1 //===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
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 performs loop invariant code motion, attempting to remove as much
11 // code from the body of a loop as possible. It does this by either hoisting
12 // code into the preheader block, or by sinking code to the exit blocks if it is
13 // safe. This pass also promotes must-aliased memory locations in the loop to
16 // This pass uses alias analysis for two purposes:
18 // 1. Moving loop invariant loads out of loops. If we can determine that a
19 // load inside of a loop never aliases anything stored to, we can hoist it
20 // or sink it like any other instruction.
21 // 2. Scalar Promotion of Memory - If there is a store instruction inside of
22 // the loop, we try to move the store to happen AFTER the loop instead of
23 // inside of the loop. This can only happen if a few conditions are true:
24 // A. The pointer stored through is loop invariant
25 // B. There are no stores or loads in the loop which _may_ alias the
26 // pointer. There are no calls in the loop which mod/ref the pointer.
27 // If these conditions are true, we can promote the loads and stores in the
28 // loop of the pointer to use a temporary alloca'd variable. We then use
29 // the mem2reg functionality to construct the appropriate SSA form for the
32 //===----------------------------------------------------------------------===//
34 #include "llvm/Transforms/Scalar.h"
35 #include "llvm/Transforms/Utils/PromoteMemToReg.h"
36 #include "llvm/Transforms/Utils/Local.h"
37 #include "llvm/Analysis/LoopInfo.h"
38 #include "llvm/Analysis/AliasAnalysis.h"
39 #include "llvm/Analysis/AliasSetTracker.h"
40 #include "llvm/Analysis/Dominators.h"
41 #include "llvm/Instructions.h"
42 #include "llvm/DerivedTypes.h"
43 #include "llvm/Target/TargetData.h"
44 #include "llvm/Support/CFG.h"
45 #include "Support/CommandLine.h"
46 #include "Support/Debug.h"
47 #include "Support/Statistic.h"
48 #include "llvm/Assembly/Writer.h"
54 DisablePromotion("disable-licm-promotion", cl::Hidden,
55 cl::desc("Disable memory promotion in LICM pass"));
57 Statistic<> NumHoisted("licm", "Number of instructions hoisted out of loop");
58 Statistic<> NumHoistedLoads("licm", "Number of load insts hoisted");
59 Statistic<> NumPromoted("licm",
60 "Number of memory locations promoted to registers");
62 struct LICM : public FunctionPass {
63 virtual bool runOnFunction(Function &F);
65 /// This transformation requires natural loop information & requires that
66 /// loop preheaders be inserted into the CFG...
68 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
70 AU.addRequiredID(LoopSimplifyID);
71 AU.addRequired<LoopInfo>();
72 AU.addRequired<DominatorTree>();
73 AU.addRequired<DominanceFrontier>(); // For scalar promotion (mem2reg)
74 AU.addRequired<AliasAnalysis>();
78 // Various analyses that we use...
79 AliasAnalysis *AA; // Current AliasAnalysis information
80 LoopInfo *LI; // Current LoopInfo
81 DominatorTree *DT; // Dominator Tree for the current Loop...
82 DominanceFrontier *DF; // Current Dominance Frontier
84 // State that is updated as we process loops
85 bool Changed; // Set to true when we change anything.
86 BasicBlock *Preheader; // The preheader block of the current loop...
87 Loop *CurLoop; // The current loop we are working on...
88 AliasSetTracker *CurAST; // AliasSet information for the current loop...
90 /// visitLoop - Hoist expressions out of the specified loop...
92 void visitLoop(Loop *L, AliasSetTracker &AST);
94 /// HoistRegion - Walk the specified region of the CFG (defined by all
95 /// blocks dominated by the specified block, and that are in the current
96 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
97 /// visit definitions before uses, allowing us to hoist a loop body in one
98 /// pass without iteration.
100 void HoistRegion(DominatorTree::Node *N);
102 /// inSubLoop - Little predicate that returns true if the specified basic
103 /// block is in a subloop of the current one, not the current one itself.
105 bool inSubLoop(BasicBlock *BB) {
106 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
107 for (unsigned i = 0, e = CurLoop->getSubLoops().size(); i != e; ++i)
108 if (CurLoop->getSubLoops()[i]->contains(BB))
109 return true; // A subloop actually contains this block!
113 /// hoist - When an instruction is found to only use loop invariant operands
114 /// that is safe to hoist, this instruction is called to do the dirty work.
