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 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
119 /// trivial: that is, that the condition controls whether or not the loop does
120 /// anything at all. If this is a trivial condition, unswitching produces no
121 /// code duplications (equivalently, it produces a simpler loop and a new empty
122 /// loop, which gets deleted).
124 /// If this is a trivial condition, return ConstantBool::True if the loop body
125 /// runs when the condition is true, False if the loop body executes when the
126 /// condition is false. Otherwise, return null to indicate a complex condition.
127 static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond,
128 bool *CondEntersLoop = 0,
129 BasicBlock **LoopExit = 0) {
130 BasicBlock *Header = L->getHeader();
131 BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator());
133 // If the header block doesn't end with a conditional branch on Cond, we can't
135 if (!HeaderTerm || !HeaderTerm->isConditional() ||
136 HeaderTerm->getCondition() != Cond)
139 // Check to see if the conditional branch goes to the latch block. If not,
140 // it's not trivial. This also determines the value of Cond that will execute
142 BasicBlock *Latch = L->getLoopLatch();
143 if (HeaderTerm->getSuccessor(1) == Latch) {
144 if (CondEntersLoop) *CondEntersLoop = true;
145 } else if (HeaderTerm->getSuccessor(0) == Latch)
146 if (CondEntersLoop) *CondEntersLoop = false;
148 return false; // Doesn't branch to latch block.
150 // The latch block must end with a conditional branch where one edge goes to
151 // the header (this much we know) and one edge goes OUT of the loop.
152 BranchInst *LatchBranch = dyn_cast<BranchInst>(Latch->getTerminator());
153 if (!LatchBranch || !LatchBranch->isConditional()) return false;
155 if (LatchBranch->getSuccessor(0) == Header) {
156 if (L->contains(LatchBranch->getSuccessor(1))) return false;
157 if (LoopExit) *LoopExit = LatchBranch->getSuccessor(1);
159 assert(LatchBranch->getSuccessor(1) == Header);
160 if (L->contains(LatchBranch->getSuccessor(0))) return false;
161 if (LoopExit) *LoopExit = LatchBranch->getSuccessor(0);
164 // We already know that nothing uses any scalar values defined inside of this
165 // loop. As such, we just have to check to see if this loop will execute any
166 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
167 // part of the loop that the code *would* execute.
168 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
169 if (I->mayWriteToMemory())
171 for (BasicBlock::iterator I = Latch->begin(), E = Latch->end(); I != E; ++I)
172 if (I->mayWriteToMemory())
177 /// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
178 /// we choose to unswitch the specified loop on the specified value.
180 unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
181 // If the condition is trivial, always unswitch. There is no code growth for
183 if (IsTrivialUnswitchCondition(L, LIC))
187 // FIXME: this is brain dead. It should take into consideration code
189 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
192 // Do not include empty blocks in the cost calculation. This happen due to
193 // loop canonicalization and will be removed.
194 if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
197 // Count basic blocks.
204 bool LoopUnswitch::visitLoop(Loop *L) {
205 bool Changed = false;
207 // Recurse through all subloops before we process this loop. Copy the loop
208 // list so that the child can update the loop tree if it needs to delete the
210 std::vector<Loop*> SubLoops(L->begin(), L->end());
211 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
212 Changed |= visitLoop(SubLoops[i]);
214 // Loop over all of the basic blocks in the loop. If we find an interior
215 // block that is branching on a loop-invariant condition, we can unswitch this
217 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
219 TerminatorInst *TI = (*I)->getTerminator();
220 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
221 if (!isa<Constant>(SI) && L->isLoopInvariant(SI->getCondition()))
222 DEBUG(std::cerr << "TODO: Implement unswitching 'switch' loop %"
223 << L->getHeader()->getName() << ", cost = "
224 << L->getBlocks().size() << "\n" << **I);
228 BranchInst *BI = dyn_cast<BranchInst>(TI);
231 // If this isn't branching on an invariant condition, we can't unswitch it.
232 if (!BI->isConditional() || isa<Constant>(BI->getCondition()) ||
233 !L->isLoopInvariant(BI->getCondition()))
236 // Check to see if it would be profitable to unswitch this loop.
237 if (getLoopUnswitchCost(L, BI->getCondition()) > Threshold) {
238 // FIXME: this should estimate growth by the amount of code shared by the
239 // resultant unswitched loops. This should have no code growth:
240 // for () { if (iv) {...} }
241 // as one copy of the loop will be empty.
