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/Support/Debug.h"
38 #include "llvm/ADT/Statistic.h"
45 Statistic<> NumUnswitched("loop-unswitch", "Number of loops unswitched");
47 class LoopUnswitch : public FunctionPass {
48 LoopInfo *LI; // Loop information
50 virtual bool runOnFunction(Function &F);
51 bool visitLoop(Loop *L);
53 /// This transformation requires natural loop information & requires that
54 /// loop preheaders be inserted into the CFG...
56 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
57 AU.addRequiredID(LoopSimplifyID);
58 AU.addRequired<LoopInfo>();
59 AU.addPreserved<LoopInfo>();
63 void VersionLoop(Value *LIC, Loop *L);
64 BasicBlock *SplitBlock(BasicBlock *BB, bool SplitAtTop);
65 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, bool Val);
67 RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
70 FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }
72 bool LoopUnswitch::runOnFunction(Function &F) {
74 LI = &getAnalysis<LoopInfo>();
76 // Transform all the top-level loops. Copy the loop list so that the child
77 // can update the loop tree if it needs to delete the loop.
78 std::vector<Loop*> SubLoops(LI->begin(), LI->end());
79 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
80 Changed |= visitLoop(SubLoops[i]);
86 /// InsertPHINodesForUsesOutsideLoop - If this instruction is used outside of
87 /// the specified loop, insert a PHI node in the appropriate exit block to merge
88 /// the values in the two different loop versions.
90 /// Most values are not used outside of the loop they are defined in, so be
91 /// efficient for this case.
93 static bool LoopValuesUsedOutsideLoop(Loop *L) {
94 // We will be doing lots of "loop contains block" queries. Loop::contains is
95 // linear time, use a set to speed this up.
96 std::set<BasicBlock*> LoopBlocks;
98 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
100 LoopBlocks.insert(*BB);
102 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
104 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
105 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
107 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
108 if (!LoopBlocks.count(UserBB))
115 bool LoopUnswitch::visitLoop(Loop *L) {
116 bool Changed = false;
118 // Recurse through all subloops before we process this loop. Copy the loop
119 // list so that the child can update the loop tree if it needs to delete the
121 std::vector<Loop*> SubLoops(L->begin(), L->end());
122 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
123 Changed |= visitLoop(SubLoops[i]);
125 // Loop over all of the basic blocks in the loop. If we find an interior
126 // block that is branching on a loop-invariant condition, we can unswitch this
128 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
130 TerminatorInst *TI = (*I)->getTerminator();
131 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
132 if (!isa<Constant>(SI) && L->isLoopInvariant(SI->getCondition()))
133 DEBUG(std::cerr << "TODO: Implement unswitching 'switch' loop %"
134 << L->getHeader()->getName() << ", cost = "
135 << L->getBlocks().size() << "\n" << **I);
139 BranchInst *BI = dyn_cast<BranchInst>(TI);
142 // If this isn't branching on an invariant condition, we can't unswitch it.
143 if (!BI->isConditional() || isa<Constant>(BI->getCondition()) ||
144 !L->isLoopInvariant(BI->getCondition()))
147 // Check to see if it would be profitable to unswitch this loop.
148 if (L->getBlocks().size() > 10) {
149 // FIXME: this should estimate growth by the amount of code shared by the
150 // resultant unswitched loops. This should have no code growth:
151 // for () { if (iv) {...} }
152 // as one copy of the loop will be empty.
154 DEBUG(std::cerr << "NOT unswitching loop %"
155 << L->getHeader()->getName() << ", cost too high: "
156 << L->getBlocks().size() << "\n");
160 // If this loop has live-out values, we can't unswitch it. We need something
161 // like loop-closed SSA form in order to know how to insert PHI nodes for
163 if (LoopValuesUsedOutsideLoop(L)) {
164 DEBUG(std::cerr << "NOT unswitching loop %"
165 << L->getHeader()->getName()
166 << ", a loop value is used outside loop!\n");
170 //std::cerr << "BEFORE:\n"; LI->dump();
171 VersionLoop(BI->getCondition(), L);
172 //std::cerr << "AFTER:\n"; LI->dump();
174 // FIXME: Why return here? What if we have:
175 // "for () { if (iv1) { if (iv2) { } } }" ?
182 /// SplitBlock - Split the specified basic block into two pieces. If SplitAtTop
183 /// is false, this splits the block so the second half only has an unconditional
184 /// branch. If SplitAtTop is true, it makes it so the first half of the block
185 /// only has an unconditional branch in it.
187 /// This method updates the LoopInfo for this function to correctly reflect the
188 /// CFG changes made.
189 BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *BB, bool SplitAtTop) {
190 BasicBlock::iterator SplitPoint;
192 SplitPoint = BB->getTerminator();
194 SplitPoint = BB->begin();
195 while (isa<PHINode>(SplitPoint)) ++SplitPoint;
198 BasicBlock *New = BB->splitBasicBlock(SplitPoint, BB->getName()+".tail");
199 // New now lives in whichever loop that BB used to.