116 void hoist(Instruction &I);
118 /// SafeToHoist - Only hoist an instruction if it is not a trapping
119 /// instruction or if it is a trapping instruction and is guaranteed to
122 bool SafeToHoist(Instruction &I);
124 /// pointerInvalidatedByLoop - Return true if the body of this loop may
125 /// store into the memory location pointed to by V.
127 bool pointerInvalidatedByLoop(Value *V) {
128 // Check to see if any of the basic blocks in CurLoop invalidate *V.
129 return CurAST->getAliasSetForPointer(V, 0).isMod();
132 /// isLoopInvariant - Return true if the specified value is loop invariant
134 inline bool isLoopInvariant(Value *V) {
135 if (Instruction *I = dyn_cast<Instruction>(V))
136 return !CurLoop->contains(I->getParent());
137 return true; // All non-instructions are loop invariant
139 bool isLoopInvariantInst(Instruction &Inst);
141 /// PromoteValuesInLoop - Look at the stores in the loop and promote as many
142 /// to scalars as we can.
144 void PromoteValuesInLoop();
146 /// findPromotableValuesInLoop - Check the current loop for stores to
147 /// definite pointers, which are not loaded and stored through may aliases.
148 /// If these are found, create an alloca for the value, add it to the
149 /// PromotedValues list, and keep track of the mapping from value to
152 void findPromotableValuesInLoop(
153 std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
154 std::map<Value*, AllocaInst*> &Val2AlMap);
157 RegisterOpt<LICM> X("licm", "Loop Invariant Code Motion");
160 FunctionPass *llvm::createLICMPass() { return new LICM(); }
162 /// runOnFunction - For LICM, this simply traverses the loop structure of the
163 /// function, hoisting expressions out of loops if possible.
165 bool LICM::runOnFunction(Function &) {
168 // Get our Loop and Alias Analysis information...
169 LI = &getAnalysis<LoopInfo>();
170 AA = &getAnalysis<AliasAnalysis>();
171 DF = &getAnalysis<DominanceFrontier>();
172 DT = &getAnalysis<DominatorTree>();
174 // Hoist expressions out of all of the top-level loops.
175 const std::vector<Loop*> &TopLevelLoops = LI->getTopLevelLoops();
176 for (std::vector<Loop*>::const_iterator I = TopLevelLoops.begin(),
177 E = TopLevelLoops.end(); I != E; ++I) {
178 AliasSetTracker AST(*AA);
185 /// visitLoop - Hoist expressions out of the specified loop...
187 void LICM::visitLoop(Loop *L, AliasSetTracker &AST) {
188 // Recurse through all subloops before we process this loop...
189 for (std::vector<Loop*>::const_iterator I = L->getSubLoops().begin(),
190 E = L->getSubLoops().end(); I != E; ++I) {
191 AliasSetTracker SubAST(*AA);
192 visitLoop(*I, SubAST);
194 // Incorporate information about the subloops into this loop...
200 // Get the preheader block to move instructions into...
201 Preheader = L->getLoopPreheader();
202 assert(Preheader&&"Preheader insertion pass guarantees we have a preheader!");
204 // Loop over the body of this loop, looking for calls, invokes, and stores.
205 // Because subloops have already been incorporated into AST, we skip blocks in
208 const std::vector<BasicBlock*> &LoopBBs = L->getBlocks();
209 for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(),
210 E = LoopBBs.end(); I != E; ++I)
211 if (LI->getLoopFor(*I) == L) // Ignore blocks in subloops...
212 AST.add(**I); // Incorporate the specified basic block
214 // We want to visit all of the instructions in this loop... that are not parts
215 // of our subloops (they have already had their invariants hoisted out of
216 // their loop, into this loop, so there is no need to process the BODIES of
219 // Traverse the body of the loop in depth first order on the dominator tree so
220 // that we are guaranteed to see definitions before we see uses. This allows
221 // us to perform the LICM transformation in one pass, without iteration.
223 HoistRegion(DT->getNode(L->getHeader()));
225 // Now that all loop invariants have been removed from the loop, promote any
226 // memory references to scalars that we can...
227 if (!DisablePromotion)
228 PromoteValuesInLoop();
230 // Clear out loops state information for the next iteration
235 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
236 /// dominated by the specified block, and that are in the current loop) in depth
237 /// first order w.r.t the DominatorTree. This allows us to visit definitions
238 /// before uses, allowing us to hoist a loop body in one pass without iteration.
240 void LICM::HoistRegion(DominatorTree::Node *N) {
241 assert(N != 0 && "Null dominator tree node?");
242 BasicBlock *BB = N->getBlock();
244 // If this subregion is not in the top level loop at all, exit.