243 DEBUG(std::cerr << "NOT unswitching loop %"
244 << L->getHeader()->getName() << ", cost too high: "
245 << L->getBlocks().size() << "\n");
249 // If this loop has live-out values, we can't unswitch it. We need something
250 // like loop-closed SSA form in order to know how to insert PHI nodes for
252 if (LoopValuesUsedOutsideLoop(L)) {
253 DEBUG(std::cerr << "NOT unswitching loop %"
254 << L->getHeader()->getName()
255 << ", a loop value is used outside loop!\n");
259 //std::cerr << "BEFORE:\n"; LI->dump();
260 Loop *NewLoop1 = 0, *NewLoop2 = 0;
262 // If this is a trivial condition to unswitch (which results in no code
263 // duplication), do it now.
264 bool EntersLoopOnCond;
265 BasicBlock *ExitBlock;
266 if (IsTrivialUnswitchCondition(L, BI->getCondition(), &EntersLoopOnCond,
268 UnswitchTrivialCondition(L, BI->getCondition(),
269 EntersLoopOnCond, ExitBlock);
272 VersionLoop(BI->getCondition(), L, NewLoop1, NewLoop2);
275 //std::cerr << "AFTER:\n"; LI->dump();
277 // Try to unswitch each of our new loops now!
278 if (NewLoop1) visitLoop(NewLoop1);
279 if (NewLoop2) visitLoop(NewLoop2);
286 /// SplitBlock - Split the specified basic block into two pieces. If SplitAtTop
287 /// is false, this splits the block so the second half only has an unconditional
288 /// branch. If SplitAtTop is true, it makes it so the first half of the block
289 /// only has an unconditional branch in it.
291 /// This method updates the LoopInfo for this function to correctly reflect the
292 /// CFG changes made.
294 /// This routine returns the new basic block that was inserted, which is always
295 /// the later part of the block.
296 BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *BB, bool SplitAtTop) {
297 BasicBlock::iterator SplitPoint;
299 SplitPoint = BB->getTerminator();
301 SplitPoint = BB->begin();
302 while (isa<PHINode>(SplitPoint)) ++SplitPoint;
305 BasicBlock *New = BB->splitBasicBlock(SplitPoint, BB->getName()+".tail");
306 // New now lives in whichever loop that BB used to.
307 if (Loop *L = LI->getLoopFor(BB))
308 L->addBasicBlockToLoop(New, *LI);
313 // RemapInstruction - Convert the instruction operands from referencing the
314 // current values into those specified by ValueMap.
316 static inline void RemapInstruction(Instruction *I,
317 std::map<const Value *, Value*> &ValueMap) {
318 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
319 Value *Op = I->getOperand(op);
320 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
321 if (It != ValueMap.end()) Op = It->second;
322 I->setOperand(op, Op);
326 /// CloneLoop - Recursively clone the specified loop and all of its children,
327 /// mapping the blocks with the specified map.
328 static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
330 Loop *New = new Loop();
333 PL->addChildLoop(New);
335 LI->addTopLevelLoop(New);
337 // Add all of the blocks in L to the new loop.
338 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
340 if (LI->getLoopFor(*I) == L)
341 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
343 // Add all of the subloops to the new loop.
344 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
345 CloneLoop(*I, New, VM, LI);
350 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
351 /// condition in it (a cond branch from its header block to its latch block,
352 /// where the path through the loop that doesn't execute its body has no
353 /// side-effects), unswitch it. This doesn't involve any code duplication, just
354 /// moving the conditional branch outside of the loop and updating loop info.
355 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
357 BasicBlock *ExitBlock) {
358 DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %"
359 << L->getHeader()->getName() << " [" << L->getBlocks().size()
360 << " blocks] in Function " << L->getHeader()->getParent()->getName()
361 << " on cond:" << *Cond << "\n");
363 // First step, split the preahder, so that we know that there is a safe place
364 // to insert the conditional branch. We will change 'OrigPH' to have a
365 // conditional branch on Cond.
366 BasicBlock *OrigPH = L->getLoopPreheader();
367 BasicBlock *NewPH = SplitBlock(OrigPH, false);
369 // Now that we have a place to insert the conditional branch, create a place
370 // to branch to: this is the exit block out of the loop that we should
373 // Split this block now, so that the loop maintains its exit block.
374 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
375 BasicBlock *NewExit = SplitBlock(ExitBlock, true);
377 // Okay, now we have a position to branch from and a position to branch to,
378 // insert the new conditional branch.