200 if (Loop *L = LI->getLoopFor(BB))
201 L->addBasicBlockToLoop(New, *LI);
202 return SplitAtTop ? BB : New;
206 // RemapInstruction - Convert the instruction operands from referencing the
207 // current values into those specified by ValueMap.
209 static inline void RemapInstruction(Instruction *I,
210 std::map<const Value *, Value*> &ValueMap) {
211 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
212 Value *Op = I->getOperand(op);
213 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
214 if (It != ValueMap.end()) Op = It->second;
215 I->setOperand(op, Op);
219 /// CloneLoop - Recursively clone the specified loop and all of its children,
220 /// mapping the blocks with the specified map.
221 static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
223 Loop *New = new Loop();
226 PL->addChildLoop(New);
228 LI->addTopLevelLoop(New);
230 // Add all of the blocks in L to the new loop.
231 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
233 if (LI->getLoopFor(*I) == L)
234 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
236 // Add all of the subloops to the new loop.
237 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
238 CloneLoop(*I, New, VM, LI);
244 /// VersionLoop - We determined that the loop is profitable to unswitch and
245 /// contains a branch on a loop invariant condition. Split it into loop
246 /// versions and test the condition outside of either loop.
247 void LoopUnswitch::VersionLoop(Value *LIC, Loop *L) {
248 Function *F = L->getHeader()->getParent();
250 DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
251 << L->getHeader()->getName() << " [" << L->getBlocks().size()
252 << " blocks] in Function " << F->getName()
253 << " on cond:" << *LIC << "\n");
255 std::vector<BasicBlock*> LoopBlocks;
257 // First step, split the preheader and exit blocks, and add these blocks to
258 // the LoopBlocks list.
259 BasicBlock *OrigPreheader = L->getLoopPreheader();
260 LoopBlocks.push_back(SplitBlock(OrigPreheader, false));
262 // We want the loop to come after the preheader, but before the exit blocks.
263 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
265 std::vector<BasicBlock*> ExitBlocks;
266 L->getExitBlocks(ExitBlocks);
267 std::sort(ExitBlocks.begin(), ExitBlocks.end());
268 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
270 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
271 LoopBlocks.push_back(ExitBlocks[i] = SplitBlock(ExitBlocks[i], true));
273 // Next step, clone all of the basic blocks that make up the loop (including
274 // the loop preheader and exit blocks), keeping track of the mapping between
275 // the instructions and blocks.
276 std::vector<BasicBlock*> NewBlocks;
277 NewBlocks.reserve(LoopBlocks.size());
278 std::map<const Value*, Value*> ValueMap;
279 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
280 NewBlocks.push_back(CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F));
281 ValueMap[LoopBlocks[i]] = NewBlocks.back(); // Keep the BB mapping.
284 // Splice the newly inserted blocks into the function right before the
285 // original preheader.
286 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
287 NewBlocks[0], F->end());
289 // Now we create the new Loop object for the versioned loop.
290 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
291 if (Loop *Parent = L->getParentLoop()) {
292 // Make sure to add the cloned preheader and exit blocks to the parent loop
294 Parent->addBasicBlockToLoop(NewBlocks[0], *LI);
295 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
296 Parent->addBasicBlockToLoop(cast<BasicBlock>(ValueMap[ExitBlocks[i]]),
300 // Rewrite the code to refer to itself.
301 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
302 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
303 E = NewBlocks[i]->end(); I != E; ++I)
304 RemapInstruction(I, ValueMap);
306 // Rewrite the original preheader to select between versions of the loop.
307 assert(isa<BranchInst>(OrigPreheader->getTerminator()) &&
308 cast<BranchInst>(OrigPreheader->getTerminator())->isUnconditional() &&
309 OrigPreheader->getTerminator()->getSuccessor(0) == LoopBlocks[0] &&
310 "Preheader splitting did not work correctly!");
311 // Remove the unconditional branch to LoopBlocks[0].
312 OrigPreheader->getInstList().pop_back();
314 // Insert a conditional branch on LIC to the two preheaders. The original
315 // code is the true version and the new code is the false version.
316 new BranchInst(LoopBlocks[0], NewBlocks[0], LIC, OrigPreheader);
318 // Now we rewrite the original code to know that the condition is true and the
319 // new code to know that the condition is false.
320 RewriteLoopBodyWithConditionConstant(L, LIC, true);
321 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, false);
324 // Try to unswitch each of our new loops now!
329 // RewriteLoopBodyWithConditionConstant - We know that the boolean value LIC has
330 // the value specified by Val in the specified loop. Rewrite any uses of LIC or
331 // of properties correlated to it.
332 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
334 // FIXME: Support correlated properties, like:
340 ConstantBool *BoolVal = ConstantBool::get(Val);
342 std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
343 for (unsigned i = 0, e = Users.size(); i != e; ++i)
344 if (Instruction *U = dyn_cast<Instruction>(Users[i]))
345 if (L->contains(U->getParent()))
346 U->replaceUsesOfWith(LIC, BoolVal);