245 if (!CurLoop->contains(BB)) return;
247 // Only need to hoist the contents of this block if it is not part of a
248 // subloop (which would already have been hoisted)
250 for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; ) {
251 Instruction &Inst = *I++;
252 if (isLoopInvariantInst(Inst) && SafeToHoist(Inst))
256 const std::vector<DominatorTree::Node*> &Children = N->getChildren();
257 for (unsigned i = 0, e = Children.size(); i != e; ++i)
258 HoistRegion(Children[i]);
261 bool LICM::isLoopInvariantInst(Instruction &I) {
262 assert(!isa<TerminatorInst>(I) && "Can't hoist terminator instructions!");
264 // We can only hoist simple expressions...
265 if (!isa<BinaryOperator>(I) && !isa<ShiftInst>(I) && !isa<LoadInst>(I) &&
266 !isa<GetElementPtrInst>(I) && !isa<CastInst>(I) && !isa<VANextInst>(I) &&
270 // The instruction is loop invariant if all of its operands are loop-invariant
271 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
272 if (!isLoopInvariant(I.getOperand(i)))
275 // Loads have extra constraints we have to verify before we can hoist them.
276 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
277 if (LI->isVolatile())
278 return false; // Don't hoist volatile loads!
280 // Don't hoist loads which have may-aliased stores in loop.
281 if (pointerInvalidatedByLoop(I.getOperand(0)))
285 // If we got this far, the instruction is loop invariant!
290 /// hoist - When an instruction is found to only use loop invariant operands
291 /// that is safe to hoist, this instruction is called to do the dirty work.
293 void LICM::hoist(Instruction &Inst) {
294 DEBUG(std::cerr << "LICM hoisting to";
295 WriteAsOperand(std::cerr, Preheader, false);
296 std::cerr << ": " << Inst);
298 if (isa<LoadInst>(Inst))
301 // Remove the instruction from its current basic block... but don't delete the
303 Inst.getParent()->getInstList().remove(&Inst);
305 // Insert the new node in Preheader, before the terminator.
306 Preheader->getInstList().insert(Preheader->getTerminator(), &Inst);
312 /// SafeToHoist - Only hoist an instruction if it is not a trapping instruction
313 /// or if it is a trapping instruction and is guaranteed to execute
315 bool LICM::SafeToHoist(Instruction &Inst) {
316 // If it is not a trapping instruction, it is always safe to hoist.
317 if (!Inst.isTrapping()) return true;
319 // Otherwise we have to check to make sure that the instruction dominates all
320 // of the exit blocks. If it doesn't, then there is a path out of the loop
321 // which does not execute this instruction, so we can't hoist it.
323 // If the instruction is in the header block for the loop (which is very
324 // common), it is always guaranteed to dominate the exit blocks. Since this
325 // is a common case, and can save some work, check it now.
326 BasicBlock *LoopHeader = CurLoop->getHeader();
327 if (Inst.getParent() == LoopHeader)
330 // Get the Dominator Tree Node for the instruction's basic block.
331 DominatorTree::Node *InstDTNode = DT->getNode(Inst.getParent());
333 // Get the exit blocks for the current loop.
334 const std::vector<BasicBlock* > &ExitBlocks = CurLoop->getExitBlocks();
336 // For each exit block, get the DT node and walk up the DT until the
337 // instruction's basic block is found or we exit the loop.
338 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
339 DominatorTree::Node *IDom = DT->getNode(ExitBlocks[i]);
342 // Get next Immediate Dominator.
343 IDom = IDom->getIDom();
345 // If we have got to the header of the loop, then the instructions block
346 // did not dominate the exit node, so we can't hoist it.
347 if (IDom->getBlock() == LoopHeader)
350 } while(IDom != InstDTNode);
357 /// PromoteValuesInLoop - Try to promote memory values to scalars by sinking
358 /// stores out of the loop and moving loads to before the loop. We do this by
359 /// looping over the stores in the loop, looking for stores to Must pointers
360 /// which are loop invariant. We promote these memory locations to use allocas
361 /// instead. These allocas can easily be raised to register values by the
362 /// PromoteMem2Reg functionality.
364 void LICM::PromoteValuesInLoop() {
365 // PromotedValues - List of values that are promoted out of the loop. Each
366 // value has an alloca instruction for it, and a canonical version of the
368 std::vector<std::pair<AllocaInst*, Value*> > PromotedValues;
369 std::map<Value*, AllocaInst*> ValueToAllocaMap; // Map of ptr to alloca
371 findPromotableValuesInLoop(PromotedValues, ValueToAllocaMap);
372 if (ValueToAllocaMap.empty()) return; // If there are values to promote...