379 new BranchInst(EnterOnCond ? NewPH : NewExit, EnterOnCond ? NewExit : NewPH,
380 Cond, OrigPH->getTerminator());
381 OrigPH->getTerminator()->eraseFromParent();
383 // Now that we know that the loop is never entered when this condition is a
384 // particular value, rewrite the loop with this info. We know that this will
385 // at least eliminate the old branch.
386 RewriteLoopBodyWithConditionConstant(L, Cond, EnterOnCond);
392 /// VersionLoop - We determined that the loop is profitable to unswitch and
393 /// contains a branch on a loop invariant condition. Split it into loop
394 /// versions and test the condition outside of either loop. Return the loops
395 /// created as Out1/Out2.
396 void LoopUnswitch::VersionLoop(Value *LIC, Loop *L, Loop *&Out1, Loop *&Out2) {
397 Function *F = L->getHeader()->getParent();
399 DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
400 << L->getHeader()->getName() << " [" << L->getBlocks().size()
401 << " blocks] in Function " << F->getName()
402 << " on cond:" << *LIC << "\n");
404 std::vector<BasicBlock*> LoopBlocks;
406 // First step, split the preheader and exit blocks, and add these blocks to
407 // the LoopBlocks list.
408 BasicBlock *OrigPreheader = L->getLoopPreheader();
409 LoopBlocks.push_back(SplitBlock(OrigPreheader, false));
411 // We want the loop to come after the preheader, but before the exit blocks.
412 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
414 std::vector<BasicBlock*> ExitBlocks;
415 L->getExitBlocks(ExitBlocks);
416 std::sort(ExitBlocks.begin(), ExitBlocks.end());
417 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
419 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
420 SplitBlock(ExitBlocks[i], true);
421 LoopBlocks.push_back(ExitBlocks[i]);
424 // Next step, clone all of the basic blocks that make up the loop (including
425 // the loop preheader and exit blocks), keeping track of the mapping between
426 // the instructions and blocks.
427 std::vector<BasicBlock*> NewBlocks;
428 NewBlocks.reserve(LoopBlocks.size());
429 std::map<const Value*, Value*> ValueMap;
430 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
431 NewBlocks.push_back(CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F));
432 ValueMap[LoopBlocks[i]] = NewBlocks.back(); // Keep the BB mapping.
435 // Splice the newly inserted blocks into the function right before the
436 // original preheader.
437 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
438 NewBlocks[0], F->end());
440 // Now we create the new Loop object for the versioned loop.
441 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
442 if (Loop *Parent = L->getParentLoop()) {
443 // Make sure to add the cloned preheader and exit blocks to the parent loop
445 Parent->addBasicBlockToLoop(NewBlocks[0], *LI);
446 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
447 Parent->addBasicBlockToLoop(cast<BasicBlock>(ValueMap[ExitBlocks[i]]),
451 // Rewrite the code to refer to itself.
452 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
453 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
454 E = NewBlocks[i]->end(); I != E; ++I)
455 RemapInstruction(I, ValueMap);
457 // Rewrite the original preheader to select between versions of the loop.
458 assert(isa<BranchInst>(OrigPreheader->getTerminator()) &&
459 cast<BranchInst>(OrigPreheader->getTerminator())->isUnconditional() &&
460 OrigPreheader->getTerminator()->getSuccessor(0) == LoopBlocks[0] &&
461 "Preheader splitting did not work correctly!");
462 // Remove the unconditional branch to LoopBlocks[0].
463 OrigPreheader->getInstList().pop_back();
465 // Insert a conditional branch on LIC to the two preheaders. The original
466 // code is the true version and the new code is the false version.
467 new BranchInst(LoopBlocks[0], NewBlocks[0], LIC, OrigPreheader);
469 // Now we rewrite the original code to know that the condition is true and the
470 // new code to know that the condition is false.
471 RewriteLoopBodyWithConditionConstant(L, LIC, true);
472 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, false);
478 // RewriteLoopBodyWithConditionConstant - We know that the boolean value LIC has
479 // the value specified by Val in the specified loop. Rewrite any uses of LIC or
480 // of properties correlated to it.
481 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
483 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
484 // FIXME: Support correlated properties, like:
490 ConstantBool *BoolVal = ConstantBool::get(Val);
492 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
493 for (unsigned i = 0, e = Users.size(); i != e; ++i)
494 if (Instruction *U = cast<Instruction>(Users[i]))
495 if (L->contains(U->getParent()))
496 U->replaceUsesOfWith(LIC, BoolVal);