump();
256 Loop *NewLoop1 = 0, *NewLoop2 = 0;
258 // If this is a trivial condition to unswitch (which results in no code
259 // duplication), do it now.
260 if (ConstantBool *V = IsTrivialUnswitchCondition(L, BI->getCondition())) {
261 UnswitchTrivialCondition(L, BI->getCondition(), V);
264 VersionLoop(BI->getCondition(), L, NewLoop1, NewLoop2);
267 //std::cerr << "AFTER:\n"; LI->dump();
269 // Try to unswitch each of our new loops now!
270 if (NewLoop1) visitLoop(NewLoop1);
271 if (NewLoop2) visitLoop(NewLoop2);
278 /// SplitBlock - Split the specified basic block into two pieces. If SplitAtTop
279 /// is false, this splits the block so the second half only has an unconditional
280 /// branch. If SplitAtTop is true, it makes it so the first half of the block
281 /// only has an unconditional branch in it.
283 /// This method updates the LoopInfo for this function to correctly reflect the
284 /// CFG changes made.
286 /// This routine returns the new basic block that was inserted, which is always
287 /// the later part of the block.
288 BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *BB, bool SplitAtTop) {
289 BasicBlock::iterator SplitPoint;
291 SplitPoint = BB->getTerminator();
293 SplitPoint = BB->begin();
294 while (isa<PHINode>(SplitPoint)) ++SplitPoint;
297 BasicBlock *New = BB->splitBasicBlock(SplitPoint, BB->getName()+".tail");
298 // New now lives in whichever loop that BB used to.
299 if (Loop *L = LI->getLoopFor(BB))
300 L->addBasicBlockToLoop(New, *LI);
305 // RemapInstruction - Convert the instruction operands from referencing the
306 // current values into those specified by ValueMap.
308 static inline void RemapInstruction(Instruction *I,
309 std::map<const Value *, Value*> &ValueMap) {
310 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
311 Value *Op = I->getOperand(op);
312 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
313 if (It != ValueMap.end()) Op = It->second;
314 I->setOperand(op, Op);
318 /// CloneLoop - Recursively clone the specified loop and all of its children,
319 /// mapping the blocks with the specified map.
320 static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
322 Loop *New = new Loop();
325 PL->addChildLoop(New);
327 LI->addTopLevelLoop(New);
329 // Add all of the blocks in L to the new loop.
330 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
332 if (LI->getLoopFor(*I) == L)
333 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
335 // Add all of the subloops to the new loop.
336 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
337 CloneLoop(*I, New, VM, LI);
342 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
343 /// condition in it (a cond branch from its header block to its latch block,
344 /// where the path through the loop that doesn't execute its body has no
345 /// side-effects), unswitch it. This doesn't involve any code duplication, just
346 /// moving the conditional branch outside of the loop and updating loop info.
347 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
348 ConstantBool *LoopCond) {
349 DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %"
350 << L->getHeader()->getName() << " [" << L->getBlocks().size()
351 << " blocks] in Function " << L->getHeader()->getParent()->getName()
352 << " on cond:" << *Cond << "\n");
354 // First step, split the preahder, so that we know that there is a safe place
355 // to insert the conditional branch. We will change 'OrigPH' to have a
356 // conditional branch on Cond.
357 BasicBlock *OrigPH = L->getLoopPreheader();
358 BasicBlock *NewPH = SplitBlock(OrigPH, false);
360 // Now that we have a place to insert the conditional branch, create a place
361 // to branch to: this is the non-header successor of the latch block.
362 BranchInst *LatchBranch =cast<BranchInst>(L->getLoopLatch()->getTerminator());
363 BasicBlock *ExitBlock =
364 LatchBranch->getSuccessor(LatchBranch->getSuccessor(0) == L->getHeader());
365 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
367 // Split this block now, so that the loop maintains its exit block.
368 BasicBlock *NewExit = SplitBlock(ExitBlock, true);
370 // Okay, now we have a position to branch from and a position to branch to,
371 // insert the new conditional branch.
372 bool EnterOnTrue = LoopCond->getValue();
373 new BranchInst(EnterOnTrue ? NewPH : NewExit, EnterOnTrue ? NewExit : NewPH,
374 Cond, OrigPH->getTerminator());
375 OrigPH->getTerminator()->eraseFromParent();
377 // Now that we know that the loop is never entered when this condition is a
378 // particular value, rewrite the loop with this info. We know that this will
379 // at least eliminate the old branch.