375 NumPromoted += PromotedValues.size();
377 // Emit a copy from the value into the alloca'd value in the loop preheader
378 TerminatorInst *LoopPredInst = Preheader->getTerminator();
379 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
380 // Load from the memory we are promoting...
381 LoadInst *LI = new LoadInst(PromotedValues[i].second,
382 PromotedValues[i].second->getName()+".promoted",
384 // Store into the temporary alloca...
385 new StoreInst(LI, PromotedValues[i].first, LoopPredInst);
388 // Scan the basic blocks in the loop, replacing uses of our pointers with
389 // uses of the allocas in question. If we find a branch that exits the
390 // loop, make sure to put reload code into all of the successors of the
393 const std::vector<BasicBlock*> &LoopBBs = CurLoop->getBlocks();
394 for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(),
395 E = LoopBBs.end(); I != E; ++I) {
396 // Rewrite all loads and stores in the block of the pointer...
397 for (BasicBlock::iterator II = (*I)->begin(), E = (*I)->end();
399 if (LoadInst *L = dyn_cast<LoadInst>(II)) {
400 std::map<Value*, AllocaInst*>::iterator
401 I = ValueToAllocaMap.find(L->getOperand(0));
402 if (I != ValueToAllocaMap.end())
403 L->setOperand(0, I->second); // Rewrite load instruction...
404 } else if (StoreInst *S = dyn_cast<StoreInst>(II)) {
405 std::map<Value*, AllocaInst*>::iterator
406 I = ValueToAllocaMap.find(S->getOperand(1));
407 if (I != ValueToAllocaMap.end())
408 S->setOperand(1, I->second); // Rewrite store instruction...
412 // Check to see if any successors of this block are outside of the loop.
413 // If so, we need to copy the value from the alloca back into the memory
416 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
417 if (!CurLoop->contains(*SI)) {
418 // Copy all of the allocas into their memory locations...
419 BasicBlock::iterator BI = (*SI)->begin();
420 while (isa<PHINode>(*BI))
421 ++BI; // Skip over all of the phi nodes in the block...
422 Instruction *InsertPos = BI;
423 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
424 // Load from the alloca...
425 LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
426 // Store into the memory we promoted...
427 new StoreInst(LI, PromotedValues[i].second, InsertPos);
432 // Now that we have done the deed, use the mem2reg functionality to promote
433 // all of the new allocas we just created into real SSA registers...
435 std::vector<AllocaInst*> PromotedAllocas;
436 PromotedAllocas.reserve(PromotedValues.size());
437 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i)
438 PromotedAllocas.push_back(PromotedValues[i].first);
439 PromoteMemToReg(PromotedAllocas, *DT, *DF, AA->getTargetData());
442 /// findPromotableValuesInLoop - Check the current loop for stores to definite
443 /// pointers, which are not loaded and stored through may aliases. If these are
444 /// found, create an alloca for the value, add it to the PromotedValues list,
445 /// and keep track of the mapping from value to alloca...
447 void LICM::findPromotableValuesInLoop(
448 std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
449 std::map<Value*, AllocaInst*> &ValueToAllocaMap) {
450 Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin();
452 // Loop over all of the alias sets in the tracker object...
453 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
456 // We can promote this alias set if it has a store, if it is a "Must" alias
457 // set, and if the pointer is loop invariant.
458 if (!AS.isForwardingAliasSet() && AS.isMod() && AS.isMustAlias() &&
459 isLoopInvariant(AS.begin()->first)) {
460 assert(AS.begin() != AS.end() &&
461 "Must alias set should have at least one pointer element in it!");
462 Value *V = AS.begin()->first;
464 // Check that all of the pointers in the alias set have the same type. We
465 // cannot (yet) promote a memory location that is loaded and stored in
467 bool PointerOk = true;
468 for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
469 if (V->getType() != I->first->getType()) {
475 const Type *Ty = cast<PointerType>(V->getType())->getElementType();
476 AllocaInst *AI = new AllocaInst(Ty, 0, V->getName()+".tmp", FnStart);
477 PromotedValues.push_back(std::make_pair(AI, V));
479 for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
480 ValueToAllocaMap.insert(std::make_pair(I->first, AI));
482 DEBUG(std::cerr << "LICM: Promoting value: " << *V << "\n");