380 RewriteLoopBodyWithConditionConstant(L, Cond, EnterOnTrue);
386 /// VersionLoop - We determined that the loop is profitable to unswitch and
387 /// contains a branch on a loop invariant condition. Split it into loop
388 /// versions and test the condition outside of either loop. Return the loops
389 /// created as Out1/Out2.
390 void LoopUnswitch::VersionLoop(Value *LIC, Loop *L, Loop *&Out1, Loop *&Out2) {
391 Function *F = L->getHeader()->getParent();
393 DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
394 << L->getHeader()->getName() << " [" << L->getBlocks().size()
395 << " blocks] in Function " << F->getName()
396 << " on cond:" << *LIC << "\n");
398 std::vector<BasicBlock*> LoopBlocks;
400 // First step, split the preheader and exit blocks, and add these blocks to
401 // the LoopBlocks list.
402 BasicBlock *OrigPreheader = L->getLoopPreheader();
403 LoopBlocks.push_back(SplitBlock(OrigPreheader, false));
405 // We want the loop to come after the preheader, but before the exit blocks.
406 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
408 std::vector<BasicBlock*> ExitBlocks;
409 L->getExitBlocks(ExitBlocks);
410 std::sort(ExitBlocks.begin(), ExitBlocks.end());
411 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
413 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
414 SplitBlock(ExitBlocks[i], true);
415 LoopBlocks.push_back(ExitBlocks[i]);
418 // Next step, clone all of the basic blocks that make up the loop (including
419 // the loop preheader and exit blocks), keeping track of the mapping between
420 // the instructions and blocks.
421 std::vector<BasicBlock*> NewBlocks;
422 NewBlocks.reserve(LoopBlocks.size());
423 std::map<const Value*, Value*> ValueMap;
424 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
425 NewBlocks.push_back(CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F));
426 ValueMap[LoopBlocks[i]] = NewBlocks.back(); // Keep the BB mapping.
429 // Splice the newly inserted blocks into the function right before the
430 // original preheader.
431 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
432 NewBlocks[0], F->end());
434 // Now we create the new Loop object for the versioned loop.
435 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
436 if (Loop *Parent = L->getParentLoop()) {
437 // Make sure to add the cloned preheader and exit blocks to the parent loop
439 Parent->addBasicBlockToLoop(NewBlocks[0], *LI);
440 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
441 Parent->addBasicBlockToLoop(cast<BasicBlock>(ValueMap[ExitBlocks[i]]),
445 // Rewrite the code to refer to itself.
446 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
447 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
448 E = NewBlocks[i]->end(); I != E; ++I)
449 RemapInstruction(I, ValueMap);
451 // Rewrite the original preheader to select between versions of the loop.
452 assert(isa<BranchInst>(OrigPreheader->getTerminator()) &&
453 cast<BranchInst>(OrigPreheader->getTerminator())->isUnconditional() &&
454 OrigPreheader->getTerminator()->getSuccessor(0) == LoopBlocks[0] &&
455 "Preheader splitting did not work correctly!");
456 // Remove the unconditional branch to LoopBlocks[0].
457 OrigPreheader->getInstList().pop_back();
459 // Insert a conditional branch on LIC to the two preheaders. The original
460 // code is the true version and the new code is the false version.
461 new BranchInst(LoopBlocks[0], NewBlocks[0], LIC, OrigPreheader);
463 // Now we rewrite the original code to know that the condition is true and the
464 // new code to know that the condition is false.
465 RewriteLoopBodyWithConditionConstant(L, LIC, true);
466 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, false);
472 // RewriteLoopBodyWithConditionConstant - We know that the boolean value LIC has
473 // the value specified by Val in the specified loop. Rewrite any uses of LIC or
474 // of properties correlated to it.
475 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
477 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
478 // FIXME: Support correlated properties, like:
484 ConstantBool *BoolVal = ConstantBool::get(Val);
486 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
487 for (unsigned i = 0, e = Users.size(); i != e; ++i)
488 if (Instruction *U = cast<Instruction>(Users[i]))
489 if (L->contains(U->getParent()))
490 U->replaceUsesOfWith(LIC, BoolVal);